Electrical Grounding Techniques
Electrical equipment, metallic structures, vessels, conduit, manholes, cable trays and fencing shall be permanently and effectively grounded in compliance with NFPA 70- Article 250. The tallest columns, stacks, steel structures, cable trays and pipe supports shall be grounded for lightning protection. Process equipment, motors and control stations in hazardous classified areas shall be grounded for the prevention of static discharge. For details see following topic.
Grounding and Lightning Protection for Substations
- Maximum resistance to ground shall not exceed 1 ohm for systems over 600 V and 5 ohms for systems below. Unless otherwise stipulated by the system supplier, computer
systems shall be 1 ohm. - The system shall consist of grounding electrodes, ground loops, ground grids (where required) and connections to the equipment requiring grounding.
- The main ground loop cables shall be a minimum 95 mm2 , bare, stranded, tinned copper.
- Cable size for outdoor equipment grounds shall be a minimum 16 mm2.
- Minimum grounding cable size shall be 6 mm2.
- Electrical equipment rated above 600 V shall have two connections to the ground system, one at each end of the equipment.
- Grounding system cable shall be stranded copper, with green or green with yellow striped PVC insulation.
- Buried connections shall be the exothermic welded type. Approved compression type connectors may be used above ground.
- Ground electrodes shall be copper or copper clad steel, a minimum of 3048 mm (10 ft) long with a minimum diameter of 3/4 inch.
- Ground loop cable shall be buried a minimum of 450 mm (18 inches).
- In non-metallic raceway systems a single insulated ground wire shall be pulled through each section of the system to be used for equipment grounding.
- Continuous cable tray systems shall have bonding jumpers installed across expansion joints and shall be connected to the grounding system at intervals not to exceed 30 m. If the tray system is not continuous from the source to utilization equipment, a separate grounding conductor is required.
- System and equipment grounding electrodes shall be interconnected by means of the main ground loop.
- A grounding grid consisting of 19 mm (3/4 in) by 3048 mm (10 ft) copper-clad steel ground rods and #3/0 AWG (95 mm2) copper cables forming a continuous loop or grid shall be used. The grid shall be installed in a uniform pattern.
- Ground rods shall be installed through a single 914 mm (3 ft) length of 203 mm (8 in) diameter concrete pipe, with removable cover to permit periodic inspection and to provide access to the ground rod clamps. The cover of the concrete pipe shall be located flush with finished grade.