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Topic Title: Zs too High. What about using an RCD?
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Created On: 23 April 2008 09:39 PM
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 23 April 2008 09:39 PM
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jimncc1701a

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Also can anyone tell me where in the regs it states that using a maximum Zs of 1666 Ohms is acceptable when using an RCD?

Is this acceptable for TN-S and TN-C-S or is it just primarily aimed at TT systems to obtain 0.4s disconnection.

I have a high Zs reading on a ring final circuit B type the Zs redaing I have is 1.30 Ohms.

Apart from splitting the ring. And turning them into two 16A radials

Can I use 413-02-16 to achieve the disconnection time?
 23 April 2008 10:13 PM
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perspicacious

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"Also can anyone tell me where in the regs it states that using a maximum Zs of 1666 Ohms is acceptable when using an RCD?"

413-02-20

Regards

BOD
 23 April 2008 11:04 PM
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sparkingchip

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I would double check your Meter.

I got a high reading yesterday, my Megger LT320 Loop Tester has lost it's accuracy again, last calibrated July 2007, reverted to the old Robin and readings are OK again!
The Megger looks nice but it is not as reliable as it's plain looking old mate.

Andy Betteridge

 24 April 2008 06:40 AM
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ebee

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Originally posted by: perspicacious

"Also can anyone tell me where in the regs it states that using a maximum Zs of 1666 Ohms is acceptable when using an RCD?"



413-02-20



Regards



BOD



Correct, however that is for a TT system.

Using an RCD to compensate for a high Zs in a TN system, whilst being a sensible precaution (failure 7% ?) and is allowed in TT systems anyway as no other option.

I would be asking why the high Zs.
If not test gear error of course, then is it High Ze or High R2.
If high R2 then is it poor/undersized cpc connection or exceptionally long ring final or radial, if so then it should be corrected.

RCDs are an absolutely wonderfull safety device when used as a back up if all else fails and as such have probably saved quite a few lives.
But to rely on them as the main or sole protection is a folly in my opinion.
With a possible failure rate of 7% too that is worrying.

OK with TT you got no choice but a front end (usually time delayed plus I delta n of x 3 feeding a downstream non delayed device) being preffered (downstreams being double pole not single pole) then this might possibly reduce to 0.49% failure rate.

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

Edited: 24 April 2008 at 06:50 AM by ebee
 24 April 2008 01:32 PM
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AJJewsbury

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413-02-20

413-02-07 might be clearer....

- Andy.
 24 April 2008 01:42 PM
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OMS

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Mmmm - I'd be careful of that one - taken out of context it could suggest that either an overcurrent device or an RCD can be utilised but we are only discussing protection from indirect contact. The misguided could be led to believe that an RCD will do the job of protecting a final circuit from overload or fault current as well

Or is it just me being a pedant again

I think 413-02-20 is the one that answers the OP in terms of the derivation of the 1666 ohm value in relation to a 30mA device

Regards

OMS

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Failure is always an option
 24 April 2008 02:14 PM
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AJJewsbury

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I think 413-02-20 is the one that answers the OP in terms of the derivation of the 1666 ohm value in relation to a 30mA device

But as ebee says, that's for TT only (413-02-01)

Shall we agree on 413-02-16 then?

so, as I see it:
413-02-07 - an RCD may be used instead of an MCB/Fuse for protection against indirect contact in an TN system (but overcurrent protection will still be needed)
413-02-08 - Zs must still be low enough for required disconnection times (in some circumstances this might require Ia>Idn)
413-02-16 - Touch voltages be < 50V in cases where device need not open, :. 1667 ohms.

- Andy.
 24 April 2008 02:39 PM
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OMS

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Shall we agree on 413-02-16 then?


Absolutely

so, as I see it:

413-02-07 - an RCD may be used instead of an MCB/Fuse for protection against indirect contact in an TN system (but overcurrent protection will still be needed)
413-02-08 - Zs must still be low enough for required disconnection times (in some circumstances this might require Ia>Idn)
413-02-16 - Touch voltages be < 50V in cases where device need not open, :. 1667 ohms.


Agreed

OMS

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 24 April 2008 05:58 PM
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Approvedspark

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I havent got my regs book with me, so bare with me, are you saying it is acceptable to have a Zs reading of 1666 on a TN system, as long as its protected by a 30 mA RCD?

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 24 April 2008 06:13 PM
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OMS

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In theory yes, although there may well be other underlying problems with the installation that need to be examined (overcurrent protection as a minimum)- any TN system exhibiting those kind of Zs values really needs looking at

As Andy pointed out above - 413-02-07

Regards

OMS

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Failure is always an option
 24 April 2008 06:37 PM
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Approvedspark

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Originally posted by: OMS

although there may well be other underlying problems with the installation that need to be examined



OMS


Well that was what I thought. Heres a question, Caravan park TT system Ze is around 4.5 ohms, the park is then wired in a tn system from the main Db, the caravans are on large ring mains wired in s.w.a. Doing a P.I.R on a caravan I get a Zs of 50 ohms, now all the vans are individually protected by 30 mA RCDs so technically this complies with the regs, but I have failed it because the Zs should really be not much higher than the Ze and I beleive the cause is rotten and poorly made of glands and the Zs reading could easily become a lot higher. Does any one agree?

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If you think you're too small to have an impact, try going to bed with a mosquito in the room.
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