First CU change - high R1+R2 reading and other problems!

[Marcosparco]

Hi Hope someone can put me on the right track here as I'm going in circles.

Ok I set out to do a straightforward CU change (at home) for my ELECSA assesment - or so I thought. I'm fairly happy with testing on new circuits but these are around 15 years old.

My overall question is what level of pre work PIR is required, how much is required to be put right v's flagged with appropriate code for a CU change?

Did what I thought was the right thing and did some pre work tests and got the following issues.

1. Polarity issue at one socket - now corrected
2. 2 way switch not working - now fixed
3. Poor ins resistance in ring, but >1 Mohm, so I believe this is within limits although advise further investigation.
4. Socket positioned over sink in celler/utility area.
5. 2 light switch backboxes too shallow to allow face plate to be tightened up fully
6. missing earths to backboxes
7. looks like 6mm main bonding to gas and no label
and last but not least....
8. R1+R2 on the one ring circuit is too high (3.59 ohm).

As it's my home I'm happy putting these right as required, but where do you draw the line, especially if it is for a customer unwilling to pay for extra work?

In some ways its good this was not straightforward ( ie typical of CU changes!?), but I'm getting bogged down, so any advice would be much appreciated thanks!

Mark

 

[telectrix]

reading your post, it looks as if you've got it all under control. item 6. though, backbox earthing is not obligatory, just a matter of personal preference. the high R1+R2 needs investigating however. could be anything from a poor connection in a socket, to a JB. likewise the low IR reading. check all sockets, look for damp.

 

[SirKit Breaker]

Marco,

Item 8, are you doing the test correctly? Dont wish to insult you but it easy to cock it up. Been there and done it.

 

[telectrix]

double check your continuity readings. make sure your leads are nulled, squeeze the crocs , and your s/c at the CU end are good.

 

[BluSparx]

Just make sure you note on your Cert that although the Ins Res is >1 mohm if it is less than 2 that it may require further investigation. I was asked why I hadn't on my Assesment

 

[IQ Electrical]

What were your end to end continuity readings on the ring?

The back boxes don't require an earth lead if they are the 'one fixed lug' type.

The socket position is more a building regulations item but you could just list it as a deviation in the appropriate box on the electrical installation certificate.

 

[Deleted member 19807]

item 5, you can get spacers with some dimmers, buy a couple of dimmers use the spacers on your house and save the dimmers for a customer with deeper boxes.

 

[Deleted member 19807]

whats the earthing as 6mm bonds are ok for a TT install.

 

[Marcosparco]

Thanks for all the replies, it never ceases to amaze me how helpful people are and how quickly they respond to questions.

Update:

Its a TNS system - so does 6mm main bonding need to be changed or coded 3/4?

ok I have to embarrass myself now. R1+R2 = (r1+r2)/4 was finally explained to us at college last night!
r1 = 0.83
r2 = 2.76
rn = 0.85

R1+R2 = 1.09 (measured previously but now treated correctly) however still too high.
Measured Ze is 0.13, so should I be using this or the suppliers figure? I've used 0800 195 1452 t contact UU previously, but it no longer connects - is there a new number or should I be ringing Eon?

Thanks again,

Mark

 

[Deleted member 19807]

you need to sort out the bad cpc connection, make sure the cpc's are not twisted together and are secured at each faceplate correctly, no point doing r1/r2 until you sort r2. what size cpc is it 1.5 or 1mm? ze 0.13 is fine for tns no need to ring anyone use this figure. bonds should be 10mm. main earth shoud be 16mm

 

[Marcosparco]

Thanks hifly,
cpc is 1.5mm. many of the cpc's are twisted together, but well secured in the faceplates which I've gone round checking - do you mean this could increase the resistance?

 

[Deleted member 19807]

Thanks hifly,
cpc is 1.5mm. many of the cpc's are twisted together, but well secured in the faceplates which I've gone round checking - do you mean this could increase the resistance?

the problem with them being twisted is you think they are both under the screw but they may not be and it makes it very hard to split the circuit to test each leg to find any faults, each cpc should be in its own g/y sleve you can then give each wire a good tug to make sure it connected correctly.

off the top of my head if its 2.5/1.5 you should be looking for cpc to be 1.4 time r1

 

[malcolmsanford]

Your r2 should be 1.67 times your r1/rn reading if it's the normal 2.5/1.5 T+E so your r2 should be approx 0.85*1.67 = 1.42 so it is a little high. Is it a spur? or just a bad connection.

As your fitting a new board don't really worry about the Zs as for a 30mA RCD your Zs will be 1666 ohms and that is what you enter on your EIC.

As your changing the board you will have to upgrade the bonding to 10mm^ and of course label it.

For you poor IR on the ring final circuit did you have anything plugged into it, or a neon still connected?

 

[chiplard]

Whenever i get a low ins res result it is nearly always something I haven't disconnected. ie, Boiler, security light, burglar alarm etc etc. I always do the first test on 250V just to ensure i dont damage anything sensitive to the higher voltage. If that test is ok then i step up to the 500v level. The customer will nearly always forget to tell you about something thats tucked away in the loft, outhouse etc.

 

[Marcosparco]

Your r2 should be 1.67 times your r1/rn reading if it's the normal 2.5/1.5 T+E so your r2 should be approx 0.85*1.67 = 1.42 so it is a little high. Is it a spur? or just a bad connection.

As your fitting a new board don't really worry about the Zs as for a 30mA RCD your Zs will be 1666 ohms and that is what you enter on your EIC.

As your changing the board you will have to upgrade the bonding to 10mm^ and of course label it.

For you poor IR on the ring final circuit did you have anything plugged into it, or a neon still connected?

Thanks all for the tips and clarification. looks like more investigation required into twisted wires and possible poor connection under boards can't access to lift at the moment.

Malcolmsanford, I understand the target calc of 1.42 for r2. I have recorded the highest reading, but in that case it wasn't a spur (!). To show further ignorance though, why does the fact there is a 30mA RCD make Zs 1666 ohms? Is it not governed by the MCB rating and rule of thumb?