Hi Zs solved by changing RCD ???

[Sav]

Hi everybody,

Ok, here is the picture.

Main house is TN-S
Ze is 0.25 ohms

There is a 16mm swa fed from a 60a isolator, which in turn is fed from 25mm tails into henley blocks.
The swa is clipped outside the side of the house, buried in the garden and along approx 25 meters into a small studio / storage area.

There, there is an 8 way c/unit. 100a main switch, 2x RCD's. 1 RCD controls the storage sockets ( 4 ) and the studio florescent tube.

The other RCD controls the sudio room sockets ( 4 ) and the storage florescent.

My mate who just completed the install, calls me and tells me that he is getting 1.17ohms when doing a Zs on the sockets !!

I go and see. The cables and sockets are in plastic trunking, so no problem there. I check again the Zs on the install at the c/unit and get 0.35ohms. Ok fine.

I remembered once, an old school sparks, who on site where we worked, had a similar problem and he changed the RCD's and got a lower Zs reading.

I pulled out 2 legrand 30ma RCD's from the van, connected them up, and low and behold Zs is now 0.43ohms on sockets !!!

I did carry out an insulation resistance test and final ring tests prior to connecting new RCD's, which all were ok, as its a new install.

Any thoughts, can this really lower the Zs ??? Do RCD's play a part when checking Zs ??

Thanks and regards,
Sav

 

[ian]

I have seen this loads of times. I occasionally help my mate out on rewires etc, and he insists in using cheep consumers units where if you add the R1 & R2 to Ze they are often 0.5 - 0.6 ohms lower than the Zs taken with the meter. This inaccuracy can only be down to the RCD. I use different consumers units from him and never see that level of difference.

Ian

 

[brucegenerator]

Had this problem on my 2391 the other day.Think it was down to a wylex consumner unit.3 phase test was fine at origin as are domestic tsts.