Testing Loop Impedance through RCD.

[Rampage]

Hi, installed some circuits in a factory. The circuits were tapped off of a busbar that goes straight to the main switchgear. From this I installed a Chint RCD, which then goes to 2x Chint TP MCB and 1x Chint SP RCD. One of the TP MCB´s goes to red commando sockets and the SP MCB goes to blue commando sockets.

Ze is 0.1 ohms, as expected.
Testing the loop impedance from the furthest socket (actually I test all sockets) the impedance is 0.62 ohms, which is too high for the MCB and way higher than the design would suggest. Both socket circuits measure around the same (they are similar length).
Connecting the L-E on the furthest socket and measuring on the downstream of the RCD gives a Loop Impedance of 0.17 ohms. so Zs = 0.27 ohms, which is fine.

So, the Chint RCD is affecting the Loop Impedance, I´ve not seen this before and I´ve installed loads of them. Checking another identicle install next to this one and the RCD is also adding around 0.4 ohms to the reading.

Loop Impedance Testerv is a Megger 1552 which was calibrated a couple of weeks ago.
No measureable resistance across the RCD using my Fluke 289 (Megger trips it on resistance when no power to RCD).

Any ideas?

 

[ipf]

There's nothing unusual about higher Zs readings whilst testing on 'no trip'....and it seems consistent with that 0.4.
R1+R2 added to Ze will give you a more realistic result.

 

[telectrix]

it's common for a RCD to affect the Zs. in this case, add the R1+R2 to the Ze to give a truer value. otherwise test with the RCD bypassed.

 

[Rampage]

Hi, just searched on Google and it's called Uplift, caused by the internals of the RCD, I´ve never experienced this before (that I know of).

Thanks.