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Topic Title: Earth-neutral link at transformer and lv switchboard ????
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Created On: 01 November 2012 11:28 AM
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 01 November 2012 11:28 AM
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mrpcook

Posts: 1
Joined: 24 February 2003

Does this sound correct ??

We have a HV/LV transformer, on the LV side the earth and neutral points are connected together, single cables connect the transformer to the lv switchboard (5-wire : three phases, neutral and separate earth). The neutral and earth bar are again connected in the LV switchboard. Can anyone shed any light on why this is done this way !!!
Thanks
 01 November 2012 12:46 PM
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OMS

Posts: 19526
Joined: 23 March 2004

it doesn't sound correct to me - you normally either have the N-E link at the transformer spillbox (or at the switchyard feeder pillar) or you have it at the LV switchboard - there is normally only 1 and if there is specific REF and UREF protection in place, more than 1 may well disable that protection.

regards

OMS

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Failure is always an option
 01 November 2012 06:35 PM
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jcm256

Posts: 1855
Joined: 01 April 2006

Was only thinking of an island situation where it might not matter, a transformer supplying only one site. Certainly, in the past I have questioned this, if by using a 3-pole change over switch the un-switched neutral is a permant connection to earth at the generator. Which is similar to your system, only stretched out a bit more?

In TN systems, a point on the power supply, generally the transformer neutral, is earthed. The exposed conductive parts of the installation are connected to the same point by a protective conductor. The system is called TN-C when the neutral function is combined with that of the protective conductor, which is then called PEN. If these conductors are separated, the system is named TN-S.
When both variants cohabit in the same installation, the term TN-C-S can be used, remembering that the TN-C system must always be upstream of the TN-System.
The impedance of the fault loop is low (it does not flow through the earth)

Regards
jcm
 02 November 2012 08:12 PM
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cookers

Posts: 204
Joined: 10 February 2012

I would concur with OMS and others.

Normally you would look to only earth the neutral at one point in this type of installation, but it looks like the "optional" link in the LV switchboard got left in place. I have seen this several times.

It will likely render any Restricted Earth Fault or Unit protection inoperative or likely make things trip when no actual protection fault has occurred, for instance you could lose the whole installation with just a small magnitude sub circuit short (it has happened to me).

This can also cause excessive currents to flow in earth conductors, throughout the system, much to the consternation of those, including myself, working on the system. You get large splashes when disconnecting earths for testing etc. However, other than the alarm it causes, normally you should live to tell the tale! Now I always measure current flow in any earth conductor before disconnection, and its not a bad idea to measure what voltage difference there is before reconnecting!

So I would remove one of the N-E links after checking if there is REF fitted and ensuring the CT's are in the right place, this can get complex depending on the protection system fitted. Advice should always be sought from the manufacturer of the REF protection system as it could be your system uses 5 CT's or something so the double N-E doesn't matter, who knows!

It is quite easy to test the unit protection by secondary after you have done this, provided of course you can turn off the installation, if you cannot turn off then you can still test that the REF relay will trip but you just have to hope you will correctly disconnect and connect the actual breaker trips!

Best of luck!
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