1. PURPOSE
1.1 This engineering standard defines uniform grounding procedures that ensure personnel safety and code conformity for different types of equipment.
2. scope
2.1 This standard applies to all plant equipment grounding systems. While directed towards Air Products’ owned and operated facilities, it shall be considered the minimum requirements for any facility design. For sale of equipment facilities, the electrical designer shall verify these requirements with the customer’s representatives before proceeding with facility design.
2.2 This standard primarily describes work associated with the aboveground installation of ground conductors and the eventual connection of those conductors to the grounding electrode system.
3. related documents
3.1 Air Products Engineering Documents
4EL64352A Substation/Plant Grounding Practices
4EL64451A Lightning Protection
STD-G307A Grounding – High Voltage Motor and Compressor
STD-G309A Grounding – Cable Trays
STD-G310A Grounding – Conduit to Cable Tray
STD-G319A Grounding – Siemens PCS7 With AC Power Feed
STD-G320A Grounding – Siemens PCS7 Without AC Power Feed
STD-P319B Typical Separately Derived Systems/Panelboard Connection Details
STD-P334A Bonding of Substation Transformer Terminal Chambers
STD-P335A Installation of Neutral Grounding Resistor on Substation Transformers
3.2 National Fire Protection Association (NFPA)
70 National Electric Code (NEC)
4. Definitions
4.1 Grounding Electrode System—A network of interconnected conductors which are mostly buried horizontally underground and in direct contact with the earth, or within and near the bottom of the concrete foundations that are in direct contact with the earth. For additional details regarding grounding electrode systems refer to 4EL64352A.
4.2 Grounding Conductor, Equipment (EGC)—The conductive path(s) installed to connect normally non-current carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both.
4.3 Bonded—The permanent joining of metallic parts and equipment of an electrical system, not identified or utilized as current-carrying components, to form an electrically conductive path that will ensure electrical continuity and the capacity to safely conduct any current likely to be imposed on the equipment to the grounding electrode system.
- EQUIPMENT GROUNDING
5.1 Non-current-carrying metal parts of equipment, raceways, and enclosures shall be grounded as follows:
5.1.1 Terminal Boxes, Pullboxes, and Junction Boxes
5.1.1.1 Metal Enclosures – Connect largest size ground conductor entering box to side wall or bottom. Connection shall be made to unpainted metal or vendor supplied ground/stud terminal via one hole crimped lug. Bond together all ground conductors terminating in a box and connect to the identified ground point. Ground conductors passing through box do not need to be terminated. Pull boxes without any terminations shall be effectively bonded to ground by the conduit system.
5.1.1.2 Nonmetallic enclosures – All metallic conduits entering nonmetallic enclosures shall be bonded together using grounding bushings and bonding jumpers sized per the largest ground conductor entering the enclosure.
5.1.2 Switchgear, 4160V Starters, Motor Control Centers, and so forth.
5.1.2.1 All equipment ground conductors entering equipment shall be connected to the ground bus.
5.1.2.2 All ground buses in equipment shall be connected to the plant grounding electrode system at two places.
5.1.3 Motors
5.1.3.1 All motors shall be grounded using an equipment ground conductor running with phase conductors. Equipment ground conductors shall be sized per NEC requirements. Equipment ground conductors shall be terminated in the motor terminal box.
5.1.3.2 For medium voltage motors (motors operating above 600 volts), refer to STD-G307A for additional requirements associated with the motor grounding.
5.1.4 Panelboards
5.1.4.1 All panelboards shall be provided with ground bus. (This bus is in addition to the neutral bus when a neutral bus is required.) Connect this bus with the equipment ground conductor from the transformer. Refer to STD-P319B.
5.1.5 Receptacles
5.1.5.1 All receptacles shall be grounding type and be grounded with a ground conductor run with the circuit conductors.
5.1.5.2 Receptacles supplying power to electronic equipment (personal computers, monitors, printers, programmable logic controllers, analyzers, and so forth) shall have a dedicated ground wire run with the circuit conductors.
5.1.6 Transformers
5.1.6.1 Substation transformers – Transformer shells shall be connected to the plant grounding electrode system at two places. The grounding conductor shall be connected to ground pads on transformer at opposite corners with two hole crimp style lugs.
- Transformer Conductor Enclosures – Metallic air terminal chambers, cable bus housings, or bus duct coverings shall be bonded to the transformer shell ground pads per STD-P334A sheets 1 and 2.
- Transformer X0 Bushing – Connect the grounding electrode conductor to the transformer secondary X0 bushing. This connection shall be continuous from grounding electrode system to X0 bushing without splices as per the NEC. Refer to standard drawing STD-P334A sheets 1 and 2 for additional details.
5.1.6.2 Substation transformers with grounding resistor. Connect the ground resistor to the grounding electrode conductor (refer to STD-P335A). The grounding electrode conductor shall be continuous without splices (except irreversible exothermic or irreversible crimp type) and shall be installed in a nonmetallic conduit to prevent the conductor from touching any metallic portion of the transformer.
5.1.6.3 Dry-type transformers. Connect the ground conductor running with line and load power conductors to the cabinet grounding point. Connect the transformer neutral (XO) bushing to the cabinet and the grounding electrode system. See standard STD-P319B.
5.1.6.4 Regulating transformers. The regulating transformer manufacturer’s wiring data should always be used to identify actual requirements for field wiring terminations. The transformer neutral (XO) shall be connected per standard STD-P319B.
5.1.7 Distributive Control Systems (DCS)
5.1.7.1 For DCS system grounding requirements, refer to standard 4EL64352A and drawings STD-G319A and STD-G320A.
5.1.8 Lightning Protection
5.1.8.1 For additional grounding requirements associated with facility lightning protection, refer to 4EL64451A.
5.1.9 Metal Framed Equipment Skids
5.1.9.1 All metal framed equipment skids shall be connected to the plant grounding electrode system at two places. The grounding conductor shall be connected to ground pads, if provided, on metal framework at opposite corners with two hole crimp style lugs. If ground pads are not provided the grounding conductors shall be connected to opposite corners of the frame base using approved methods as described in standard 4EL64352A section 8.6.
- EQUIPMENT GROUNDING CONDUCTORS
6.1 Conduit and Cable Tray Systems
6.1.1 Air Products does not rely on conduit, raceway systems and cable tray as a grounding conductor; however, these systems must be effectively bonded to the grounding electrode system, using bonding jumpers, clamps, straps, and so forth.
6.2 A copper ground wire shall be run in the same raceway or trench with the ac power circuit conductors for all electrical equipment. Sizing of the ground wire shall be as identified in this standard, as identified in 4EL64352A or as required by the NEC.
6.2.1 Cable tray systems shall be bonded to each other and to the grounding electrode system (refer to STD-G309A and STD-G310A). All AC multi-conductor power and control cables run in cable tray shall have an integral ground wire as part of the cable assembly.
6.2.2 When multi-conductor power cables (either low voltage or medium voltage) are run in parallel the equipment ground conductor shall be sized per NEC requirements. If the equipment ground conductor installed as an integral part of the multi-conductor power cable assembly is not of sufficient size, an additional ground conductor of sufficient size shall be run in the cable tray with the multi-conductor power cables.
6.2.2.1 When multiple sets of power cables requiring separate ground conductors (as identified in paragraph 6.2.2) are installed in the same raceway, cable or cable tray, a single equipment grounding conductor may be used.
6.2.3 When single conductor power cables (either low voltage or medium voltage) are installed in cable trays, an additional ground conductor shall be run in the cable tray with the power cables.