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Topic Title: High Zs, commercial.
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Created On: 02 October 2013 09:54 PM
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 02 October 2013 09:54 PM
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geov

Posts: 186
Joined: 22 February 2004

Hi guys,
I've seen conflicting advice about this issue, so interested in views. EICR on a community "business centre" (circa 1993) done by others, has highlighted high Zs readings on a number of circuits. For example, a couple of ring mains finals on 61009's, 32 amp c types (make unknown), measured respectively Zs 0.67ohms (max permitted 0.58) and 0.85 (max permitted 0.58).

There is another example affecting a lighting circuit, but I don't have the readings to hand.

It's a TN-S with the "Ze" recorded as 0.10, although this was with the earth and bonds in place.

These have been given a Code 1. I have seen others state that a max Zs of 1667ohms can be applied in such cases, in which case no remedial action would have to be taken - do you agree.....or?

I have been asked for my opinion by the electrician who has been asked to quote for the remedials, but I'm not sure how best to advise. He hopes to go to the site this week, to actually see the install first hand.

Regards.
 02 October 2013 10:52 PM
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daveparry1

Posts: 6210
Joined: 04 July 2007

1667 ohms can be used if there's a 30m/a rcd in circuit but apparently only on a TT system, the figure is somewhat less on a TN system. This was brought to my attention on here recently on a post and I think it was either Andy or OMS that corrected me on it. From memory I think the figure was something like 1260 ohms on a TN system provided a 30m/a rcd is in place,

Dave.
 02 October 2013 10:52 PM
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Fm

Posts: 675
Joined: 24 August 2011

If its on a 61009 device whats the problem?
Lighting circuit,run a larger cable or run a seperate earth conductor to lower zs
 03 October 2013 12:26 AM
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kj scott

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Compliance with Regulation 411.3.2.1 will be achieved by use of the 30 mA RCD.
Without reference to specific manufacturer's data; table 3A (300ms disconnection) guides us to a maximum Zs of 7666. If you wished to consider a maximum touch voltage of 50V this value would reduce to 1666; however this should not be an issue if equipotential bonding is present together with a TN earthing arrangement.
Also the maximum Zs for a 32 type C device is 0.72, as opposed to the ritual 80% value; on closer view of the circuit and test conditions the 0.67 may well be acceptable; although close.

-------------------------
http://www.niceic.biz

Edited: 13 October 2013 at 10:24 AM by kj scott
 03 October 2013 08:30 AM
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Parsley

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411.4.5 The characteristics of the protective devices (see Regulation 411.4.4) and the circuit impedances shall fulfil the following requirement:
Zs × Ia <U0

7666 x 0.03=230V
1667 x 0.03=50V

The lower Zs reading may be OK though depending on what loads were being used at the time of the test.

Regards
 03 October 2013 10:09 AM
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AJJewsbury

Posts: 11468
Joined: 13 August 2003

1667 ohms can be used if there's a 30m/a rcd in circuit but apparently only on a TT system, the figure is somewhat less on a TN system.

Almost....

For ADS:
For 0.2s disconnection (TT final circuits <=32A etc), 2x Idn is required = 60mA. 230V/60mA = 3833 Ohms.
For 0.4s disconnection (e.g TN final circuits <=32A etc) 1x Idn is sufficient = 30mA. 230V/30mA = 7666 Ohms.

But TT (but not TN) has a additional requirement that 50V above true earth cannot be allowed to persist (for more than 5s) - e.g. due to leakage currents. 30mA RCD needs 30mA to guarantee disconnection in 5s, so 50V/30mA = Ra (rather than Zs) of 1667 Ohms.

So for TN with 30mA RCD protection, max Zs = 7666 Ohms.
For TT, it needs to satisfy both Zs <= 3833 Ohms and Ra <= 1667 Ohms.

Given that in a TT system, Ra is by far the biggest part of Zs, it's a pretty good approximation (and errs on the side of safety) to say Zs <= 1667 Ohms instead of Ra <= 1667 Ohms.

So we end up with:
TN: Zs <= 7666 Ohms
TT: Zs <= 1667 Ohms.

- Andy.
 03 October 2013 01:36 PM
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daveparry1

Posts: 6210
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Thanks for explaining that in detail Andy, I didn't quite understand it last time!

Dave.
 03 October 2013 03:34 PM
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geov

Posts: 186
Joined: 22 February 2004

All, thanks for the responses. I'll digest tonight and may come back with more questions!

Regards
 03 October 2013 04:07 PM
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dg66

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Theres also a possible issue with volt drop

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Regards

Dave(not Cockburn)
 03 October 2013 04:28 PM
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AJJewsbury

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For example, a couple of ring mains finals on 61009's, 32 amp c types (make unknown), measured respectively Zs 0.67ohms (max permitted 0.58) and 0.85 (max permitted 0.58).

You seem to be using the 80% figures - they're only a rule of thumb as they presume that the conductors of the entire loop are at room temperature. Is that really the case? I would have thought that the DNO's supply conductors at least were at operating temperature (or something close to it).

BS 7671 allowed 0.72 Ohms for a 32A C type MCB (or RCBO if the designer has elected not to rely on the RCD element for ADS) - so it's possible that the 0.67 figure might comply - if for instance the circuit was fully loaded at the time of the test. Even if it wasn't, but R1+R2 was relatively small and so Zdb largeish, it still might comply (presuming the submain was warm even if the final circuit wasn't).

Appendix 14 gives some explanation - basically if the check fails at the first attempt, you're meant to re-calculate the rule of thumb, applying the 80% (or more appropriate figure) only to the unloaded part of the circuit (R1+R2 for instance), leaving the full 100% allowance for loaded parts (e.g. the supplier's lines).

- Andy.
 04 October 2013 10:43 AM
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geov

Posts: 186
Joined: 22 February 2004

"There's also a possible issue with volt drop" - thanks Dave, this is going to be checked.

Andy - as ever, thanks for your helpful insight/input.

Regards.
 04 October 2013 05:33 PM
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psychicwarrior

Posts: 220
Joined: 18 October 2010

i realise this is about high Zs and also i will defer to greater minds than mine, but i always thought a final circuit must be 'designed' correctly and couple with its protective device correctly and one couldn't rely on RCD device for a poor r1+r2......TT especially. am i wrong ?
 07 October 2013 12:03 PM
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AJJewsbury

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and one couldn't rely on RCD device for a poor r1+r2

But you can - see reg 411.4.4 (ii). (Although you might still have to think about L-N faults, especially where the conductors are smaller than the protective device - e.g. ring finals)
- Andy.
 10 October 2013 11:35 AM
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psychicwarrior

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Joined: 18 October 2010

@AJJ

sorry Andy, i was not ever of the understanding an RCD cannot be used as fault protection (with the NOTE 1 caveat of course)

but i do concede that by using an RCD and regarding earth fault, i stand corrected; obviously r1+r2 is involved in the earth fault loop impedance, so an RCD by its nature does take care of higher values.

i have no idea why i said what i said above... duh!..... i could give a 'reason' but it alters nothing about the fact lol
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