More building regs questions v.2

[ppelec100]

So having a look at the electricians guide to the building regulations, section 7 safe working.
7.3 is about pre work tests and is states that electricians should carry out the following test before any installation work is carried out including additions and alterations to existing installations also rewires and new installations...

a) polarity
b) effectiveness of earthing
c) operation of RCD's

My 2 questions are as follows:

Q1) Why is the insulation resistance test not included? I know if the IR was 0 ohms it would be tripping, but lets say if it was like 1 mega ohm... surely this should be investigated as it is low and additions and alterations will be affected if that 1 mega ohm then went below?

Q2) How far do you go with the effectiveness of the earthing? Is it just an R1+R2 test for continuity of cpc or is it also a Zs reading?

Thanks for any answers once again, just trying to learn for the future. ?

 

[telectrix]

IMO the 3 things mentioned are more to do with ensuring your safety before starting work. Other testing comes later.

 

[snowhead]

As Tel, the clue is in the title of the chapter,
section 7 safe working

Subsequent chapters will have different procedures to conduct

 

[telectrix]

just to expand.....

a) polarity........ ensures that if you switch a MCB off to work on a circuit, you are switching L and not N
b) effectiveness of earthing. .....ensures that you aren't going to get a belt off metallic enclosures etc.
c) operation of RCD's. will save you life if you make a cock-up.

now where's my tinfoil hat to ensure the aliens can't suck out my brain?

 

[pc1966]

Those 3 items are all about ensuring that fault protection works. Not that the installation is free of faults!

a) Goes beyond the MCB being a means of isolation. Correct polarity is critical (in the UK for sure) so that your OCPD is in the line conductors (not neutral) otherwise you have no meaningful over current protection to earth. A fault very likely will cause a fire as your 2.5mm or whatever is trying to take out the DNO's 100A fuse. That is why incorrect polarity is a C1 for EICR.

b) Effective earth is also fundamental to safety. Even with RCD protection you must have a usable earth otherwise the RCD is not "additional" protection, since it won't trip on a typical fault until some poor sod completes the circuit. Should that be in somewhere like a shower the resulting shock current/time even from a 30mA instant RCD is getting in to the dangerous region.

c) Is kind of self-explanatory. But you also have to remember in a TT system you are almost certainly dependent on the RCD action to clear a fault as your earth rod is unlikely to be low enough impedance to trip the MCB action. RCD are far more complicated than a fuse, or thermal-magnetic MCB, so testing them under simulated leakage conditions is needed as you can't be sure they are working simply because you can manually flip the breaker on/off.

But you are right that you should also be testing the insulation out before energising final circuits in all cases.

 

[telectrix]

basically, broken down into 3 sections.
1. before working.
2. before energising.
3 . after energising.

 

[Lister1987]

just to expand.....

a) polarity........ ensures that if you switch a MCB off to work on a circuit, you are switching L and not N
b) effectiveness of earthing. .....ensures that you aren't going to get a belt off metallic enclosures etc.
c) operation of RCD's. will save you life if you make a cock-up.

now where's my tinfoil hat to ensure the aliens can't suck out my brain?

Waste of tinfoil Tel ?

Goes back to way tests are done on the prescribed order; to ensure the following test has a safe way of going wrong

 

[ppelec100]

IMO the 3 things mentioned are more to do with ensuring your safety before starting work. Other testing comes later.

As Tel, the clue is in the title of the chapter,
section 7 safe working

Subsequent chapters will have different procedures to conduct

Those 3 items are all about ensuring that fault protection works. Not that the installation is free of faults!

basically, broken down into 3 sections.
1. before working.
2. before energising.
3 . after energising.

Okay thanks I should of noted that the section 7 is about safe working and this is mainly for the safety of the person carrying out the work.

So where will I find info on the pre-checks to do on a circuit to determine if the circuit is adequate to carry out any alterations or additions to?

I'm assuming an R1+R2 test should be done as a pre-check which is basically part of the 7.3 as well as after works being carried out to make sure there is a continuity of CPC.

For a ring circuit the end to end test will need to be done too? Will the step 2 and step 3 test need to be done (figure of 8 of end to ends)?

Should an IR test be done a as a pre-check to ensure the circuit is adequate enough for additions or alterations too? As I wouldn't feel too keen on making an alteration or addition to a circuit which has a IR of 1M ohm, although it satisfies the minimum requirement, it would still require some investigation right?

 

[Lister1987]

Okay thanks I should of noted that the section 7 is about safe working and this is mainly for the safety of the person carrying out the work.

So where will I find info on the pre-checks to do on a circuit to determine if the circuit is adequate to carry out any alterations or additions to?

I'm assuming an R1+R2 test should be done as a pre-check which is basically part of the 7.3 as well as after works being carried out to make sure there is a continuity of CPC.

For a ring circuit the end to end test will need to be done too? Will the step 2 and step 3 test need to be done (figure of 8 of end to ends)?

Should an IR test be done a as a pre-check to ensure the circuit is adequate enough for additions or alterations too? As I wouldn't feel too keen on making an alteration or addition to a circuit which has a IR of 1M ohm, although it satisfies the minimum requirement, it would still require some investigation right?

Sequence of tests will be on part 6 or big blue, or in GN3 and on-site guide.

 

[pc1966]

I'm assuming an R1+R2 test should be done as a pre-check which is basically part of the 7.3 as well as after works being carried out to make sure there is a continuity of CPC.

For a ring circuit the end to end test will need to be done too? Will the step 2 and step 3 test need to be done (figure of 8 of end to ends)?

Should an IR test be done a as a pre-check to ensure the circuit is adequate enough for additions or alterations too?

Short answer is yes - test first or repent later when problems come up!

Never assume something is correct before you start, otherwise you might make a change and then spend your time trying to fix your work when really the fault was elsewhere.

As I wouldn't feel too keen on making an alteration or addition to a circuit which has a IR of 1M ohm, although it satisfies the minimum requirement, it would still require some investigation right?

I would agree with you there, anything below a few Meg is suspect unless you have a very good reason why it might be like that (e.g. very extensive industrial circuit with lots of dust and damp).