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Topic Title: Isolators on TN-C-S supplies
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Created On: 11 July 2013 04:50 PM
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 11 July 2013 04:50 PM
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geov

Posts: 165
Joined: 22 February 2004

Don't know why I have not spotted this before and would welcome thoughts/views.

On TNCS supplies which have had DNO supplied isolators, I have seen examples where the neutral has gone via the isolator and others where it bypasses the isolator and goes straight from meter to CU. Is there a right or wrong way?

Regards.
 11 July 2013 05:04 PM
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AJJewsbury

Posts: 11218
Joined: 13 August 2003

In BS 7671 terms it's acceptable not to switch/isolate N on a TN system - the main exception being the "main switch" for a domestic S.P. supply which has to interrupt both live conductors. Presumably the CU contains the 'main switch' so the DNO's isolation doesn't have to provide that function.

If the isolator was upstream of the N-PE link you'd certainly not want it to open the N (CNE), but hopefully not even the DNO's would do that!

So I guess both are right.
- Andy.
 11 July 2013 05:06 PM
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Parsley

Posts: 960
Joined: 04 November 2004

537.2.1.1 Every circuit shall be capable of being isolated from each of the live supply conductors. In a TN-S or TN-C-S system, it is not necessary to isolate or switch the neutral conductor where it is regarded as being reliably connected to Earth by a suitably low impedance.

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 11 July 2013 06:01 PM
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geov

Posts: 165
Joined: 22 February 2004

Thanks guys, I was overthinking it. As you say Andy, the DP isolation occurs at the CU.
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