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Topic Title: Zs too high
Topic Summary: RCBO
Created On: 12 May 2011 09:35 AM
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 12 May 2011 09:35 AM
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charlieroad

Posts: 150
Joined: 24 September 2007

Hi,

If the Zs in a circuit exceeds the max in the regs is it acceptable to use an RCD or RCBO?
 12 May 2011 09:43 AM
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dg66

Posts: 1670
Joined: 11 January 2008

In short yes,but in my opinion new circuits shouldnt be designed with this in mind.

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Dave(not Cockburn)
 12 May 2011 09:55 AM
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OMS

Posts: 19602
Joined: 23 March 2004

There is a school of thought that suggests if you are using an RCBO you should still design for a limiting Zs for the overcurrent part of the breaker as the RCD is generally only providing "additional protection"

If Zs is that high, you might also want to check the predicted L-N loop to determine protection from short circuits

As DG66 points out - I certainly wouldn't be designing for this condition on new works

Regards

OMS

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Failure is always an option
 12 May 2011 10:02 AM
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charlieroad

Posts: 150
Joined: 24 September 2007

Thanks,
If the Zs is noted as high in a PIR even though the circuit is protected by an RCD or RCBO would there be a code 2 for requires attention?
 12 May 2011 10:15 AM
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dg66

Posts: 1670
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On a PIR my personal view is if the max Zs is only just above the max tabulated value and an RCD is fitted then i wouldnt worry too much and make a note on the report,obviously if the measured value is way above the max tabulated value then i would code 3 requires further investigation as to why the readings are so way out,you've seen the situation its your call.

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Dave(not Cockburn)
 12 May 2011 11:01 AM
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pmenetwork

Posts: 353
Joined: 20 September 2009

--------------no problems there, PSCC will be ok, RCCD will trip
unless the loop is horrendous and overload well covered if trip sized correctly-------------









----------------------------------------------------
pmesystemdesign@btinternet.com
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 12 May 2011 11:11 AM
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charlieroad

Posts: 150
Joined: 24 September 2007

Thanks for the responses. There have been alterations to an existing circuit and the MCB has been changed to an RCBO which I expect is due to the high Zs. The latest PIR shows a code 2 for the high Zs.

regards
 12 May 2011 11:23 AM
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dg66

Posts: 1670
Joined: 11 January 2008

Cant see a code 2 myself,no code 4 either as it seems compliant,hence possibly a code 3 dependant on the situation.

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Dave(not Cockburn)
 12 May 2011 11:34 AM
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Avatar for OMS.
OMS

Posts: 19602
Joined: 23 March 2004

Originally posted by: pmenetwork

--------------no problems there, PSCC will be ok, RCCD will trip

unless the loop is horrendous and overload well covered if trip sized correctly-------------
----------------------------------------------------

pmesystemdesign@btinternet.com

----------------------------------------------------


I'd forgotten you DNO boys could predict L-N loops without even visiting site

How can you possibly know that PSCC will be high enough to trip the MCB part of the RCBO.

Zs tells you that the L-E loop is high in this case - it doesn't tell you if it's high on E or on L. If it's the latter it's quite possible that N will be high as well

You surely can't be contesting the suggestion that a check on the L-N loop is worthwhile

Before you reply, perhaps best to make sure you aren't arguing with a fool though - you wouldn't want to look foolish would you

regards

OMS

-------------------------
Failure is always an option
 12 May 2011 11:46 AM
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pmenetwork

Posts: 353
Joined: 20 September 2009

--------------the difference, is the experience I have gained over many
years, You can tell from reading this situation from this poster that
this query is no big deal.
as you say, the poster didnt tell us the figures, if they were that bad
the installation would have many issues.

and another thing- I have seen the apalling abuse that DC is getting,
its a disgrace, this is the IET I am surprised people havnt been
reprimanded.

I saw the word "stupid" used for people who believe a particular site is
being used misleadingly. who is the real stupid one?

the "Zs too high" poster is satisfied, by the way by our answers, and has
thanked us.
 12 May 2011 12:19 PM
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Avatar for OMS.
OMS

Posts: 19602
Joined: 23 March 2004

Originally posted by: pmenetwork

--------------the difference, is the experience I have gained over many

years, You can tell from reading this situation from this poster that

this query is no big deal.

Ahhh - I forgot you guys can tell me ze depending on what part of town we're in. As for being no big deal, the circuit has been altered you don't know if that lateration has added significant cable lengths that are presenting as high zs but could be masking low PSCC and appalling volt drop issues.

as you say, the poster didnt tell us the figures, if they were that bad

the installation would have many issues.

It would, but it's generally a case that at this stage no one knows


and another thing- I have seen the apalling abuse that DC is getting,

its a disgrace, this is the IET I am surprised people havnt been

reprimanded.

There's a moderator button bottom left of each post - fill your boots


I saw the word "stupid" used for people who believe a particular site is

being used misleadingly. who is the real stupid one?

As I said, there's a moderator button - it won't be the first time a few of us have had our collars felt by the mods - i guess you haven't had that pleasure yet

the "Zs too high" poster is satisfied, by the way by our answers, and has

thanked us.

Indeed - but I guess you've heard the phrase "Ignorance is bliss" - it may well be that this is an unknown unknown scenario



May I conlude by your response that you aren't feeling foolish at the moment

OMS

-------------------------
Failure is always an option
 12 May 2011 01:31 PM
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kj scott

Posts: 2144
Joined: 02 April 2006

Originally posted by: charlieroad

Thanks for the responses. There have been alterations to an existing circuit and the MCB has been changed to an RCBO which I expect is due to the high Zs. The latest PIR shows a code 2 for the high Zs.



regards


If a Zs value is high, then you need to establish whether it is due to circuit conditions, supply conditions, or a fault. You can then descide the appropriate course of action. Just installing an RCD or RCBO may not be the solution in a deteriorating installation.

It would appear that PME is psychic.
Relying on experience is fine where the experience is valid and gained from a correct source. However we are seeing the result of inadequate/incorrect knowledge gained from 'past experience', in another long running thread. Spending a long time doing something the wrong way does not make you competent.

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