Earthing arrangements of switched alternative generator sets

[Ian1981]

Regulation 551.4.3.2.1 suggests that when supplying earthing for a switched alternative generator that fault protection should not rely upon the TN building supply earthing ie an earth electrode would need to be installed for the generator.
My question is should the electrode also be connected to the buildings MET?
Thanks

 

[paul291277]

I think "rely upon" is the key phrase so surely there is no harm.

The extra path to earth will reduce EFLI in all circuits but it is unlikely that will be enough to affect breaking capcity of MCB's but will need to be retested (well that's just my slightly drunk amateur response anyway)

 

[marconi]

Could you provide some more information and context please for folk to consider in their response?

a. Single or three phase?
b. kVA of generator?
c. Alternative supply to what type of building?
d. Any special considerations due to the location of the site or its purpose?
e. Current earthing system of installation?
f. Permanent(ie fixed) generator or an installation to be ready to connect to a mobile generator(eg Aggreko) delivered to site?
g. Is parallel operation with the mains required?
h. Is the installation to be powered at the same time by both mains and generator power? eg: some sporting or entertainment events?

 

[Ian1981]

It’s to supply the lift to a block of flats if the power is lost for emergency services (tho I wouldn’t imagine the fire service would use it or residents if there’s a fire but that is what’s been requested by the client)
Building is TN-CS generator is permanently fixed.
3 phase.
Kva I’m not sure at this point in time

 

[marconi]

http://www.mitsubishielectric.com/e...ators/nexiez-lite/pdf/nexiez-lite_catalog.pdf

Page 7 of 8 says 12kVA for a circa 1000kg lift and 5kVA for a circa 500kg lift.

What extra kVA capacity you need will depend on how the lift motor is started and controlled.

kVA of generator is important because it determines the PSSC and PEFC and thence how to achieve ADS.

 

[shaun1]

[QUOTE="marconi, post: 1323849, member:

kVA of generator is important because it determines the PSSC and PEFC and thence how to achieve ADS.[/QUOTE]

Do you know of any good sources of information on this subject as its something I would like to learn more about. I only recently realised that the PSSC of a generator is actually quite low as it is only capable of delivering a slightly larger current than it's rated capacity.

Also, with regards to earthing arangements, I have always undestood that the suppliers earth should not be relied upon for alternative supplies, as it can't be guarenteed in the event of a failure of the main supply. Using this earth could also present a danger to persons working on the distribution network. But the earthing systems should be interconnected to make sure everything remains at the same pontential. But again, I would also be interested in more information on this.

 

[westward10]

This is very interesting as the lift must be considered an evacuation route as per BS9999 Annex G. This is generally provided where the footfall of disabled persons would saturate suitable refuge areas, a hospital being a good example.

 

[marconi]

What you will require is the generator to provide an earth referenced neutral, so the generator star point must be bonded to an earth electrode in close proximity to the generator. If the star point was not earthed then an earth fault on one of the lines would raise the potential of the neutral with respect to earth. Since the generator casing could become energised it too must be bonded to the earth electrode.

Now you need to consider how to achieve ADS in the event of a short circuit and earth fault - the supply's and fault loop impedances matter here.

If the impedances are so high that insufficient earth fault current flows to operate the ADS such as fuses or mcbs quickly enough you have to employ RCDs to shut off the generator supply. In this event the generator and installation will become a TT provided that the installation MET is connected to an earth electrode. One should not rely on what the supplier has in place external to the building to regularly earth the neutral and cpc.

But since the generator and installation MET are in close proximity, and within the equi-potential zone the two separate earth electrodes can and should be one and the same to reduce the earth fault loop impedance and the touch voltage. Thus, bond the generator earth electrode and MET together. One cannot get a lower earth fault loop impedance by anything other than connecting the MET to the generator star point bonded to terra firma by an earth electrode. So TT becomes TN-S

If the EFLI are low then the generator and installation can form a TN-S, with ADS by fuses or MCBs and generator earth electrode and MET bonded together.

That's my reasoning and answer. I have assumed the generator is within the footprint of the building or closely adjacent to it.

At all times the electrical wiring in the building must preserve the automatic and fast means whilst it is energised to detect and shut off supply in the event of an earth fault irrespective of source of supply.

See this too:

OC 482/2: Electrical safety of independent low-voltage ac - http://www.hse.gov.uk/foi/internalops/ocs/400-499/oc482_2.htm

 

[westward10]

If this is an evacuation lift it must be installed in compliance with BS9999 Annex G that being it must have the same fire protection as that of a refuge area.

 

[Ian1981]

If you read regulation number 551.4.3.2.1 it states
Protection by automatic disconnection of the supply shall not rely upon the connection to the earthed point if the supply for distribution of electricity when the generator is operating as a switched alternative to a TN system.
A suitable means of earthing shall be provided.
So to me that says earth it via an electrode.

A lift company shall be installing the lift and we will provide any provisions they require.

Thanks for the input some food for thought

 

[marconi]

In para2 ofmy post #8 I should have been clearer by writing:

Now you need to consider how to achieve ADS for the generator in the event of a short circuit and earth fault - the generator source and fault loop impedances matter here.

 

[Ian1981]

Giving it more thought the electrode will be for referencing the neutral to earth.
Once the generator is switched over the earthing arrangement will be the generators TN-S arrangement so will not be using the buildings TN-CS arrangement and will satisfy that particular regulation