continuity of radial circuits

[voltage]

Hi all

First off some info about me im early 30s :/ had to retrain so went back to college and undertook a domestic installers course and also the bs7671 17th edition which i passed with a healty 91% got all my certs etc.
I've now applied to the NICEIC to come out and inspect me on a major which will be a consumer unit change at my mum and dads house. The choice of consumer unit ive opted for is a MK dual RCD 17th edition board.

Regards to testing ive bought the NICEIC dvd on inspection and testing initial verification for domestic properties.

so i just want to run some things past you guys to see if im correct.


continuity of radial circuit protective conductors

I've safely isolated the supply and locked it off ive tested it with a martindale voltage indicator and proving unit to prove its dead.

the way i would use would be to dissconnect the gas and water bonding conductors so not to include them.

now say im testing the lighting circuit downstairs i would link the line and cpc from lighting circuit into connector block together set my megger on ohms for continuity and test switch line and cpc at each celling rose on the circuit with switch on then switch off to prove polarity. Iwould take the highest reading and this would be my R1 + R2 which goes on the cert ? is this way correct ?


Say im testing a shower circuit would i use the same procedure as above but instead of celling rose i test from consumer unit to 45amp double pole switch and at shower point aswell ? if someone could explain this one to me i would be very gratefull.

 

[Sintra]

I will always calculate Zs.

This way you have a measured Ze and a measured (R1+R2) - in theory the highest possible value for the circuit to ensure it meets the required value for the protective device.

By measurement you will include all other parallel paths that may in future be removed.

Not in any book you will read but it satisfies me.

I for one have always measured Zs and always will.

You say that in theory the calculated value will always be the highest, well if all the switches, protective devices, RCD's etc are all in good condition then you are correct.
If you have an MCB for example that is faulty and has a high impedance across its terminals then this will give a higher reading than any calculation.

Another example could be a BMS panel or the like were the impedance of the components in the panel could not be calculated and therefore a measured Zs would be the only way.

Remember a measured Zs test is the only test that actually tests the complete circuit and IMO needs to be carried out.

 

[chiplard]

I for one have always measured Zs and always will.

You say that in theory the calculated value will always be the highest, well if all the switches, protective devices, RCD's etc are all in good condition then you are correct.
If you have an MCB for example that is faulty and has a high impedance across its terminals then this will give a higher reading than any calculation.

Another example could be a BMS panel or the like were the impedance of the components in the panel could not be calculated and therefore a measured Zs would be the only way.

Remember a measured Zs test is the only test that actually tests the complete circuit and IMO needs to be carried out.

Fair point.

But when measuring Zs do you always check every outlet or just the one that is furthest away, or come to think of it, each outlet on a double socket.

These too could be faulty and cause the Zs to be too high at even the nearest point.

Also the no trip facility for testing an RCD protected circuit will not always give an accurate reading.

 

[jazzygazza]

dont know where you are getting that info from mate, they ask you as the qualifying manager to go out to 2 jobs, i have done niceic qualification with 2 companies, they come out spend just over an hour going through insurances,paperwork, registration numbers and how sheets are filled in , some even go through bits of the regs too, then you go to 2 site visits of completed jobs and go through paperwork and how installation was tested and always good to ask questions as they are happy to help if you get something wrong, so dont worry that way

 

[kels bels]

Hi everyone!

I'm struggling with expected results.... once you've got the R1+R2 result, I've heard that anything over 0.8 ohms indicates a loose termination. But during my training I was taught to expect anything between 0.01 and 2 ohms as a good result.

Can anyone help clarify what is a good result?

Thanks, Kelly.x

 

[malcolmsanford]

Your R1 + R2 value will be determined by the length of the cable if you had a 10m radial and a 20m radial you would expect that if the cable was of the same CSA then you would have approx half the value in the 10m circuit.

You can always work out your R1 + R2 by calculation by using the resistivity values in the OSG from table 9A on pg 164

R1 + R2 = mΩ/m x Length/1000 just to compare your measured value.

As for "expected good result" it depends on above factors.

This 0.01 to 2 Ω was your tutor referring to the expected result of the 10mm bonding conductor and you have got them mixed up.

 

[ezzzekiel]

Hi everyone!

I'm struggling with expected results.... once you've got the R1+R2 result, I've heard that anything over 0.8 ohms indicates a loose termination. But during my training I was taught to expect anything between 0.01 and 2 ohms as a good result.

Can anyone help clarify what is a good result?

Thanks, Kelly.x

Circuit readings will vary depending on csa and length of circuit you appear a little confused unfortunately most books tell you how to carry out tests but not how to interprit the results this is an experience thing.

For example in a standard domestic house a cooker would give around .09 ohms so if youre getting no or much higher reading then you know theres a open or loose connection