measure Zs or not?

[the_pasty]

Hi,

When testing a TT installation, if Ze is well below 200 ohms and r1 +r2 less than 1ohm for each circuit, is there really any need to measure Zs for each circuit? I can understand the need to measure Zs in a TNCS installation for example because there are tabulated values which it must be within to ensure disconnection times but for a TT installation protected by an RCD i dont see the point. I ask because i brought this up at work and i was told by my supervisor that it must be measured but I wasn't given a reasonable explanation for doing so.
Thanks

 

[securespark]

If you're filling in an install cert, you must do it anyway because the cert asks for that info? Why cut corners? - it shows you've done the job properly.

 

[the_pasty]

Thanks for the reply. I do it because the install cert requires it but i still dont see the point of measuring Zs in the circumstances i mentioned.I'm sure if it came to court i could argue successfully that it is not necessary.

 

[sparkybird]

I agree - Zs readings of 200+ ohms are a bit meaningless! However, I would note on the cert the reasons why you did not record them. Of course you already have filled in the R1 + R2 values

 

[securespark]

I'd be worried with a Zs of more than 200 Ohms.

I mean, the max according to OSG is 200 Ohms for Ze. When installing rods, I don't get much higher than 45 Ohms.

I know in theory you have a max of 1666Ohms, beyond which correct RCD operation is not guaranteed, but I like to stick to max Zs of 200.

NIC say 100 for Ze, IIRC.