Instrumentation and Control Earthing & Grounding Technical Requirements

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Instrumentation and Control Earthing & Grounding Technical Requirements

1. International Standard NEC, Article 250 is applied to Electrical safety ground and instrumentation circuit ground.
2. Manufacturer recommendation and technical requirements (e.g., distributed control systems and ESD Systems) for grounding and earthing system of instrumentation and control to be followed on priority basis.
3. Safety Ground: All exposed non-current-carrying metallic parts that could become energized with hazardous potentials must be reliably connected to the equipment grounding circuits. Therefore, all metal equipment and enclosures within a panel or series of panels (i.e., instrument cases, hinged doors, racks, etc.) shall be bonded with bonding jumpers and connected to a safety ground bus with a minimum copper wire size of 4 mm² cross-sectional area. Two copper conductors, 25 mm² minimum, shall be connected from the safety ground bus to a single tie point on the safety ground grid in a closed loop configuration. Safety ground connections must be made such that when a case-grounded instrument is removed, the integrity of the rest of the safety ground system is maintained.
4. Enclosures for field instruments shall be grounded as follows:
   1. Instruments Operating at Greater Than 50 Volts – The enclosures for instrument devices operating at 120 V AC or 125 VDC shall be grounded per SAES-P-111.
   2. Instruments Operating at Less than 50 VDC – The enclosures of instrument devices operating at 50 VDC or less may be grounded using one of the following options:
      (a) 25 mm² ground wire.
      (b) Connecting the enclosure to a grounded instrument stand or other supporting structure, provided that the instrument device is properly fastened and the mounting clamp is mechanically and electrically in intimate contact with the stand.
      (c) Using the conduit as a ground conductor, provided that the conduit system is continuous and properly grounded. A bonding jumper shall be used across any flexible conduit at the instrument end. All conduit fittings shall be listed as suitable for grounding.
      (d) Using the cable armor (assuming armored cable is used) provided that the following criteria are met:
      1. The armor construction is suitable as a grounding path per the NEC.
      2. The cable glands, on each end of the armored cable, shall be designed to bond the armor to the gland (i.e., listed as suitable for grounding).
      3. The armored cable runs in one continuous length from a properly grounded junction box to the device being grounded, i.e., no splices are permitted.
      4. The armored cable is not in direct contact with the soil for any portion of the run.”
5. Instrument DC & Shield Ground:
   The purpose of instrument DC & Shield ground bus bar is to reduce the effect of electrical interference upon the signal being transmitted. A DC & Shield ground bus bar shall be provided within each cabinet for consolidating instrument signal commons and cable shield drain wires. This ground bus shall be isolated from the safety ground system and from the body of the cabinet except at the plant reference point as shown in Library Drawing DC-950150.
6. Each instrument signal common shall be connected to the isolated instrument DC & Shield ground bus with copper wire sized to carry the expected fault current or 1.5 mm², whichever is larger. Two insulated copper conductors, 25 mm² minimum, shall be connected from the instrument DC & Shield ground bus within each cabinet to a single tie point on the Master Instrument Ground bar (MIG) within the control building in a closed loop configuration. The resistance from the isolated instrument DC & Shield ground bus to the plant ground grid shall be less than 1 ohm.
7. Grounding within Control and Process Interface Buildings: In cases where there are many ground buses such as in control or process interface buildings, isolated instrument DC & Shield ground bars from all cabinets shall be consolidated into a Master Instrument Ground Bus (MIG) located within that building. Similarly, AC ground bus bars shall be consolidated into a Master Safety Ground Bus (MSG).
8. The Master instrument ground bar shall be connected to the PAS Master Reference Ground (MRG). The MRG should then be connected in a loop configuration to a single point on the plant grounding grid as shown in Library Drawing DC-950150.
9. Master Safety ground shall be connected directly to the plant grounding grid, as mandated by IEEE 1100. The connection to the plant grid shall be in a loop configuration as detailed in Library Drawing DC-950150.
10. Special Considerations:
    Some equipment (data highways, computers, distributed control systems, etc.) may require special provisions for grounding. Manufacturers’ recommendations should be carefully evaluated at all times.”
11. Ground Fault Detection:
    When critical control systems, i.e., emergency shutdown (ESD) systems, utilize fully floating DC power where both positive and negative buses are isolated from earth ground, a selective ground fault detection system shall be incorporated to detect leakage current from field I/O wiring to ground.
12. Due care must be taken to ensure that circuits from one ground fault selector switch will not be cross-connected to circuits from any other ground fault selector switch (e.g., at common annunciator points, lamp test connections).