HIPOT Testing
DECEMBER 1, 2011 10 COMMENTS
What is HIPOT Testing (Dielectric Strength Test):
Hipot Test is short name of high potential (high voltage) Teat and It also known as Dielectric Withstand Test. A hipot test checks for “good isolation.” Hipot test makes surety of no current will flow from one point to another point. Hipot test is the opposite of a continuity test.
Continuity Test checks surety of current flows easily from one point to another point while Hipot Test checks surety of current would not flow from one point to another point (and turn up the voltage really high just to make sure no current will flow).
Importance of HIPOT Testing:
The hipot test is a nondestructive test that determines the adequacy of electrical insulation for the normally occurring over voltage transient. This is a high-voltage test that is applied to all devices for a specific time in order to ensure that the insulation is not marginal.
Hipot tests are helpful in finding nicked or crushed insulation, stray wire strands or braided shielding,
conductive or corrosive contaminants around the conductors, terminal spacing problems, and tolerance errors in cables. Inadequate creepage and clearance distances introduced during the manufacturing process.
HIPOT test is applied after tests such as fault condition, humidity, and vibration to determine whether any
degradation has taken place.
The production-line hipot test, however, is a test of the manufacturing process to determine whether the construction of a production unit is about the same as the construction of the unit that was subjected to type testing. Some of the process failures that can be detected by a production-line hipot test include, for example, a transformer wound in such a way that creepage and clearance have been reduced. Such a failure could result from a new operator in the winding department. Other examples include identifying a pinhole defect in insulation or finding an enlarged solder footprint.
As per IEC 60950, The Basic test Voltage for Hipot test is the 2X (Operating Voltage) + 1000 V
The reason for using 1000 V as part of the basic formula is that the insulation in any product can be subjected to normal day-to-day transient over voltages. Experiments and research have shown that these over voltages can be as high as 1000 V.
Test method for HIPOT Test:
Hipot testers usually connect one side of the supply to safety ground (Earth ground). The other side of the supply is connected to the conductor being tested. With the supply connected like this there are two places a given conductor can be connected: high voltage or ground.
When you have more than two contacts to be hipot tested you connect one contact to high voltage and connect all other contacts to ground. Testing a contact in this fashion makes sure it is isolated from all other contacts.
If the insulation between the two is adequate, then the application of a large voltage difference between the two conductors separated by the insulator would result in the flow of a very small current. Although this small current is acceptable, no breakdown of either the air insulation or the solid insulation should take place.
Therefore, the current of interest is the current that is the result of a partial discharge or breakdown, rather
than the current due to capacitive coupling.
Time Duration for HIPOT Test:
The test duration must be in accordance with the safety standard being used.
The test time for most standards, including products covered under IEC 60950, is 1 minute.
A typical rule of thumb is 110 to 120% of 2U + 1000 V for 1–2 seconds.
Current Setting for HIPOT Test:
Most modern hipot testers allow the user to set the current limit. However, if the actual leakage current of the product is known, then the hipot test current can be predicted.
The best way to identify the trip level is to test some product samples and establish an average hipot current. Once this has been achieved, then the leakage current trip level should be set to a slightly higher value than the average figure.
Another method of establishing the current trip level would be to use the following mathematical formula: E(Hipot) / E(Leakage) = I(Hipot) / 2XI(Leakage)
The hipot tester current trip level should be set high enough to avoid nuisance failure related to leakage current and, at the same time, low enough not to overlook a true breakdown in insulation.
Test Voltage for HIPOT Test:
The majority of safety standards allow the use of either ac or dc voltage for a hipot test.
When using ac test voltage, the insulation in question is being stressed most when the voltage is at its peak, i.e., either at the positive or negative peak of the sine wave.
Therefore, if we use dc test voltage, we ensure that the dc test voltage is under root 2 (or 1.414) times the ac test voltage, so the value of the dc voltage is equal to the ac voltage peaks.
For example, for a 1500-V-ac voltage, the equivalent dc voltage to produce the same amount of stress on the insulation would be 1500 x 1.414 or 2121 V dc.
Advantage / Disadvantage of use DC Voltage for Hipot Test:
One of the advantages of using a dc test voltage is that the leakage current trip can be set to a much lower value than that of an ac test voltage. This would allow a manufacturer to filter those products that have marginal insulation, which would have been passed by an ac tester.
when using a dc hipot tester, the capacitors in the circuit could be highly charged and, therefore, a safe-discharge device or setup is needed. However, it is a good practice to always ensure that a product is discharged, regardless of the test voltage or its nature, before it is handled.
It applies the voltage gradually. By monitoring the current flow as voltages increase, an operator can detect a potential insulation breakdown before it occurs. A minor disadvantage of the dc hipot tester is that because dc test voltages are more difficult to generate, the cost of a dc tester may be slightly higher than that of an ac tester.
The main advantage of the dc test is DC Voltage does not produce harmful discharge as readily occur in AC. It can be applied at higher levels without risk or injuring good insulation. This higher potential can literally “sweep-out” far more local defects.
The simple series circuit path of a local defect is more easily carbonized or reduced in resistance by the dc leakage current than by ac, and the lower the fault path resistance becomes, the more the leakage current increased, thus producing a “snow balling” effect which leads to the small visible dielectric puncture usually observed. Since the dc is free of capacitive division, it is more effective in picking out mechanical damage as well as inclusions or areas in the dielectric which have lower resistance.
Advantage / Disadvantage of use AC Voltage for Hipot Test:
One of the advantages of an ac hipot test is that it can check both voltage polarities, whereas a dc test charges the insulation in only one polarity. This may become a concern for products that actually use ac voltage for their normal operation. The test setup and procedures are identical for both ac and dc hipot tests.
A minor disadvantage of the ac hipot tester is that if the circuit under test has large values of Y capacitors,
then, depending on the current trip setting of the hipot tester, the ac tester could indicate a failure. Most safety standards allow the user to disconnect the Y capacitors prior to testing or, alternatively, to use a dc hipot tester. The dc hipot tester would not indicate the failure of a unit even with high Y capacitors because the Y capacitors see the voltage but don’t pass any current.
Step for HIPOT Testing:
Only electrically qualified workers may perform this testing.
Open circuit breakers or switches to isolate the circuit or Cable that will be hi-pot tested.
Confirm that all equipment or Cable that is not to be tested is isolated from the circuit under test.
The limited approach boundary for this hi-pot procedure at 1000 volts is 5 ft. (1.53m) so place barriers
around the terminations of cables and equipment under test to prevent unqualified persons from crossing this boundary.
Connect the ground lead of the HIPOT Tester to a suitable building ground or grounding electrode conductor. Attach the high voltage lead to one of the isolated circuit phase conductors.
Switch on the HIPOT Tester. Set the meter to 1000 Volts or pre decide DC Voltage. Push the “Test” button on the meter and after one minute observe the resistance reading. Record the reading for reference.
At the end of the one minute test, switch the HIPOT Tester from the high potential test mode to the voltage measuring mode to confirm that the circuit phase conductor and voltage of HIPOT Tester are now reading zero volts.
Repeat this test procedure for all circuit phase conductors testing each phase to ground and each phase to each phase.
When testing is completed disconnect the HIPOT Tester from the circuits under test and confirm that the circuits are clear to be re-connected and re-energized.
To PASS the unit or Cable under Test must be exposed to a minimum Stress of pre decide Voltage for 1 minute without any Indication of Breakdown. For Equipments with total area less than 0.1 m2, the insulation resistance shall not be less than 400 MΩ. For Equipment with total area larger than 0.1 m2 the measured insulation resistance times the area of the module shall not be less than 40 MΩ⋅m2.
Safety precautions during HIPOT Test:
During a HIPOT Test, There may be at some risk so to minimize risk of injury from electrical shock make sure HIPOT equipment follows these guidelines:
1. The total charge you can receive in a shock should not exceed 45 uC.
2. The total hipot energy should not exceed 350 mJ.
3. The total current should not exceed 5 mA peak (3.5 mA rms)
5. If the tester doesn’t meet these requirements then make sure it has a safety interlock system that guarantees you cannot contact the cable while it is being hipot tested.
For Cable:
1. Verify the correct operation of the safety circuits in the equipment every time you calibrate it.
2. Don’t touch the cable during hipot testing.
3. Allow the hipot testing to complete before removing the cable.
4. Wear insulating gloves.
5. Don’t allow children to use the equipment.
6. If you have any electronic implants then don’t use the equipment.
Star-Delta Starter
MARCH 16, 2012 41 COMMENTS
Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges. To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed. Two methods are used for reduction of starting voltage are star delta starting and auto transformer stating.
Working Principal of Star-Delta Starter:
This is the reduced voltage starting method. Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings as illustrated in the figure below. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three. After a period of time the winding are reconfigured as delta and the
motor runs normally.
Star/Delta starters are probably the most common reduced voltage starters. They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply.
Traditionally in many supply regions, there has been a requirement to fit a reduced voltage starter on all motors greater than 5HP (4KW). The Star/Delta (or Wye/Delta) starter is one of the lowest cost electromechanical reduced voltage starters that can be applied.
The Star/Delta starter is manufactured from three contactors, a timer and a thermal overload. The contactors are smaller than the single contactor used in a Direct on Line starter as they are controlling winding currents only. The currents through the winding are 1/root 3 (58%) of the current in the line.
There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and that only carries star current while the motor is connected in star. The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating.
Star-delta Starter Consists following units:
1) Contactors (Main, star and delta contactors) 3 No’s (For Open State Starter) or 4 No’s (Close Transient Starter).
2) Time relay (pull-in delayed) 1 No.
3) Three-pole thermal over current release 1No.
4) Fuse elements or automatic cut-outs for the main circuit 3 Nos. 5) Fuse element or automatic cut-out for the control circuit 1No.
Power Circuit of Star Delta Starter:
The main circuit breaker serves as the main power supply switch that supplies electricity to the power circuit.
The main contactor connects the reference source voltage R, Y, B to the primary terminal of the motor U1,
V1, W1.
In operation, the Main Contactor (KM3) and the Star Contactor (KM1) are closed initially, and then after a period of time, the star contactor is opened, and then the delta contactor (KM2) is closed. The control of the contactors is by the timer (K1T) built into the starter. The Star and Delta are electrically interlocked and preferably mechanically interlocked as well. In effect, there are four states:
The star contactor serves to initially short the secondary terminal of the motor U2, V2, W2 for the start sequence during the initial run of the motor from standstill. This provides one third of DOL current to the motor, thus reducing the high inrush current inherent with large capacity motors at startup.
Controlling the interchanging star connection and delta connection of an AC induction motor is achieved by means of a star delta or wye delta control circuit. The control circuit consists of push button switches, auxiliary contacts and a timer.
Control Circuit of Star-Delta Starter (Open Transition):
The ON push button starts the circuit by initially energizing Star Contactor Coil (KM1) of star circuit and Timer Coil (KT) circuit.
When Star Contactor Coil (KM1) energized, Star Main and Auxiliary contactor change its position from NO to NC.
When Star Auxiliary Contactor (1)( which is placed on Main Contactor coil circuit )became NO to NC it’s complete The Circuit of Main contactor Coil (KM3) so Main Contactor Coil energized and Main
Contactor’s Main and Auxiliary Contactor Change its Position from NO To NC. This sequence happens in a friction of time.
After pushing the ON push button switch, the auxiliary contact of the main contactor coil (2) which is connected in parallel across the ON push button will become NO to NC, thereby providing a latch to hold the main contactor coil activated which eventually maintains the control circuit active even after releasing the ON push button switch.
When Star Main Contactor (KM1) close its connect Motor connects on STAR and it’s connected in STAR until Time Delay Auxiliary contact KT (3) become NC to NO.
Once the time delay is reached its specified Time, the timer’s auxiliary contacts (KT)(3) in Star Coil circuit will change its position from NC to NO and at the Same Time Auxiliary contactor (KT) in Delta Coil Circuit(4) change its Position from NO To NC so Delta coil energized and Delta Main Contactor becomes NO To NC. Now Motor terminal connection change from star to delta connection.
A normally close auxiliary contact from both star and delta contactors (5&6)are also placed opposite of both star and delta contactor coils, these interlock contacts serves as safety switches to prevent simultaneous activation of both star and delta contactor coils, so that one cannot be activated without the other deactivated first. Thus, the delta contactor coil cannot be active when the star contactor coil is active, and similarly, the star contactor coil cannot also be active while the delta contactor coil is active.
The control circuit above also provides two interrupting contacts to shutdown the motor. The OFF push button switch break the control circuit and the motor when necessary. The thermal overload contact is a protective device which automatically opens the STOP Control circuit in case when motor overload current is detected
by the thermal overload relay, this is to prevent burning of the motor in case of excessive load beyond the rated capacity of the motor is detected by the thermal overload relay.
At some point during starting it is necessary to change from a star connected winding to a delta connected winding. Power and control circuits can be arranged to this in one of two ways – open transition or closed transition.
What is Open or Closed Transition Starting
(1) Open Transition Starters.Discuss mention above is called open transition switching because there is an open state between the star state and the delta state.
In open transition the power is disconnected from the motor while the winding are reconfigured via external switching.
When a motor is driven by the supply, either at full speed or at part speed, there is a rotating magnetic field in the stator. This field is rotating at line frequency. The flux from the stator field induces a current in the rotor and this in turn results in a rotor magnetic field.
When the motor is disconnected from the supply (open transition) there is a spinning rotor within the stator and the rotor has a magnetic field. Due to the low impedance of the rotor circuit, the time constant is quite long and the action of the spinning rotor field within the stator is that of a generator which generates voltage at a frequency determined by the speed of the rotor. When the motor is reconnected to the supply, it is reclosing onto an unsynchronized generator and this result in a very high current and torque
transient. The magnitude of the transient is dependent on the phase relationship between the generated voltage and the line voltage at the point of closure can be much higher than DOL current and torque and can result in electrical and mechanical damage.
Open transition starting is the easiest to implement in terms or cost and circuitry and if the timing of the changeover is good, this method can work well. In practice though it is difficult to set the necessary timing to operate correctly and disconnection/reconnection of the supply can cause significant voltage/current transients.
In Open transition there are Four states:
1. OFF State: All Contactors are open.
2. Star State: The Main [KM3] and the Star [KM1] contactors are closed and the delta [KM2] contactor is open. The motor is connected in star and will produce one third of DOL torque at one third of DOL current.
3. Open State: This type of operation is called open transition switching because there is an open state between the star state and the delta state. The Main contractor is closed and the Delta and Star contactors are open. There is voltage on one end of the motor windings, but the other end is open so no current can flow. The motor has a spinning rotor and behaves like a generator.
4. Delta State: The Main and the Delta contactors are closed. The Star contactor is open. The motor is connected to full line voltage and full power and torque are available
(2) Closed Transition Star/Delta Starter.
There is a technique to reduce the magnitude of the switching transients. This requires the use of a fourth contactor and a set of three resistors. The resistors must be sized such that considerable current is able to flow in the motor windings while they are in circuit.
The auxiliary contactor and resistors are connected across the delta contactor. In operation, just before the star contactor opens, the auxiliary contactor closes resulting in current flow via the resistors into the star connection. Once the star contactor opens, current is able to flow round through the motor windings to the supply via the resistors. These resistors are then shorted by the delta contactor. If the resistance of the resistors is too high, they will not swamp the voltage generated by the motor and will serve no purpose.
In closed transition the power is maintained to the motor at all time. This is achieved by introducing resistors to take up the current flow during the winding changeover. A fourth contractor is required to place the resistor in circuit before opening the star contactor and then removing the resistors once the delta contactor is closed. These resistors need to be sized to carry the motor current. In addition to requiring more switching devices, the control circuit is more complicated due to the need to carry out resistor switching
In Close transition there are Four states:
1. OFF State. All Contactors are open
2. Star State. The Main [KM3] and the Star [KM1] contactors are closed and the delta [KM2] contactor is open. The motor is connected in star and will produce one third of DOL torque at one third of DOL current.
3. Star Transition State. The motor is connected in star and the resistors are connected across the delta contactor via the aux [KM4] contactor.
4. Closed Transition State. The Main [KM3] contactor is closed and the Delta [KM2] and Star [KM1] contactors are open. Current flows through the motor windings and the transition resistors via KM4.
5. Delta State. The Main and the Delta contactors are closed. The transition resistors are shorted out. The Star contactor is open. The motor is connected to full line voltage and full power and torque are available.
Effect of Transient in Starter (Open Transient starter)
It is Important the pause between star contactor switch off and Delta contactor switch is on correct. This is because Star contactor must be reliably disconnected before Delta contactor is activated. It is also important that the switch over pause is not too long.
For 415v Star Connection voltage is effectively reduced to 58% or 240v. The equivalent of 33% that is obtained with Direct Online (DOL) starting.
If Star connection has sufficient torque to run up to 75% or %80 of full load speed, then the motor can be connected in Delta mode.
When connected to Delta configuration the phase voltage increases by a ratio of V3 or 173%. The phase currents increase by the same ratio. The line current increases three times its value in star connection.
During transition period of switchover the motor must be free running with little deceleration. While this is
happening “Coasting” it may generate a voltage of its own, and on connection to the supply this voltage can randomly add to or subtract from the applied line voltage. This is known as transient current. Only lasting a few milliseconds it causes voltage surges and spikes. Known as a changeover transient.
Size of each part of Star-Delta starter
(1) Size of Over Load Relay:
For a star-delta starter there is a possibility to place the overload protection in two positions, in the line or in the windings.
Overload Relay in Line:
In the line is the same as just putting the overload before the motor as with a DOL starter.
The rating of Overload (In Line) = FLC of Motor.
Disadvantage: If the overload is set to FLC, then it is not protecting the motor while it is in delta (setting is x1.732 too high).
Overload Relay in Winding:
In the windings means that the overload is placed after the point where the wiring to the contactors are split into main and delta. The overload then always measures the current inside the windings.
The setting of Overload Relay (In Winding) =0.58 X FLC (line current).
Disadvantage: We must use separate short circuit and overload protections. (2) Size of Main and Delta Contractor:
There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These are AC3 rated at 58% of the current rating of the motor.
Size of Main Contactor= IFL x 0.58
(3) Size of Star Contractor:
The third contactor is the star contactor and that only carries star current while the motor is connected in star. The current in star is 1/ √3= (58%) of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor rating.
Size of Star Contactor= IFL x 0.33
Motor Starting Characteristics of Star-Delta Starter:
Available starting current: 33% Full Load Current.
Peak starting current: 1.3 to 2.6 Full Load Current.
Peak starting torque: 33% Full Load Torque.
Advantages of Star-Delta starter:
The operation of the star-delta method is simple and rugged
It is relatively cheap compared to other reduced voltage methods.
Good Torque/Current Performance.
It draws 2 times starting current of the full load ampere of the motor connected
Disadvantages of Star-Delta starter:
Low Starting Torque (Torque = (Square of Voltage) is also reduce).
Break In Supply – Possible Transients
Six Terminal Motor Required (Delta Connected).
It requires 2 set of cables from starter to motor.
It provides only 33% starting torque and if the load connected to the subject motor requires higher starting torque at the time of starting than very heavy transients and stresses are produced while changing from star to delta connections, and because of these transients and stresses many electrical and mechanical break-down occurs.
In this method of starting initially motor is connected in star and then after change over the motor is connected in delta. The delta of motor is formed in starter and not on motor terminals.
High transmission and current peaks: When starting up pumps and fans for example, the load torque is low at the beginning of the start and increases with the square of the speed. When reaching approx. 80-85 % of the motor rated speed the load torque is equal to the motor torque and the acceleration ceases. To reach the rated speed, a switch over to delta position is necessary, and this will very often result in high
transmission and current peaks. In some cases the current peak can reach a value that is even bigger than for a D.O.L start.
Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-delta starter.
Low Starting Torque: The star-delta (wye-delta) starting method controls whether the lead connections from the motor are configured in a star or delta electrical connection. The initial connection should be in the star pattern that results in a reduction of the line voltage by a factor of 1/√3 (57.7%) to the motor and the current is reduced to 1/3 of the current at full voltage, but the starting torque is also reduced 1/3 to 1/5 of the DOL starting torque .
The transition from star to delta transition usually occurs once nominal speed is reached, but is sometimes performed as low as 50% of nominal speed which make transient Sparks.
Features of star-delta starting
For low- to high-power three-phase motors.
Reduced starting current
Six connection cables
Reduced starting torque
Current peak on changeover from star to delta
Mechanical load on changeover from star to delta
Application of Star-Delta Starter:
The star-delta method is usually only applied to low to medium voltage and light starting Torque motors.
The received starting current is about 30 % of the starting current during direct on line start and the starting
torque is reduced to about 25 % of the torque available at a D.O.L start. This starting method only works when the application is light loaded during the start. If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position.
Impact of Floating Neutral in Power Distribution
JULY 28, 2012 12 COMMENTS
Introduction:
If The Neutral Conductor opens, Break or Loose at either its source side (Distribution Transformer, Generator or at Load side (Distribution Panel of Consumer), the distribution system’s neutral conductor will “float” or lose
its reference ground Point. The floating neutral condition can cause voltages to float to a maximum of its Phase volts RMS relative to ground, subjecting to its unbalancing load Condition.
Floating Neutral conditions in the power network have different impact depending on the type of Supply, Type of installation and Load balancing in the Distribution. Broken Neutral or Loose Neutral would damage to the connected Load or Create hazardous Touch Voltage at equipment body. Here We are trying to
understand the Floating Neutral Condition in T-T distribution System.
What is Floating Neutral?
If the Star Point of Unbalanced Load is not joined to the Star Point of its Power Source (Distribution
Transformer or Generator) then Phase voltage do not remain same across each phase but its vary according to the Unbalanced of the load.
As the Potential of such an isolated Star Point or Neutral Point is always changing and not fixed so it’s called Floating Neutral.
Normal Power Condition & Floating Neutral Condition
Normal Power Condition:
On 3-phase systems there is a tendency for the star-point and Phases to want to ‘balance out’ based on the ratio of leakage on each Phase to Earth. The star-point will remain close to 0V depending on the distribution of the load and subsequent leakage (higher load on a phase usually means higher leakage).
Three phase systems may or may not have a neutral wire. A neutral wire allows the three phase system to use a higher voltage while still supporting lower voltage single phase appliances. In high voltage distribution situations it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).
3 Phase 3 Wire System:
Three phases has properties that make it very desirable in electric power systems. Firstly the phase currents tend to cancel one another (summing to zero in the case of a linear balanced load). This makes it possible to eliminate the neutral conductor on some lines. Secondly power transfer into a linear balanced load is constant.
3 Phase 4 Wire System for Mix Load:
Most domestic loads are single phase. Generally three phase power either does not enter domestic houses or it is split out at the main distribution board.
Kirchhoff’s Current Law states that the signed sum of the currents entering a node is zero. If the neutral point is the node, then, in a balanced system, one phase matches the other two phases, resulting in no current through neutral. Any imbalance of Load will result in a current flow on neutral, so that the sum of zero is maintained.
For instance, in a balanced system, current entering the neutral node from one Phase side is considered positive, and the current entering (actually leaving) the neutral node from the other side is considered negative.
This gets more complicated in three phase power, because now we have to consider phase angle, but the concept is exactly the same. If we are connected in Star connection with a neutral, then the neutral conductor will have zero current on it only if the three phases have the same current on each. If we do vector analysis on this, adding up sin(x), sin(x+120), and sin(x+240), we get zero.
The same thing happens when we are delta connected, without a neutral, but then the imbalance occurs out in the distribution system, beyond the service transformers, because the distribution system is generally a Star Connected.
The neutral should never be connected to a ground except at the point at the service where the neutral is initially grounded (At Distribution Transformer). This can set up the ground as a path for current to travel back to the service. Any break in the ground path would then expose a voltage potential. Grounding the neutral in a 3 phase system helps stabilize phase voltages. A non-grounded neutral is sometimes referred to as a “floating neutral” and has a few limited applications.
Floating Neutral Condition:
Power flows in and out of customers’ premises from the distribution network, entering via the Phase and leaving via the neutral. If there is a break in the neutral return path electricity may then travel by a different path. Power flow entering in one Phase returns through remaining two phases. Neutral Point is not at ground Level but it Float up to Line Voltage. This situation can be very dangerous and customers may suffer serious electric shocks if they touch something where electricity is present.
Broken neutrals can be difficult to detect and in some instances may not be easily identified. Sometimes broken neutrals can be indicated by flickering lights or tingling taps. If you have flickering lights or tingly taps in your home, you may be at risk of serious injury or even death.
Voltage Measurement between Neutral to Ground:
A rule-of-thumb used by many in the industry is that Neutral to ground voltage of 2V or less at the receptacle is okay, while a few volts or more indicates overloading; 5V is seen as the upper limit.
Low Reading: If Neutral to ground voltage is low at the receptacle than system is healthy, If It is high, then you still have to determine if the problem is mainly at the branch circuit level, or mainly at the panel level.
Neutral to ground voltage exists because of the IR drop of the current traveling through the neutral back to
the Neutral to ground bond. If the system is correctly wired, there should be no Neutral to Ground bond except at the source transformer (at what the NEC calls the source of the Separately Derived System, or SDS, which is usually a transformer). Under this situation, the ground conductor should have virtually no current and therefore no IR drop on it. In effect, the ground wire is available as a long test lead back to the Neutral to ground bond.
High Reading: A high reading could indicate a shared branch neutral, i.e., a neutral shared between more than one branch circuits. This shared neutral simply increases the opportunities for overloading as well as for one circuit to affect another.
Zero Reading: A certain amount of Neutral to ground voltage is normal in a loaded circuit. If the reading is stable at close to 0V. There is a suspect an illegal Neutral to ground bond in the receptacle (often due to lose strands of the neutral touching some ground point) or at the subpanel. Any Neutral to ground bonds other than those at the transformer source (and/or main panel) should be removed to prevent return currents flowing through the ground conductors.
Various Factors which cause Neutral Floating:
There are several factors which are identifying as the cause of neutral floating. The impact of Floating Neutral is depend on the position where Neutral is broken
1) At The Three Phase Distribution Transformer:
Neutral failure at transformer is mostly failure of Neutral bushing.
The use of Line Tap on transformer bushing is identified as the main cause of Neutral conductor failure at transformer bushing. The Nut on Line Tap gets loose with time due to vibration and temperature difference resulting in hot connection. The conductor start melting and resulting broke off Neutral.
Poor workmanship of Installation and technical staff also one of the reasons of Neutral Failure.
A broken Neutral on Three phases Transformer will cause the voltage float up to line voltage depending upon the load balancing of the system. This type of Neutral Floating may damage the customer equipment connected to the Supply.
Under normal condition current flow from Phase to Load to Load to back to the source (Distribution
Transformer). When Neutral is broken current from Red Phase will go back to Blue or Yellow phase resulting Line to Line voltage between Loads.
Some customer will experience over voltage while some will experience Low voltage.
2) Broken Overhead Neutral conductor in LV Line:
The impact of broken overhead Neutral conductor at LV overhead distribution will be similar to the broken at transformer.
Supply voltage floating up to Line voltage instead of phase Voltage. This type of fault condition may damage customer equipment connected to the supply.
A broken Neutral of service conductor will only result of loss of supply at the customer point. No any damages to customer equipments.
4) High Earthing Resistance of Neutral at Distribution Transformer:
Good Earthing Resistance of Earth Pit of Neutral provide low resistance path for neutral current to drain in earth. High Earthing Resistance may provide high resistance Path for grounding of Neutral at Distribution Transformer.
Limit earth resistance sufficiently low to permit adequate fault current for the operation of protective devices in time and to reduce neutral shifting.
5) Over Loading & Load Unbalancing:
Distribution Network Overloading combined with poor load distribution is one of the most reason of Neutral failure.
Neutral should be properly designed so that minimum current will be flow in to neutral conductor.
Theoretically the current flow in the Neutral is supposed to be zero because of cancellation due to 120 degree phase displacement of phase current.
IN= IR<0 + IY<120 + IB<-120.
In Overloaded Unbalancing Network lot of current will flow in Neutral which break Neutral at its weakest Point.
6) Shared neutrals
Some buildings are wired so that two or three phases share a single neutral. The original idea was to duplicate on the branch circuit level the four wire (three phases and a neutral) wiring of panel boards. Theoretically, only the unbalanced current will return on the neutral. This allows one neutral to do the work for three phases. This wiring shortcut quickly became a dead-end with the growth of single-phase non-linear loads. The problem is that zero sequence current
From nonlinear loads, primarily third harmonic, will add up arithmetically and return on the neutral. In addition to being a potential safety problem because of overheating of an undersized neutral, the extra neutral current creates a higher Neutral to ground voltage. This Neutral to ground voltage subtracts from the Line to Neutral voltage available to the load. If you’re starting to feel that shared neutrals are one of the worst ideas that ever got translated to copper.
7) Poor workmanship & Maintenance :
Normally LV network are mostly not given attention by the Maintenance Staff. Loose or inadequate tightening of Neutral conductor will effect on continuity of Neutral which may cause floating of Neutral.
How to detect Floating Neutral Condition in Panel:
Let us Take one Example to understand Neutral Floating Condition.We have a Transformer which Secondary is star connected, Phase to neutral = 240V and Phase to phase = 440V.
Condition (1): Neutral is not Floating
Whether the Neutral is grounded the voltages remain the same 240V between phase & Neutral and 440V between phases. The Neutral is not Floating.
Condition (2): Neutral is Floating
All Appliances are connected: If the Neutral wire for a circuit becomes disconnected from the household’s main power supply panel while the Phase wire for the circuit still remains connected to the panel and the circuit has appliances plugged into the socket outlets. In that situation, if you put a voltage Tester with a neon lamp onto the Neutral wire it will glow just as if it was Live, because it is being fed with a very small current coming from the Phase supply via the plugged-in appliance(s) to the Neutral wire.
All Appliances are Disconnected: If you unplug all appliances, lights and whatever else may be connected to the circuit, the Neutral will no longer seem to be Live because there is no longer any path from it to the Phase supply.
Phase to Phase Voltage: The meter indicates 440V AC. (No any Effect on 3 Phase Load)
Phase to Neutral Voltage: The meter indicates 110V AC to 330V AC.
Neutral to Ground Voltage: The meter indicates 110V.
Phase to Ground Voltage: The meter indicates 120V.
This is because the neutral is “floats” above ground potential (110V + 120V = 230VAC). As a result the output is isolated from system ground and the full output of 230V is referenced between line and neutral with no ground connection.
If suddenly disconnect the Neutral from the transformer Neutral but kept the loading circuits as they are, Then Load side Neutral becomes Floating since the equipment that are connected between Phase to Neutral will become between Phase to Phase ( R to Y,Y to B), and since they are not of the same ratings, the artificial resulting neutral will be floating, such that the voltages present at the different equipments will no longer be 240V but somewhere between 0 (not exactly) and the 440 V (also not exactly). Meaning that on one line Phase to Phase, some will have less than 240V and some will have higher up to near 415. All depends on the impedance of each connected item.
In an unbalance system, if the neutral is disconnected from the source, the neutral becomes floating neutral and it is shifted to a position so that it is closer to the phase with higher loads and away from the phase with smaller load. Let us assume an unbalance 3 phase system has 3 KW load in R-phase, 2 KW load in Y-phase and 1 KW load in B-phase. If the neutral of this system is disconnected from the main, the floating neutral will be closer to R-phase and away from B-phase. So, the loads with B-phase will experience more voltage than usual, while the loads in R-phase will experience less voltage. Loads in Y-phase will experience almost same voltage. The neutral disconnect for an unbalanced system is dangerous to the loads. Because of the higher or lower voltages, the equipment is most likely to be damaged.
Here we observe that Neutral Floating condition does not impact on 3 Phase Load but It impacts only 1 Phase Load only
How to Eliminate Neutral Floating:
There are Some Point needs to be consider to prevent of Neutral Floating.
a) Use 4 Pole Breaker/ELCB/RCBO in Distribution Panel:
A floating neutral can be a serious problem. Suppose we have a breaker panel with 3 Pole Breaker for Three Phase and Bus bar for Neutral for 3 Phase inputs and a neutral (Here we have not used 4 Pole Breaker). The voltage between each Phase is 440 and the voltage between each Phase and the neutral is 230. We have single breakers feeding loads that require 230Volts. These 230Volt loads have one line fed by the breaker and a neutral.
Now suppose the Neutral gets loose or oxidized or somehow disconnected in the panel or maybe even out where the power comes from. The 440Volt loads will be unaffected however the 230V loads can be in serious trouble. With this Floating neutral condition you will discover that one of the two lines will go from 230Volts up to 340 or 350 and the other line will go down to 110 or 120 volts. Half of your 230Volt equipment will go up in high due to overvoltage and the other half will not function due to a low voltage condition. So, be careful with floating neutrals.
Simply use ELCB, RCBO or 4 Pole Circuit Breaker as income in the 3ph supply system since if neutral opens it will trip the complete supply without damaging to the system.
b) Using Voltage Stabilizer:
Whenever neutral fails in three phase system, the connected loads will get connected between phases owing to floating neutral. Hence depending on load resistance across these phases, the voltage keeps varying between 230V to 400V.A suitable servo stabilizer with wide input voltage range with high & low cutoff may help in protecting the equipments.
c) Good workmanship & Maintenance :
Give higher Priority on Maintenance of LV network . Tight or apply adequate Torque for tightening of Neutral conductor in LV system
Conclusion:
A Floating Neutral (Disconnected Neutral) fault condition is VERY UNSAFE because If Appliance is not working and someone who does not know about the Neutral Floating could easily touch the Neutral wire to find out why appliances does not work when they are plugged into a circuit and get a bad shock. Single phase Appliances are design to work its normal Phase Voltage when they get Line Voltage Appliances may Damage .Disconnected Neutral fault is a very unsafe condition and should be corrected at the earliest possible by troubleshooting of the exact wires to check and then connect properly.
What is Earthing
NOVEMBER 27, 2011 21 COMMENTS
Introduction:
The main reason for doing earthing in electrical network is for the safety. When all metallic parts in electrical equipments are grounded then if the insulation inside the equipments fails there are no dangerous voltages present in the equipment case. If the live wire touches the grounded case then the circuit is effectively shorted and fuse will immediately blow. When the fuse is blown then the dangerous voltages are away.
Purpose of Earthing:
(1) Safety for Human life/ Building/Equipments:
To save human life from danger of electrical shock or death by blowing a fuse i.e. To provide an alternative path for the fault current to flow so that it will not endanger the user
To protect buildings, machinery & appliances under fault conditions.
To ensure that all exposed conductive parts do not reach a dangerous potential.
To provide safe path to dissipate lightning and short circuit currents.
To provide stable platform for operation of sensitive electronic equipments i.e. To maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment .
(2) Over voltage protection:
Lightning, line surges or unintentional contact with higher voltage lines can cause dangerously high voltages to the electrical distribution system. Earthing provides an alternative path around the electrical system to minimize damages in the System.
(3) Voltage stabilization:
There are many sources of electricity. Every transformer can be considered a separate source. If there were not a common reference point for all these voltage sources it would be extremely difficult to calculate their relationships to each other. The earth is the most omnipresent conductive surface, and so it was adopted in the very beginnings of electrical distribution systems as a nearly universal standard for all electric systems.
(1) Plate type Earthing:
Generally for plate type earthing normal Practice is to use
Cast iron plate of size 600 mm x600 mm x12 mm. OR
Galvanized iron plate of size 600 mm x600 mm x6 mm. OR
Copper plate of size 600 mm * 600 mm * 3.15 mm
Plate burred at the depth of 8 feet in the vertical position and GI strip of size 50 mmx6 mm bolted with the plate is brought up to the ground level.
These types of earth pit are generally filled with alternate layer of charcoal & salt up to 4 feet from the bottom of the pit.
(2) Pipe type Earthing:
For Pipe type earthing normal practice is to use
GI pipe [C-class] of 75 mm diameter, 10 feet long welded with 75 mm diameter GI flange having 6 numbers of holes for the connection of earth wires and inserted in ground by auger method.
These types of earth pit are generally filled with alternate layer of charcoal & salt or earth reactivation compound.
Method for Construction of Earthing Pit (Indian Electricity
Board):
Excavation on earth for a normal earth Pit size is 1.5M X 1.5M X 3.0 M.
Use 500 mm X 500 mm X 10 mm GI Plate or Bigger Size for more Contact of Earth and reduce Earth Resistance.
Make a mixture of Wood Coal Powder Salt & Sand all in equal part
Wood Coal Powder use as good conductor of electricity, anti corrosive, rust proves for GI Plate for long life.
The purpose of coal and salt is to keep wet the soil permanently.
The salt percolates and coal absorbs water keeping the soil wet.
Care should always be taken by watering the earth pits in summer so that the pit soil will be wet.
Coal is made of carbon which is good conductor minimizing the earth resistant.
Salt use as electrolyte to form conductivity between GI Plate Coal and Earth with humidity.
Sand has used to form porosity to cycle water & humidity around the mixture.
Put GI Plate (EARTH PLATE) of size 500 mm X 500 mm X 10 mm in the mid of mixture.
Use Double GI Strip size 30 mm X 10 mm to connect GI Plate to System Earthling.
It will be better to use GI Pipe of size 2.5″ diameter with a Flange on the top of GI Pipe to cover GI Strip from EARTH PLATE to Top Flange.
Cover Top of GI pipe with a T joint to avoid jamming of pipe with dust & mud and also use water time to time through this pipe to bottom of earth plate.
Maintain less than one Ohm Resistance from EARTH PIT conductor to a distance of 15 Meters around the EARTH PIT with another conductor dip on the Earth at least 500 mm deep.
Check Voltage between Earth Pit conductors to Neutral of Mains Supply 220V AC 50 Hz it should be less than 2.0 Volts.
Factors affecting on Earth resistivity:
(1) Soil Resistivity:
It is the resistance of soil to the passage of electric current. The earth resistance value (ohmic value) of an earth pit depends on soil resistivity. It is the resistance of the soil to the passage of electric current.
It varies from soil to soil. It depends on the physical composition of the soil, moisture, dissolved salts, grain size and distribution, seasonal variation, current magnitude etc.
In depends on the composition of soil, Moisture content, Dissolved salts, grain size and its distribution, seasonal variation, current magnitude.
(2) Soil Condition:
Different soil conditions give different soil resistivity. Most of the soils are very poor conductors of electricity when they are completely dry. Soil resistivity is measured in ohm-meters or ohm-cm.
Soil plays a significant role in determining the performance of Electrode.
Soil with low resistivity is highly corrosive. If soil is dry then soil resistivity value will be very high.
If soil resistivity is high, earth resistance of electrode will also be high.
(3) Moisture:
Moisture has a great influence on resistivity value of soil. The resistivity of a soil can be determined by the quantity of water held by the soil and resistivity of the water itself. Conduction of electricity in soil is through water.
The resistance drops quickly to a more or less steady minimum value of about 15% moisture. And further increase of moisture level in soil will have little effect on soil resistivity. In many locations water table goes down in dry weather conditions. Therefore, it is essential to pour water in and around the earth pit to maintain moisture in dry weather conditions. Moisture significantly influences soil resistivity
(4) Dissolved salts:
Pure water is poor conductor of electricity.
Resistivity of soil depends on resistivity of water which in turn depends on the amount and nature of salts dissolved in it.
Small quantity of salts in water reduces soil resistivity by 80%. common salt is most effective in improving conductivity of soil. But it corrodes metal and hence discouraged.
(5) Climate Condition:
Increase or decrease of moisture content determines the increase or decrease of soil resistivity.
Thus in dry whether resistivity will be very high and in monsoon months the resistivity will be low.
(6) Physical Composition:
Different soil composition gives different average resistivity. Based on the type of soil, the resistivity of clay soil may be in the range of 4 – 150 ohm-meter, whereas for rocky or gravel soils, the same may be well above 1000 ohm-meter.
(7) Location of Earth Pit :
The location also contributes to resistivity to a great extent. In a sloping landscape, or in a land with made up of soil, or areas which are hilly, rocky or sandy, water runs off and in dry weather conditions water table goes down very fast. In such situation Back fill Compound will not be able to attract moisture, as the soil around the pit would be dry. The earth pits located in such areas must be watered at frequent intervals, particularly during dry weather conditions.
Though back fill compound retains moisture under normal conditions, it gives off moisture during dry weather to the dry soil around the electrode, and in the process loses moisture over a period of time. Therefore, choose a site that is naturally not well drained.
(8) Effect of grain size and its distribution:
Grain size, its distribution and closeness of packing are also contributory factors, since they control the manner in which the moisture is held in the soil.
Effect of seasonal variation on soil resistivity: Increase or decrease of moisture content in soil determines decrease or increase of soil resistivity. Thus in dry weather resistivity will be very high and during rainy season the resistivity will be low.
(9) Effect of current magnitude:
Soil resistivity in the vicinity of ground electrode may be affected by current flowing from the electrode into the surrounding soil.
The thermal characteristics and the moisture content of the soil will determine if a current of a given magnitude and duration will cause significant drying and thus increase the effect of soil resistivity
(10) Area Available:
Single electrode rod or strip or plate will not achieve the desired resistance alone.
If a number of electrodes could be installed and interconnected the desired resistance could be achieved. The distance between the electrodes must be equal to the driven depth to avoid overlapping of area of influence. Each electrode, therefore, must be outside the resistance area of the other.
(11) Obstructions:
The soil may look good on the surface but there may be obstructions below a few feet like virgin rock. In that event resistivity will be affected. Obstructions like concrete structure near about the pits will affect resistivity. If the earth pits are close by, the resistance value will be high.
(12) Current Magnitude:
A current of significant magnitude and duration will cause significant drying condition in soil and thus increase the soil resistivity.
Measurement of Earth Resistance by use of Earth Tester:
For measuring soil resistivity Earth Tester is used. It is also called the “MEGGER”.
It has a voltage source, a meter to measure Resistance in ohms, switches to change instrument range, Wires to connect terminal to Earth Electrode and Spikes.
It is measured by using Four Terminal Earth Tester Instrument. The terminals are connected by wires as in illustration.
P=Potential Spike and C=Current Spike. The distance between the spikes may be 1M, 2M, 5M, 10M, 35M, and 50M.
All spikes are equidistant and in straight line to maintain electrical continuity. Take measurement in different directions.
Soil resistivity =2πLR.
R= Value of Earth resistance in ohm.
Distance between the spikes in cm.
π = 3.14
P = Earth resistivity ohm-cm.
Earth resistance value is directly proportional to Soil resistivity value
In this method earth tester terminal C1 & P1 are shorted to each other and connected to the earth electrode (pipe) under test.
Terminals P2 & C2 are connected to the two separate spikes driven in earth. These two spikes are kept in same line at the distance of 25 meters and 50 meters due to which there will not be mutual interference in the field of individual spikes.
If we rotate generator handle with specific speed we get directly earth resistance on scale.
Spike length in the earth should not be more than 1/20th distance between two spikes.
Resistance must be verified by increasing or decreasing the distance between the tester electrode and the spikes by 5 meter. Normally, the length of wires should be 10 and 15 Meter or in proportion of 62% of ‘D’.
Suppose, the distance of Current Spike from Earth Electrode D = 60 ft, Then, distance of Potential Spike
would be 62 % of D = 0.62D i.e. 0.62 x 60 ft = 37 ft.
Four Point Method:
In this method 4 spikes are driven in earth in same line at the equal distance. Outer two spikes are connected to C1 & C2 terminals of earth tester. Similarly inner two spikes are connected to P1 & P2 terminals. Now if we rotate generator handle with specific speed, we get earth resistance value of that place.
In this method error due to polarization effect is eliminated and earth tester can be operated directly on A.C.
GI Earthing Vs Copper Earthing:
As per IS 3043, the resistance of Plate electrode to earth (R) = (r/A) X under root(P/A).
Where r = Resistivity of Soil Ohm-meter.
A=Area of Earthing Plate m3.
The resistance of Pipe electrode to earth (R) = (100r/2πL) X loge (4L/d).
Where L= Length of Pipe/Rod in cm
d=Diameter of Pipe/Rod in cm.
The resistivity of the soil and the physical dimensions of the electrode play important role of resistance of Rod with earth.
The material resistivity is not considered important role in earth resistivity.
Any material of given dimensions would offer the same resistance to earth. Except the sizing and number of the earthing conductor or the protective conductor.
Pipe Earthing Vs Plate Earthing:
Suppose Copper Plate having of size 1.2m x 1.2m x 3.15mm thick. soil resistivity of 100 ohm-m,
The resistance of Plate electrode to earth (R)=( r/A)X under root(π/A) = (100/2.88)X(3.14/2.88)=36.27 ohm
Now, consider a GI Pipe Electrode of 50 mm Diameter and 3 m Long. soil resistivity of 100 Ohm-m,
The resistance of Pipe electrode to earth (R) = (100r/2πL) X loge (4L/d) = (100X100/2X3.14X300) X loge
From the above calculation the GI Pipe electrode offers a much lesser resistance than even a copper plate electrode.
As per IS 3043 Pipe, rod or strip has a much lower resistance than a plate of equal surface area.
Length of Pipe Electrode and Earthing Pit:
The resistance to earth of a pipe or plate electrode reduces rapidly within the first few feet from ground (mostly 2 to 3 meter) but after that soil resistivity is mostly uniform.
After about 4 meter depth, there is no appreciable change in resistance to earth of the electrode. Except a number of rods in parallel are to be preferred to a single long rod.
Amount of Salt and Charcoal (more than 8Kg) :
To reduce soil resistivity, it is necessary to dissolve in the moisture particle in the Soil.
Some substance like Salt/Charcoal is highly conductive in water solution but the additive substance would reduce the resistivity of the soil, only when it is dissolved in the moisture in the soil after that additional quantity does not serve the Purpose.
5% moisture in Salt reduces earth resistivity rapidly and further increase in salt content will give a very little decrease in soil resistivity.
The salt content is expressed in percent by weight of the moisture content in the soil. Considering 1M3 of Soil, the moisture content at 10 percent will be about 144 kg. (10 percent of 1440 kg). The salt content shall be 5% of this (i.e.) 5% of 144kg, that is, about 7.2kg.
Amount of Water Purring:
Moisture content is one of the controlling factors of earth resistivity.
Above 20 % of moisture content, the resistivity is very little affected. But below 20% the resistivity increases rapidly with the decrease in moisture content.
If the moisture content is already above 20% there is no point in adding quantity of water into the earth pit, except perhaps wasting an important and scarce national resource like water.
Length Vs Diameter of Earth Electrode:
Apart from considerations of mechanical strength, there is little advantage to be gained from increasing the earth electrode diameter with the object in mind of increasing surface area in contact with the soil.
The usual practice is to select a diameter of earth electrode, which will have enough strength to enable it to be driven into the particular soil conditions without bending or splitting. Large diameter electrode may be more difficult to drive than smaller diameter electrode.
The depth to which an earth electrode is driven has much more influence on its electrical resistance characteristics than has its diameter.
Maximum allowable Earth resistance:
Major power station= 0.5 Ohm.
Major Sub-stations= 1.0 Ohm
Minor Sub-station = 2 Ohm
Neutral Bushing. =2 Ohm
Service connection = 4 Ohm
Medium Voltage Network =2 Ohm
L.T.Lightening Arrestor= 4 Ohm
L.T.Pole= 5 Ohm
H.T.Pole =10 Ohm
Tower =20-30 Ohm
Treatments to for minimizing Earth resistance:
Remove Oxidation on joints and joints should be tightened.
Poured sufficient water in earth electrode.
Used bigger size of Earth Electrode.
Electrodes should be connected in parallel.
Earth pit of more depth & width- breadth should be made
Difference between Unearthed Cable & Earthed Cables
JANUARY 2, 2013 2 COMMENTS
Introduction:
In HT electrical distribution, the system can be earthed or unearthed. The selection of earthed/unearthed cable will depend on system. If distribution system is earthed then we have to use cable which is manufactured for earthed system. (Which the manufacturer specifies). If the system is unearthed then we need to use cable which is manufactured for unearthed system. The unearthed system requires high insulation level compared to earthed System.
For earthed and unearthed XLPE cables, the IS 7098 part2 1985 does not give any difference in specification. The insulation level for cable for unearthed system has to be more.
Earthed System:
Earlier the generators and transformers were of small capacities and hence the fault current was less. The star point was solidly grounded. This is called earthed system.
In Three phases earthed system, phase to earth voltage is 1.732 times less than phase to phase voltage. Therefore voltage stress on cable to armor is 1.732 times less than voltage stress between conductors to conductor.
Where in unearthed system, (if system neutral is not grounded) phase to ground voltage can be equal to phase to phase voltage. In such case the insulation level of conductor to armor should be equal to insulation level of conductor to conductor.
In an earthed cable, the three phase of cable are earthed to a ground. Each of the phases of system is grounded to earth. Examples: 1.9/3.3 KV, 3.8/6.6 KV system
Unearthed System:
Today generators of 500MVA capacities are used and therefore the fault level has increased. In case of an earth fault, heavy current flows into the fault and this lead to damage of generators and transformers. To reduce the fault current, the star point is connected to earth through a resistance. If an earth fault occurs on one phase, the voltage of the faulty phase with respect to earth appears across the resistance. Therefore, the voltage of the other two healthy phases with respect to earth rises by 1.7 times. If the insulation of these phases is not designed for these increased voltages, they may develop earth fault. This is called unearthed system.
In an unearth system, the phases are not grounded to earth .As a result of which there are chances of getting shock by personnel who are operating it. Examples : 6.6/6.6 KV, 3.3/3.3 KV system.
Unearthed cable has more insulation strength as compared to earthed cable. When fault occur phase to ground voltage is √3 time the normal phase to ground voltage. So if we used earthed cable in unearthed System, It may be chances of insulation puncture. So unearthed cable are used. Such type of cable is used in 6.6 KV systems where resistance type earthing is used.
Nomenclature:
In simple logic the 11 KV earthed cable is suitable for use in 6.6 KV unearthed system. The process of manufacture of cable is same. The size of cable will depend on current rating and voltage level.
Voltage Grade (Uo/U) where Uo is Phase to Earth Voltage & U is Phase to Phase Voltage.
Earthed system has insulation grade of KV / 1.75 x KV.
For Earthed System (Uo/U): 1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33 kV.
Unearthed system has insulation grade of KV / KV.
For Unearthed System (Uo/U): 3.3/3.3 kV and 11/11 kV.
3 phase 3 wire system has normally Unearthed grade cables and 3 phase 4 wire systems can be used earthed grade cables, insulation used is less, and cost is less.
Thumb Rule:
As a thumb rule we can say that 6.6KV unearthed cable is equal to 11k earthed cable
i.e. 6.6/6.6kvUnearthedcable can be used for 6.6/11kv earthed system. because each core of cable have the insulation level to withstand 6.6kv so between core to core insulation level will be 6.6kv+6.6kv = 11kv
For transmission of HT, earthed cable will be more economical due to low cost where as unearthed cables
are not economical but insulation will be good.
Generally 6.6 kV and 11kV systems are earthed through a neutral grounding resistor and the shield and armor are also earthed, especially in industrial power distribution applications. Such a case is similar to an unearthed application but with earthed shield (some times called solid bonding). In such cases, unearthed cables may be used so that the core insulation will have enough strength but current rating is de-rated to the value of earthed cables. But it is always better to mention the type of system earthing in the cable
specification when ordering the cables so that the cable manufacturer will take care of insulation strength and de rating.
Over Current Relay(Type-Application-Connection):
JANUARY 1, 2013 2 COMMENTS
Types of protection:
Protection schemes can be divided into two major groupings:
1. Unit schemes
2. Non-unit schemes
