earth loop reading

[IanMichael]

struggling with a earth loop reading.

have a split load consumer unit. 100A breaker with 3 light circuits and boiler on one side and a 63a 30ma rccb with 3 ring mains and cooker circuits on the other side. originally i got called out for a tripping mcb and rcd on the kitchen ring when they overloaded the ring. did a quick check on the Zs and got 4.56 from every socket in the kitchen - then did a check on the other rings on the RCD, all about the same figure. did a Ze test (all good 0.18). stripped down the neutral busbar and earths from the mcbs and did a Zs test on the busbar (4.56 again) changed RCD but still same problem. meggered the rcd circuits - all good apart from kitchen ring L-PE 85.4Ohms on 500v - but still shouldnt cause such high results on the other Zs? any suggestions?? apparantly no pictures hooks or nails have been banged in either.

 

[SparkyNinja]

and did a Zs test on the busbar (4.56 again)

Could you explain this a little easier?

What terminations did you connect to?

 

[IanMichael]

basically my zs reading at all sockets is around the 4.56 mark (only on the rcd side of things) the ze is 0.18. the insulation resistance readings between l-n and l-pe are all fine. i dont know why i have such a high zs reading on my sockets???

 

[SparkyNinja]

what were the R1+R2 readings like?

how about the end to end ones? r1, rn etc?

 

[IanMichael]

didnt test end to ends to confirm the ring, - the installation is only a 1.5 yrs old. i did strip out all the mcbs on the rcd side of things (including neutrals and earths) and the did a Zs test on the RCCB using the busbar. still had the same problem of a high zs 4.56 (which is leading me to believe that its not a problem with the wiring on the MCB's ) ??????

 

[GaryMo]

Sometimes you can get spurious results when measuring earth fault loop impedance on circuits protected by RCD's due to the meter limiting the test current to 15mA so as not to trip the 30mA RCD.

For a sanity check you need to measure R1+R2 and add this to your measured Ze reading of 0.18 ohms. This then becomes your calculated Zs.

meggered the rcd circuits - all good apart from kitchen ring L-PE 85.4Ohms on 500v

Megger is a brand of test equipment, not an actual test. It's an Insulation Resistance test.

Is that 85.4 ohms or 84.5 Mohms?

 

[securespark]

Are you selling your services??

 

[GreenElectrics]

Have you checked the tightness of all the connections in the consumer unit?
had a similar problem and found the neutral "flying lead" was loose on the outgoing side of the RCD module before it returned to neutral bar.

 

[IanMichael]

yes - tried the neutral flylead as well - will try r1, r2 values and end to ends next week. is there such a thing as a faulty busbar???? its crabtree cu, with a concealed busbar.

 

[GaryMo]

yes - tried the neutral flylead as well - will try r1, r2 values and end to ends next week. is there such a thing as a faulty busbar???? its crabtree cu, with a concealed busbar.

r1 and r2 values are end-to-end resistance measurements for the line and cpc conductors which form part of a ring final circuit.

You need to measure R1+R2 which should be similar to (r1+r2)/4 for a healthy ring final with no spurs.

 

[SparkyNinja]

and did a Zs test on the busbar (4.56 again)

Could you explain this a little easier?

What terminations did you connect to?

I asked this question because if you are getting a Zs of 4.56 WITHIN the consumer unit as it sounds, then it is in there that the problem lies and the internal connections should be checked as GreenElectrics has suggested.