Will Zs measurements be redundant under the 17th Edition?

[mijo]

Think the only way out of this information impasse is to approach manufacturers and ask for the max fault current that their RCD's and RCCBO's will withstand WITHIN STATED RCD OPERATION TIMES i.e. mS.
I suspect in many cases these withstand currents will be less than the earth fault loop impedance will let through in the event of an earth fault .
Then one relies on the MCB part of the RCCBO or a Fuse to break the fault current at much longer operating times and ELZ becomes highly important.
It is not unknown for max withstand current for RCD to be say 200 amps and min operating current for MCB to be 300 amps (30 amp type C MCB) ELZ 0.64ohms is imperative.
Regards Mijo

 

[a.walk]

Ok chaps

just watched a joint NICEIC/Megger DVD on intitial verification

They state on there that where circuits are RCD protected, the Zs reading need only be taken with a low resolution meter capable of measuring 1667 ohms

I also have in front of me a copy of "Guide to the Wiring Regulations BS7671:2008"
published by no less than the ECA

Page 63 has the following quote under the heading "Irrelevant ELI specification"

"A common source of misunderstanding is that of either specifying or measuring values of ELI where the circuit also has an RCD fitted. ELI measurement under these circumastances is a futile exercise. The circuit will have been checked for continuity, and this is what is needed, together with, of course, functional checks of the RCD...........and for clarification, circuits with RCBO's do not need to meet the specified ELI values"

So back to the original post - will Zs values be irrelevant under the 17th edition? (for RCD protected circuits)


Well I think they will
NICEIC think they will
Megger think they will
The ECA think they will

and for information, the IET often adopt the ECA's reccomendations

*shakey takes a bow*

Hate to say i am right chaps, but, ........erm........point proved i think

Absolutely agree!! Continuity test will tell you all about the circuit (loose connections, circuit lenght etc). the rule is simple anyway if you have measured R1+R2 and also Ze you automatically have a Zs value without measuring.