Bonding a Separately Derived Generator
A small commercial building installs a 100 kW standby generator with a 4-pole transfer switch. The generator is a separately derived system because the transfer switch switches the neutral. The generator output is 347/600 V three-phase, four-wire. The electrician has bonded the generator neutral to the generator frame at the generator, and has also left the main service bonding jumper in place at the service. The grounding electrode conductor from the generator runs to a nearby ground rod driven at the generator pad, and the building steel is bonded at the service only. The electrician asks you to sign off so the generator can be commissioned. You notice the neutral conductor between the generator and the transfer switch is the same size as the phase conductors.
1. Evaluate the grounding and bonding arrangement for this separately derived system and identify any deficiencies.
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Because the transfer switch switches the neutral, the generator is a separately derived system and must be grounded and bonded as such. The generator neutral must be bonded to the generator frame and to the grounding electrode system at the generator, and the bonding jumper must be at that point only.
The key question is whether there is more than one neutral-to-ground bond on the generator source. If the main service bonding jumper remains in place and the generator neutral is bonded at the generator, the two systems are only connected when the transfer switch is in one position, so the neutrals are separated by the 4-pole switch. That part can be acceptable if the transfer switch truly switches the neutral and the generator neutral is not bonded again downstream.
The grounding electrode conductor must connect the generator to the grounding electrode system, not just a single isolated rod. A single rod at the pad may not meet the resistance requirement, and the building steel should be part of the electrode system. The neutral conductor sized the same as phase conductors is generally acceptable for a full-size neutral, but I would verify it against the calculated neutral load and the applicable table.
I would not sign off until the electrode system connection is verified and the single-point bonding is confirmed.
2. What tests or documentation would you require before commissioning, and why?
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I would require a continuity test confirming the neutral-to-ground bond exists at the generator and only at the generator for the separately derived source. I would require verification that the transfer switch switches the neutral and that the neutral is not bonded at the transfer switch enclosure.
I would require a grounding electrode resistance test or documentation that the electrode system meets the maximum resistance, and confirmation that the building steel and any underground metal water pipe are bonded into the electrode system. I would also require the generator manufacturer's grounding diagram and the engineered design showing the neutral sizing.
Documentation should include the as-built single-line diagram, the transfer switch wiring diagram, and the electrician's bonding and grounding checklist. The reason for each item is to prove there is exactly one neutral-to-ground bond per source and a low-impedance fault path back to the source, so that fault current will trip the overcurrent device rather than energize the frame.