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Topic Title: HIGH Zs ON MCCB FINAL CIRCUIT
Topic Summary: COMPLIED WITH 16TH EDITION
Created On: 31 December 2010 08:58 PM
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 31 December 2010 08:58 PM
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John Peckham

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What are the Forum members views on a final circuit supplied from an MCCB <32A that had a satisfactory measured Zs as it fed fixed equipment in compliance with the 16th Edition. Now the same installation is PIRed and the circuit has an unsatisfactory Zs as the disconnection time has been reduced to 0.4s. How would you Code this?

PS Please do not say "the regs. are not retrospective" and/or you need to know what Edition of the Regs. was in force at the time of the installation!

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John Peckham

http://www.astutetechnicalservices.co.uk/
 31 December 2010 09:27 PM
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michaelbrett

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John

I think you need to look at the type of circuit and assess the risk.

My immediate thoughts are code 4. ('Complied yesterday - not today' kind of thing.)

If it is a socket outlet circuit, then maybe code 2. My thought process is that if it is a socket outlet, then the load profile is unknown and there could be a higher risk.

Hope this makes sense.

Regards

Mike
 31 December 2010 09:33 PM
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GeoffBlackwell

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My view is it was considered safe under the criteria used to judge these things when it was constructed - in those terms it is as safe as it has always been.

The criterion used to judge these things now has changed and it does not meet these new requirements.

So it is a safe as it was but it does not meet the requirements for an installation constructed now.

So what .

BS 7671 requires that you assess the installation against the current requirements - it is then for you to judge what recommendations you wish to make (Appendix 6 is only informative).

You could take the view that you will use a code 4 style recommendation if you wish to make such a value judgement that is, of course, at variance with current recommended practice. You could do what I do - and that is to inform the customer of the facts, explain that safety standards have moved on, and leave it to them to decide how they wish to proceed - simples - and limited liability to boot .

Regards

Geoff Blackwell
 31 December 2010 10:01 PM
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John Peckham

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Mike

The circuit was fixed equipment lighting in fact so the disconnection time was 5s under the 16th.

Geoff

I did in fact talk the chief engineer for these premises through my findings. My dilema was I had previously PIRed this installation twice before and now I was testing exactly the same circuits unaltered or extended and now reporting a defect. I did in fact opt for a Code 4 however I have been turning this over in my mind finding reasons for a 2 or a 4.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 31 December 2010 10:14 PM
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Martynduerden

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What is the actual disconnection time John you are getting?

As it is lighting (dependant on switching) I would be leaning towards a code 4, otherwise code 2 and pass it over to the duty holder.

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Martyn.

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 31 December 2010 10:21 PM
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GeoffBlackwell

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John here is one of my standard report items - please note that I generally charge mega bucks for this so consider it as your Xmas present .

The standard for Electrical Installations BS 7671:2008 (The Wiring Regulations) has recently been revised.

This report generally assesses the installation against the requirements of the new standard. The installation was constructed to an earlier edition than this and some of the items listed here are as a result of changes in the new standard - these will usually be marked code 4. These new requirements are not retrospective but, in many cases, they do provide an increase in safety.

An example of this is the use of Residual Current Devices (RCDs) to supplement other protective measures. This is called Additional Protection and it has now been included in BS 7671.

You may wish to consider improving your installation by adding some of the safety features that must now be used on new installation work.

If you have had previous inspections carried out on this installation to earlier editions of the Wiring Regulations you may notice that some test results that were previously acceptable are now shown as not meeting the standard - this is because of changes in the standard.


Regards

Geoff Blackwell
 01 January 2011 12:32 AM
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Testit

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Is this to spur on a discussion? lol

Code 4....

or

If you think the undisclosed disconnection time is unsafe due to the disconnection time then a code 2.. but since we allow 5s disconnection for distribution, which can run through buildings.. is it unsafe?

What is the actual disconnection time with the Zs of the circuit? and is the environment such that you think it may pose more immediiate danger of a shock... does it have skilled personnel to change the bulbs? or is it done by the secretary to gain attention from the boss?..

One deliberates I suppose....

That'll be £300 please.. stuck here on new years eve cos of some P*at

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 01 January 2011 07:17 AM
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ebee

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Well the only way to make electricity safe is not to have it.
So when we talk about safety it is really about relative safety not absolute safety.
"as safe as"
Our perception of safety as such therefore changes over time - usually for the better (safer).
What we considered to be reasonably safe yersterday might not been seen as so safe today.
The further we go back the more this tends to show.
One hunderd years ago what was standard practice might well horrify us today.
The fact the regs have change must give us a code 4 at least john.
Code 2 maybe - you use your judgement especially on such a small timescale as 16th Edition to 17th Edition.
Over a larger timescale you might start to wonder about a code 1.
tough call innit?



Happy New Year

Just think back say 30 years ago, only yesterdayish to a great many of us - how things were so different back then, not just electrically but in all walks of life, view those things in todays standard, wow - mindblowing.
It's a miricale we ever made it safely through to today.

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

Knotted cables cause Lumpy Lektrik
 01 January 2011 10:22 AM
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John Peckham

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Martyn

I don't have the time current curves for the MCCBs only the max Zs for 0.4 and 5s.

The issues for me as GB points out is it was OK/Safe/Satisfactory/compliant a couple of years ago it is the change in the standard that now makes it non-compliant.

For it to be unsafe I/you would have to believe a 5s disconnection time was not safe. If so why is my 16A circuit unsafe with a 5 sec disconection time when a 40A circuit is safe? Both feeding fixed equipment.

Ebee

How do you get to a Code 1? Again why is a final circuit (>32A)feeding some fixed equipment safe witha 5s disconnection time and a final circuit (<32A) not safe. Do you get smaller shocks from bigger pieces of kit?

I do have a problem with 5s disconnection times being safe in the 1st place. I suspect it is more to do with practical design than safety but I stand to be corrected?

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John Peckham

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 01 January 2011 10:37 AM
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perspicacious

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Sooner or later, non compliances on an installation will result in the installation becoming unsatisfactory

Think about it, it is only a matter of time.............

Insert your own dates in the xxxx

When did "bonding" become a requirement (xxxx) and when did it become "unsatisfactory" on a PIR? (xxxx)

When did it become a requirement for a socket-outlet rated at 32 A or less which may reasonably be expected to supply portable equipment for use outdoors to be provided with supplementary protection (xxxx) and when did it become "unsatisfactory" on a PIR? (xxxx)

Regards

BOD
 01 January 2011 11:17 AM
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ebee

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Sorry John I didn't make myself clear.
I was not saying that this particular defect could be a code 1 but rather that some things that used to be compliant years ago could become code 2s or even 1s in future.
Bods example of bonding not being a requirement years ago and lack of it being unsat is agood example of changes in perception

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 01 January 2011 11:51 AM
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rikhill

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Well it is definately a Code 4 (Doesn't comply with current regs) but I can see your dilemma with regards to 'safety' and whether it deserves a code 2.

From your description of the issues/circuit I think I would leave it at code 4.

On a related note this is my biggest concern with the proposed new reporting standard - I find the Code 4 items the most useful way of quickly identifying the competence of the person preparing a PIR. With your example above you could be perfectly justified in leaving such an item off the report completely if you didn't believe it affected safety; which feels wrong to me.
 01 January 2011 04:23 PM
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Testit

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I do have a problem with 5s disconnection times being safe in the 1st place. I suspect it is more to do with practical design than safety but I stand to be corrected?


Totally agree. I have this issue with shallow buried tails I find everywhere... But it depends on the actual disconnection time... The standard is a transitional issue as things progress to new levels of protection... I would be issuing a code 4.. I believe you utilise the same software for certs.. easycert... this offers a code 4 non compliant as well as code 4 non compliant and requires attention in the coding and definitions.... Not immediately unsafe, but not currently compliant and should really be upgraded, but not requiring a code 2 or fail..... along with some guidance notes to the customer re shck protection... job done!

Edit: 5s I dont think offers enough protection... but its not a fail and a code 4 that should be given some attention if funding permits...

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 01 January 2011 07:00 PM
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michaelbrett

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Originally posted by: John Peckham

Mike



The circuit was fixed equipment lighting in fact so the disconnection time was 5s under the 16th.



Geoff



I did in fact talk the chief engineer for these premises through my findings. My dilema was I had previously PIRed this installation twice before and now I was testing exactly the same circuits unaltered or extended and now reporting a defect. I did in fact opt for a Code 4 however I have been turning this over in my mind finding reasons for a 2 or a 4.




John

I am inclined to agree with Geoff Blackwell comments.

However, If circuit parameters have not changed but Zs is now out of spec due to faster disconnection time requirements this could be viewed as an issue.

As you are registered with the NICEIC, you might be best advised to follow their recommendations and add a note as suggested by Geoff in the 'Summary of the Inspection' box in the PIR highlighting this issue.

Sometimes a discussion with the local NICEIC Engineer can be helpful in reaching a conclusion.

Personally, If I'm signing a report, I would be inclined to be risk-adverse and also add the notes (or something similar) as suggested by Geoff.

I would be interested to know how you propose to proceed.

Regards

Mike
 02 January 2011 03:43 PM
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John Peckham

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Mike

It has gone in as a Code 4 with a comment that it did comply with a 5s disconnection time required by the 16th Edition.

My thinking was it is no longer compliant as the max. Zs for 0.4s disconnection had been exceeded. However 5s disconnection is still permitted under the 17th Edition for final circuits >32A so it cannot be dangerous or potentialy dangerous.

BOD is throwing in some attempted diversions as to when things became unaccepatble. That would be a useful debate for another thread. The issue of lack of bonding being unsatisfactory could be clouded by the fact that it used to be an NICEIC Code 1 then downgraded to a Code 2 when the status of Code 2s was increased.

Whilst it was my decision I am also risk averse so I will not go against the written advice of the great and the good of the ESC,IET etc.
If these bodies had said that a 5s disconection time was no longer accepatable then I would have Code 2ed the defect.

Think about this scenario if you have a 20A CPD final circuit supplying fixed equipment that has a Zs value exceeding the max. Zs for 0.4s. This is non compliant and has to be reported on a PIR. Assuming the conductor CSA is large enough to "correct" the defect you fit a 40A device. Hey presto the installation is now compliant as the measured Zs meets the Zs for 5s ADS. The circuit is now compliant but is it safer?

Now to question the good book itself I have a problem understanding why a 5s disconnection time is consider accepatable? On a TN system wwhy two different standards for ADS times if the prime consideration is safety? I could understand it if it was for circuits suppling equipment you could hold or pick up but not for fixed plant.

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John Peckham

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 02 January 2011 04:20 PM
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GeoffBlackwell

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Now to question the good book itself I have a problem understanding why a 5s disconnection time is consider acceptable? On a TN system why two different standards for ADS times if the prime consideration is safety? I could understand it if it was for circuits supplying equipment you could hold or pick up but not for fixed plant.


Well where shall we start .

How about the addiction we in Northern Europe and North America have to making things go bang - when all we really want to do is minimise the risks of serious injury or death from electric shock hazards created by circuit faults occurring within an installation.

If we must make things go bang - which of course is not the only way of providing protection - then we need enough current to flow to do it. Therein lays the problem - if you want a big bang for a large fuse or circuit breaker you need lots of amps, and this implies that you have a very low Zs.

Simple enough for any circuit rated at 32A or less but it gets harder to achieve the higher up the scale you go. In fact it gets so hard that on very large circuits we have to consider abandoning the big bang theory altogether .

Now we can trace the 0.4 second disconnection time (TN) and the 0.2 second time (TT) to the work of Biegelmeier and Lee, but where does 5 seconds come from. Well the urban myth I usually quote is that when confronted with Table 41.A (now 41.1) the Chief Engineer of a UK based multi national realised that none of the installations in his factories could meet a blanket 0.4 second disconnection time. A quick calculation on the back of a fag packet showed that even the old BS 3036 fuses might be ok if 5 seconds was used .

Now why has the criterion changed? i.e. the move from 'fixed appliances' to distribution circuits and final circuits over 32A.

I have my views on it but what do others think?

Regards

Geoff Blackwell
 02 January 2011 04:31 PM
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slittle

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Geoff,

Nothing wrong with a big bang. It generally assists with fault finding as well because the flash marks are easy to see

Back to John's original question. I would have gone for a 4 as well. I think it's hard to say it was unsafe but agreed it doesn't comply with the current edition of our beloved BS7671.

Stu
 02 January 2011 05:11 PM
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rocknroll

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but where does 5 seconds come from.


Its the relationship between the magnitude of the voltage and the time it may be allowed to persist.

The theory is that the critical condition between touch voltage and the maximum disconnecting time is 50v for 5s, so with regard to all of the overcurrent devices (although the 3036 is suspect) if the 50v for 5s condition is met then all the shorter disconnection times for higher voltages will be met.

Just to point out that the majority of devices used are current limiting and are manufactured to operate in max of 0.6s even with a high Zs to protect cables and equipment, but then we run into fuse and MCB technology.

regards

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"Take nothing but a picture,
leave nothing but footprints!"
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"Oh! The drama of it all."
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"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
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 02 January 2011 05:23 PM
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rogersmith7671

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Where Supplementary equipotential bonding exists which includes all simultaneously accessible exposed conductive-parts and extraneous-conductive-parts including, where practicable the main metallic reinforcement of constructional reinforced concrete, and that, the equipotential bonding system is connected to the protective conductors of all equipment including those of socket-outlets. Then automatic disconnection according to 411.3.2.2, TAB 41.1 et,al is not absolute if it cannot be achieved.

Regards
 02 January 2011 05:29 PM
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John Peckham

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I don't know GB why 5s is considered safe.

If the R1 and R2 values are the same the potential touch voltage would be in the order of 115V for 5 seconds. The B&L curves say 100V touch voltage a current of 62mA for a maximum of 400ms.

I think the Chief Engineer back of a fag packet economic calculation may not be far from the truth.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
IET » Wiring and the regulations » HIGH Zs ON MCCB FINAL CIRCUIT

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