CPC sizes historical

[flying_fox]

Hi (first post)

Doing an EICR on a commercial property built circa 1980 (probably deigned 1978/1979).

The majority of the wiring appears to be from that initial date but some work has been done since installation.

The wiring is mostly run as separate cores in conduit and trunking.

Looking at single phase 32 amp ring circuits, being run from consumer units, the CPC appears to be 1mm^2 rather than 1.5mm^2 (measured using vernier for confirmation).

1. Was this a recommended/prefered size for the CPC?
   
2. Was this size of CPC correctly sized in accordance with the regulations applicable at the time of installation?
   
3. Could this size be correct if specified by the designer with supporting calculations?
   
4. What version of the regs were pertinent at that time?
   

Measuring the PFC at the board indicated about 1KA.

Any assistance/information would be appreciated.

Thanks in advance.

 

[Steve3948]

I have never seen a 6491X - 1.0mm

 

[Andy™]

Hi (first post)Doing an EICR on a commercial property built circa 1980 (probably deigned 1978/1979).

The majority of the wiring appears to be from that initial date but some work has been done since installation.

The wiring is mostly run as separate cores in conduit and trunking.

Looking at single phase 32 amp ring circuits, being run from consumer units, the CPC appears to be 1mm^2 rather than 1.5mm^2 (measured using vernier for confirmation).

1. Was this a recommended/prefered size for the CPC?
   
2. Was this size of CPC correctly sized in accordance with the regulations applicable at the time of installation?
   
3. Could this size be correct if specified by the designer with supporting calculations?
   
4. What version of the regs were pertinent at that time?
   

Measuring the PFC at the board indicated about 1KA.

Any assistance/information would be appreciated.

Thanks in advance.

are you sure your competent to be doing EICR's?

7671 definition

A person who possess sufficient technical knowledge, relevant practical skills and experience for the nature of electrical work undertaken......

are you sure its 1mm

 

[Adrian]

[quote name='Andy

 

[Doc Hudson]

Was this a recommended/preferred size for the CPC?Was this size of CPC correctly sized in accordance with the regulations applicable at the time of installation?

Could this size be correct if specified by the designer with supporting calculations?

What version of the regs were pertinent at that time?

I am assuming you do not have a copy of the current wiring regulations? As there is a page giving previous editions and dates of previous versions. There is a complete chapter '54' on earthing arrangement and CPC's subsection 543 specifically is Protective conductors and starts with Cross-Sectional Area. Inspection and testing generally requires a greater level of experience and knowledge than just being able to install and test new jobs. Or is this a Students college question?

Doc H.

---------- Post Auto-Merged at 15:49 ---------- Previous post was made at 15:46 ----------

Thread moved due to nature of questions.

Doc H.

 

[flying_fox]

Thank you for the responses.

Somewhat surprised by one of the replies but I have managed to achieve 2391 but I freely admit that I am still learning and was seeking advice from those with more knowledge than myself .

The sizing of the wires may be an imperial equivalent but I had assumed that as the original schedules I have seen are metric that I was working with a metric wire (at what date could I reasonably expect to discount the use of imperial cable sizes and what would have been the minimum size of an imperial equivalent required for a 32amp ring main).

As Steve stated about the size of the cables normally being the same size (eg 2.5mm^2) had this been the situation I would have been happy with circuit as found

When looking at the distribution board it is not obvious that the earth wires used vary in size from those of the radial circuits that supply lights and and other low current devices (5, 6, 10 amp circuits)

As a matter of due diligence I will be looking at the suspect wiring to confirm or amend my original opinion of the size of wiring used.

I suspect that I may be looking at partial historic rewire as some T&E has been used in places but inspection of the whole cable lengths is somewhat problematic and not all junction boxes are accessible.

Circuit continuity of conductors and CPC at the sockets appears to be correct in that loop resistances are close to each other when breaking the circuit at the point of inspection. However trying to match the corresponding CPCs at the distribution board is proving problematic as multiple earth paths appear to be present (543.2.9 is apparently not satisfied).

This is more for information:-

With regards to use of 3036 - no this is a circuit using an MCB Type B as the current limiting device.

With regards to the use of 1mm^2 on sockets could you provide me with a pointer as to where this is appropriate (possible uses I consider would 1.5 T&E supplying a single spur, multiple sockets on a radial where that radial is protected by a suitable circuit protective device, electrically linking a metal flushbox to the CPC [if you consider that the metal fixing screws do not provide a suitable conductive path]). Not sure about the non-compliance of 1mm^2 when a 3036 is used so would welcome a pointer as to where to look in the regs. The only reference I have found so far relates to cable grouping and methods of fixing but I do seem to recall that 3036 were prohibited in certain other circumstances but cannot at the moment recall where.

Doc H:- Thanks for response. Yes I do have current version of regs but forgot to check - apologies.

To amend my original question "If designed in accordance with 14th Edition as Amended in 1976 would a CPC of 1mm^2 be a recommended/preferred size for the CPC?

Could this size of CPC be considered to be correctly sized in accordance with the regulations applicable at the time of installation if specified by the designer with supporting calculations?"

With regards to Chapter 54

How would I apply the Adiabatic equation so that it relates to the CPC in a Ring Circuit?

Worst case Assume one leg of the ring is dead and use a fault current of 160Amp with an intitial temp of 70deg C for bunched cables would imply that the CPC is OK if disconnection occurs in less than 0.4 Secs but not if it exceeds this time.

This not an exam question but something that I am requesting help on.

I have access to the previous PIR which I suspect was carried out with wiring in the present configuration. This document assumes that the CPC sizing is 1.5mm and that circuits are correctly configured, something with which I am not in total agreement at the present time.

Thanks in advance.

 

[Doc Hudson]

Thread moved (again) apologies. But we have in the past had students posing questions as if they were real work questions. It is generally better all round if members have a reasonable idea of what level to write their replies at.

Doc H.

---------- Post Auto-Merged at 22:00 ---------- Previous post was made at 21:55 ----------

Could the installation have previously been using metal conduit and/or trunking as a CPC with additional wire added at a later date?

Doc H.

 

[flying_fox]

Hi Doc

Doesn't appear to have been metal conduit used as the CPC. Metal conduit would be a possible explanation of the existing earthing situation.

From what I understand it is possible that metal conduit may have been fitted into the structure of the building for some circuits but I am not 100% certain of this although if had been used I suspect it was on the radial circuits.

All the exposed conduit appears to be in plastic 25mm round tube connecting into white plastic similar rectangular section to Marshall's.

The wiring in the rooms appears to be run in skirting trunking with some updates to the wiring running in a non-prescribed fashion.

The previous PIRs appear to have made some assumptions which one might reasonably make under normal circumstances but I suspect these were not accurate.

Schedules do not reflect the wiring as fitted although they are indicative, previous PIR assigns certain items to circuits from which they are not presently fed.

The measured values of Zs at sockets on the same ring main varies significantly although most are within defined limits. For speed of testing I am using a Plug in lead connected to the meter rather than opening up every socket and connecting to the wires so I would expect a little variation on the results due to contact resistance within the socket.

My intention is to update schedules to reflect situation as is and to identify, to the best of my abilities, any areas that are of potential cause for concern.

If the ring circuits are not adequately protected by the existing CPC I would be considering recommending either de-rating the circuits or remedial work to bring them up to current standard. Due to the age of the fuseboards de-rating may not be possible.

Thanks again.

 

[springcrocus]

Is this any use?

 

[flying_fox]

Hi Springcrocus.

Thanks for PDF - MID 1975 detail about 2.5/1.0 T&E may give an indication of why 1.0 would have been used if somebody hadn't been totally conversant with regulation changes at the time.

PDF in general will be useful for future reference.

Many thanks,

 

[flying_fox]

Reading through the PDF re 2001 changes to supplementary bonding - this is a possible explanation as to the reason for multiple earth paths at the DB. This then raises the question as to how to verfiy that 543.2.9 is satisfied?

 

[park1]

Hi (first post)Doing an EICR on a commercial property built circa 1980 (probably deigned 1978/1979).

The majority of the wiring appears to be from that initial date but some work has been done since installation.

The wiring is mostly run as separate cores in conduit and trunking.

Looking at single phase 32 amp ring circuits, being run from consumer units, the CPC appears to be 1mm^2 rather than 1.5mm^2 (measured using vernier for confirmation).

1. Was this a recommended/prefered size for the CPC?
   
2. Was this size of CPC correctly sized in accordance with the regulations applicable at the time of installation?
   
3. Could this size be correct if specified by the designer with supporting calculations?
   
4. What version of the regs were pertinent at that time?
   

Measuring the PFC at the board indicated about 1KA.

Any assistance/information would be appreciated.

Thanks in advance.

If you have your 2391 then you should be able to use the Adiabatic to confirm compliance.

 

[steptoe]

Tell you what, if you are really doing a PIR on this then it does indeed appear you are way out of your depth!

You have already said you didnt think to look in BGB, WTF? That should be the first place you look, not on am internet forum!

We do like to try and help, especially where grey areas exist, or you simply cant find the info,

But to not even try doing it, you at you have 2391 and understand the adiabactic, so go and all it out, stop pretending you are more competent than you really are.

 

[Paultrim]

A bit harsh, the chap is looking for info, we're all students at the end of the day and those that think they know it all are the poorer for it. I never imagine I

know everything and always enjoy listening to other peoples/fellow engineers advice and experience

 

[Andy™]

A bit harsh, the chap is looking for info, we're all students at the end of the day and those that think they know it all are the poorer for it. I never imagine Iknow everything and always enjoy listening to other peoples/fellow engineers advice and experience

this is basic stuff that anyone carrying out a EICR should be aware of. if you dont, then your not competent, in which case your not working to 7671.

the person who he should have been asking is his mentor whilst training to be a spark (although he probably never had time in the 5 day course)

 

[flying_fox]

Assumptions:-Energy loss by conduction, radiation and convection is zero

 

[flying_fox]

Correction to post

Where I is given by Vo/Ze. If Vo is taken as 230V and Ze is 1.44 Ohmn = 159.722 Ohmn

should read

Where I is given by Vo/Ze. If Vo is taken as 230V and Ze is 1.44 Ohmn, I = 159.722 Amps

 

[park1]

Assumptions:-Energy loss by conduction, radiation and convection is zero