Switching neutral in a transfer switch

[iwire]

Re: Switching neutral in a transfer switch

I understand what you are saying about the elevated voltages from the earth and the grounded surfaces.

But and I am not trying to be a wiseguy, is there any evidence to show that people have been injured by this series of events, and if so any idea why the code has not required the grounded conductor switched?

 

[Ed MacLaren]

Re: Switching neutral in a transfer switch

I also prefer the SDS configuration for a generator installation, for the same reasons outlined above.

Ed: What is the purpose of the green wire from the transfer switch to the generator frame, when the neutral ground tie is at the first switch?

I don't know whether the NEC requires an EGC between the generator and transfer switch or not, but it does provide a more direct ground fault clearing path. (Sketch C) This is another advantage of the SDS configuration.

Ed

 

[hornetd]

Re: Switching neutral in a transfer switch

We keep tripping over the wording on separately derived systems. What Bennie and others keep arguing is that there should be no conductive pathway between the two systems. My ongoing problem is that no matter how well they argue for that approach the US NEC does not permit it.

What I want is an answer to how I could possibly build an isolated electrical system in any building that is served by utility power.

If I use bennies solution of putting the transfer switch ahead of the service disconnect I have to have a service rated transfer switch. So I do the connection of the main bonding jumper to the neutral on the load side of the switch. If I make the connection on the load side have I not violated the provision of 250.24(A)(1) and 250.24(B).

250.24 Grounding Service-Supplied Alternating-Current Systems.
(A) System Grounding Connections. A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service, in accordance with 250.24(A)(1) through (A)(5).
(1) General. The connection shall be made at any accessible point from the load end of the service drop or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means.
(B) Grounded Conductor Brought to Service Equipment. Where an ac system operating at less than 1000 volts is grounded at any point, the grounded conductor(s) shall be run to each service disconnecting means and shall be bonded to each disconnecting means enclosure. The grounded conductor(s) shall be installed in accordance with 250.24(B)(1) through (B)(3).

I don't see anything in the code that would permit me to unbond the utility's grounded conductor from the enclosure of the service disconnecting means regardless of whether that enclosure is the transfer switch cabinet or another cabinet.

If anyone can find a way that is code compliant to interrupt the inevitable conductive path of all code compliant installations there would be a use for such techniques on every livestock ranch or farm. The problem is that it is not in the utilities' interest to permit us to disconnect there neutral from the premises grounding electrode as they are using the earth as a parallel conductor for their neutral.
--
Tom

[ March 29, 2003, 02:55 PM: Message edited by: hornetd ]

 

[bennie]

Re: Switching neutral in a transfer switch

The original concern for distribution transformers was the possibility of high voltage appearing on the premises wiring system.

The MGN system was adopted to provide a short circuit on the end user secondary, in the event of a winding to winding fault.

This connection is a solid electrical connection to supply conductors originating in another system.

A transformer can not be a source for a premises wiring system and qualify as a source for a separately derived system, when it is fed by a grounded supply.

 

[roger]

Re: Switching neutral in a transfer switch

Hey Bennie, just a thought, maybe there should be a term such as secondary voltage system (SVS) to describe a system that would have some common connection, even if just grounding/bonding.

I really don't like the way I worded that.

We could still use the term seperately derived system (SDS) to describe truly isolated systems, ie health care type isolation transformers.

Roger

 

[bennie]

Re: Switching neutral in a transfer switch

Hi Roger: There is a real explanation of a separately derived system, but the official version is not real

From my 1905 engineering book... An isolation transformer is one with an ungrounded secondary, from a grounded primary, with primary and secondary voltage the same.

A transformer source for a separately derived system has an ungrounded primary, with a grounded secondary.

 

[russ]

Re: Switching neutral in a transfer switch

bennie:
I see if you had no equipment ground, you would have no problem.
I work in an area that is all conduit for the most part so the equipment ground is always there.

Toms post sums up the rest.

Russ

 

[bennie]

Re: Switching neutral in a transfer switch

Russ: This is the reason for designing the transfer switch ahead of the neutral/ground tie at the service.

I understand that all of the quality brand transfer switches are service rated as a standard.

This is why the first switch can contain the interconnection.

 

[goldstar]

Re: Switching neutral in a transfer switch

Bob (Iwire),

I remember going 'round and 'round with the manufacturer's engineer on this. She insisted that the neutral float and not be bonded to ground. I'll have to check the generator next time I'm at the gas station. Thanks for the "heads-up".

 

[Ed MacLaren]

Re: Switching neutral in a transfer switch

This is the reason for designing the transfer switch ahead of the neutral/ground tie at the service.

This is clearly not practical in the numerous installations where the emergency generator is only required to pick up a portion (critical loads) of the total load in a facility.

During a storm, or other events, when an active line conductor breaks, the first contact is often the multi-ground neutral conductor.

This contact will impress the high voltage on the metal surfaces of the premises.

This is also one reason for the quick response of generator units in coming on line.

This is the only reason for disconnecting the utility neutral.

If this actually was a safety concern, and the real reason for "disconnecting the utility neutral", what about the vast majority of installations (including your house and mine) where the utility neutral is continuous (unswitched and unfused) right through to the branch circuits and devices?

Ed

 

[Nick]

Re: Switching neutral in a transfer switch

I know this is a little astray from the subject but, when installing an emergency back up with an un switched grounded conductor, has anyone ever complied with or seen compliance with 700.8(B)?

 

[bennie]

Re: Switching neutral in a transfer switch

Ed: There is nothing to prevent the ground/neutral to elevate to the voltage of the conductor making contact.

I have seen two buildings where the service equipment was blown off the walls. One took a 230 KV hit.

I call these events back door faults.

I think many of these incidents are written off as lightning strikes.

[ March 30, 2003, 12:11 PM: Message edited by: bennie ]

 

[bennie]

Re: Switching neutral in a transfer switch

Nick: I never thought of the requirement mentioned in 700.8.

Does this mean that every pole in the world with a secondary rack, and a ground electrode, will require a sign?