Need confirmation on Zs results.

[stevethesparks]

Hi all, after considering replacing a friends old part rcd protected CU for a Wylex Hi Integrity unit, I have found the ring main to be the only ring in the house. The Continuity readings are R1 = 1.03, RN = 1.03, R2 = 1.87, these readings appear to me to be perfectly acceptable for the size of the ring. The Ze is 0.27 of which may improve slightly by changing the short 10mm Main earth to a 16mm on a TN-S system. The Measured Zs at the furthest socket outlet is 0.62, although this was using triplock and so may not be as accurate a figure as possible. The calculated Zs would be 3.17 of which is above the required 1.44 (1.15) minimum as would be using a type B 32 amp MCB. Firstly should I have much concern between the calculated and measured Zs and which should I go by, I understand it is the measured? Also as all MCB's are 30mA RCD protected, I have seen on other posts that under these circumstances a slightly higher than required Zs could be, lets say be ignored. I would just like some input and views on my findings and what others may consider apart from splitting the ring. Many thanks.

 

[telectrix]

you say R1 etc. do you mean r1 etc. where r1 is the end to end resistance of the RFC. if so, you need to divide r1+r2 by 4 when calculating Zs.

 

[chrish_1977]

Normally you would calculate if there was a limitation to you getting an actual reading, always go with what your tester tells you, an if you want to check your results, calculate what you should get to what your actual results are

 

[keniff]

I make it 1 ohm on the calculated zs but it is Sunday morning after all :/

 

[stevethesparks]

Further to my above post, |I have just realised I didn't divide by 4.......DOH

 

[telectrix]

yep. if he's using r1 +r2 instead of R1+R2, his calc.will be out. i make it R1+R2 as 0.725, then add Ze.

 

[stevethesparks]

Thank you all for your prompt replies. Why is it I always realise my mistakes only when posted and not before. And yes my typo should have been r1 not R1 etc, Thank again.

 

[Deleted member 9648]

Hi all, after considering replacing a friends old part rcd protected CU for a Wylex Hi Integrity unit, I have found the ring main to be the only ring in the house. The Continuity readings are R1 = 1.03, RN = 1.03, R2 = 1.87, these readings appear to me to be perfectly acceptable for the size of the ring. The Ze is 0.27 of which may improve slightly by changing the short 10mm Main earth to a 16mm on a TN-S system. The Measured Zs at the furthest socket outlet is 0.62, although this was using triplock and so may not be as accurate a figure as possible. The calculated Zs would be 3.17 of which is above the required 1.44 (1.15) minimum as would be using a type B 32 amp MCB. Firstly should I have much concern between the calculated and measured Zs and which should I go by, I understand it is the measured? Also as all MCB's are 30mA RCD protected, I have seen on other posts that under these circumstances a slightly higher than required Zs could be, lets say be ignored. I would just like some input and views on my findings and what others may consider apart from splitting the ring. Many thanks.

Just to clarify here....your redings appear as expected for an obviously large ring circuit. I'd agree that a slightly higher than permitted Zs could still be acceptable if an RCD is in place,but this would normally be when changing a type B for a C for example,or when one long spur is giving a high reading and the rest of the ring is ok.

 

[Octopus]

Did you not connect the ends at the CU and go round all the sockets to check the R1+R2?

 

[pmf]

Normally you would calculate if there was a limitation to you getting an actual reading, always go with what your tester tells you, an if you want to check your results, calculate what you should get to what your actual results are

This all day long ^^^^

 

[MDJ]

Did you not connect the ends at the CU and go round all the sockets to check the R1+R2?

+1, answer please

 

[chelseaspark197]

What was rcd trip time?

 

[stevethesparks]

I have yet to go back as I was running out of time. However, that shall be done and the highest reading logged. Had I done that it may have clicked that I hadn't divided by four

 

[stevethesparks]

Max trip time was 144ms@1x and 21ms@5X

 

[Octopus]

Max trip time was 144ms@1x and 21ms@5X

Thats hign, what was the other RCD's time?