Grounding of emergency generator's neutral

[arneykaner]

Emergency generator, 480/277V, 3Ph, 4W serves
a commercial building. Question:
Can the generator neutral be grounded twice - at the generator terminal and at the emergency system service entrance disconnect (prior to transfer switch).

This is how the normal system grounding is done:
at the secondary of the utility transformer, and at the main service disconnect in the building.
Can we handle a generator source the same way as the utility source?

Your arguments (why it is possible or why it is not) are highly appreciated.

Thank you

 

[Ed MacLaren]

Re: Grounding of emergency generator's neutral

Most codes permit only one grounding electrode system connection to the grounded (neutral) conductor of any system, within a building or structure.

1. Is the generator within the building it is serving?

2. Is the generator feeder configured as a separately derived system (SDS)?
See this recent topic. http://www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=7;t=000094

Ed

[ April 18, 2003, 06:10 PM: Message edited by: Ed MacLaren ]

 

[tom baker]

Re: Grounding of emergency generator's neutral

The bonding of a generartor grounded conductor is dependant upon the type of transfer switch. If the neutral is switched, then it becomes a sep derived service and the grounded conductor must be bonded. If the grounded conductor (neutral) is not switched, then the grouned conductor must not per Art 250 be regrounded. See the definition of Sep derieved service in Art. 100.

 

[bennie]

Re: Grounding of emergency generator's neutral

Is the transfer switch being installed on the supply, or load side, of the main?

 

[iwire]

Re: Grounding of emergency generator's neutral

Hi Bennie, how are you doing today?

This subject of transfer switches on the load or line side of the main is interesting to me.

I have never seen a generator set up to take the whole building in one transfer switch.

If someone wanted to pay the expense of that large of a generator to run the whole building you would still need separate transfer switches to keep the Article 700 wiring separate from the article 701 and 702 wiring.

In my area there is usually a generator dedicated to Article 700 loads only, and if any one wants to power say a DATA center they will add another just for that use.

Bob

 

[bennie]

Re: Grounding of emergency generator's neutral

Hi Bob: Load transfer is a lot more complex than just activating a switch. Some loads are not compatible with an out of sync change of power source.

Resistive loads don't care. Inductive loads hate to see an out of sync voltage hit its windings.

Some loads, without ride through provisions, are allowed to completely shut down before restarting.

All of this, and many more functions, must be considered when alloting transfer of power.

 

[dereckbc]

Re: Grounding of emergency generator's neutral

Arney, the neutral cannot be grounded twice. In some cases it cannot be grounded at all at the generator.

As stated already it depends on what you do at the transfer switch, and if the primary service has GFP.

Here is a simple guideline.

If the generator is permanently installed on premises or the primary service has GFP, install the generator as an SDS system and switch the neutral if it is supplied.

If the generator is portable, and the primary service does not have GFP, it makes no difference if the generator is SDS or not.

 

[bennie]

Re: Grounding of emergency generator's neutral

Arney: Does this generator supply the entire load?

[ April 19, 2003, 05:32 PM: Message edited by: bennie ]

 

[arneykaner]

Re: Grounding of emergency generator's neutral

Originally posted by Ed MacLaren:
Most codes permit only one grounding electrode system connection to the grounded (neutral) conductor of any system, within a building or structure.

1. Is the generator within the building it is serving?

2. Is the generator feeder configured as a separately derived system (SDS)?
See this recent topic. http://www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=7;t=000094

Ed

1. Generator is outside of building and generator
service conductors have main service disconnect.
2. The transfer switch is has (4) poles, so neutral is switched, so it is a separrately derived system

 

[arneykaner]

Re: Grounding of emergency generator's neutral

Originally posted by bennie:
Arney: Does this generator supply the entire load?

Actually, I have two buildings with the arrangement, as I discribed. In both cases, there
are(4)pole Transfer Switches (switched neutral).
But in one case, the generator has the same capacity as utility transformer and serves the entire building load. In another case, generator
serves only selected emergency load (but still has main service disconnect in the building)

 

[arneykaner]

Re: Grounding of emergency generator's neutral

Originally posted by bennie:
Is the transfer switch being installed on the supply, or load side, of the main?

On the load side of both - normal main service
disconnect and emergency main service disconnect

 

[arneykaner]

Re: Grounding of emergency generator's neutral

Originally posted by iwire:
Hi Bennie, how are you doing today?

This subject of transfer switches on the load or line side of the main is interesting to me.

I have never seen a generator set up to take the whole building in one transfer switch.

If someone wanted to pay the expense of that large of a generator to run the whole building you would still need separate transfer switches to keep the Article 700 wiring separate from the article 701 and 702 wiring.

In my area there is usually a generator dedicated to Article 700 loads only, and if any one wants to power say a DATA center they will add another just for that use.

Bob

You are right, but with one exception: if there is
no qualified Life Safety system load in the building. My case is a water lift (booster) pump station, which is not occupied, therefore no qualified Life Safety load.

 

[arneykaner]

Re: Grounding of emergency generator's neutral

Originally posted by dereckbc:
Arney, the neutral cannot be grounded twice. In some cases it cannot be grounded at all at the generator.

As stated already it depends on what you do at the transfer switch, and if the primary service has GFP.

Here is a simple guideline.

If the generator is permanently installed on premises or the primary service has GFP, install the generator as an SDS system and switch the neutral if it is supplied.

If the generator is portable, and the primary service does not have GFP, it makes no difference if the generator is SDS or not.

Dereck, I completely disagree with you in terms of what is permitted/not permitted by Codes, but
I do agree with you on the issue of engineering
approach. I know that your type of approach is quite popular with the Electrical Utility people,
and there are very interisting considerations about ground fault protection. If you are interested to discuss it further, you are welcome
to call me (312)577-3269, 9 to 5, Central Time.

Just couple of things:
1. under NEC 480/277V system must be grounded.
2. Grounding through an impedance/resistance to limit ground fault current looks attractive, but brings in an issue of system stability in case of short circuit.

 

[bennie]

Re: Grounding of emergency generator's neutral

During a power outage, the premises wiring is a separately derived system powered by a generator, with no electrical connection to the other supply conductors originating in another system.

The premises wiring, when supplied by the utility transformer, is not a separately derived AC system.

The rules in section 250.30 apply to grounding separately derived AC systems. This pertains to the premises wiring not to the generator.

The source of power is on the supply side of its disconnect and can be grounded more than at one point.

 

[arneykaner]

Re: Grounding of emergency generator's neutral

Originally posted by tom baker:
The bonding of a generartor grounded conductor is dependant upon the type of transfer switch. If the neutral is switched, then it becomes a sep derived service and the grounded conductor must be bonded. If the grounded conductor (neutral) is not switched, then the grouned conductor must not per Art 250 be regrounded. See the definition of Sep derieved service in Art. 100.

Thank you, Tom.

I think the same way.

 

[bennie]

Re: Grounding of emergency generator's neutral

Arney: Simply stated... The generator neutral can be grounded at the generator and at the service.

The generator is a rotary transformer and can be connected as if it is a transformer secondary.

The neutral should be internally connected to the core of the stator. This is to prevent, winding to core fault current, from appearing at the main switch.

These faults will be detected as a phase failure, by the control system, and will dump the exciter.

 

[bennie]

Re: Grounding of emergency generator's neutral

Arney: For substantiation of my remarks refer to...250.30 (1): Attention is called to the words "this connection shall be made at any point on the separately derived system from the source to the first system disconnect".