I'm installing a replacement generator (1500kW, 480/277V output) in a system that is 480V, 3-wire (no 277V loads). The ATS is 3-pole, no neutral, and the utility feed is 3-wire. In the generator connection box, there is a neutral bus (no ground bus), and there is a chassis ground lug on the generator case. The manufacturer is telling me to bond my Equipment Grounding Conductor to the chassis ground, and make no connection to the neutral. But doesn't the EGC have to be referenced to the generator ground point? If there is a ground fault, don't I need a low-impedance path to ground? Why not bond the EGC to the generator neutral bus?
Generator Grounding/Bonding
- Thread starter msteiner
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Since your ATS is not a four pole (switched nuetral), the generator is not SDS and does not require the neutral to be grounded, that is done at your service entrance to provide a fault path. Therefore you run 5 circuit conductors from your generator 3-P, 1-N and an EGC.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Just a dumb terminology question here: if the utility feed is "3 wire", for what is presumably a 3 phase system, does this mean that no neutral has been run with the service conductors, presumably implying an ungrounded or corner grounded system? Or does "3 wire" simply not include a grounded messenger or other grounded conductor since it isn't used to carry current?
-Jon
-Jon
Good question Winnie, I should have read the post closer myself.
Is this a true Delta 3-wire system with GFD? Or is this a standard 4-wire system being used as Delta downstream?
If it is a true 3-Delta service, then all you run from the generator is 3-P and an EGC. Your switchgear should have GFD relays to protect the conductors. If a fault is detected, an alarm is souded for maintenance crew to isolate and repair, but it does not take the generator or service off-line as single ground fault posses no real harm. It is only when and if a second fault occurs that becomes a porlem, at which time the switch gear should operate and open the circuit.
Is this a true Delta 3-wire system with GFD? Or is this a standard 4-wire system being used as Delta downstream?
If it is a true 3-Delta service, then all you run from the generator is 3-P and an EGC. Your switchgear should have GFD relays to protect the conductors. If a fault is detected, an alarm is souded for maintenance crew to isolate and repair, but it does not take the generator or service off-line as single ground fault posses no real harm. It is only when and if a second fault occurs that becomes a porlem, at which time the switch gear should operate and open the circuit.
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The utility transformer is a delta-wye. The neutral is bonded to ground at the transformer secondary breaker at the substation, then the neutral is not carried any further. So it behaves as a 3-wire + G system. With no neutral at the ATS to bond to, it would seem that the generator neutral must be bonded to ground AT the generator breaker.
Here's another perspective: Consider the generator as separately derived. Normally, a SDS would require neutral switching at the ATS. My understanding of this requirement is that the two ground systems (utility and generator) need to be electrically discontinuous. However, because there are no neutrals run to the ATS, there is nothing to switch. So the generator neutral is bonded to ground at the generator, and the utility neutral is bonded to ground at the xfmr. The result is the same situation you would have if a 4-pole transfer switch was installed, and the neutral was switched. The sticking point is the fact that there are no neutral conductors in the system--but they are certainly not required. Thoughts??
- Location
- Illinois
- Occupation
- retired electrician
Is the substation on the load side of the service disconnect? 250.24(C) required the grounded conductor to be run to the service disconnect even if it is not needed for the building.
Don
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