‘ CONTENTS Section Page SCOPE 4. . . REFERENCES 4. . . International Practices 4. . . Other Literature 4. . . BACKGROUND 5. . . DEFINITIONS 5. . . Charging Current 5. . . Equipment Grounding 5. . . Ground 5. . .
High Resistance Grounded System 5. . .
Low Resistance Grounded System 5. . .
Low Voltage System 5. . .
Solidly Grounded System 5. . .
System Grounding 6. . .
System Neutral 6. . .
Ungrounded System 6. . .
THE UNGROUNDED SYSTEM 6. . .
THE GROUNDED SYSTEM 7. . .
Neutral Point Grounding 7. . .
The Solidly Grounded System 7. . .
The High Resistance Grounded System 8. . .
Other System Grounding Methods 9. . .
SOLIDLY GROUNDED SYSTEM DESIGN 10. . .
Fault Current Magnitude – General 10. . .
Maximum (Bolted) Line-To-Ground Faults 11. . .
Adequate Ground-Fault Protection per IP 16-4-1 11. . .
Maximum Distance Tables – IP 16-4-1 11. . .
Clarifications on Use of Maximum Distance Tables 11. . .
When System Impedance Cannot be Neglected 12. . .
Impedance-Related Adjustments of Table Distances 12. . .
Examples of Percent Reductions in Maximum Distances 13. . .
Use of Tables 1A & 1B – Steel Conduit 13. . .
Use of Table 1A/1B Distance-Data – Without Interpolation or Extrapolation 14. . .
Interpolation of Table 1A/1B Distance Data 15. . .
Extrapolation of Table 1A/1B Distance Data 16. . .
Use of Tables 2A & 2B – Aluminum Conduit 16. . .
Use of Tables 3A, 3B, & 4 – Ground Return Cable or Wire 17. . .
CONTENTS (Cont)
Section Page
Methods for Estimating Reff, Xeff, or Zeff . . . 20
Xeff for Ground-Return Wire or Cable 20. . .
GROUND FAULT PROTECTIVE DEVICES 22. . .
GROUND FAULT PROTECTION DEVICE APPLICATION 23. . .
GROUND FAULT PROTECTIVE DEVICE COORDINATION 24. . .
PROTECTIVE DEVICE APPLICATION FOR TYPICAL SYSTEM CONFIGURATIONS 24. . .
Use of Ground Sensor (50GS) Relays 27. . .
SYSTEM AND COMPONENT DESIGN AND APPLICATION – HIGH RESISTANCE GROUNDED SYSTEM 29. . .
Fault Current Magnitude 29. . .
Determining System Charging Current 31. . .
Ground Resistor Sizing 31. . .
Ground Fault Detection and Location Devices 32. . .
APPENDIX 33. . .
Examples of Calculating Line-to-Ground Fault Currents 33. . .
Example 1 33. . . Example 2 34. . . Example 3 34. . . Example 4 34. . . Example 5 35. . . Example 6 35. . .
Methods for Calculating Effective Impedance (Zeff) . . . 36
Zeff for Circuits Using Aluminum Conduit as Ground Return Conductor 36. . .
Zeff of Circuit in Non-Metallic Conduit or Having Ground Return Conductor within Cable Assembly 36. . . FIGURES
Figure 1 Ungrounded System Equivalent Circuit 6. . .
Figure 2 Solidly-Grounded System Equivalent Circuit 7. . .
Figure 3 High-Resistance Grounded System Equivalent Circuit 8. . .
Figure 4 Low-Resistance Grounded System Equivalent Circuit 9. . .
Figure 5 Low-Reactance Grounded System Equivalent Circuit 10. . .
Figure 6 Ground Sensor Protection 22. . .
Figure 7 Residual Ground Fault Protection 23. . .
Figure 8 Neutral Ground Circuit Protection 23. . .
Figure 9 Secondary-Selective Substation Ground-Fault Protection 25. . .
Figure 10 Radial Substation (Dedicated Primary Feeder) – Ground Relaying 26. . .
Figure 11 Radial Substation (Tapped Primary Feeder) – Ground Relaying 26. . .
Figure 12 Use of 50GS Relay for Motors 27. . .
Figure 13 Coordination of 50GS Relay with Contactor Interrupting Capacity 28. . .
Figure 14 High Resistance Grounded System Under Normal Conditions 30. . .
Figure 15 High Resistance Grounded System – Fault on Phase A 30. . .
Figure 16 High Resistance Grounded System Using Distribution Transformer with Secondary Resistor 32. . .
Revision Memo 12/96 Highlights of this revision are:
1. Updated “References”. 2. Revised “Background”. 3. Revised most “Definitions”.
4. Revised discussion of charging current limits in “The High Resistance Grounded System”.
5. Completely replaced old section entitled “System and Component Design and Application Solidly Grounded Systems” with new section entitled “Solidly Grounded System Design”, including explaining use of “Maximum Distance Tables” in IP 16-4-1.
6. Revised section on “Fault Current Magnitude” in “System and Component Design and Application – High Resistance Grounded System”.
SCOPE
This section covers the low-voltage (600 volts and lower) system ground fault protection practices normally used in Exxon plants. The section includes information on the methods normally used for grounding the low voltage system neutral and on the protective system and component design and application.
REFERENCES INTERNATIONAL PRACTICES
IP 16-2-1 Power System Design
IP 16-4-1 Grounding and Overvoltage Protection
IP 16-12-1 Switchgear, Control Centers and Bus Duct OTHER LITERATURE
Beeman, D., “Industrial Power Systems Handbook,” McGraw Hill (1955).
IEEE (Institute of Electrical and Electronic Engineers) Standard 142-1991 “Recommended Practice for Grounding Of Industrial and Commercial Power Systems.”
The National Electrical Code, 1996 Edition (NFPA 70-1996). National Fire Protection Association.
IEEE Transactions on Industry Applications VOL. IA-13 No. 5 Sept/Oct. 1977 “The Reality of High Resistance Grounding,” J. R. Dunki-Jacobs.
IEEE Proceedings 1964 Industrial-Commercial Power Systems Technical Conference “Industrial Power Systems Grounding Practice,” L. W. Manning.
AIEE (American Institute of Electrical Engineers) Transactions Part II, (Applications and Industry) Volume 79, May 1960 “Determination of Ground Fault Current on Common Alternating Current Grounded Neutral Systems in Standard Steel or Aluminum Conduit,” J. A. Geiger, O. C. Davidson and R. W. Brendel.
AIEE Transactions Part II (Applications and Industry) Vol. 73, July 1954, “Iron Conduit Impedance Effects in Ground Circuit Systems,” A. J. Bisson and E. A. Rochau.
Dunki-Jacobs, J. R., “The Impact of Arcing Ground Faults on Low Voltage Powers Systems” General Electric Co., publication GET-6098 (1970).
Dunki-Jacobs, J. R. and Savoie, P. J., “A Guide to Ground Fault Protection” General Electric Co. periodical “Industrial Power Systems” December, 1972, March, 1973, June, 1973.
Grissom, S. B., “Grounding of Power System Neutrals” Westinghouse Electric Co. publication Electrical Transmission and Distribution Reference Book, Fourth Edition (1964) Chapter 19.
Shields, F. J., “System Grounding for Low Voltage Power Systems” General Electric Co. publication GET3548.
“System and Equipment Grounding” and “Capacitance Constants” General Electric Co. publication Industrial Power Systems Data Book, Section 3 and Appendix C.
Wagner, C. L., “Effect of Grounding Impedance on the Magnitude of Transient Overvoltage Due to Arcing Ground Faults” Westinghouse Electric Corp. Report No. 60 – 166 (June, 1960).
BACKGROUND
Per Exxon practices, low-voltage system neutrals are usually solidly grounded; but high-resistance grounding may be specified where permitted by regulations. Proposals for ungrounded systems and for systems grounded through Petersen coils need the approval of Owner’s Engineer. The National Electric Code of the USA requires solid grounding of low-voltage systems that have line-to-neutral loads. Ungrounded operation is not normally considered in the design of new low-voltage systems because of the risk of excessive overvoltages during arcing ground faults. Low-resistance grounding is not considered for low-voltage systems because it would require sensitive ground-fault protective devices throughout the system, which would be costly.
Ground-fault protection for Exxon’s solidly grounded low-voltage systems is provided by phase overcurrent devices to the extent permitted by Exxon practices and by regulatory requirements. Ground-fault relays are installed when phase overcurrent devices do not provide adequate ground-fault protection, and when local regulations require ground-fault relays on certain size circuits.
DEFINITIONS CHARGING CURRENT
For an individual feeder, charging current is the steady-state per-phase current that flows in the feeder due to the inherent capacitances of the feeder and of any electrical equipment connected to the feeder. For individually-shielded cable conductors, the charging current of the conductor is due only to the line-to-ground capacitive reactance, Xc0, of the conductors. For individual
cables that are not shielded, and for bare-conductor overhead lines, both the line-to-ground and the line-to-line capacitances of the conductors give rise to charging current. For the purposes of this practice, only the line-to-ground charging current, due to Xc0, is relevant.
For an electrical system, the system’s steady-state per-phase charging current (herein called Ic0) is the sum of all the charging currents associated with that phase. The mathematical quantity that equals three times the system’s per-phase charging current to ground is sometimes called the system’s total charging current to ground (1 to 2 amperes for most low voltage systems). This quantity (3 times Ic0) is equal to the capacitive component of the fault-point ground-fault current during a line-to-ground fault. EQUIPMENT GROUNDING
An intentional connection to ground from non-current-carrying metal parts of a wiring system or from non-current-carrying metal parts of other electrical apparatus connected to the electrical system; e.g., metal enclosures, motor frames, etc.
GROUND
The term used for the earth and for a conducting connection to the earth – where “earth” is a voltage reference plane which, in theory, is large enough and conductive enough that currents passing through it do not cause a resistive voltage drop. A connection to ground can be intentional as in system and equipment grounding, or it can be unintentional as in a ground fault. The purpose of an intentional connection to ground is to establish and maintain the potential of earth at the connection point (for reference or safety purposes), and to conduct ground-fault current into or out of the earth. References to “ground” or “earth” may be to the earth soil, or to a conducting body, in intimate contact with the earth, that has essentially the same potential as the earth; e.g., ground rods, buried lengths of bare wire, buried metallic water pipes, reinforcing bars in building footings, etc.
HIGH RESISTANCE GROUNDED SYSTEM
A system with a high-value resistor intentionally connected between the system neutral and ground. Normally the resistance value is such that ground fault current is limited to 10 amperes or less.
LOW RESISTANCE GROUNDED SYSTEM
A system which has a low value resistor intentionally connected between the system neutral and ground. Normally the resistance value is such that ground fault current is within a range of 25 amperes to several thousand amperes.
LOW VOLTAGE SYSTEM
In this section, a system with line-to-line voltages of 1000 volts or less; e.g., 480 V in the U.S.A., 380 V in Europe. SOLIDLY GROUNDED SYSTEM
A system grounded through an adequate ground connection in which no impedance has been inserted intentionally; e.g., a system supplied by a wye connected transformer whose neutral is connected directly to ground by a metallic conductor.
DEFINITIONS (Cont) SYSTEM GROUNDING
An intentional connection, either solid or through impedance, between ground and at least one point in the current-carrying parts of the system. The connection point in the system is usually at the neutral of a power transformer or generator. When system grounding through impedance is used to limit ground fault current in our plants, the impedance is normally resistive and is sized to provide either a high-resistance or low-resistance grounded system.
SYSTEM NEUTRAL
A point in the current-carrying circuit of a system that is usually used for grounding purposes; e.g., the neutral of a wye connected transformer.
UNGROUNDED SYSTEM
A system without an intentional connection between ground and a point in its current-carrying circuits, except for potential indicating or measuring devices, or other high impedance devices.
THE UNGROUNDED SYSTEM
An ungrounded system is one which has no intentional connection between the system and ground. The system is coupled to ground via the distributed capacitance-to-ground of the system conductors, such as the windings of motors and transformers, and the cables of the distribution system. Therefore, the so-called ungrounded system is actually a “capacitively” grounded system. A ground fault on one phase of the system will produce a small flow of current at the fault point supplied from the distributed capacitance of the other two phases. Figure 1 illustrates the ungrounded system circuit.
FIGURE 1
UNGROUNDED SYSTEM EQUIVALENT CIRCUIT
The chief advantage of the ungrounded system is that it permits continued operation of a circuit which has a single-line-to-ground fault (the most common fault type) without serious equipment damage or system upset. Ground fault sensing equipment can be used to detect ground faults and to sound an alarm. Orderly shutdowns can be scheduled in order to locate and repair faulted circuits. There are serious disadvantages associated with the ungrounded system. These are:
If a ground fault is not located and removed promptly, the possibility exists that the continuous current flow at the ground fault point, although small, will escalate the minimum damage fault into a double line-to-ground or three-phase fault which can cause serious damage. A typical maximum value of this small continuous current flow at the fault point for a 480-volt ungrounded system is 1 to 2 amperes.
A ground fault on one line causes full line-to-line voltage to appear throughout the system between the two unfaulted lines and ground. This voltage is 73% above the normal value, but conductor and equipment insulation is rated normally to withstand this value. However, if this voltage is applied for long periods, it may result in failure of insulation which may have deteriorated due to age or physical damage.
THE UNGROUNDED SYSTEM (Cont)
Ungrounded systems are vulnerable to line-to-ground transient overvoltages (as much as six times normal). The overvoltages can place abnormally high stresses on conductor and equipment insulation. These stresses can cause aged insulation to fail and new insulation to weaken prematurely. More frequent equipment and cable failures can result, and such failures may occur simultaneously in equipment on different circuits.
While ground fault detection is relatively simple, ground fault location can be difficult and time consuming. Ground-fault location techniques include: de-energizing individual circuits one at a time until the faulted circuit is located; or using a tone or pulse generator and a coupling sensor. This methods are generally inconvenient, and location of grounded circuits may be postponed for long periods.
The well-maintained ungrounded system, in which the first ground fault is promptly located and removed, has the capability for greater service continuity than the grounded system. However, experience indicates that for most plants the grounded neutral system provides adequate service continuity and has other advantages including freedom from transient overvoltages.
THE GROUNDED SYSTEM NEUTRAL POINT GROUNDING
The most desirable and common method of grounding a system is to make the connection to ground at the system’s neutral point. There are other methods such as line grounding and mid-phase grounding, but these methods are rarely used in new systems and will not be covered in this practice.
The circuit points for system neutral grounding are obtained in different ways depending on winding configuration of the power sources. In a wye-connected system, the neutral points of the source transformer and/or generator windings are brought outside the equipment enclosure and connected to ground. For a delta-connected power source or for wye-connected sources where the neutral point is not available, zig-zag or wye-delta grounding transformers are used to provide the neutral points for system grounding.
Exxon normal practice is to use power transformers with wye-connected secondary windings, or wye-connected generators in systems where there is power generation at the low voltage level. The systems are grounded at the neutral points of these wye-connected sources.
THE SOLIDLY GROUNDED SYSTEM
In solidly grounded systems, the neutral points of one or more power sources are connected to ground without intentional insertion of an impedance. The magnitude of ground fault current is about the same as three-phase fault current. Figure 2 illustrates the solidly grounded system circuit.
FIGURE 2
THE GROUNDED SYSTEM (Cont) The advantages of the solidly grounded system are:
The magnitudes of transient and steady-state line-to-ground overvoltages are kept within safe limits. For systems having ground fault current approximately equal to three-phase fault current, line-to-ground voltages on unfaulted phases during faults are close to the normal line-to-ground value.
With properly applied protective devices, ground faults are quickly detected and removed. This fast fault clearing minimizes system upsets and equipment damage.
The magnitude of ground current is usually high enough so that most phase protective devices are sensitive enough to detect ground faults. As a result, separate ground-fault protection is not always needed.
The disadvantages of solid grounding are:
Although ground faults are quickly isolated, high magnitudes of ground fault current flow before the fault is cleared. Damage at the fault point can be severe, especially if first line protection fails.
Ground faults cause service interruptions because faulted circuits are disconnected automatically by the protective devices.
A severe flash hazard to personnel exists where arcing grounds occur. These arcing ground can be self-sustaining and very damaging to equipment if protective devices are not sensitive enough to detect them.
Because the solid grounding method successfully controls overvoltages and provides enough fault current to allow protective devices to quickly and selectively remove faults, it is the most widely used grounding method for low voltage systems. International Practice 16-2-1 requires that all low voltage systems be solidly grounded unless high-resistance grounding is specified, or unless alternative methods (ungrounded system or Petersen coil) are approved by the Owner’s Engineer.
THE HIGH RESISTANCE GROUNDED SYSTEM
A high resistance grounded system is achieved by inserting a high ohmic value resistance between one or more system neutral points and ground. This resistance is sized to limit ground fault current through the grounding resistance to a value equal to or slightly greater than three times the per-phase system capacitive charging current to ground. The quantity equal to three times the per-phase charging current to ground is sometimes called the total charging current to ground (1 to 2 amperes for most low voltage systems), and is equal to the capacitive component of the ground-fault current during a line-to-ground fault. Protective devices are arranged for detection and alarm. Experience with high resistance grounded systems shows that the total system capacitive charging current to ground for the initial installation and including future expansions should not exceed 3.53 A (preferred) up to 7.07 A (the recommended maximum) where faulted circuits are not disconnected automatically by protective devices. Figure 3 illustrates the high resistance grounded system circuit. The circuit labeled “Thevenin Equivalent” is an approximate equivalent based on the assumption that the system and transformer reactances are very small compared to RN and XCO. High resistance grounding is discussed further later in this practice.
FIGURE 3
THE GROUNDED SYSTEM (Cont) The advantages of high resistance grounding are:
Unscheduled equipment shutdowns caused by line-to-ground faults are avoided because automatic tripping is not used.
Since very little ground current is allowed to flow, fault point damage is usually small.
Line-to-ground transient overvoltages, due to inductive-capacitive resonance and repetitive restriking of arcing faults, are limited to safe values (250% of normal or lower). The effectiveness of the system in limiting resonant overvoltages is a function of the ability of the neutral resistance to absorb the energy stored in the system capacitance-to-ground at the instant the fault occurs.
Arcing grounds are usually self-extinguishing, with limited energy released by the arc.
Flash hazard for line-to-ground faults is reduced greatly compared to solidly grounded systems.
The disadvantages of high resistance grounding include:
Ground fault location can be difficult and time-consuming. However, there are detection systems available which trained personnel can use to shorten fault location time.
If a second ground fault occurs on another phase before the first fault is located and cleared, a double line-to-ground fault results with the subsequent damage and automatic tripping of one or two circuits.
System insulation is subjected to higher than normal voltage when a single ground fault occurs. System insulation on the two unfaulted phases will see line-to-line voltage as in the ungrounded system, this may result in failure of weak insulation if the fault remains for longer periods.
Although solid grounding is the most widely used low voltage grounding method, high resistance grounding is sometimes used where an immediate service interruption, due to a ground fault, is undesirable.
OTHER SYSTEM GROUNDING METHODS
Low resistance and low reactance grounding of the system neutral are two other methods which can be used on low voltage systems. However, these methods have not been used in Exxon plants and their use is generally uncommon.
Low resistance grounding is accomplished by inserting a low ohmic value resistance between the system neutral point and ground. The resistance value is selected to limit ground fault currents to 20% of three-phase fault current and lower, with a minimum of about 400 amperes. Protective devices are arranged to trip when ground faults occur. Figure 4 illustrates the low-resistance grounded system circuit.
FIGURE 4
LOW-RESISTANCE GROUNDED SYSTEM EQUIVALENT CIRCUIT
This method has the advantages that transient overvoltages are kept within safe limits and due to the lower fault current and the use of ground fault protective devices, fault damage is reduced compared to damage on a solidly grounded system.
THE GROUNDED SYSTEM (Cont)
The disadvantages of the low resistance grounded system are the additional cost of ground fault protective devices required to provide selective protection, the more severe flash hazard compared with that of a high resistance grounded system, and the unscheduled equipment shutdowns caused by the automatic tripping of faulted circuits.
To date, low resistance grounding has not found wide application in low voltage systems because of the high cost of obtaining selective ground fault protection. This method may find wider acceptance for low voltage applications when small and inexpensive ground fault relays are developed. These relays must be sensitive and compatible with circuit protection used on motor controllers, small feeders, and branch circuits.
Low reactance grounding is accomplished by inserting a low ohmic value reactor between a system neutral point and ground. The reactor is sized to limit ground-fault current levels to between 25% and 100% three-phase fault levels (usually closer to 100%). Figure 5 illustrates the low reactance grounded system circuit.
FIGURE 5
LOW-REACTANCE GROUNDED SYSTEM EQUIVALENT CIRCUIT
This method is not often used in low voltage systems. It does have some applications, however, such as when low voltage generation is present. In a solidly grounded system with generators, ground fault current may be larger than three-phase fault current (for close-in, bolted faults on the generator). To reduce fault current, reactance grounding is sometimes used.
Since ground-fault current levels are approximately equal to three-phase fault levels, this method is similar to the solid grounding method as far as advantages and disadvantages are concerned.
SOLIDLY GROUNDED SYSTEM DESIGN FAULT CURRENT MAGNITUDE – GENERAL
In order to design an effective protective scheme for a solidly grounded system, the designer must know the range of possible ground fault currents. This range is generally considered to lie between the maximum bolted line-to-ground fault and a line-to-ground fault with arc resistance at the end of an outgoing feeder. The maximum ground fault current must be checked against the line-to-ground fault-interrupting capability of interrupting devices. The minimum fault current due to an arcing ground fault at the end of a feeder is used to determine the adequacy of the feeder’s ground fault protection.
The general equation for calculating ground-fault current is:
ILG 3ELN
Z1 Z2 Z0 Eq. (1)
where: IL–G = Single-line-to-ground fault current
EL–N = System line-to-neutral voltage
Z1 = System equivalent positive sequence impedance
Z2 = System equivalent negative sequence impedance Z0 = System equivalent zero sequence impedance
SOLIDLY GROUNDED SYSTEM DESIGN (Cont)
Note: All sequence impedances are in complex form (Z1 = R1 + jX1), and can be in either ohms or per unit, depending on the calculation. In the general case, Z0 includes the R0 + jX0 of phase conductors and windings between the relevant neutral and the fault point, plus three times any impedances that exist only in the ground return path between the fault point and the relevant neutral; e.g., 3ZN (neutral grounding impedance), 3ZF (fault impedance), 3RGR (ground return path resistance). X0 of the phase conductors takes account of the inductive reactance between the phase conductors and the ground return path.
The use of this equation or modifications of this equation are discussed below.
Example 1 in the Appendix is a calculation of ground fault current using this equation for a bolted fault at the end of a feeder on a solidly grounded system. Example 1 makes the simplifying assumption that all impedances upstream of the faulted cable are small enough that they can be assumed to be zero.
MAXIMUM (BOLTED) LINE-TO-GROUND FAULTS
Although bolted line-to-ground faults rarely occur, they are important because they cause maximum ground current flow. The magnitude of this current depends on the line-to-neutral source voltage and the impedances of the sequence networks, per Equation 1. For the maximum line-to-ground fault current at a main source bus of a solidly grounded system, the impedances of the ground return path are all assumed to be zero. Typically, the maximum ground fault current in our solidly grounded systems is somewhat greater than the maximum three-phase fault current.
ADEQUATE GROUND-FAULT PROTECTION PER IP 16-4-1
IP 16-4-1 specifies the basis for determining if circuit ground fault protection is adequate, per the following statement:
“The combined impedance of the ground return path and the supply circuit line conductors shall be low enough to insure operation of the circuit overcurrent protective device in less than two seconds on a single line-to-ground fault at the load end of the circuit. An arc voltage of 40 volts in phase with the line-to-neutral source voltage shall be assumed at the point of fault.”
Our general practice is not to provide ground-fault relays on low-voltage circuits if the above basis is satisfied by the phase overcurrent devices. When ground fault relays are applied, the basis in the statement is almost always satisfied by high-sensitivity 50GS relays. The adequacy of ground fault protection should be verified if a 51N relay protects a low-voltage circuit. If local regulations regarding ground fault protection are more restrictive than IP 16-4-1, the local requirements shall be followed. For example, some codes may require ground-fault relays on all motors of a certain size or larger.
The adequacy of ground-fault protection in low-voltage solidly-grounded systems should be determined, to the extent possible, via the use of the “Maximum Distance” tables in IP 16-4-1. These tables are discussed below.
MAXIMUM DISTANCE TABLES – IP 16-4-1
IP 16-4-1 provides tables for use in determining the maximum circuit distances allowed by our practices for various cable installations in solidly grounded low-voltage systems. The maximum distances in the tables are based on the “two second, 40-volt arc” criterion discussed above, and are presented as a function of the rating of the overcurrent-device, device operating speed, and the sizes of conduits, cables, and ground-return conductors.
The notes associated with the tables describe how to use the tables; however, clarification is provided below regarding the tables and their use. (Because the “A” tables deal only in feet and the “B” tables deal only in meters, references to distances in these tables are presented in only the units used in the specific table.)
Clarifications on Use of Maximum Distance Tables
Clarifications on the use of the “Maximum Distance” tables in IP 16-4-1 are provided as follows:
The tables assume that the impedance upstream of the cable circuit is negligibly small compared to the cable impedance. This assumption is usually valid, but if the upstream impedance is not negligible compared to the cable-circuit impedance, the tables are not directly applicable, and calculations must be performed to take account of upstream impedances. See “When System Impedance Cannot Be Neglected” below.
The word “Rating” in the heading of each table means “Relay Pickup” (expressed in line-side amperes) when a relay is involved. The word “Rating” is correct for fuses and molded case circuit breakers.
SOLIDLY GROUNDED SYSTEM DESIGN (Cont)
The notes in the tables refer to protective device operation at a nominal setting of 10 times their rating. The words “at a nominal setting of” mean “at a current of”. For example, a fuse characterized by Factor B in Table 1 must operate in less than 2 seconds at a current of v 6 times its rating. In Tables 2 and 3, the words “divided by device setting” at the end of the next to last sentence of Note 1 mean “divided by the multiple of device rating at which device operates in just under 2 seconds.”
The actual maximum-total-clearing time characteristic of a protective device must be used to determine the maximum circuit length allowed under the 2-seconds rule of IP 16-4-1. Note 1 of Tables 2 and 3 indicates how to adjust the table distances upward for protective devices that totally clear a fault within 2 seconds at multiples of rating less than 10. The table distances would have to be adjusted downward if there were an MCCB protecting a feeder (without contactor switching), and the MCCB had a 2-second maximum-total-clearing time at a multiple of rating greater than 10. The distances in Tables 1A and 1B cannot be adjusted by the current-ratio method of Note 1 in Tables 2 and 3. Distance data in Table 1A or 1B must be interpolated or extrapolated when it becomes necessary to adjust distance data in these tables. Use of the distance tables is discussed below.
WHEN SYSTEM IMPEDANCE CANNOT BE NEGLECTED
Distance data in the “Maximum Distances” tables is based on the assumption that the impedance upstream of the cable is negligible. A ground fault relay must be added if the actual circuit distance is greater than the maximum allowable distance per the tables. However, if the actual circuit distance is less than the distance determined using the tables, the effect of upstream impedances on table distance should be evaluated if either of the following conditions apply:
The actual circuit distance is between 90% and 100% of the maximum distance determined using the tables, or,
The load on a feeder is greater than 10% of the local transformer’s OA capacity. Before doing this comparison, adjust the transformer OA capacity downward. If the total impedance upstream of the local transformer is more than 5% of the local transformer impedance; i.e., adjust the transformer capacity downward by the ratio of the transformer’s impedance divided by the sum of the transformer impedance plus the upstream impedance.
IMPEDANCE-RELATED ADJUSTMENTS OF TABLE DISTANCES
When the effect of impedance upstream of the cable circuit should be evaluated (per the above), either a short circuit calculation should be performed (as discussed later), or the following method should be used to adjust distances in any of the tables to account for impedance upstream of the cable.
For each maximum distance (Lmax) in any of the tables, there is an inherent total-circuit maximum impedance (Zmax), which is equal to 237 V divided by the 2-second-clearing current used in the table. The 2-second-clearing current used in Tables 2 and 3 is equal to the Device Rating times 10. The 2-second-clearing current used in Tables 1A and 1B is equal to the Device Rating times either 10 or 6 or 4, depending on the Trip Setting Factor used to determine Lmax. To the extent source impedance. ZS, upstream of the cable, uses up some of the total-circuit maximum impedance, the maximum cable-circuit distance in the table must be adjusted downward. The adjusted maximum distance can be closely approximated via the following equation:
LȀmax + Lmax
ǒ
1 – ZS ZmaxǓwhere: Lmax is the maximum distance for a cable circuit found by using the table L’max is the adjusted maximum distance
ZS is the magnitude of the impedance upstream of the cable circuit, expressed in ohms and referenced to the low-voltage side of the transformer
Zmax is the magnitude of the maximum allowable impedance upon which Lmax is based. Zmax is equal to 237V divided by the 2-second-clearing current used in the table (as explained in the paragraph above).
SOLIDLY GROUNDED SYSTEM DESIGN (Cont)
The upstream impedance, ZS, for a line-to-ground fault can be determined via symmetrical components (Z1 plus Z2 plus Z0, all divided by 3). However, when the impedances upstream of the local transformer are negligible, ZS can be assumed to be equal to ZT (the impedance of the local transformer).
Note: The biggest impedance-related reductions in table distances occur with the smaller transformers and the higher-rated protective devices. The following table shows example percent reductions in table distances for two local transformer sizes (5% impedance) and two protective device ratings (2-second operation at 10 times rating). The local transformers are assumed to be the only significant upstream impedance.
EXAMPLES OF PERCENT REDUCTIONS IN MAXIMUM DISTANCES ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ 500 kVA TRANSFORMERÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ 1000 kVA TRANSFORMER ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 125 A Device ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ 12% ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ 6% ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 70 A Device ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ 7% ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ 3.5%
The following example illustrates the need for evaluating the effect of upstream impedance on distances found in the Maximum Distance tables:
A 500 kVA, 480 V transformer, with ZT = 5%, has an impedance of 0.023 ohms reflected to the 480 V secondary side. Assume a 480V, 100 HP motor, a 2-inch steel conduit ground return, a 400-foot circuit length, and a 150 A protective device that totally clears a fault in 2 seconds at 1500 A (10 times rating). Assume all other upstream impedances are negligible. The maximum allowable circuit distance in Table 1A is 450 feet (150 A device, 2-inch conduit, TSF = A); however, the load is more than 10% of the transformer rating and the effect of the transformer impedance should be checked. The inherent impedance, Zmax, in the table is 0.158 ohms (237 V divided by 1500 A). Per the calculation below, the adjusted maximum distance is 384 feet, which is about a 15% reduction in allowable distance. Since the actual circuit length exceeds this adjusted limit, a ground fault relay should be added.
LȀmax + Lmax
ǒ
1 – ZSZmax
Ǔ
+450ǒ
1 – 0.0230.158
Ǔ
+450 (0.854) + 384 feetThe adjustment factor of 0.854 applies only to the distance of 450 feet because Zmax is different for each different distance in any of the tables.
USE OF TABLES 1A & 1B – STEEL CONDUIT
Tables 1A and 1B are to be used when rigid steel conduit is the ground return conductor. The distances in these tables are a function of the conduit size, and the protective device’s “Trip Setting Factor” (TSF) and nominal rating – which together define the current a given device requires to operate within 2 seconds.
The size of the line conductor is not a factor in the steel-conduit distance tables. Note 2 of these tables gives percent increases in maximum distances when there is a ground-return conductor in the conduit. Distances in Tables 1A and 1B cannot be adjusted per the ratio method described in Note 1 of Tables 2 and 3, because the impedance of steel conduit varies with current. Depending on actual circuit and protective-device parameters, one of the following uses of the data in Tables 1A and 1B will apply:
The distance data in the tables may be used without interpolation or extrapolation when it is obvious that a ground fault relay is either needed or not needed, per the guidelines in the next section, entitled “Use of Table 1A/1B Distance-Data – Without Interpolation or Extrapolation”.
If it is not obvious from the table whether or not a ground fault relay should be added per the guidelines in the next section, the following procedures should be used:
+ The distance data in the tables should be interpolated if the protective device’s actual 2-second clearing current is within the range of 2-second-clearing currents inherent in the table (see below).
+ The distance data in the tables should be extrapolated if the actual 2-second clearing current is outside the range of 2-second-clearing currents inherent in the table.
SOLIDLY GROUNDED SYSTEM DESIGN (Cont)
+ In all cases, the distance data in the tables may have to be adjusted if the impedance upstream of the cable is not negligible (as explained above).
USE OF TABLE 1A/1B DISTANCE-DATA – WITHOUT INTERPOLATION OR EXTRAPOLATION
To use the distance data in Tables 1A and 1B without interpolation or extrapolation, use the following procedures and guidelines (substitute “pickup” for “rating” when the protective device is a relay):
1. Determine the actual multiple of device rating at which the circuit’s protective device totally clears a fault in 2 seconds. 2. Given the circuit’s conduit size and phase protective-device rating, compare the actual circuit distance against the distances
found in Table 1A or 1B, per the following guidelines:
When there is no data in the table for the device rating and conduit size, interpolation or extrapolation of distance data will be required, unless otherwise noted.
If the actual multiple of device rating (per 1. above) is greater than 10:
+ The circuit does need a separate ground fault relay if the actual circuit distance is greater than the distance for TSF = A.
+ If the actual circuit distance is less than the distance for TSF = A, interpolation or extrapolation of the table data will be required.
If the actual multiple of device rating is less than 10 and greater than 6, compare the actual circuit distance to the distances for Trip Setting Factors (TSFs) A and B:
+ The circuit does not need a separate ground-fault relay if the actual circuit distance is less than 90% of the smaller distance (TSF = A). Note: If there is no distance data for the device rating, the circuit does not need a separate ground-fault relay if the actual circuit distance is less than 90% of the “TSF = A” distance for the next smaller conduit size that has distance data for the given device rating.
+ The circuit does need a separate ground-fault relay if the actual circuit distance is greater than the larger distance (TSF = B).
+ If the actual distance is between the larger distance and 90% of the smaller distance, interpolation is required and the effect of upstream impedances should be checked.
If the actual multiple of device rating is less than 6 and greater than 4, compare the actual circuit distance to the distances in the table for TSF = B and TSF = C:
+ The circuit does not need a separate ground-fault relay if the actual circuit distance is less than 90% of the smaller distance (for TSF = B). Note: If there is no distance data for the device rating, the circuit does not need a separate ground-fault relay if the actual circuit distance is less than 90% of the “TSF = B” distance for the next smaller conduit size that has distance data for the given device rating.
+ The circuit does need a separate ground-fault relay if the actual circuit distance is greater than the larger distance (for TSF = C).
If the actual distance is between the larger distance and 90% of the smaller distance, interpolation is required and the effect of upstream impedances should be checked.
If the actual multiple of device rating is less than 4:
+ The circuit does not need a separate ground fault relay if the actual circuit distance is less than 90% of the distance for TSF = C. Note: If there is no distance data for the device rating, the circuit does not need a separate ground-fault relay if the actual circuit distance is less than 90% of the “TSF = C” distance for the next smaller conduit size that has distance data for the given device rating.
+ If the actual circuit distance is greater than 90% of the distance for TSF = C, interpolation or extrapolation of the table data will be required, and the effect of upstream impedances should be checked.
3. For a circuit which already has a ground fault relay, with an operating time of 2 seconds, or less, at a current that is 4 times pickup or less, perform the following check. Check that the actual circuit distance does not exceed the table distance in the following column and row:
In the column of the device rating that equals (or next exceeds) the relay’s line-side pickup current; and
SOLIDLY GROUNDED SYSTEM DESIGN (Cont)
It is unlikely that the actual circuit distance will exceed the table distance, but if it does, calculate the ground-fault current (from impedance data), and use it to determine if the relay clears the fault in less than 2 seconds. In the absence of any other ground-return impedance data for the fault calculation, use 0.0006 ohms per foot for cable-circuits in 2-inch and smaller steel conduit, and use 0.0005 ohms per foot for circuits in 2.5-inch and larger steel conduit. If the ground fault protection is found to be inadequate per this analysis, a more sensitive ground fault relay should be used if available.
INTERPOLATION OF TABLE 1A/1B DISTANCE DATA
Interpolation or extrapolation of Table 1A and 1B distance data is required when the limiting distance cannot be determined per the guidelines in the preceding section. When interpolation is not possible, the data should be extrapolated if accuracy is possible (see the “Extrapolation” section below). If extrapolation is not considered accurate enough, a ground fault calculation must be done.
The key points regarding interpolation of distance data in Tables 1A and 1B are the following:
Interpolation or extrapolation of data in Tables 1A and 1B is possible because all of the distances in the tables for a given conduit size are points on a curve of distance versus 2-second-clearing current, regardless of protective device rating.
The 2-second clearing current inherent in Tables 1A and 1B for any given distance is equal to the Device Rating (above the distance) multiplied by the factor (10, 6, or 4) associated the Trip Setting Factor (left of the distance).
Interpolation of the distance data inherent in Tables 1A and 1B for a given conduit size is possible when the actual 2-second-clearing current of a protective device is within the range of 2-second-clearing currents inherent in the table for that size conduit.
The range of 2-second-clearing currents inherent in the tables for a given conduit size is from 4 times the smallest device rating with distance data, up to 10 times the highest device rating with distance data.
Given the actual protective device’s 2-second-clearing current, it is important to interpolate between the two nearest data points; i.e., interpolate between the distances whose 2-second-clearing currents are nearest to the device’s actual 2-second-clearing current. The two nearest 2-second-clearing currents in the table may not be on the same TSF line. Prior to interpolating between two distance-versus-current points, check that the two nearest data points have been found by ensuring that there is no distance in the table (for the given conduit size) that is between the two distances chosen.
For example, for a 3-inch steel conduit and a device with a 2-second-clearing current of 1450-A, the nearest currents in the table for a 3-inch conduit are 1400 A (350 A rating, TSF = C, 690 feet) and 1500 A (250 A rating, TSF = B, 660 feet). Since there is no other distance between 690 and 660 feet for 3-inch conduit, it is certain that the two nearest data points have been selected for interpolation. With the actual 2-second clearing current of 1450 A being midway between 1400 A and 1500 A, interpolation yields the maximum distance of 685 feet, midway between 690 feet and 660 feet.
Even when there is no distance data in the table for a given device rating and conduit size, interpolation is possible when the actual 2-second-clearing current of the device falls within the range of 2-second-clearing currents inherent in the table for the conduit size. For example, a 300 A device protecting a circuit that is in a 4-inch steel conduit has no explicit distance data in Tables 1A (or 1B). However, there is inherent distance data for 4-inch conduits in Tables 1A and 1B for any device that has a 2-second-clearing current between 1600 A (4 times 400 A) and 6000 A (10 times 600 A).
The two scenarios which follow illustrate use of the distance versus current data inherent in Tables 1A and 1B:
Assume a 175 A device totally clears a fault in 2 seconds at 1800 A, and that the associated circuit is 565 feet long in a 4-inch steel conduit. There is no distance data in Table 1A for this scenario. Since 1800 A is exactly 4 times a 450 A rating (for which there is distance data), the maximum distance of 630 feet can be found at the intersection of the 450 A device-rating and TSF = C. Since the actual circuit distance is less than 90% of the table distance, the effect of upstream impedance does not have to be checked, and no ground fault relay is needed.
Assume the 175 A device in the above scenario totally clears a fault in 2 seconds at 1900 A. Since 1900 A is not a 4, 6 or 10 times multiple of any device rating in the table, the maximum distance must be found by interpolation between the distances associated with the two operating currents nearest to 1900 A. In this case, the interpolation must be done between 630 feet for 1800 A (4 times 450 A), and 590 feet for 2000 A (4 times 500 A). These two points are the nearest to the devise operating current of 1900 A because there is no distance in the table for 4-inch conduit that is between 630 feet and 590 feet. Since 1900 A is midway between 1800 A and 2000 A, interpolation yields the limiting distance of 610 feet, which is midway between 630 feet and 590 feet. Because the actual circuit distance of 565 feet is greater than 90% of the distance found using the tables, the effect of upstream impedances should be checked:
SOLIDLY GROUNDED SYSTEM DESIGN (Cont)
+ Assume a 1000 kVA transformer with an impedance of 0.0115 reactive ohms, and a heater with impedance of 0.002 resistive ohms. The resultant upstream impedance magnitude is 0.0117 ohms. The maximum impedance for a clearing current of 1900 A is 0.125 ohms (= 237V/1900A). The distance adjustment factor is 0.9064 (= 1 – 0.0117/0.125) per the equation in the section (above) entitled “When System Impedance Cannot Be Neglected”. Therefore the adjusted maximum allowable distance is 553 feet (= 0.9064 times 610 feet). Since the actual circuit distance is 570 feet, a ground fault relay must be added.
+ If the transformer in this example had a 2000 kVA rating, with a resultant upstream impedance of 0.007 ohms, the adjustment factor would be 0.944 (= 1 – 0.007/0.125), and the adjusted maximum distance would be 576 feet. Since the actual circuit distance is 565 feet, there is no need for a ground fault relay.
EXTRAPOLATION OF TABLE 1A/1B DISTANCE DATA
When a device’s 2-second-clearing current is outside the relevant data range in Tables 1A and 1B, graphical extrapolation of the distance versus current data can provide reasonable results.
USE OF TABLES 2A & 2B – ALUMINUM CONDUIT
Tables 2A and 2B indicate maximum allowable circuit distances when rigid aluminum conduit is used as the ground return. The distances in these tables are a function of the conduit size, the protective device’s nominal rating, and the size of the line conductor. The distances are for protective devices that operate in two seconds at a current of 10 times the device rating.
Where the distance is blank for a given aluminum-conduit size, line-conductor size, and device rating, multiply any one of the published distances (for the given conduit and line size) by the ratio of the “device rating” for that published distance, divided by the actual device rating or actual relay pickup (in line-side amperes). For example, if a 300 A device protects a 95 square mm line conductor in a 2-inch conduit, there is no distance entry for this situation. Therefore, multiply the Table 2B distance of 285 meters (for a 250 A device, 2-inch conduit, and 95 square mm line) by 250/300.
If the protective device operates in 2 seconds at a higher multiple of rating than 10 times rating, the distance per the table should be decreased via multiplication by the ratio of 10 divided by the higher multiple of rating.
Protection is adequate if the actual circuit distance is less than the applicable distance in the tables, or is less than an adjusted distance derived from the tables, as discussed in the following paragraphs. Distances in the tables can be adjusted upward when the protective device operates in less than 2 seconds at a current ten times its rating or pickup or setting.
If a phase overcurrent device does not provide adequate protection based on the actual or adjusted table distance, a ground fault relay must be added to the circuit. The adequacy of protection provided by a ground fault relay must be checked in the same way as for a phase fault device, using the ground-fault relay’s line-side pickup (for 51N) or line-side setting (50 GS) in place of the “Device Nominal Rating” in the tables.
If the circuit distance is greater than the distance per the table and the phase protective device operates within 2 seconds at a current less than 10 times its rating, the distance in the table can be increased via the following adjustment: multiply the table distance by the ratio of 10 divided by the multiple of device rating at which the device operates within 2 seconds. For example, if a 250 A fuse operates in 2 seconds at 7.33 times its rating, multiply the distance in the table by 1.364 (= 10 7.33).
If a smaller line conductor is used than is given in the table for a given size aluminum conduit:
For the conduit size and device rating, check if the actual circuit distance is greater than the shortest distance (or shortest adjusted distance per previous paragraph) – in which case the protection is not adequate and a ground-fault relay is required.
If the above check does not establish the need for a ground fault relay, check if the actual circuit distance is less than the published (or adjusted) distance for the same size line conductor in the next smaller size conduit (for the same device rating) – in which case the protection is adequate.
SOLIDLY GROUNDED SYSTEM DESIGN (Cont)
If the actual circuit distance is between the above distances, the designer should perform a calculation to determine the circuit impedance taking account of the smaller line conductor, and adjust the maximum permissible length accordingly, using Equation 8 below. Since the combined line and conduit impedance is essentially all resistance, this impedance can be adjusted using line resistances from manufacturer’s data, or using line resistance data that can be back-calculated from the distance tables by using Equation 7 below.
If the circuit is protected by a ground fault relay, Note 1 of these tables calls for a distance calculation using actual impedances. Protection is adequate if the actual circuit distance is less than the distance in the table for the actual conduit and line size at the smallest device rating that equals or exceeds the ground fault relay pickup. If the actual circuit distance exceeds this table distance, perform the following calculation, which uses the impedances inherent in the distance tables. This calculation assumes that the ground-fault relay is either instantaneous (50GS) or has a time dial setting (51N) such that the relay operates in 2 seconds or less at 4 times pickup.
For the actual aluminum conduit size and line size, read any Lmax and its corresponding device rating (IR1) in Table 2A or 2B. This Lmax is based on an operating current of 10 times the device rating, IR1. If we call IGFR the ground-fault relay pickup or instantaneous
setting, and if 4xIGFR assures relay operation in 2 seconds or less, the calculated maximum length, LGFR, for the ground fault relay application is:
LGFR Lmax x 10IR1
4IGFR Eq. (2)
where: LGFR is the calculated maximum distance allowed with a ground fault relay.
Lmax is any maximum distance per Table 2A or 2B for the size conduit and line conductor in the circuit.
IR1 is the device rating (in primary side amperes) corresponding to Lmax in the table. “10” is the multiple of device rating on which the table is based.
IGFR is the ground fault relay pickup (or instantaneous setting) in line-side amperes. “4” is the multiple of IGFR that ensures relay operation (accounting for CT error, etc.).
For example, for a 1-inch conduit, a 70 A MCCB, and a 10 Awg line conductor, the maximum distance from Table 2A is 310 feet for a 70 A device (which operates within 2 seconds at 10 times its rating). If the actual circuit distance is 1600 feet, a ground fault relay, 50GS, must be added. Assume the 50GS relay picks up at 10 A (1 A relay setting with a 50/5 CT).
The highest published distance for 1-inch conduit and a 10 Awg line is 1070 feet for a 20 ampere device. Since the circuit distance is 1600 feet, we need to do a calculation of the maximum allowable distance using Equation 2. Using Lmax = 1070 feet and IR1 = 20 A from Table 2A, and IGFR = 10 A for the 50 GS:
1070 x 10 x 20
4 x 10 5350 feet
Therefore, the ground fault relay provides adequate protection.
USE OF TABLES 3A, 3B, & 4 – GROUND RETURN CABLE OR WIRE
Tables 3A and 3B are for circuits not in steel or aluminum conduit, with the ground return conductor within the cable assembly. The distances in these tables are a function of the protective device’s nominal rating, and the sizes of the line and ground return conductors. The distances are for phase protective devices that operate in two seconds at a current of 10 times the device rating. Table 4 provides adjustment factors to decrease the Table 3A and B distances if the ground return conductor is outside of the cable assembly. These adjustment factors are a function of the spacing between the ground return conductor and the phase conductor. In the rest of this section, references to Table 3A and 3B distances mean the distances adjusted by Table 4 factors if applicable.
If the protective device operates in 2 seconds at a higher multiple of rating than 10 times rating, the distance in the table should be decreased via multiplication by the ratio of 10 divided by the higher multiple of rating. For example, if a device operates in 2 seconds at 11 times its rating, multiply the table distance by 10/11.
SOLIDLY GROUNDED SYSTEM DESIGN (Cont)
Where the distance is blank for a given line-conductor size, ground-return conductor size, and device rating, multiply any one of the published distances (for the line size and ground-return conductor size) by the ratio of the “device rating” for that published distance, divided by the actual device rating or actual relay pickup (in line-side amperes). For example, a 50 A device protects a 25 square mm line conductor with a 25 square mm ground-return conductor, but there is no distance entry for this situation. There is a distance of 225 meters published for a 70 A device, and 25 square mm line and ground-return conductors. Therefore, multiply 225 meters by 70A/50A to determine that the maximum allowable distance for 25 square mm line and ground-return conductors is 315 meters when the circuit is protected by a 50 A device that operates in 2 seconds at 10 times its rating.
Protection is adequate if the actual circuit distance is less than the applicable distance in the tables, or is less than an adjusted distance derived from the tables, as discussed in the following paragraphs. Distances in the tables can be adjusted upward when the protective device operates in less than 2 seconds at a current ten times its rating or pickup or setting.
If a phase overcurrent device does not provide adequate protection based on the actual or adjusted table distance, a ground fault relay must be added to the circuit. The adequacy of protection provided by a ground fault relay must be checked in the same way as for a phase fault device, using the ground-fault relay’s line-side pickup (for 51N) or line-side setting (50 GS) in place of the “Device Nominal Rating” in the tables.
If the circuit distance is greater than the distance in the table and the phase protective device operates within 2 seconds at a current less than 10 times its rating, the distance in the table can be increased via the following adjustment: multiply the table distance by the ratio of 10 divided by the multiple of device rating at which the device operates within 2 seconds. For example, if a 250 A fuse operates in 2 seconds at 7.33 times its rating, multiply the distance in the table by 1.364 (= 10 B 7.33).
If the ground return conductor is larger than any in the table for a given size line conductor:
First check if the actual circuit distance is less than the longest published distance (or longest adjusted distance per the previous paragraph) for the line conductor size and device rating – in which case the protection is adequate and a ground-fault relay is not required.
If the check above does not determine that the protection is adequate, the designer should perform a calculation to determine the impedance due to the larger ground-return conductor, and adjust the maximum permissible length accordingly, using Equation 8 below. Since the combined line and ground-return conductor impedance is essentially all resistance, this impedance can be adjusted using line resistances from manufacturer’s data, or using line resistance data that can be back-calculated from the distance tables by using Equation 7 below.
If the circuit is protected by a ground fault relay, Note 1 of these tables calls for a distance calculation using actual impedances. Protection is adequate if the actual circuit distance is less than the distance in the table for the actual line and ground-return conductor sizes at the smallest device rating that equals or exceeds the ground fault relay pickup. If the actual circuit distance exceeds this table distance, perform the following calculation, which uses the impedances inherent in the distance tables. This calculation assumes that the ground-fault relay is either instantaneous (50GS) or has a time dial setting (51N) such that the relay operates in 2 seconds or less at 4 times pickup.
For the actual line and ground-return conductor sizes, read any Lmax and its corresponding device rating (IR1) in Table 3A or 3B. This Lmax is based on an operating current of 10 times the device rating, IR1. If we call IGFR the ground-fault relay pickup (or instantaneous setting), and if 4xIGFR assures relay operation in 2 seconds or less, the calculated maximum length, LGFR, for the ground fault relay application is obtained using Equation 2, repeated below:
LGFR +Lmax x10 @ IR1
4@ IGFR Eq. (2)
where: LGFR is the calculated maximum distance allowed with a ground fault relay
Lmax is any maximum distance per Table 3A or 3B for the line and ground-return conductor sizes in the circuit. IR1 is the device rating (in primary side amperes) corresponding to Lmax in the table.
“10” is the multiple of device rating on which the table is based.
IGFR is the ground fault relay pickup (or instantaneous setting) in line-side amperes. “4” is the multiple of IGFR that ensures relay operation (accounting for CT error, etc.)
SOLIDLY GROUNDED SYSTEM DESIGN (Cont)
For example, for a 70 A MCCB, and 10 Awg line and ground-return conductors, the maximum distance from Table 3A is 160 feet for a 70 A device (that operates within 2 seconds at 10 times its rating). If the actual circuit distance is 1600 feet, a ground fault relay, 50GS, must be added. Assume the 50GS relay picks up at 10 A (1 A relay setting with a 50/5 CT).
The highest published distance for 10 Awg line and ground-return conductors is 560 feet for a 20 ampere device. Since the circuit distance is 1600 feet, we need to do a calculation of the maximum allowable distance using Equation 2:
560 x 10 x 20
4 x 10 +2800 feet
Therefore, the ground fault relay provides adequate protection since the circuit distance is 1600 feet. GROUND-FAULT CALCULATIONS
IP 16-4-1 indicates that the calculation of ground fault current, when required, should involve the use of specific circuit impedances, and that the fault clearing time should be determined for the calculated current and the characteristic of the actual protective device. If specific impedance data is not available, this design practice provides a method for estimating the circuit impedance.
When calculation is required for an end-of-circuit ground fault with a 40-volt arc voltage drop, Equation 1 would be modified as follows for a rigorous solution:
IL – G + 3 x (EL – N – 40)
Z1 )Z2 ) ZȀ0 Eq. (3)
where: The variables are in amperes, volts and ohms.
The 40-volts arc drop at the fault point is subtracted from the line-to-neutral voltage, with both at the same phase angle of zero-degrees.
Z1 and Z2 are the system positive and negative sequence impedances. (In the general case, the system includes the cable and the upstream source impedances.)
Z0’ = Z0 – 3ZF,
The components of Z0’ are as follows:
+ Per above, Z0’ does not include 3ZF, the fault impedance term, the effect of which has been accounted for by the 40-volts drop in the numerator.
+ 3ZN is zero in this case because the system is solidly grounded.
+ Z0’ includes three times the circuit’s ground-return-path cold resistance, RGR. + Z0’ includes the hot resistance of the phase conductor, RPC, (as does Z1 and Z2). + Z0’ includes the zero-sequence inductive reactance of circuit components, including (in
the general case) the X0 of relevant source generators and transformers, and the X0C between the cable phase conductors and the ground return path. See Equation 4. Equation 3 can be used when impedance data for the installation is available or can be reasonably estimated, and should be used when the source impedance upstream of the cable is not negligible.
When the sum of the sequence impedances of the source system (upstream of a cable) is very small compared to the sum of the sequence impedances of the cable (say 1:10), the source impedance can be assumed to be zero with reasonable accuracy, and only the cable sequence impedances need to be used in Equation 3. This would result in the following equation:
IL – G + 3 x (EL – N – 40)
3 RPC ) 3 RGR ) j (X1C ) X1C ) X0C) Eq. (4)
where: The denominator of Equation 4 represents the sum of the sequence impedances of the cable and its ground return path (the source impedances having been assumed to be negligibly small). RPC is the phase conductor hot a-c resistance, from the source to the fault.
RGR is the ground return conductor cold a-c resistance, from the fault back to the relevant source neutral. X1C is the positive sequence reactance of the cable, which is equal to X2C, the negative sequence
reactance of the cable.
SOLIDLY GROUNDED SYSTEM DESIGN (Cont) Example 2 in the Appendix uses Equation 4.
If RGR and/or X0C for the cable installation is not available, the following calculation techniques may be employed to estimate the cable/ground-return impedance for use in the denominator of either Equation 3 or Equation 4.
Rewriting Equation 4: IL–G + EL – N – 40 L
ƪ
(R2 eff) )(X 2 eff)ƫ
1ń2 + EL – N– 40 L (Zeff) Eq. (5)where: L = Length of conductor from source to fault point, meters.
Reff = Effective resistance (hot) of one phase conductor from source to fault point plus the resistance (cold) of the ground return path, ohms per meter.
Xeff = Effective circuit inductive reactance, accounting for effect of both the phase conductor and the ground return path, ohms per meter.
Zeff = Effective impedance magnitude of the cable phase conductor and ground return circuit, ohms per meter.
The fault current, IL–G, calculated using Equations 3, 4 or 5 would be used to check that the device providing ground-fault protection operates quickly enough to satisfy IP 16-4-1 or local regulations.
METHODS FOR ESTIMATING Reff, Xeff, OR Zeff
Methods for estimating Reff, Xeff, or Zeff in Equation 5 (when RGR and/or X0C data are not available) are presented in the following paragraphs:
Xeff For Ground-Return Wire or Cable
For a ground-return wire within or external to a cable assembly, and not in a metallic conduit/pipe Reff = RPC + RGR
where: RPC is the a-c resistance (hot) of one phase conductor from source to fault, and RGR is the resistence (cold) of ground return wire from source to fault point, all in ohms per meter.
Xeff = 4f x 10–7 (0.5 + ln4 S 2
d1 d2) ohms per meter Eq. (6)
where: Xeff = Reactance between two round conductors, with diameters d1 and d2, separated by distance S. This approximates the average of the three sequence reactances of the cable/ground-return circuit. S = Distance between centers of phase and ground return conductor, meters.
d1 = Diameter of phase conductor, meters. f = System frequency
d2 = Diameter of ground return conductor, meters. ln = Natural logarithm (base e)
If the data for Equation 6 is not available, a calculation can be made to extract the impedance used in the IP 16-4-1 distance tables, which impedance can be used for Zeff. Solving Equation 5 for Zeff, and substituting Lmax for L, and “trip current” for ILG:
Zeff + EL*N * 40
Lmax x trip current ohms per meter Eq. (7)
where: Lmax is the distance found using the distance tables in IP 16-4-1.
“Trip current” is the fault current at which the overcurrent-device operates in less than 2 seconds; i.e., the trip current upon which the distance Lmax is based. For example; for a 50 A device rating, the trip current is 500 A for “Factor A” in Tables 1A and 1B.
