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Process Industry Practices

Process Control

PIP PCCEL001

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been prepared from the technical requirements in the existing standards of major industrial users, contractors, or standards organizations. By harmonizing these technical requirements into a single set of Practices, administrative, application, and engineering costs to both the purchaser and the manufacturer should be reduced. While this Practice is expected to incorporate the majority of requirements of most users, individual applications may involve requirements that will be appended to and take precedence over this Practice. Determinations concerning fitness for purpose and particular matters or application of the Practice to particular project or engineering situations should not be made solely on information contained in these materials. The use of trade names from time to time should not be viewed as an expression of preference but rather recognized as normal usage in the trade. Other brands having the same specifications are equally correct and may be substituted for those named. All Practices or guidelines are intended to be consistent with applicable laws and regulations including OSHA requirements. To the extent these Practices or guidelines should conflict with OSHA or other applicable laws or regulations, such laws or regulations must be followed. Consult an appropriate professional before applying or acting on any material contained in or suggested by the Practice.

© Process Industry Practices (PIP), Construction Industry Institute, The University of Texas at Austin, 3208 Red River Street, Suite 300, Austin, Texas 78705. PIP member companies and subscribers may copy this Practice for their internal use.

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Process Industry Practices

Process Control

PIP PCCEL001

Instrumentation Electrical Requirements

Table of Contents

1. Introduction ...2

1.1 Purpose ...2

1.2 Scope ...2

2. References...2

2.1 Process Industry Practices ...2

2.2 Industry Codes and Standards ...2

2.3 Government Regulations ...4

3. Definitions...4

4. General...5

5. Environmental ...6

6. Instrument Signal

Compatibility ...6

7. Wire and Cable Systems...7

7.1 General...7 7.2 Instrument Wiring ...7 7.3 Segregation/Separation Requirements ...11 7.4 Cable Tray ...11 7.5 Conduit Systems...11 7.6 Junction Boxes ...12

8. Terminations ... 15

8.1 General ... 15

8.2 Milliamp Signals - Typically 4 to 20 mA ... 15

8.3 Voltage Signals - 100 mV or Greater... 15

8.4 Voltage Signals - Less than 100 mV... 16

8.5 Thermocouple Signals ... 16

9. Instrument Power Systems ... 16

9.1 General ... 16

9.2 Branch Circuits Design ... 17

10. Grounding ... 18

11. Intrinsically Safe Instrument

Systems... 19

12. Non-Incendive Systems ... 19

13. Control Panel and Cabinet

Wiring ... 20

14. Installation... 20

Figure 1:

Typical Instrument Systems Power/Grounding Requirements

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1.

Introduction

1.1 Purpose

The purpose of this Practice is to provide electrical requirements for process measurement and control systems.

1.2 Scope

This Practice provides the requirements for the equipment selection, system design, and installation of electrical supply and wiring to support process measurement and control systems.

2.

References

Applicable requirements in the latest edition (or the edition indicated) of the following industry codes, standards, and references shall be considered an integral part of this Practice. Short titles will be used herein when appropriate.

2.1 Process Industry Practices (PIP)

PIP ELSWC03 - 600 Volt Power and Control Cable

PIP ELSWC03D - Data Sheet for 600 Volt Power and Control CablePIP ELSWC05 - 300 Volt Instrumentation Tray Cable

PIP ELSWC05D - Data Sheet for 300 Volt Instrumentation Tray CablePIP PCCGN001 - General Instrument Design Checklist

PIP PCCGN002 - General Instrument Installation CriteriaPIP PCESS001 - Safety Systems Guidelines

PIP PCIEF000 - Instrumentation Fabrication DetailsPIP PCIEL000 - Instrumentation Electrical Details

PIP PCSCB001 - Control Building Considerations SpecificationPIP PCSEL003 - Instrument Junction Boxes Specifications

2.2 Industry Codes and Standards

• American Petroleum Institute (API)

– API RP500 - Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class 1, Division 1 and Division 2

– API RP505 - Recommended Practice for the Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class 1, Zone 0, Zone 1, and Zone 2

– API RP552 - Transmission Systems

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• Institute of Electrical and Electronic Engineers (IEEE)

– IEEE 518 - IEEE Guide for the Installation of Electrical Equipment to Minimize Electrical Noise Inputs to Controllers from External Sources – IEEE 1100 - Recommended Practice for Powering and Grounding Sensitive

Electronic Equipment, The Emerald Book • ISA

– ISA RP12.1 - Definitions and Information Pertaining to Electrical Instruments in Hazardous (Classified) Locations

– ISA RP12.6 - Wiring Practices for Hazardous (Classified) Locations Instrumentation Part 1: Intrinsic Safety

– ISA RP60.8 - Electrical Guide for Control Centers

– ISA S12.1 - Definitions and Information Pertaining to Electrical Instruments in Hazardous (Classified) Locations

– ISA S12.6 - Installation of Intrinsic Safe Systems for Hazardous (Classified) Locations

– ISA S12.10 - Area Classification in Hazardous (Classified) Dust Locations – ISA S12.12 - Nonincendive Electrical Equipment for Use in Class I and II

Division 2 and Class III Divisions 1 and 2 Hazardous (Classified) Locations – ISA S50.1 - Compatibility of Analog Signals for Electronic Industrial Process

Instruments

– ISA S84.01 - Application of Safety Instrumented Systems for the Process Industries

– ISA Comprehensive Dictionary of Measurement and Control • National Electric Manufacturers Association (NEMA)

– NEMA 250 - Enclosures for Electrical Equipment (1000 Volts Maximum) – NEMA ICS6 - Enclosures for Industrial Control and Systems

– NEMA VE2 - Metal Cable Tray Installation Guidelines • National Fire Protection Association (NFPA)

– NFPA 70 - National Electrical Code (NEC)

– NFPA 496 - Purged and Pressurized Enclosures for Electrical Equipment – NFPA 497 - Recommended Practice for the Classification of Flammable

Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas

– NFPA 499 - Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas

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2.3 Government Regulations

Federal Standards and Instructions of the Occupational Safety and Health

Administration, including any requirements by state or local agencies with jurisdiction, shall apply.

• United States Department of Labor, Occupational Safety and Health Administration (OSHA)

– OSHA 29 CFR 1910 - Occupational Safety and Health Standards

3.

Definitions

AC Safety Ground: The grounding system required by NEC Article 250 to provide protection for personnel and electrical equipment. The Instrument Ground Bus is connected to the Safety Ground per the NEC requirements.

Control Room: The location where direct operation of the unit or plant is performed and where operations personnel are in constant attendance

Field Instrument Enclosure: The term used in the generic sense to indicate a cabinet or building that houses instruments and/or wiring termination external to Instrument Rooms or Control Buildings

Field Junction Box (JB): See Junction Box

Field-Powered Devices: Instruments located in the field that are powered from sources other than the Basic Process Control System (BPCS) and/or Safety Instrumented System (SIS). These are sometimes referred to as 4-wire devices.

Interface Box or Marshalling Cabinet: See Junction Box

Instrument Rooms: The term used in the generic sense for a walk-in type structure, including rack rooms, remote instrument enclosures, or any completely enclosed structure that houses control equipment

Instrument Ground Bus (IGB): Grounding system connected to a high-quality earth ground, independent from the AC Safety Ground, for all instrumentation signals. The IGB is tied to the AC Safety Ground at only one point, near a high-quality earth ground, per NEC and Figure 1 of this Practice. The IGB is isolated from all other grounds.

Junction Box (JB): A protective enclosure around connections between electric wires or cables (ISA Comprehensive Dictionary). The junction box may be a Field Junction Box, usually referred to as JB, or it may be within a control building or instrument enclosure, where it is referred to as an Interface Box (IB). The IB is sometimes called a Marshalling Cabinet or Termination Cabinet.

Owner: Principal end user

Uninterruptible Power Supply (UPS): A power supply system that provides uninterrupted power to the connected equipment for a specified period of time, even during failure of the

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primary power source. The UPS is typically a rectifier with batteries and an inverter system to provide alternating current (AC). A UPS is often used to provide reliable power to the

instrumentation power distribution system. (Refer to NEC Article 645-11.)

4.

General

4.1 All process measurement and control equipment shall be suitable for the electrical area classification in which they are installed.

4.2 All techniques used to comply with area classification requirements shall comply with NEC Chapter 5 or the authority having jurisdiction.

4.3 Equipment and enclosures that require purging to comply with area class requirements shall be purged in accordance with NFPA 496. Use of purging to reduce area

classification shall require Owner’s approval. For environmental purges, see Section 5.4.

4.4 Instruments shall not be installed in Zone 0, Zone 1, or Division 1 areas unless approved by Owner.

4.5 Instrument electrical equipment shall be rated for continuous energized duty. 4.6 Instrument cabinets or panels that contain more than one power source shall have a

caution sign identifying the separate power sources.

4.7 Redundant instruments shall be supplied from separate circuits and from separate branches where practical for each instrument. Separate I/O cards should be considered.

4.8 Indicator lights (non-annunciator lights used to visually communicate messages) on control panels shall be uniformly color-coded within each operating unit and,

preferably, across each site. A recommended color code for panel indicating lights is: 4.8.1 Green - Normal state, operating, usually energized

4.8.2 Red - Abnormal state, out of service, usually de-energized, or potentially dangerous

Comment: This convention is opposite to electrical power indicating light convention and is not allowed on UL- (Underwriters Laboratories, Inc.) labeled devices.

4.9 Each device of 50 V (AC or DC) or greater shall have its own individual lockable disconnect switch with appropriate labeling. Either local or remote lockout is permissible.

4.10 All electrical equipment including instrumentation shall be designed and installed in accordance with NEC and OSHA 1910 Subpart S.

4.11 Any box, including junction boxes, containing voltages higher than 50 V (AC or DC) shall have a caution sign on the outside of the box.

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4.13 Field instruments directly connected to flammable or explosive process fluids shall be designed with multiple barriers to prevent entry of vapors into the cable/conduit system. If the additional seal is required, a drain, vent, or other device is required so that the failure of the primary seal is obvious. Refer to NEC Article 501-5(f)(3).

Comment: The use of a sealing fitting with a drain, provided that the drain is on the instrument side, will satisfy the above-mentioned NEC requirement. 4.14 Minimum Design Capacity Allowance shall be provided as follows:

4.14.1 Design capacity allowance is the capacity available at the completion of design.

4.14.2 Control building cabinets and consoles shall be designed to accommodate a minimum of 20% additional equipment.

4.14.3 Field junction boxes and field cables shall be designed and installed to accommodate a minimum of 20% additional field devices.

4.14.4 Field wire raceways, exclusive of conduits, shall be designed for a minimum of 20% future additions.

4.14.5 Cable tray from the process area into the control room and instrument buildings shall be sized for a minimum of 40% future additions.

4.14.6 Power panels shall have at least 20% spare circuit breakers or fuses installed at design completion (refer to Section 9.2.5).

4.15 Input and output circuits of controllers that are not current limited shall be protected by fuses.

5.

Environmental Considerations

5.1 All instrument equipment shall be enclosed in environmentally appropriate housings. 5.2 NEMA 12 rated enclosures shall be used as a minimum for dry, indoor locations. For

indoor areas that contain oil or dust, NEMA 13 is the minimum rating (refer to NEMA ICS6 and NEMA 250).

5.3 NEMA 4 shall be the minimum outdoor rating for enclosures, and NEMA 4X shall be used in wet locations or where corrosion is an issue. Explosion-proof and

dust-ignition-proof enclosures shall be rated for the environment in which they are installed. If NEMA 7 enclosures are required outdoors, dual-rated enclosures (NEMA 7 and NEMA 4 or 4X) shall be used (refer to NEMA ICS6 and NEMA 250). See Section 4.3 on the use of purging for reduction of the electrical area classification.

5.4 With the Owner’s approval, enclosure purging for environmental purposes shall be considered where the ambient atmosphere contains corrosive contaminates or where other severe conditions exist. Use of purge media other than instrument air shall require specific Owner’s approval.

6.

Instrument Signal Compatibility

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of 4 mA to 20 mA DC in accordance with ISA S50.1. Other analog signals for existing instrumentation systems may be allowed with Owner’s approval.

6.2 Digital communications between smart transmitters and control systems shall adhere to vendor recommendations.

6.3 Power supply voltage shall nominally be 24 VDC.

7.

Wire and Cable Systems

7.1 General

The installation of all wiring and cables, including low-voltage instrument cables, shall comply with NEC requirements for the type of cable or wiring being used and the location in which it is being installed.

7.1.1 The routing for cables from field areas to the control room and instrument room in new plant areas is in overhead cable trays. Cable trays should not be routed in fire hazardous areas (e.g., over pumps, under process air fans, etc.). 7.1.2 For fire protection of instrument cables, refer to PIP PCCGN002 or to

Owner-supplied procedure.

7.1.3 Cables that are laid directly under raised floors shall be specifically approved for this method of installation by NEC Article 354. AC power wiring (120 VAC and higher) routed in the raised floor space shall utilize conduit, metal-enclosed raceway, metal clad cable, or cable tray, with adequate separation from the signal wiring.

7.1.4 The use of the area under a raised floor for an HVAC plenum shall require Owner’s approval. The wiring shall be installed in accordance with NEC Article 300-222. Any wiring in the HVAC plenum shall be “Plenum Rated” and non-toxic, unless installed in an enclosed conduit system.

7.1.5 For Safety Instrumented Systems (SIS) wiring, refer to ISA S84.01 and PIP PCESS001.

7.1.6 If the room is classified as an information technology equipment room, it shall meet all the requirements of NEC Article 645.

Comment: Process control electronic equipment alone does not require the room to be classified as a computer/data-processing room.

7.2 Instrument Wiring

All cables for general instrument use, both single pair and multiple wire, shall meet the requirements of PIP ELSWC03 and PIP ELSWC05. All instrument wiring shall be listed for the specific application for which it is being used.

7.2.1 Wiring

7.2.1.1 Each wire within a field cable, including spares entering a JB or interface cabinet (marshalling panel), shall be terminated.

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7.2.1.2 Splitting multiple conductor cables among multiple devices/boxes in the field is not permitted.

7.2.1.3 Protection against back electromotive force (EMF) shall be

provided for inductive loads such as relays, solenoids, etc. This may be accomplished within the equipment card or by one of the

methods below:

a. For DC loads, a diode may be installed electrically across the coil.

b. For AC inductive loads, a metal oxide varistor (MOV) may be installed across the coil.

7.2.1.4 Lightning protective devices shall be used when any of the following conditions exist (see API RP552, Section 16 and NEC, Article 280):

a. Electronic systems are located in large, open areas, such as tank farms.

b. Instrument wire and cable runs exceed 1500 feet. c. Instrument cables run aerially on poles.

7.2.1.5 Each pair or triad shall be permanently tagged and identified at both ends. Wire markers, tubular heat shrink, or other permanently affixed markers shall be used to ensure permanence of the marking. Machine printing is preferred for clarity.

7.2.1.6 In general, cables shall meet the design criteria listed below and shall be selected in accordance with Table 1.

a. Single Pair Instrument Signal Cable with an Overall Shield (SPISCO)

Application: This type covers the minimum requirements for single circuit cable for analog (e.g., 4 - 20 mA DC, 1 - 5 VDC, 0 - 100 mVDC) or low-voltage (50 volts DC or less), discrete signals for instrumentation and control signal transmission. Type: The pairs shall be 2 copper conductors, minimum 16 AWG, with an overall shield.

b. Single Pair Thermocouple Extension Cable with an Overall Shield (SPTECO)

Application: This type covers the minimum requirements for single pair cable made up of individually shielded, thermocouple extension wires twisted into pairs.

Type: Each pair shall be 2 solid alloy, thermocouple extension wires per ISA MC96.1 with an overall shield. The individual wires and outer jackets shall be color coded per

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c. Single Triad Instrument Signal Cable with an Overall Shield (STISCO)

Application: This type covers the minimum requirements for single circuit cable where three wires are required (e.g., RTDs and vibration) for instrumentation and control signal

transmission.

Type: The triads shall be 3 copper conductors, minimum 16 AWG, with an overall shield.

d. Multi-Pair Instrument Signal Cable with an Overall Shield (MPISCO)

Application: This type covers the minimum requirements for multiple circuit cable for high-level analog (e.g.,

4 - 20 mA DC, 1 - 5 VDC) or low-voltage digital (50 volts DC or less) signals used for instrumentation and control signal transmission. This type of cable is not to be used for signals below 100 millivolts.

Type: The cable shall be 12 or 24 pairs with an overall shield. Each pair shall be 2 copper conductors, minimum 20 AWG. e. Multi-Pair Thermocouple Extension Cable, Individually Shielded

with Overall Shield (MPTECI)

Application: This type covers the minimum requirements for multiple pair cable made up of individually shielded,

thermocouple extension wires twisted into pairs.

Type: The cable shall be 12 or 24 pairs with an overall shield. Each pair shall be 2 solid alloy, thermocouple extension wires per ISA MC96.1 with an individual shield. Each pair shall have an identifying number and shall be insulated from other pair shields. Conductors shall be minimum 20 AWG. The individual wires and outer jackets shall be color coded per ANSI/ISA MC96.1.

f. Multi-Pair Instrument Signal Cable Individually Shielded Pairs with Overall Shield (MPISCI)

Application: This type covers the minimum requirements for multiple individually shielded, circuit cable for low-level analog (e.g., 0 - 100 mVDC). With Owner’s approval, it may be used for high-level analog (e.g., 4 - 20 mA DC, 1 - 5 VDC) or for low-voltage digital (50 volts DC or less) signals used for instrumentation and control signal transmission.

Type: The cable shall be 12 or 24 pairs with an overall shield. Each pair shall be 2 copper conductors, minimum 20 AWG, with an individual shield insulated from other pair’s shields.

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g. Multi Triad Instrument Signal Cable with an Overall Shield (MTISCO)

Application: This type covers the minimum requirements for triads (three wire) multiple circuit cable for high-level analog (e.g., 4 - 20 mA DC, 1 - 5 VDC) or low-voltage digital (50 volts DC or less) signals used for instrumentation and control signal transmission. This type of cable is not to be used for signals below 100 Millivolts.

Type: The cable shall be 12 or 24 pairs with an overall shield. Each pair shall be 2 copper conductors, minimum 20 AWG. h. Multi-Triad Instrument Signal Cable Individually Shielded

Triads with Overall Shield (MTISCI)

Application: This type covers the minimum requirements for multiple individually shielded circuit cable for low-level analog (e.g., 0 - 100 mVDC). With Owner’s approval, it may be used for high-level analog (e.g., 4 - 20 mA DC, 1 - 5 VDC) or for low-voltage digital (50 volts DC or less) signals used for instrumentation and control signal transmission.

Type: The cable shall be 12 or 24 triads with an overall shield. Each triad shall be 2 copper conductors minimum 20 AWG, with an individual shield insulated from other pair’s shields.

7.2.2 Circuit Impedance

7.2.2.1 Wire resistance of single device, 4 - 20 mA instrument loops can usually be neglected for one-way distances less than 1000 feet. Where multiple devices are used in applications where the one-way loop distance is over 1000 feet and are wired in series in a

4 - 20 mA loop, the total circuit impedance shall be evaluated for proper operation.

7.2.2.2 120 VAC wiring to solenoid valves, relays, and other electro-mechanical devices shall be sized to have a voltage drop of less than 5% from the voltage source transformer to the control device at the rated holding current.

7.2.2.3 For long runs where wiring distributed capacitance is in parallel with the control device, designer shall perform calculations to ensure correct operability of each circuit.

Comment: AC input as discussed above may not de-energize due to the distributed capacitance effect. The use of rectifiers and DC relays should be considered.

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7.2.2.4 DC wiring to solenoid valves, relays, and other electromechanical devices should be sized to have a voltage drop of less than 10%.

Comment: Voltage drop calculations are not required for low-wattage (1.4 watts) DC solenoid valves that use multiple conductor cables, 20 AWG twisted pairs, and have lengths less than 700 feet. For distances over 700 feet, the use of higher voltage devices or larger wire size should be considered.

7.2.2.5 When low-energy devices such as pilot-operated solenoids, pilot lights, etc., are driven by an electronic amplifier, the “off position” leakage current must be low enough to allow the load to be “turned-off” when de-energized.

7.3 Segregation/Separation Requirements

7.3.1 Instrument wiring shall be segregated according to wiring class and circuit type in accordance with Table 1 and separation distances in accordance with Table 2.

7.3.2 Generally, each circuit type shall be run in a separate tray/conduit, except as noted in Table 2.

7.3.3 Intrinsically safe systems and fire detection systems shall be segregated from other wiring and shall have dedicated junction boxes and marshalling panels. 7.3.4 If different circuit types have to be terminated in the same junction box, each

circuit type shall enter the junction box in a separate cable or conduit and shall be terminated on terminal strips that are physically separated according to circuit type. Plastic or metal barriers that are labeled with the circuit type or power level shall have separate terminal strips.

7.4 Cable Tray

7.4.1 Cable tray systems shall be utilized to route multi-pair and multi-wire cables from field junction boxes or remote I/O enclosures to instrument buildings (see NEMA VE2).

7.4.2 Cable tray systems shall be grounded in accordance with NEC Table 318-7(b)(2).

7.4.3 Cable tray fill shall be based on NEC Tables 318-9, 318-9(c), and 318-10.

7.5 Conduit Systems

7.5.1 Instruments installed in NEC Class I, Division 2, or Division 1 with Owner’s approval that by design rely on a single compression seal diaphragm or tube, shall require an additional seal that meets the conditions of the process fluid. (See 4.13).

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7.5.2.1 Field instruments shall be wired from the field junction box to the field instrument by a dedicated, individual cable. Single pair cables in conduit or trays shall be the preferred wiring method.

7.5.2.2 Termination of conduit with flex conduit in Division 2 or approved flex for Division 1 area shall be used to provide isolation of the conduit system from vibration to protect against thermal expansion and for maintenance. The length shall be limited to 24 inches unless otherwise approved by Owner.

7.5.2.3 Conduit wiring capacity shall be sized in accordance with NEC. Comment: When performing this sizing calculation, care must

be taken to allow for the reduced area of the conduit fitting.

7.5.2.4 In highly corrosive environments, aluminum, PVC, or PVC-coated conduit may be used where approved by Owner. When PVC is used, the use of a grounding wire and additional supports are required in accordance with NEC Article 347.

7.5.2.5 Fiber and data highways shall meet equipment and cable vendor’s installation requirements and NEC Article 770.

7.5.2.6 Redundant data highways shall have separate routings with maximum practical physical separation to minimize the possibility of a single event causing the simultaneous loss of both wiring systems. Redundant data highway routings shall be approved by Owner.

7.6 Junction Boxes

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Table 1

Wire and Cable Requirements for Instrument Circuits

(For control panel wiring, see PIP PCSCP001. For 300 volt instrument tray cable, see PIP ELSWC02. For 600 volt power and control cable, see PIP ELSWC03.)

A. NEC Class 1 Circuits (Notes 1 and 3)

Instrument Class Circuit Example Conduit Wiring Tray Cable

A. 120 VAC or Less Switches, Solenoids, Relays 600 V THWN, Insulated, 14 AWG Minimum

600 V TC, Insulated

B. NEC Article 727 Circuits, Conduits, and Cable Tray Installations (Notes 1 and 11)

B1. Instrument Circuit Class and Type

Circuit Example Single Pairs or Triads Multiple Pair or Triad Cable

B2. 24 VDC or Less (Notes 4 and 8)

4 - 20 mA DC RTDs, Weigh Cells, Solenoids, Alarms, Switches, Relays, Secondary Motor Control, Digital Process Transmitter

300 V Shielded Twisted Pairs or Triads

CABLE TYPES: SPISCO, STISCO (Notes 2, 3, 5, 9, and 10)

300 V ITC PVC Insulated Twisted Pairs or Triads, Overall Shield

CABLE TYPES: MPISCO, MTISCO (Notes 2, 3, 4, 9, and 10) B3. Frequency or

Pulse Train Digital Communication (Note 6)

Speed, Vibration, Turbine Meter

300 V Shielded Twisted Pairs or Triads

CABLE TYPES: SPISCO, STISCO (Notes 2, 3, 9, and 10)

Individually Shielded Pairs Tray Cable or Triads CABLE TYPES: MPISCO, MTISCO (Notes 2, 3, 5, 9, and 10) B4. Thermocouple Measurement (Note 8)

Thermocouples 300 V ITC, 16 AWG, Individually Shielded Twisted Pair

CABLE TYPES: SPTECO

(Notes 2, 3, 9, and 10)

300 V ITC, 20 AWG, Individually Shielded Twisted Pairs, Overall Shield

CABLE TYPES: MPTECO

(Notes 2, 3, 9, and 10)

C. Data Highways and Other High-Speed Circuits

Data Highways (Notes 7 and 10)

EIA-422A Data Highways/ High-Speed Communication

Follow Cable and System Manufacturer’s

Recommendation

Follow Cable and System Manufacturer’s

Recommendation Notes:

1. Type A circuits shall be separated from all other types of instrument circuits by either conduit tray dividers or separate trays. 2. 20 AWG minimum wire size for multi-pair cable; 16 AWG minimum wire size for single pair cable. A larger wire size shall be used if

required by the calculations.

3. Wire sizes listed in the table and notes are minimum requirements. Actual wire size shall be based on load current and voltage drop requirements. The use of parallel wires to meet the current requirements is not allowed.

4. Higher DC levels require Owner’s approval.

5. Type ITC cable shall not be installed on either non-power-limited circuits or powered-limited circuits operating at more than 150 volts or more than 5 amperes.

6. Vibration signals shall be run in galvanized steel conduit, flexible conduit, or armored cables from the probe to the local transducer. 7. Maximum separation between redundant highways shall be obtained within the operating plant. The use of a single cable tray for

primary and redundant highways is not acceptable.

8. Type B1 and B3 circuits may be routed in the same conduits and trays.

9. For a description of the six-letter designators, see Section 6.2.1.7 and refer to the listings in PIP ELSWC05 and PIP ELSWC05D.

10. Differences in the manufacturer-recommended cable and these requirements shall be resolved with the Owner. 11. Physical barriers are also required to separate intrinsically safe wiring from other wiring.

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Table 2

Segregation and Minimum Spacing Requirements for Parallel Runs (For wiring level definitions, see Note 1. For units of measurement in inches, see Note 3.)

Steel-Conduit-to-Steel-Conduit Spacing Wiring Levels 1 2 3 4 5 1 0 1 3 12 12 2 1 0 3 9 12 3 3 3 0 0 0 4 12 9 0 0 0 5 12 12 0 0 0

Tray-to-Tray Spacing or Tray-to-Conduit Spacing

Wiring Levels 1 2 3 4 5 1 0 1 6 26 26 2 1 0 6 18 26 3 6 6 0 6 12 4 26 18 6 0 0 5 26 26 12 0 0 Notes: 1. Wiring Levels:

Level 1 - High Noise Susceptibility Application

1. 4 - 20 mA, 24 VDC instrument signals, other analog < 50 VDC and digital process transmitters 2. Discrete, 24 VDC instrument signals

3. +/- 50 VDC and common buses feeding analog instrument signals 4. +/- 24 VDC and common buses feeding digital instrument signals 5. Thermocouple and RTD circuits

6. Telephone circuits

Level 2 - Medium Noise Susceptibility Application 1. Light and switching circuits 24 VDC or less 2. Analog signals > 50 VDC < 28 VAC ripple Level 3 - Low Noise Susceptibility Application 1. 120/240 VAC feeders < 20 amps

2. Light and switching circuits 24 VDC or greater

3. 120/240 VDC relay, contactor and circuit breaker coils < 20 amps 4. Analog signals > 50 VDC < 28 VAC ripple

Level 4 - Medium Power Application

1. Primaries and secondary of transformers > 5 KVA 2. AC and DC buses 0 - 800 volts with currents > 20 amps Level 5 - High Power Application

1. AC and DC buses > 1 kW or currents > 800 amps, or both 2. This table is based on IEEE 518.

3. This table does not apply to the use of non-metallic conduit.

4. Cable tray spacing is defined as the minimum distance (in inches) between the top of one tray and the bottom of the tray above or between the sides of adjacent trays. This spacing also applies to the distance between trays and power equipment less than 100 KVA.

Conduit spacing is defined as the minimum distance (in inches) between the outside surfaces of conduits. This spacing also applies to the distance between conduits and power equipment less than 100 KVA.

5. When unlike signal levels must cross, in trays they shall cross at 90-degree angles. Where it is not possible to maintain spacing, a grounded metal barrier should be placed between unlike levels at the crossover point.

6. Levels 3 and 4 may be run in a common tray if separated by a barrier.

7. When separate trays are impractical, Levels 1 and 2 may be combined in a common tray, provided a grounded metal barrier separates the levels.

8. Trays for all levels were based on metal, solidly grounded, with good ground continuity. 9. Only cables having the same voltage class (Table 1) may be run together.

10. Trays and conduits containing Levels 1 and 2 shall not be routed parallel to high-power equipment enclosures of 100 KVA and larger at a spacing of less than 5 feet for trays and 2-1/2 feet for conduit.

11. Where the spacing listed in Table 2 is difficult to maintain, parallel runs should be minimized and in no case be run parallel to each other for a distance greater than 5 feet.

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8.

Terminations

8.1 General

8.1.1 All field instruments wiring shall be landed on terminal strips in field junction boxes. See NEC Article 110-14. Devices with attached pigtails may be spliced in conduit fittings with Owner’s approval. See

NEC Article 370-16.

8.1.2 Individual circuit wires shall not be spliced.

8.1.3 Connections to terminal strips shall be limited to two wires per screw. If the wires are solid or ferruled, then only one wire shall be connected per screw. 8.1.4 When terminating lugs are used on stranded wire, they shall be of the locking

type and limited to their design capability (i.e., two wires maximum per lug). 8.1.5 Terminal shall be tightened to the torque required by manufacturer.

8.1.6 All wires (including spares and shield drain wires) of each cable shall be landed on both ends in continuous order on terminals, dressed out neatly, and labeled.

8.1.7 Overall cable shield drain wires and individual pair shields shall be grounded to the IGB in the instrument building in which the cables terminate. Grounded thermocouples shall be installed per

Section 8.5 with shield wire grounded in thermocouple head.

8.1.8 All wire pairs, triads, and cables (including spares) used to connect instrument components and systems shall have uniform identification labels on each wire and cable at each point of termination.

8.2 Milliamp Signals - Typically 4 - 20 mA

8.2.1 Each signal pair shield shall be connected to the IGB and shall be isolated from ground elsewhere.

8.2.2 Signal (power) isolation shall be required for all devices powered from sources other than the Basic Process Control System (BPCS) and SIS that are located in the instrument building.

8.2.3 The black wire shall be positive at the power source and the white wire shall be negative.

8.2.4 Individual drain wires shall be isolated from ground at the field device and where exposed in conduit or cable seal fittings by use of electrical tape or heat shrink tubing.

8.3 Voltage Signals - 100 mV or Greater

The cabling for these measurement signals, sometimes called high-level signals, shall meet the same requirements as mA signals. (See Section 8.2.)

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8.4 Voltage Signals - Less than 100 mV

These measurement signals, sometimes called low-level signals, shall meet the same requirements as thermocouples. (See Section 8.5.)

8.5 Thermocouple Signals

8.5.1 Thermocouple extension wire shall be used between the thermocouple and the transmitter, multiplexer, or other monitoring device.

8.5.2 Thermocouple extension wire shall be the same type of material as the thermocouple. Specifically designed extension wire may be used instead of exotic materials with Owner’s approval.

8.5.3 The number of thermocouple extension cable junctions shall be minimized. 8.5.4 For grounded thermocouples, individual shield wires shall be grounded only at

the thermocouple connection in the head.

Comment: Shielding of thermocouple signals is extremely important to prevent noise.

8.5.5 If the ambient temperature is above 190º F (in rotating equipment or near furnaces), thermocouple wires shall have high-temperature insulation such as extended polytetra-fluorethylene.

9.

Instrument Power Systems

9.1 General

9.1.1 Control system instrument power requirements generally shall be 120 VAC and/or 24 VDC. Use of other voltages requires Owner’s approval.

9.1.2 AC power for instrument systems and supervisory control computers shall be supplied from UPS. The UPS shall be supplied from both a primary and a secondary power source.

9.1.3 For a typical power distribution system for the BPCS and SIS that requires a UPS, see Figure 1.

9.1.4 Alarms shall be installed in the control room to indicate: a. The loss of the secondary source

b. A transfer to the secondary source c. UPS trouble

d. UPS on batteries e. Low-battery alarm

9.1.5 A dedicated isolation instrument power transformer shall be provided for UPS bypass power.

9.1.6 The instrument power system shall be separate and distinct from non-critical circuits such as heating and lighting.

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9.1.7 Operating time capability (power from batteries of UPS) required after the total outage of AC supply power shall be set by the Owner considering unit response time and needs of units served.

Comment: A minimum of 30 minutes is a typical time for instrument systems power backup. Where additional time is required, consideration should be given to a backup generator as a secondary source for UPS.

9.1.8 Provisions for testing the UPS with process running shall be provided.

9.2 Branch Circuits Design

9.2.1 Circuit breakers or fuses shall be used to safeguard principal branches from subordinate branch faults. Each instrument supply shall be protected by an individual fuse or a current-limiting device.

9.2.2 Circuit protective devices shall be coordinated to ensure that the device nearest the overload or fault will open first, minimizing the possibility of initiating a UPS transfer and isolating the fault from the rest of the system. Circuits and loops shall be protected in related process systems.

Comment: UPS power distribution panels usually require either fast-acting electronic circuit breakers or fuses to allow circuit protective device coordination.

9.2.3 Power connections to redundant control system components and modules (e.g., DC power supplies, BPCS control modules, etc.) shall be taken from separate branch circuits and separate supplies (minimum of different breakers or distribution panels) to reduce common mode failures.

Three-phase UPS output is desirable and allows redundant components to be connected to separate phases, minimizing the impact of voltage depression on faulted phases before clearing.

9.2.4 Fast-acting single pole breakers or fuses shall be used for the 120 VAC, single-phase 2-wire circuits. Each circuit shall have an individual, unswitched neutral wire. Where only one single-phase inverter is supplied, fuses may be required to clear faults in adequate time to prevent loss of power supplies during a fault.

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9.2.5 Design and installation of the neutral wire shall satisfy the following requirements:

Grounding: The main bonding jumper at the derived source or the first panel downstream of the derived source establishes the AC neutral ground point. All circuit neutral wiring shall be insulated. Color Coding: Neutral wires shall be white except when there is more than

one, in which case, each shall be color coded to satisfy NEC Article 210.5.

Splicing: Splicing of the neutral wire is prohibited except when necessary at a field device.

Connection: The neutral wire connected to a device shall always be the one assigned to the source that serves that device, never one from some other circuit source.

9.2.6 Assignment of computer circuits shall meet the following requirements: a. Each computer system or subsystem requiring power shall be on a

separate circuit. The circuit may be wired with type “SO” service cord (NEC Table 400-4). Computer cabinets and devices shall be wired either to the computer power panel or connected to a receptacle using a “twist-lock” plug. UPS receptacles shall be distinguishable from other

receptacles.

b. High-power peripheral devices, such as line printers, shall be on a separate circuit and shall be connected via a wall- or floor-mounted receptacle, providing an isolated AC Safety Ground per NEC Article 250-74, Exception 4.

c. Peripheral devices (e.g., CRTs, gateways, etc.) and other equipment essential to normal process control shall be assigned to the highest reliability power available (such as UPS power).

d. Devices that are not essential for normal process control may be assigned to lower reliability power.

e. No spare wall or floor outlets are to be installed in the computer power system unless clearly marked as to the service.

10.

Grounding

10.1 Grounding electronic systems (e.g., BPCS, SIS, and Programmable Logic Controller (PLC)) shall follow the criteria below and the manufacturer’s recommendations. Any deviation shall be reviewed by the manufacturer’s technical representative and approved by Owner. See Figure 1 (INSTGND001).

10.2 Two grounding systems shall be required in the control room or instrument building: a. AC Safety Grounding for personnel safety

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The IGB and AC Safety Grounds shall join electrically at a single point near a high-quality earth ground (IEEE 1100, API RP 552, and Figure 1).

10.3 All electrical equipment shall be grounded to the AC Safety Ground for personnel protection to satisfy NEC Article 250.

10.4 The IGB wiring shall be installed to provide a stable voltage reference. The grounding system(s) should provide ground resistance level(s) consistent with supplier

recommendations and should meet the NEC requirements.

10.5 Ground systems shall be installed to provide a ground system that meets vendor requirements. Since various suppliers have different grounding requirements, care must be taken to develop a scheme that meets those requirements, meets the NEC requirements, has testing capabilities, and has supplier approval. (See IEEE 1100.)

Comment: A removable link may be provided between the AC Safety Ground and the IGB to allow testing of the IGB.

10.6 All IGB bars, plates, and wire shall be isolated from building AC Safety Ground path except at the one point specified in Section 10.2 above.

10.7 The following signal and equipment connections shall be made to the IGB: a. Instrument power supplies (negative leads)

b. BPCS, PLC, SIS, and other control modules (signal common, logic ground, network ground, etc.)

c. Instrument (individual and overall cable) shields

10.8 Signals originating from instruments powered by sources external to the control building (e.g., a chromatograph in an analyzer building) often are electrically

referenced to another ground potential. These instruments shall have isolated outputs, or the output signals shall be electrically isolated when brought into the BPCS, PLC, SIS, and other control modules.

10.9 Ground loops shall be avoided.

Comment: Ground loops exist when extraneous, unwanted currents flow in a wire causing an offset in the instrument reading. Care must be taken to avoid multiple grounds.

Comment: When instrumentation wiring is of the shielded type, shields should be grounded at only one place.

11.

Intrinsically Safe Instrument Systems

11.1 Use of intrinsically safe systems shall require Owner’s approval.

11.2 Refer to NEC Article 500-4(e) and ISA RP12.6 for requirements of intrinsically safe systems.

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12.2 All non-incendive wiring shall be installed in accordance with NEC Article 500-4(f) and ISA RP 12.12.

13.

Control Panel and Cabinet Wiring

All control panels, consoles, and equipment cabinets shall be wired in accordance with PIP PCSCP001.

14.

Installation

All instrumentation wiring installations shall be in accordance with installation details in PIP PCIEL000, unless otherwise approved by Owner. All Type B circuits shall use Instrument Tray Cable (ITC).

15.

Data Highways

Primary and backup data highway cables shall follow different routing and preferentially enters buildings and/or enclosures from opposite sides.

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References

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