High Zs on RFC for a domestic PIR

[freedomrun]

Hi all, just completed a PIR on a domestic install. The Zs on the RFC is a bit high for a B32 MCB, i'm getting several readings between & around 1.16 ohms (some just below and a few higher), up to the worst being 1.43 ohms. The cct is RCD protected with 30mA protection. The readings were taken with the circuit energised. Also, my reading of the end to end readings of the RFC gave a higher than calculated value for the cpc - could this be the reason for my high Zs maybe? I'm going to give a CODE 3 to investigate the high cpc value further. I seem to remember reading that Zs figures can be higher when taken live, should i repeat the Zs as a dead test? Cheers.

 

[PC Electrics]

I think I'd give this a code 2 tbh. it needs attention, not just investigation at some spurious future date.

You say you took e-e readings but don't say what they were. Is the ring over length with spurs too? or just old accessories and maybe loose connections? And what was Ze? There are other possibilities too.

 

[monkey5]

as above, what was the end to end readings. id give the terminal screws a tighten can make a huge difference it if slightly loose.

wayne

 

[tom1]

what size is the cpc,

have you tested r1 r2 and ze, non trip loop test can add a bit on to the reading.

 

[Robin Spark]

Hi all, just completed a PIR on a domestic install. The Zs on the RFC is a bit high for a B32 MCB, i'm getting several readings between & around 1.16 ohms (some just below and a few higher), up to the worst being 1.43 ohms. The cct is RCD protected with 30mA protection. The readings were taken with the circuit energised. Also, my reading of the end to end readings of the RFC gave a higher than calculated value for the cpc - could this be the reason for my high Zs maybe? I'm going to give a CODE 3 to investigate the high cpc value further. I seem to remember reading that Zs figures can be higher when taken live,

Zs are normally lower than Ze + R1 +R2 due to parallel paths unless you have a dodgy socket adaptor on your tester or the sockets themselves are knackered.

should i repeat the Zs as a dead test? Cheers.

If you can do this, then can you tell the rest of us how to do it as well,... new one on me:tongue in cheek

 

[freedomrun]

TN-S, Ze = 0.15 ohms & (L-L = 0.48) (N-N = 0.48) & (cpc-cpc = 1.21) (all 2.5mm T&E) No SPURS, RFC is in a modest 2 Bed Semi.

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To get the Zs by dead test .... I would add the Ze to the R1+R2 by doing the fig 8 test and the CU or SKT outlet. Would this not be acceptable?

 

[steptoe]

CPC = 1.OMM POSSIBLY,?

IS IT OLDER CABLE?

THIS WOULD BE QUITE NORMAL READINGS IN THAT CASE.

sorry about the caps, :Blushing

 

[freedomrun]

I would say from the age off the property it is probably 1960's - could be a 1.0mm cpc i suppose.

 

[PC Electrics]

Other things being equal that's a 1mm cpc, I'd say. Early 70's install, yes?

Your R1+R2 calcs to about 0.45, so Zs should be about 0.6. What results did you get when testing for R1+R2?

Check connections and state of sockets (incl flicking the switches a few times). Look for corrosion whilst you're behind sockets too.

if no joy drop to a 20A MCB'?

 

[Robin Spark]

Did you do R1+Rn and R1+R2 tests? If no spurs as you say, were your results consistent around the ring?

 

[PC Electrics]

Sorry to be pedantic Robin, but I think you mean R1+Rn and R1+R2. We already have r1, r2 and rn above.

 

[Robin Spark]

Ooops me bad, sorry trying to multi task at the moment!

 

[freedomrun]

It could well be an early 70's build. I've only taken Zs readings at each Socket outlet when the circuit was energised and my readings ranged from 0.83 to 1.43 ohms, but most readings are around the 1.25 Ohm area. It did cross my mind about dropping the B32 to a B25 or B20.

 

[freedomrun]

I've now decided to use the Maximum figure as stated on table 41.3 for the B32 MCB, therefore my Zs values are within the regs after all.

 

[Zeespark]

41.3 in the BRB/BGB or GN3?

AndyGuinness

 

[Andy™]

Hi all, just completed a PIR on a domestic install.

I would say from the age off the property it is probably 1960's - could be a 1.0mm cpc i suppose.

if you have done the PIR, then you should know if tis 1.5 or 1mm earth, as this is also required for schedule of test results. if your unable to work out if its 1 or 1.5mm then IMO you are not competent to be carrying out PIR's