Can anyone explain why certain authorities insist to use 4Pole circuit breaker for generators and transformers?

1) What type of action/ protection is expected from the 4P breaker fitted on generator when its neutral is solidly grounded (TN-S system for 3P4W distribution)?
2) Is it recommended to link the neutral and body earthing at the source (generator or transformer)?

Appreciate your input

Regards

Prakash

You might be changing a TN-S system to a TN-C-S system if you only use a 3-Pole.
Exemption could be sought (in the past) from the supply authority if the transformer supplies only one consumer (Yourself). Generators are usually supplied (for this reason) with the neutral/earth link missing and is up to the designer to sort out the earthing arrangements.

Hi,

Thanks for the response, but you haven't answered my question.
My question is: purpose of using 4Pole breaker for generator/ transformer as main for islolation or protection?

Is it for isolating the unit completetly out of the cuircuit?. Then how it is possible, as the neutral is solidly grounded

Is it for protectiing the generator. Then how?

Regards

Prakash

Prakash

REC's or supply authorities will rarely assume anything when it comes to private equipment connected to their network. For clarity I'm assuming you are referring to a basic non synchronising changeover pair.

You say "What type of action/ protection is expected from the 4P breaker fitted on generator when its neutral is solidly grounded?" well you assume that the neutral will always be solidly grounded, is that guaranteed? Will the generator earth always stay the same? Could the neutral ever have is potential raised above earth? Do you see where I'm going with this?

By making all changeover pairs four pole this problem can be eliminated.

It also solves a problem if the generator or local transformer has any earth fault protection, by eliminating parallel earth paths. I can't count the number of times I've been to site with tripping earth fault protection on a generator only to find the changeover is three pole.

As far as your second question is concerned, well it depends, if it's a single set or transformer, then yes, probably, but maybe not. No single answer just as there is no single design.

Chris

-------------------------
Is the search for the Higgs Boson mass hysteria?

If the generator or transformer has restricted earth fault protection, then in my experience the neutral is earthed at the switchboard. A 4 pole circuit breaker then clears an earth fault on the generator immediately. If the circuit breaker is 3 pole, the REF protection trips the circuit breaker and the field suppression switch. The fault current is not cleared immediately but decays as the field decays.

With a transformer, the REF protection should trip both the primary and secondary circuit breakers so the fault is cleared immediately. However if the transformer is out of service for maintenance, an earth fault on the system could make the transformer windings live till the fault is cleared.

Possibly these are the reasons.
Best wishes
John

http://www.flickr.com/photos/5...N06/5407437740/


Take a look at the link above. This explains in my experience why 4-pole CBs are used for generator sets i.e. to stop multiple neutral earth links in an installation.

In answer to the OPs 2nd question: Again, it depends. You should only link (normally seperate) neutrals and earths in ONE place. Hence the 4-pole being used to prevent muliple connected links.

Regards,

-------------------------
Everyone loves a fireman - but hates the fire inspector.

Edited: 12 February 2012 at 03:31 PM by timothyboler

My memory is that G59 allows you to use either 3 pole or 4 pole, and gives advice and diagrams for either.

There are disadvantages and advantages to either solution.

If you have UPS supported circuits on the system connected to a 4 pole generator changeover, you need to assess what impact this will have on your UPS protected circuits as you will lose the system neutral/earth reference during changeover, so for a short time the UPS neutral is not referenced to earth and will float depending on system impedances.

This is unlikely to cause failure, but any RCD's connected downstream of UPS will likely trip (a bit inconvenient!).

Usual solution is some sort of UPS isolation transformer, which gives an independent neutral reference. However isolation transformers are expensive and also can cause other problems.

Or you can go for 3 pole changeover!