Grounding of Portable Generators
2014 NEC Article 250.52 lists eight (8) potential grounding electrodes that can be used:
1. Metal underground water pipe
2. Metal frame of the building or structure
3. Concrete-encased electrode
4. Ground ring
5. Rod and Pipe electrodes
6. Other listed electrodes
7. Plate electrodes
8. Other local metal underground systems or structures
Each of the grounding electrodes has specific requirements for installation to achieve the lowest possible ground resistance for that type. The requirements in 250.53 (A)(1) to be installed “below permanent moisture level if practical” speaks more to their long term reliable function as these types of installations are rarely maintained as are other electrodes and systems.
In their case a 25 ohm value to earth is required. Where a single rod, pipe or plate electrode cannot meet this requirement they must be supplemented by an additional grounding electrode.
This also speaks to the importance of moisture (water) in achieving and maintaining low earth resistance. It should be noted, except for single Rod, Pipe and Plate electrode ground installations, that resistance or conductivity to earth is not required to meet a specific ohmic value.
The NEC does not require this parallel configuration to achieve 25 ohms or less. The 25 ohm requirement is not mandated on other types of grounding electrodes. The assumption has been that the other types of electrodes will achieve values lower than 25 ohms.
It should also be noted that the NEC does not specify types of soil where ground resistivity will vary during their long useful life. IEEE guides give greater insight into those requirements where achieving a low resistance value is critical to function or increased safety. The NEC is a minimum requirement. The origin of the 25 ohm requirement in the NEC goes back to the early 1900’s. The factual history is not clear but this was a time when early inventors such as Edison, Varley and Westinghouse were using the earth as one of the circuit conductors. Electrical systems and configurations have greatly changed since that time to increase user safety and system reliability. In those days an 8 foot ground rod would usually penetrate into the water table.
It is important to note why we earth (ground) our electrical equipment and systems for today’s needs when evaluating the RU Grounded Ground Bond Device.
OSHA 29 CFR 1926.404(f)(3)(1), 2014 NEC 250.34 and SJVBU 1389a section 4.7.5 require portable and vehicle mounted generators to be grounded (earthed) under the following conditions:
1. Generators greater than 5kW single phase shall have one or more approved connections to ground (earth) by means of grounding electrode(s) described in NEC Article 250.
2. When receptacle (plug sockets) mounted on the generator frame or vehicle are used to provide power to cord and plug connected equipment.
3. Non-current carrying metal parts of the equipment and the equipment grounding conductor of the receptacles are bonded to the generator frame.
4. For vehicle mounted generators, the frame of the generator is bonded to the frame of the vehicle.
5. Any neutral conductor is bonded to the generator frame.
Before considering why we ground our electrical systems and equipment it is important to understand the difference between grounding (aka earthing) and bonding. The 2014 NEC Article 100 provides effective definitions as:
Connected to ground (the earth) or a conductive body that extends the ground connection. Grounding and bonding applies to electrical systems and equipment to achieve safe use and operation.
1) Electrical System Grounding: “will limit the voltage imposed by lightning, line surges or unintentional contact with higher-voltage lines and will stabilize the voltage to ground during normal operation”. The type of system grounding where the RU Grounded device is intended to be utilized is a solidly grounded system on the portable generators.
2) Grounding of Electrical Equipment: “normally non-current carrying conductive materials connected to earth to limit the voltage to ground on these materials”. NOTE: Regardless of the type of system grounding employed, all electrical equipment must be grounded to ensure worker safety. The RU Grounded device can be utilized to ground both the system and the equipment.
Connected to establish electrical continuity or conductivity.
1) Bonding of Electrical Equipment: “normally non-current carrying conductive materials connected together and the electrical supply source to establish an effective ground fault current path”.
2) Bonding of Electrically Conductive Materials and Other Equipment: “normally noncurrent carrying conductive materials likely to become energized connected together and to the electrical supply source to establish an effective ground fault path”.
3) Effective Ground Fault Current Path: “a low impedance path capable of carrying the maximum ground fault current likely to be imposed to facilitate the operation of over current devices. THE EARTH SHALL NOT BE CONSIDERD AS AN EFFECTIVE GROUND FAULT CURRENT PATH”.