Impedance readings on a ring

[EddieB]

Testing a domestic ring main recently R1 end to end was .4 Rn was .4 the cpc which was probably 1mm (oldish cables 1975ish) read .2. possible reasons for this??

On another issue how do test the loop impedance at a fixed appliance or a light, to fulfill the measured loop impedance column requirement on the schedule of tests. will the calc R1 + R2 suffice?

 

[WarrenG]

Can you say how you are testing these circuits as I would expect different readings to what you have given?

 

[EddieB]

At the Consumer unit I split the ring and test end to end on all 3 legs I would have expected the CPC to have a reading of about 1/2 again due to the smaller csa

 

[fog]

I always use guidance notes 3 for testing explains and shows how all testing should be done, i have noticed i always get different readings if i test again and always make sure the meters test leads are making a good connection.

 

[WarrenG]

At the Consumer unit I split the ring and test end to end on all 3 legs I would have expected the CPC to have a reading of about 1/2 again due to the smaller csa

Me too about 1.67 times. Everything unplugged?

If a higher reading is found then I'm afraid further investigation is required to check its a fall ring, no inter connections, any connections, breaks etc.

The loop Zs is taken at the furthest point of the circuit and to do the test you will have to check your instructions for your own meter as some require a two lead connection and others three. You should also check to see your meter has a D-lok to avoid nuisence tripping of RCD's.

PS as Fog has said GN3 is your best bet.

 

[DanBrown]

The reason for a difference resistance reading (ohms) between the live conductors and earth, is the simple fact it is smaller in CSA. Rather than having a guess, like you said, half it.. what you should do is a simple calculation. For example on a ring circuit the line conductors are usually 2.5mm and 1.5mm for cpc.
To calculate this, you basically 2.5/1.5 = 1.666. So the CPC resistance should be roughly 1.666 times greater (in theory)

Hope this helps...

 

[EddieB]

Thank you all , however the reading for the cpc is lower not higher, I would expect to to find the cpc come in at about .66 not .2 which is what I'm reading. I will check to see if everything is unplugged but if its split at the consumer unit I cannot see if an item plugged in would make that much difference to the cpc impedance

 

[Had8Lives]

Hmm, sounds like the CPC may have been split and rewired in a single of larger size. Further Investigation required.