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Topic Title: Max Zs for RCD in a TNCS system
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Created On: 30 March 2011 04:38 PM
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 30 March 2011 04:38 PM
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leckie

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Afternoon all

I posted yesterday regarding recording the max Zs on an EIC for a TT system, 1667 ohms for a 30mA rcd.

On a TNCS system does the same apply? I've always recorded values from table 41.3.
 30 March 2011 04:49 PM
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AJJewsbury

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It depends on what the designer intended - with TN systems you often have the choice of using overcurrent devices or RCDs for ADS. Having an RCD there (say for additional protection) doesn't necessarily mean it's being relied on for ADS (protection against indirect contact). Many people prefer to use overcurrent devices (better reliability, both from failures of the device and also failures in supply) than RCDs, so will expect MCB/fuse Zs values even if an RCD is present. But using an RCD is permitted (and can be a useful get-out-of-jail free card) if Zs is higher.
- Andy.
 30 March 2011 04:57 PM
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leckie

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Well thats what Ive got on a cert I am reviewing. The Zs is too high to comply with table 41.3, but the device is an RCBO.

So I know that RCBO covers for ADS but I wondered what value of max Zs should be recorded on the cert.

Ive never seen a cert saying 1667 on a TNCS system but should that in fact be what is recorded when, as in this case, the RCBO is being used to achieve ADS?
 30 March 2011 05:04 PM
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AJJewsbury

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Ive never seen a cert saying 1667 on a TNCS system but should that in fact be what is recorded when, as in this case, the RCBO is being used to achieve ADS?

I'd say yes.
- Andy.
 30 March 2011 05:07 PM
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daveparry1

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If it's tncs or tns I would make every effort to get the Zs down to the figure in the regs,
Dave.
 30 March 2011 05:15 PM
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leckie

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In actual fact i will Dave, the RCBO is a C type, swapping to a B will then comply with 41.3. It was more of a question of what you would actually do
if there was no alternative in terms of filling in the certificate.

It would actually comply with the regs.......just not 41.3, though i think a recorded max Zs of 1667 would stand out a bit
 30 March 2011 05:48 PM
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daveparry1

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Yes I think it would look a bit odd especially if somone who'd had little experience with TT systems looked at it!
Dave.
 30 March 2011 05:49 PM
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OMS

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Originally posted by: leckie

In actual fact i will Dave, the RCBO is a C type, swapping to a B will then comply with 41.3. It was more of a question of what you would actually do

if there was no alternative in terms of filling in the certificate.

It would actually comply with the regs.......just not 41.3, though i think a recorded max Zs of 1667 would stand out a bit


I'd be a bit wary on a TN-C-S system as P-N faults will be of the same magnitude as P-E faults - if Zs is low then so will the L-N loop be and you could be in a position where serious damage to conductors will occur due to "long" disconnection times (you may be making a demand on the thermal rather than magentic part of the breaker)

regards

OMS

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Failure is always an option
 30 March 2011 05:58 PM
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leckie

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You do mean if Zs is high dont you?

Anyway Ive had a hard day so I'm off for a

But I will swap it tomorrow and check the calc - sure it was fine when I did it.
 30 March 2011 06:05 PM
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OMS

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Originally posted by: leckie

You do mean if Zs is high dont you?

Anyway Ive had a hard day so I'm off for a

But I will swap it tomorrow and check the calc - sure it was fine when I did it.


Sorry yes - if Zs is high - P-N fault current will be low (or lower) than earth fault current - so a thermal check would be a good idea

Regards

OMS

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Failure is always an option
 30 March 2011 06:49 PM
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kj scott

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Table 41.3 only relates to EN 60898 devices; and the overcurrent charecteristics of EN 61009 devices; not earth fault conditions where the RCD element of the device will precede.

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http://www.niceic.biz
 30 March 2011 09:45 PM
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leckie

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You will have to explain that to me, I am not that bright
 30 March 2011 09:49 PM
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leckie

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Hang on, I think I do understand. In fact thats what has already been pointed out by oms?
 30 March 2011 10:10 PM
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pmenetwork

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dont worry leckie, some of the posters have been under a lot of pressure
this week trying to understand earthing systems.
there is no need to be "wary of PME", it is our most used earth return,
tried and trusted for over 40 years. Perfectly complimented with a 30ma rccd, it provides one of our most safe systems of protection.
1667? we all know thats the limit, wont happen in a PME network of course, but that is the max, although I understand many "guide" books
and indeed a self styled "national inspection" outfit, advise otherwise.
 30 March 2011 10:18 PM
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OMS

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Originally posted by: pmenetwork

dont worry leckie, some of the posters have been under a lot of pressure this week trying to understand earthing systems.

there is no need to be "wary of PME", it is our most used earth return, tried and trusted for over 40 years. Perfectly complimented with a 30ma rccd, it provides one of our most safe systems of protection.

Well that's debateable for many reasons, but my point was that you are likley to see very similar fault levels for both P-E faults and P-N faults. If Zs is that high then there is inadequate earth fault current to disconnect a circuit in defined time. If the short circuit is similar or lower (and an RCD won't see short circuit current) then you are likley to thermally stress a cable as you will be reliant in part on the thermal tripping mechanism of the breaker - but I guess you didn't spot that

1667? we all know thats the limit, wont happen in a PME network of course, but that is the max, although I understand many "guide" booksand indeed a self styled "national inspection" outfit, advise otherwise.

Missed the point a tad there I suspect



OMS

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 30 March 2011 10:19 PM
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kj scott

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Rather the system imposed upon the consumer; by the then Nationalised distributor. OMS is correct in his observation that caution is recommentded in a TN-C-S sytem with high Zs values.
btw; 1667 is not an accepable value; as it breaches the requirements of BS 7671:2008.

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 30 March 2011 10:47 PM
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AJJewsbury

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I'd be a bit wary on a TN-C-S system as P-N faults will be of the same magnitude as P-E faults

Unless reduced CSA c.p.c.s are being used ....
- Andy.
 30 March 2011 11:11 PM
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pmenetwork

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PME has never been imposed on anyone, it is there if the legal requirements are met.
Correct design ensures low Zs.

Nationalised industries?
Research, Development, Training, no private "rip off courses"
that we see today
.............and of course someone always at the end of the phone
to help us with a query.
thats why I and people like me know the difference between supply systems and others on this forum are struggling today.
where are the Nic? the other trade asss? the iet "helpline"
I was part of the Nat Ind, and proud of it.
 30 March 2011 11:25 PM
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OMS

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Originally posted by: AJJewsbury

I'd be a bit wary on a TN-C-S system as P-N faults will be of the same magnitude as P-E faults


Unless reduced CSA c.p.c.s are being used ....

- Andy.


True Andy - but the effects of bonding and other parallel paths tends to even that out when you have a high measured Zs - SC currents are likley to be even lower then earth fault curents in a lot of cases

Regards

OMS

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Failure is always an option
 30 March 2011 11:28 PM
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Avatar for OMS.
OMS

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Originally posted by: pmenetwork

PME has never been imposed on anyone, it is there if the legal requirements are met.

Correct design ensures low Zs.

Nationalised industries?

Research, Development, Training, no private "rip off courses" that we see today

.............and of course someone always at the end of the phone

to help us with a query.

thats why I and people like me know the difference between supply systems and others on this forum are struggling today.

where are the Nic? the other trade asss? the iet "helpline"

I was part of the Nat Ind, and proud of it.


Awww didums - did that nasty Mrs T take the gravy train away then

OMS

-------------------------
Failure is always an option
IET » Wiring and the regulations » Max Zs for RCD in a TNCS system

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