Generator Bonding
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hillbilly1
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- North Georgia mountains
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- Owner/electrical contractor
If it has a four pole transfer switch, yes (bond at the generator). You will also need it to be connected to the grounding electrode also.
hillbilly1
Senior Member
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- North Georgia mountains
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- Owner/electrical contractor
The four pole switch breaks the neutral along with the phases, normally used in conjunction with ground fault switchgears, If this generator is 480, the main on it may also be ground fault, in which for either system to work correctly the neutrals have to be isolated from each other.
brian john
Senior Member
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- Leesburg, VA
Even with out ground fault, grounding a generator with a 3-pole ATS is wrong and will result in a parallel path of the neutral and the GEC. When the generator is supporting the load the GEC and neutral will carry current. When the generator is off there is a good chance the GEC and neutral will have current on them, commonly called circulating current.
If you do not ground the neutral at the generator with a 4-pole ATS then when the generator is connected to the load you will have an ungrounded system.
This is based on a 3-phase wye system.
If you do not ground the neutral at the generator with a 4-pole ATS then when the generator is connected to the load you will have an ungrounded system.
This is based on a 3-phase wye system.
tryinghard
Senior Member
- Location
- California
EEC said:
If this generator is the sole or whole source it should be treated as a separately derived system because it would be the only source. If this is the case you need to bond the neutral at either the generator or the first disconnect (which is the transfer switch). The grounding electrode conductor must connect at the same location. The bonding usually takes place at the transfer switch. Notice ?Separately Derived Systems? in definitions and 250.30.
If this is a second source to supply portions of distribution and the grounded ? neutral ? conductor does not open with the transfer switch it is not a separately derived system (SDS) because the ground fault path remains available to either source in the event of a short. This is most common with multiple sources.
It?s all about the fault path back to source to allow extremely high current as fast as possible which enables overcurrent protection to function ? trip the breaker. Fault current is not trying to go to earth it is trying to return to its source.
Returning unused current does not follow the path of least resistance it follows ALL paths to its source, so it is important to control neutral current trying to return to its source solely on a conductor. If neutral bonding happens incorrectly or in multiple locations a parallel path of dangerous touch voltage will exist on conductive items other than the actual conductor. This is why the bond must take place at either the source or the first disconnect.
A transformer (if not an auto) is a SDS and the bonding usually happens at the transformer with the XO tap, if it is a grounded system, but not past the first disconnect or at both locations. If there is no XO tap like 480V 3W delta none of the three transformers are grounded it is simply 3 phases and this is considered an ?ungrounded system? for this reason. Not because there is no electrode, grounding electrode conductor, or equipment grounding these are all still required. A system is qualified as ?grounded? or ?ungrounded? only by the existence of one of its transformers connected to earth ? usually through the XO tap.
Still not clear
Still not clear
The generator is an emergency source that is tapped into two 400 amp service rated disconnects. Then from there too two ATS, one for elevator 3 phase solid nuetral ATS and the other 4 pole switched neutral ATS for building emergency loads, ie lighting etc.
Whats wrong with bonding at the two disconnects and not at generator?
Still not clear
The generator is an emergency source that is tapped into two 400 amp service rated disconnects. Then from there too two ATS, one for elevator 3 phase solid nuetral ATS and the other 4 pole switched neutral ATS for building emergency loads, ie lighting etc.
Whats wrong with bonding at the two disconnects and not at generator?
chris kennedy
Senior Member
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- Miami Fla.
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- 60 yr old tool twisting electrician
IMO you can't have both. For a 3 pole solid neutral ATS the neutral floats in the genny. For a 4 pole ATS the neutral is bonded at the genny and as tryinghard stated, is also connected to a GES at the genny.EEC said:
tryinghard
Senior Member
- Location
- California
EEC said:
You can bond the neutral at either just fine, nothing wrong with doing this at the first disconnects.
Why are you switching the neutral for the building loads?
brian john
Senior Member
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- Leesburg, VA
chris kennedy said:
In the "technical" sense I do not think you can call this floating, as the neutral is solidly grounded just no at the generator.
chris kennedy
Senior Member
- Location
- Miami Fla.
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- 60 yr old tool twisting electrician
brian john said:
Bad choice of terms. But I hope OP catches my drift.
hillbilly1
Senior Member
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- North Georgia mountains
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- Owner/electrical contractor
Is the service ground fault protected? If so that is probally why a four pole was ordered.
tryinghard
Senior Member
- Location
- California
EEC said:
If you have one supply source (generator) feeding a tap to two disconnects the GEC and bonding has to take place at the same location, so if your bonding at the first disconnect the GEC must terminate there as well, at both disconnects. You cannot bond at either ATS in your case this would be a violation of 250.142(B).
You have to supply an EGC from each disconnect to both ATS?s as per 250.118.
You have one ATS not switching the neutral and another ATS switching the neutral from the same source feeder tap but these will both need EGC.
brian john
Senior Member
- Location
- Leesburg, VA
EEC said:
MAny engineers prefer this method with GFP and a generator.
As long as you keep the grounding straight either system will operate.
Chris in this case the neutral connections do sort of float (in the air) the connections are just hanging.
hillbilly1
Senior Member
- Location
- North Georgia mountains
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- Owner/electrical contractor
I have a drawing that I have done explaining in simple terms how and why the bonding should be done, but the file was too large for this forum, Basically the ground fault relay is tripped by a predetermined amount of current that flows through the grounding system, not the neutral (at least not until after the current transformer encircling all four of the conductors) ( four wire systems) The CT under normal conditions "sees" no or little current when there is no fault to ground. When a fault to ground happens this current returns outside of the CT causing current to flow in the ct, and when it reaches the set point, the relay is activated tripping the breaker or switch. Any circulating currents caused by the neutral being grounded past the CT can cause nusiance tripping. Bonding the service on the load side can also render the ground fault inoperable as the fault current would then flow through the CT instead of around it. Sorry this is so long, a picture would have shortened this post.EEC said:
brian john
Senior Member
- Location
- Leesburg, VA
There are 3 styles of GFP typically utilized at 480/277 VAC ssyetms, Ground return, a single CT on the neutral ground bond. This system is also common in Main tie main.
The system you noted Zero Sequence, by far more common, but it matters not whetee you utilized a 3-pole ATS or 4-pole ATS, just so you ground/bond properly.
Residual which utilizes 4 seperate CT's.
All GFP systems will be affected by downstrean grounds on the neutral.
ZERO SEQUENCE All amperages noted are figments of my imagination. Top picture all OK, bottom "A" phase short.
The system you noted Zero Sequence, by far more common, but it matters not whetee you utilized a 3-pole ATS or 4-pole ATS, just so you ground/bond properly.
Residual which utilizes 4 seperate CT's.
All GFP systems will be affected by downstrean grounds on the neutral.
ZERO SEQUENCE All amperages noted are figments of my imagination. Top picture all OK, bottom "A" phase short.
tryinghard said:
In the situation with (1) switched neutral and (1) solid neutral, The only bonding taking place will be at the main service. The ground and neutral are seperated even at the (2) generator disconnects. Otherwise you'll have parallel paths on the ground wire with the solid neutral feeder. The fault current path with the switched neutral will flow back to the main service bond and then continue back to the generator via the neutral of the solid neutral feeder and trip the generator breaker on that feeder of the fault.
Rick
hillbilly1
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- North Georgia mountains
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- Owner/electrical contractor
Yes thats just like the drawing that I do, I didn't mention the other types, as I was trying to keep it simple. We use a lot of the motorized breaker type of transfer switches, and I have a hard time keeping our guys from trying to bond on the load side of the switch instead of the properly bonding on the line side. (4 pole GF type)brian john [IMG said:http://i159.photobucket.com/albums/t127/brianjohn2580/GFPDRAW1.jpg[/IMG]
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