High ZS values on rewire

[gafc156]

Hi all,

Only recently joined the forum, and thought i would get some views on the following.

Just wondered if anybody has had the following issues when fitting a new ccu.

Recently completed a rewire, including new new metal ccu, completed all dead tests ok, however upon live tests i got high zs readings on all circuits.

When i emailed the manufacturer`s technical support with my concerns i was told "most electricians in the know remove the rcd`s" or bypass them to conduct live tests.

I have never heard/had to remove an rcd for testing, in order to get lower zs readings.

zs =1.26
Ze= 0.22
Ring circuit r1/r2=0.38

Thanks

 

[Simonslimline]

Probably from using low current no trip Zs. Try testing with the RCD's bypassed using high loop.

I have had trouble with RCD's on multiple occasions.

Edit - I only scanned your post but it seems the CU manufactures have advised the same.

 

[davesparks]

This is why low current Zs testing is a last resort, not the automatic go-to response for carrying EFLI testing

 

[TJ Anderson]

Easy enough to prove actually. Carry out low current Zs at both sides of RCD and you can see what it is adding to your reading. I record this info on EIC that the RCD is adding X to cct Zs as it could take it over max Zs for OCPD even though it is the RCD that is adding it and ironically, the same RCD then additionally protects circuit upto a max Zs of 1667 ohms, so don't worry too much. I still record actual results though.

 

[Ryan102]

As others said, I would consider testing both sides of the RCD to ensure that's the problem, as long as the RCD is fine your result is still within the required maximum Zs. Nothing you can do about it really if it is the RCD, The R1 R2 is a reasonable figure.

 

[Emm Eff]

Easy enough to prove actually. Carry out low current Zs at both sides of RCD and you can see what it is adding to your reading. I record this info on EIC that the RCD is adding X to cct Zs as it could take it over max Zs for OCPD even though it is the RCD that is adding it and ironically, the same RCD then additionally protects circuit upto a max Zs of 1667 ohms, so don't worry too much. I still record actual results though.

Are you saying that a RCD protected circuit doesn't need to comply with Max Zs for the associated MCB?

 

[Leesparkykent]

Its not ideal as its a bit of a get out of jail free card on TN systems for bad design Imo. But saying that it does comply...you are using the RCD for earth fault protection just like you would for a TT system with a high Ra. 50V is considered the safe touch voltage and you have a 30mA RCD so 50/0.03 equals 1667Ohms.

 

[malcolmsanford]

The MCB side of an RCD circuit is purely for overload protection. The fault protection is given by the RCD, and the maximum Zs for a 3omA RCD is 1667.

 

[Emm Eff]

The MCB side of an RCD circuit is purely for overload protection. The fault protection is given by the RCD, and the maximum Zs for a 3omA RCD is 1667.

Interesting, thanks.

 

[malcolmsanford]

Have a look at table 41.5 in the Reg, well I think it is 41.5 lol..........I haven't got my regs with me at the moment

 

[Emm Eff]

Have a look at table 41.5 in the Reg, well I think it is 41.5 lol..........I haven't got my regs with me at the moment

Ive seen the figure quoted for earth electrode testing, and the formula makes sense.