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Topic Title: Zs of a TT supply taken off a TNC-S supply
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Created On: 09 March 2016 07:07 PM
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 09 March 2016 07:07 PM
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Adrian8077

Posts: 34
Joined: 20 October 2015

Hi all, in the process of wiring an outbuilding, have ran a 16mm swa from a 40a mcb in the house to a new DB in outbuilding. Have fitted an earth rod and connected as TT, but I'm unsure about testing Zs of the supply from the house, do I just run a temp earth cable, or use the SWA to get an R2 so I can confirm the Zs is ok for the 40a mcb ?

Ady
 09 March 2016 07:28 PM
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broadgage

Posts: 2380
Joined: 07 August 2007

I would measure the earth loop impedance in the usual way between the armouring and the phase conductor, so as to prove that the 40 amp OCPD at the supply end will open promptly in case of an earth fault near the far end of the SWA.

A completely separate test will be required of the outgoing circuits in the outbuilding to prove that the local earth rod gives a low enough earth loop impedance to reliably/promptly operate the RCD(s) in the outbuilding.
 09 March 2016 07:45 PM
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daveparry1

Posts: 7728
Joined: 04 July 2007

I agree with Broadgage.
 09 March 2016 08:17 PM
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Adrian8077

Posts: 34
Joined: 20 October 2015

Hi, thanks for replies, have tested in the outbuilding using the earth rod, was only when I got home I realised those reading would be no good for the supply from the house

Ady
 09 March 2016 11:59 PM
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mapj1

Posts: 9356
Joined: 22 July 2004

The fault path you are testing for is the one that answers the killer exam question
" would a spade through the SWA trip the MCB or kill the gardener first?"
You may assume the gardener is 1000 ohms, and can survive mains voltage for no more than 0.2 seconds, or half of it for a bit longer, as per IEC 479 with no permanent damage to his or her heart rhythm.
Bonus points will be awarded for knowing the exact naughty word the gardener would use during this experiment.
To that end, as others have already said, the only sensible loop back test is armour to live, no parallel paths permitted - but that is why.



-------------------------
regards Mike
 10 March 2016 06:41 PM
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Adrian8077

Posts: 34
Joined: 20 October 2015

Interesting question, but wouldn't it depend on construction of spade ?
 10 March 2016 08:42 PM
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mapj1

Posts: 9356
Joined: 22 July 2004

yes But the regs are worded in a way that is safe in the worst case
- so for example,
1) most people are higher resistance than assumed, unless wet which is the worst case,
2) on TN the voltage on the spade or other implement is assumed to be ~ half the supply, being a division between the live and CPC wiring. On TT, as the Ze is normally higher it is assumed to be the full voltage. In both cases bonding will bring it down, but the worst case of no bonding is assumed.
3) it is assumed the RCD needs the full 30mA to trip it, and the MCB is at the upper edge of specs, and it all passes via the victim, neither of these is usually true either, as the but it is the worst case.
So continue the meme and assume the worst kind of spade too.!!

-------------------------
regards Mike
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