Ze low Zs high ?

[sweed73]

Hi everyone, fitted a new 2 way CU today 64A RCD main switch. during testing confirmed Ze at 0.06 Ohms on no trip. Zs at shower approx 1.4 Ohms , 10mm cable approx 5 meters length. After working backwards discovered Zs on the load side of the RCD was 1.04 Ohms. Left installation de energized till I can get back tomorrow. Anyone come across this ? I know there is a difference between the trip, no trip results which is why I did the Ze on no trip as well. I suspect the internals are faulty and was going to change it out and compare.

 

[somersetsparks]

should have switched it on and off a few times, may just be some sort of protective coating on the contacts of the rcd

 

[telectrix]

it's quite common for a RCD to affect the Zs. i was on a call-out last year where Ze was 0.2ohms, and on the outgoing side of the RCD was 340ohms!!!!

 

[sweed73]

I'm assuming its a resistance across the contacts, tried to take a reading on a unit at home using continuity on my multifunction tester but just trips the RCD prob as it sees the current across one set of contacts and not the other. Will try operating the RCD unit a good few times tomorrow before changing it to see if it helps.

 

[singingfraggle]

take a zs reading on the incoming side of the rcd and then one on the outgoing side of the same rcd note the difference take that off your end of line zs reading and you will find that its not actually far out from ze +r1r2 if everything ok.
had the same problem with my 1710 mft and depending on make of mcb the values can vary drasticly