RCD tripping

[AlarKris]

I have a strange RCD tripping problem, we have a dual RCD consumer unit, when the main switch is turned off both RCD s activate, they have activated individually but not at the same time. There is no continuity between the neutrals and both sides have an insulation resistance of >25 meg, we have replaced both Rcds which are now Schneider , yet the problem still persists, everything tests fine. So I'm either missing something obvious or not, any help would be gratefully received, thanks

 

[GMES]

Hi and welcome to the forum.

 

[Octopus]

How long ago was the CU installed?

 

[AlarKris]

6 months ish, it was a new build, we wired it.

 

[Octopus]

Any recent DIY? Somebody been fiddling with the wiring.

It would be highly unlikely for what you outline without something significant being done!

Best you compare the test results from 6 months ago with now

 

[AlarKris]

Any recent DIY? Somebody been fiddling with the wiring.

It would be highly unlikely for what you outline without something significant being done!

Best you compare the test results from 6 months ago with now

we have they've not altered , barely at all, and no building work has taken place, the wiring is as we left it, you can see why we are stumped, I will ask about picture locations etc....

 

[tony mc]

You say with the Main Switch off both RCD's activate does the main switch not isolate the CU ?

 

[AlarKris]

As we turn off the main switch both RCDs trip simultaneously, it's a bugger, nothing seems to have changed and the test results are good.

 

[tony mc]

More information is needed with regards the fault /faults please !

 

[Octopus]

As we turn off the main switch both RCDs trip simultaneously, it's a bxgger, nothing seems to have changed and the test results are good.

Then don't turn off the main switch!

What tests have you conducted? Please be specific!

 

[tony mc]

How tight are the connections in the main switch and associated connections ?

 

[AlarKris]

Then don't turn off the main switch!

What tests have you conducted? Please be specific!

Insulation resistance, phase -earth, phase to phase,
rcd test including ramp, ze and zs, all connections are tight and clean

 

[Octopus]

Insulation resistance, phase -earth, phase to phase,
rcd test including ramp, ze and zs, all connections are tight and clean

And the values are?

 

[Deleted member 26818]

The RCDs operating when the Main switch is switched off, could be due to capacitance.
Not sure if there is a solution, not sure if a solution is necessary?

 

[Richard Burns]

As above if the RCDs trip on main switch turning off then they just need to be turned on again before you switch on again.

However saying that when you switch off the main switch the RCDs trip so you have changed the RCDs is a bit like saying I turned the kettle on and the circuit breaker tripped, I have changed the circuit breaker and the kettle turning on still trips the circuit breaker.

Have you tried a new main switch?

 

[Toonlad]

Are you throwing the switch under normal load. Or have you tried it will all the ccts off and only the 2 rcds. As said earlier has the DP isolator been swapped out.

Me personally wouldnt get to hooked up on it. Just switch them back on again , as Richard said.

 

[SJD]

As you turn the main switch off, you break both L and N, but not both at exactly the same time. For a very short period of time (I'm talking a small fraction of a second here), perhaps you have the L still connected, but the N already broken.

Now imagine you have a circuit, or some connected equipment, with a low-ish insulation resistance from N to E. In normal operation, it doesn't cause a problem. But if only L is connected, and N is broken, perhaps enough current can flow (say 30mA) via the E instead, and cause tripping.

Have you checked IR, of N to E - both the installation and any loads that are connected when this problem shows up?

I had something like this recently where I could guarantee to trip the RCD with the main switch, turned out to be some lift shaft lights having low IR from N to E.