Testing a new Consumer Unit - Expected results, and what bad results mean

[kels bels]

Hi all

Read through lots of posts and got some really amazing help with my NICEIC assessment coming up. I've been installing CU's and testing for some time now but have fortunately never come across bad test results.

Now I'm trying to get my head around them for my NICEIC assessment and I have a couple of questions:-

(On a standard 3 bedroom house)

Continuity of CPC - if I'm getting higher results than should be expected, what could this mean?

Insulation Resistance - if I'm getting say 7 Mohms as a reading, what does this mean? What could I suggest to the customer??

Zs - If readings are higher than max. permittable, but the circuits are all RCD protected, should anything be done? What does it mean when I get high readings?

Ze - If readings are higher than max. permittable, what does this mean? And what can be done to fix it?

Thank you for all your help and support!!!

 

[telectrix]

Hi all

Read through lots of posts and got some really amazing help with my NICEIC assessment coming up. I've been installing CU's and testing for some time now but have fortunately never come across bad test results.

Now I'm trying to get my head around them for my NICEIC assessment and I have a couple of questions:-

(On a standard 3 bedroom house)

Continuity of CPC - if I'm getting higher results than should be expected, what could this mean? poor connections

Insulation Resistance - if I'm getting say 7 Mohms as a reading, what does this mean? What could I suggest to the customer?? nothing wrong with 7Meg.

Zs - If readings are higher than max. permittable, but the circuits are all RCD protected, should anything be done? What does it mean when I get high readings? if you've just installed a CU only, nothing to do but record the readings on your cert.

Ze - If readings are higher than max. permittable, what does this mean? And what can be done to fix it? if it's a TN ststem, contact DNO

Thank you for all your help and support!!!

my answers, in bold italic

 

[kels bels]

Thank you!!!

So if there's nothing wrong with 7Mohms, why am I getting this result instead of 299Mohms as I'm getting on other circuits?

Kelly.x

 

[malcolmsanford]

sounds like you have a load still on the circuit, what type of circuit is it

 

[telectrix]

. or a bit of earth leakage on a fridge/freezer/boiler etc.could be a neon on a FCU

 

[kels bels]

That makes sense! Thank you - something still plugged in in garage!

 

[white armour]

Hi kb, with regards to your high reading of cpc on rfc testing, is the cpc 1.5 or 1.0 mm? They used to use the smaller csa in the 70s (I think) this may give you a higher reading than for a 1.5mm cpc.

Regards wa

 

[ezzzekiel]

As youve been installing and testing for a while now then what would you have done if you came across bad results?

 

[kels bels]

I've been working with a Senior Electrician who has been training me!! So anything difficult I might come across he'd have been able to help..... but I don't work with him now, and I'm going it alone, so want to be 100% confident on what i'm doing and that i understand it.

 

[ezzzekiel]

a lot comes with experience but providing your results are within limits of bs7671 then its a pass, from there its best to post your results to get a better explanation of any readings you are uncertain about

tels original answer gives a good basic explanation to start from, but for now as you take a reading try to understand what you are recording and not just writing it down,

a higher reading on continuity means a loose or open connection

a lower ir reading means a million things (general age, detieoration, nipped cable, items still connected)

A high zs could be a pass if rcd protected but id be looking at why its high (unless tt in which case a high ze effects result - zs =ze+r1+r2) whih is why rcd were always used on tt systems

a high ze would be a dno fault and would be reported to them as an emergency

 

[Paul001]

Ive come across a DNO TNC-S supply that has not had the link put across the neutral and CPC and the flat has been using the gas and water as the Main earth . If i didnt take the main earth out to do the Ze as you should do the reading would have been ok .

Over time you will get used to the readings and what you should expect so you know when somethings wrong .

 

[kels bels]

I have a TNS system, 1.2kA PFC, Ze 0.15

Supply tails 16mm, Main Earthing CSA 6mm

Is this acceptable? Are there any regulations that state minimum CSA for Main Earthing??

Main Bonding is 4mm so I understand that's undersized.

 

[malcolmsanford]

Regs section Chapter 54.

In a TN-S or TT system you can have a main earthing conductor of 2.5mm providing that you can prove it's effectiveness by the adiabatic equation from reg 543.1.3.

If you do not want to do this then you must be guided by reg 543.1.4 and table 54.7 which for supply tails ≤16 will be a 10mm conductor.

As for the bonding conductor then yes the 4mm is too small. For a TN-S the smallest size it can be is 6mm reg 544.1.1