What test to carry out before consumer unit change?

[ppelec100]

What test should I carry out before changing a consumer unit to ensure that it is eligible for one?
I’m guessing R1 + R2 to make sure it has cpc at circuits. Say for lighting do I check every single light fitting or...?
IR test I’m assuming of course so there are no trips.

I’m not a testing qualified just installation. So this info will help me out for a CU coming up. I will be getting someone to test it and cert it afterwards.

Thanks for any help.

 

[Dustydazzler]

You can test at the furthest point if you can work out which it is , or I sometimes just do a bit of random testing at some of the more easy accessible points

As for IR , yes it is often wise to do some IR testing to avoid and tripping issues later

 

[ppelec100]

You can test at the furthest point if you can work out which it is , or I sometimes just do a bit of random testing at some of the more easy accessible points

As for IR , yes it is often wise to do some IR testing to avoid and tripping issues later

Thank you.
I forgot to mention end to ends for ring circuits. Do I need to do the figure of 8 test step 2&3 of ring continuity) too, or is this just for new installations for polarity check?
Are these test sufficient for a consumer unit change?

 

[pc1966]

Remember the checks before a CU change are really about establishing what extra work is likely to be involved in making the installation safe & reliable overall. After the CU change you have all of the usual tests to be done anyway, but you really want to be able to quote the customer a realistic price first and explain why so they don't get a nasty surprise later and you get a reputation for not sticking to the expected quote.

So global IR (L+N to E) is a good and simple one:

• If above a couple of Mohm at 500V all is fine
• If not, but OK at 250V probably a SPD in some extension cable - not a big risk.
• If a few tens of kOhm it might be a RCD socket outlet or similar, but it could be more trouble like soaked/trashed cable or wet junction box.
• If few ohms or less then a N - E short that has to be found and fixed

Checking CPC continuity is another important point:

• For any RFC the end-end loop R is a very good start, though does not prove any spurs are correct, but otherwise high confidence
• For any radial a R1+R2 check at furthest point is good test (for radial sockets again not proving any other branches are OK)
• Light circuits - always a worry as pre-1966 wiring might lack CPCs (see BPG#1 for info on that case) or sadly more likely is some muppet did not connect up CPCs thinking lights were double-insulated or similar. Again R1+R2 test or wander lead to a couple of light fittings will find if serious issue or not.

Checking the means of earthing / supply Ze is important so any assumptions on design values for final Zs, etc, are acceptable.

Last but by no means least, the danger of linked circuits:

• The issue of a "borrowed neutral" between light circuits, typically a badly implemented up/down system. Not only dangerous but going to trip any RCBO/RCD.
• A cross-linked pair of RFC (where two RFC are in one giant loop fed from two MCBs) which is a major safety risk as well since 64A equivalent breaker way above 2.5mm cable's safe ratings

 

[Dustydazzler]

Thank you.
I forgot to mention end to ends for ring circuits. Do I need to do the figure of 8 test step 2&3 of ring continuity) too, or is this just for new installations for polarity check?
Are these test sufficient for a consumer unit change?

I do end to end on all rings
I do IR tests where possible
and R1 R2 at easy positions

Ze
Zs
RCD trip times

personally don't bother with figure 8

 

[Petej999]

What test should I carry out before changing a consumer unit to ensure that it is eligible for one?
I’m guessing R1 + R2 to make sure it has cpc at circuits. Say for lighting do I check every single light fitting or...?
IR test I’m assuming of course so there are no trips.

I’m not a testing qualified just installation. So this info will help me out for a CU coming up. I will be getting someone to test it and cert it afterwards.

Thanks for any help.

What test should I carry out before changing a consumer unit to ensure that it is eligible for one?
I’m guessing R1 + R2 to make sure it has cpc at circuits. Say for lighting do I check every single light fitting or...?
IR test I’m assuming of course so there are no trips.

I’m not a testing qualified just installation. So this info will help me out for a CU coming up. I will be getting someone to test it and cert it afterwards.

Thanks for any help.

Would you know and understand the results if you were told how and what tests to do?

 

[ppelec100]

Remember the checks before a CU change are really about establishing what extra work is likely to be involved in making the installation safe & reliable overall. After the CU change you have all of the usual tests to be done anyway, but you really want to be able to quote the customer a realistic price first and explain why so they don't get a nasty surprise later and you get a reputation for not sticking to the expected quote.

So global IR (L+N to E) is a good and simple one:

• If above a couple of Mohm at 500V all is fine
• If not, but OK at 250V probably a SPD in some extension cable - not a big risk.
• If a few tens of kOhm it might be a RCD socket outlet or similar, but it could be more trouble like soaked/trashed cable or wet junction box.
• If few ohms or less then a N - E short that has to be found and fixed

Checking CPC continuity is another important point:

• For any RFC the end-end loop R is a very good start, though does not prove any spurs are correct, but otherwise high confidence
• For any radial a R1+R2 check at furthest point is good test (for radial sockets again not proving any other branches are OK)
• Light circuits - always a worry as pre-1966 wiring might lack CPCs (see BPG#1 for info on that case) or sadly more likely is some muppet did not connect up CPCs thinking lights were double-insulated or similar. Again R1+R2 test or wander lead to a couple of light fittings will find if serious issue or not.

Checking the means of earthing / supply Ze is important so any assumptions on design values for final Zs, etc, are acceptable.

Last but by no means least, the danger of linked circuits:

• The issue of a "borrowed neutral" between light circuits, typically a badly implemented up/down system. Not only dangerous but going to trip any RCBO/RCD.
• A cross-linked pair of RFC (where two RFC are in one giant loop fed from two MCBs) which is a major safety risk as well since 64A equivalent breaker way above 2.5mm cable's safe ratings

Thank you for this very in depth answer!

I did some fault finding to try find a nuisance trip of the house which I will be changing the CU. I’ve narrowed it down to fridge freezer, fish tank filter or pump as I put them in an extension lead for the time being on a socket fed by the RCD that wasn’t tripping and it tripped same day.

It is currently a RCD split board installed by Mr bodger. I’ve recommend they get it redone and changed to RCBO board. They also use so many extensions. Every socket has a fully occupied extension lead minimum 4 plugs. Anyways I did IR test with everything unplugged and it was clear.

Will need to do R1 + R2 tests tho.

I did end to end and like you mentioned about the split ring being fed by 2x 32a MCB that’s exactly what I found. I also found another ring which had been split but this ring had one of its LNE leg just connector blocked floating inside the CU.

Can I just check the Ze or does Zs need to be carried out too before the CU change?

When finding out if there are borrowed neutrals, I’m guessing I would check continuity between neutral and other neutral conductors at the board. For example disconnect 1 neutral and test between that conductor and the neutral bars?

 

[ppelec100]

I do end to end on all rings
I do IR tests where possible
and R1 R2 at easy positions

Ze
Zs
RCD trip times

personally don't bother with figure 8

RCD trip times would be done after of course right?

 

[Dustydazzler]

RCD trip times would be done after of course right?

YEs sorry

First 3 dead

Last 3 live

 

[ppelec100]

Would you know and understand the results if you were told how and what tests to do?

Yes I would. I learnt testing as part of my apprenticeship. If I don’t understand I would do some reading in the books or some research about it. I feel pretty confident but even so I’m hesitant sometimes and like to verify and gain further info from other more skilled and experienced electricians.

R1 + R2 I would get a reading if there is continuous cpc, would operate the light switch to get the reading and off to remove it. If I get no reading then there’s no continuous earth/ break somewhere.

IR test between conductors of say L to N would mean there is a short between them somewhere.