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?

 

[stevethesparks]

There is only one RCD on the current board

 

[telectrix]

if you test the RCD again , you'll probably get lower readings. if it's not tripped for a while it could have got a bit sticky.

 

[Octopus]

There is only one RCD on the current board

In which case its not a hi integrity as I think of hi integrity as 2 x RCD's + a few MCB/RCBO ways!

 

[stevethesparks]

I am contemplating putting in a Hi Integrity one in place of the current one. I did actually mention that in my fist sentence. I was doing some checks before hand as he has just moved in.

 

[Guitarist]

Depending on the RCD type, you have upto 300ms at 1x to play with, so while higher than we usually find, it's not outside acceptable tolerance.

 

[ARK]

the earth loop can be massive with an rcd in line, well over a 1000 ohms and still comply

 

[Guitarist]

the earth loop can be massive with an rcd in line, well over a 1000 ohms and still comply

One would never accept a high earth loop reading on a TN system though, simply because it has an RCD in-line. If it's high, then there is either a fault, or the system wasn't designed very well.

 

[Jay Sparks]

the earth loop can be massive with an rcd in line, well over a 1000 ohms and still comply

Put your tin hat on and get ready.........

 

[pencilpusher]

the earth loop can be massive with an rcd in line, well over a 1000 ohms and still comply

Not for me I'm afraid. I always design to meet the MCB disconnection times on TN systems. I certainly don't put all my faith in the RCD to be happy with resistances that high.

E54 will have something to say about that too, I'm sure!

 

[Engineer54]

In which case its not a hi integrity as I think of hi integrity as 2 x RCD's + a few MCB/RCBO ways!

I don't, a high integrity CU is an all RCBO board! lol!!

 

[Engineer54]

Not for me I'm afraid. I always design to meet the MCB disconnection times on TN systems. I certainly don't put all my faith in the RCD to be happy with resistances that high.

E54 will have something to say about that too, I'm sure!

What can you say to statements like that?? He's just another brainwashed fella that considers RCDs as the electricians cure all for badly designed and/or installed electrical work that don't/can't meet primary Zs values!!

 

[Silly Sausage]

the earth loop can be massive with an rcd in line, well over a 1000 ohms and still comply

That is my nomination for 'Post of the Month' :smilielol5:

What are the odds that someone would pass a circuit with a 4 digit reading?!?!

 

[Deleted member 9648]

the earth loop can be massive with an rcd in line, well over a 1000 ohms and still comply with meeting required disconnection times

If you'd included the bit in red you'd be correct in theory.....in practice though an earth loop that high (and the reasons for it being that high) is not going to comply with anything else.