Generator & Transformer Grounding In NEC

[timm333]

In transformer, the Y point of the secondary is connected to ground. Can we connect the body of the transformer with the same Y-point (for equipment grounding)?

Also can we ground the generator in the same way as transformer (connect the Y point of generator to ground)?

 

[NCTHAI]

Power ground (Neutral)and protection ground (Body) connection should be separate... You can not connect them together.

 

[samundra24E]

If you connect them to the same point charge present in transformer has the chance to travel in the form of current to the neutral of the transformer.

 

[musaUKPN]

Does it mean that no bonding between power neutral and body grounding should exist? If so how can we bond the two?

musa UKPN

 

[jghrist]

Power ground (Neutral)and protection ground (Body) connection should be separate... You can not connect them together.

This is not true at the source of a separately derived system like the secondary of a delta-wye transformer. The neutral is bonded to ground there. The generator neutral can be bonded to ground, and it becomes a separately derived system. If it is, then the neutral has to be switched in an ATS so that the two ground points don't get connected to the same neutral. If the generator neutral is not bonded to ground, it is not a separately derived system and the neutral can't be switched at the ATS. The key is to have only one ground on the neutral to avoid problems with ground fault protection.

There are other considerations such as impedance for ground fault currents to consider that would make grounding the generator neutral desirable.

 

[timm333]

Please refer to Fig 1 on page 1 of the following link:

http://mercedeelectric.com/Documents/00585847.pdf

Here the Y point of the generator is neutral and it looks that it is connected to the Y point of the utility transformer. The transfer switch in this figure is 3-pole.

In our case we have a main transformer (12.47kV – 480V) on left and generator (480V) on right and the transfer switch in between them. The transfer switch is service entrance type (with switched neutral).

The problem is that we have another small transformer (480-120/208V) downstream of the transfer switch. This second transformer feeds all the panelboard loads.

So should I disregard the upstream neutral in the 4-pole transfer switch and use only three poles of the transfer switch and keep the fourth pole as spare?

And then I can take the Y point of the smaller downstream transformer as the neutral (and ground) for all the panelboard loads. Will it be acceptable? Comments will be appreciated.

 

[jghrist]

The secondary smaller downstream transformer is also a searately derived system and the neutral needs to be bonded to a grounding electrode at the secondary. This is independent of whether or not the generator is a separately derived system.

If there is a ground fault on the 208 V secondary, the fault current needs to have a metallic path back to the source, which is the 480-120/208 V transformer. The bond from ground to the transformer neutral provides this metallic connection.

In your case, you will have three separately-derived systems. Two will be 480 V - one served from the secondary of the 12.47 kV - 480 V transformer and one served from the generator. These will be connected as shown on Fig. 4 of your reference. The third separately-derived system is the 208 V secondary.

 

[timm333]

Thanks Jghrist, a couple of more questions…

In both Fig 1 and Fig 4 of the above reference, the five wires (3-Phases+G+N) are going towards the load. But in our case the load is a downstream transformer which already has its own neutral. So how should I deal with this situation?

Also I understand that GFP shown in Fig 1 – Fig 4 in the above reference is not required in our case. Because as per NEC 230.95, the GFP is required only for currents larger than 1000A; but in our case the current is less than 1000A, so GFP should not be required. Is it correct?

Thanks for help...

 

[jghrist]

In both Fig 1 and Fig 4 of the above reference, the five wires (3-Phases+G+N) are going towards the load. But in our case the load is a downstream transformer which already has its own neutral. So how should I deal with this situation?

I don't know what you mean by the transformer already having its own neutral. Do you mean the 208 V neutral? This doesn't have anything to do with the 480 V wires from the ATS. Is the 480-120/208 transformer primary connected delta? If so, it doesn't need a neutral.

Also I understand that GFP shown in Fig 1 – Fig 4 in the above reference is not required in our case. Because as per NEC 230.95, the GFP is required only for currents larger than 1000A; but in our case the current is less than 1000A, so GFP should not be required. Is it correct?

GFP is required if the service disconnect is rated 1000 A or more; it doesn't matter if the current is less. If NEC doesn't require GSP, that doesn't mean it is prohibited. It can be required by specifications or installed just to provide better ground fault protection.

 

[timm333]

Jghrist, Yes I meant neutral of 208V; and the 480-120/208V is primary connected delta.

I have attached the diagram here. Please have a look and let me know your comments. Thanks

 

[jghrist]

You cannot bond the neutral to ground at the ATS. The neutral is bonded at the 12 kV - 480 V transformer secondary and can only be bonded at one point.

You need an equipment grounding conductor run to the 480-120/208 V transformer from the ATS to ground the transformer enclosure. Otherwise, there is no metallic return path for a 480 V ground fault at the transformer.

You are not grounding the generator neutral at the generator, so it is not a separately derived system. As such, you are correct not to switch the neutral at the ATS. An alternative would be to ground the generator neutral, making it a separately derived system and use the ATS neutral switch.

There is a possible problem with not grounding the generator neutral. If the generator is running, and there is a ground fault, the return current would have to flow through the grounding conductor to the 12.47 kV - 480 V transformer and back through the neutral to get back to the generator neutral. This path may have a high impedance and may make fault detection ineffective. If the generator and the 480 V service are close together, this may not be a problem.

Why is the fire pump not powered by the generator? If the normal service is reliable enough that an alternate source is not required, why have a generator at all?

 

[timm333]

We have two fire pumps, one is electrical on main utility transformer, and the second is standby pump. This standby pump is diesel operated, so it is not connected to the generator.

Please see the revised attached diagram now. The changes are in red color.

I have now used all four poles of the transfer switch. The first three poles of transfer switch are three phases (L1,L2,L3) and they go to the delta primary of the downstream dry type transformer. But where shall the switched neutral of the transfer switch (4th pole) go? Will it be spare?