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Working with a spark, now when we change a CU no new circuits just old for new, We do all end to ends on rings, IR tests and Zs making sure all the new protective devices that disconnection times are met, would you do R1+R2?

If we do a CU change and add new circuits we do R1+R2 on the new circuits what do you guys do?
 
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Welchyboy1

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no i dont really think its required for existing circuits i just do zs testing and calculate r1r2

Maybe for lighting if my meter sometimes trips 6a mcbs on zs

Obviously test rings for continuity

For a new install i would dead test r1r2 first
 

Ian1981

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no i dont really think its required for existing circuits i just do zs testing and calculate r1r2

Maybe for lighting if my meter sometimes trips 6a mcbs on zs

Obviously test rings for continuity

For a new install i would dead test r1r2 first
How do you calculate your R1+R2 as that should only be done by measurement only other than at the design stage?
 

Ian1981

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Working with a spark, now when we change a CU no new circuits just old for new, We do all end to ends on rings, IR tests and Zs making sure all the new protective devices that disconnection times are met, would you do R1+R2?

If we do a CU change and add new circuits we do R1+R2 on the new circuits what do you guys do?
R1+R2 is for initial verification so if they are new circuits then this must be done, existing then not necessarily required tho not precluded.
Existing then I just make sure all exposed conductive parts are connected to earth with the R2 method and then just measure the Zs but it’s whatever makes you happy
 

Pete999

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How do you calculate your R1+R2 as that should only be done by measurement only other than at the design stage?
Measure Z E and ZS and take it from there ZE = ZS +(R1+R2) Transpose the formulae Bob's yer Uncle simples as Sergei would say.
 

Andy78

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Measure Z E and ZS and take it from there ZE = ZS +(R1+R2) Transpose the formulae Bob's yer Uncle simples as Sergei would say.
I was always taught this was incorrect and not permitted due to connected potential parallel paths when taking a live Zs reading giving a falsely low reading. R1+R2 measured dead and in isolation of other parts of the installation then worked with Ze into Zs always gives a highest and worst case value so can be trusted, but not the other way round.
 

Ian1981

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Measure Z E and ZS and take it from there ZE = ZS +(R1+R2) Transpose the formulae Bob's yer Uncle simples as Sergei would say.
That is an incorrect method as parallel paths are present
 

ipf

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Measure Z E and ZS and take it from there ZE = ZS +(R1+R2) Transpose the formulae Bob's yer Uncle simples as Sergei would say.
For a board change, I agree with you, Pete.....but with rcd involvement, how can it be guaranteed you have a true Zs reading.
The extent of testing should be agreed with the client, really, and allowed for accordingly.
 

Baddegg

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I thought zs=Ze+ (R1+R2)...but Ze does not = Zs-(R1+R2) or something like that possibly maybe......I’m feeling really uncomfortable with the new look.....the only other time I’ve felt like this was when they spruced up my local! Took me ages to get over that....
 
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