RCDs - Should they trip without MCB

[iantj]

We have been in house for around 6 years and have had intermittent problems with the RCD. Initially, put down to new house being wet and also sometimes caused by garden electrics (also protected by own RCD)... all of which we could understand (if not happy).

Recently seems to be tripping due to overload - but without tripping any MCB.
( Scenario - using Lawnmower. Start mower, power goes. Turn off TV/Video - on standby and water heater. Use Lawnmower with no problem )

I am going to try replacing the RCD anyway, but two questions that (perhaps dumbly) I haven't yet found clear answers to anywhere.

#1 Is it common for RCD to trip "randomly" without MCB tripping ?
#2 Should it be possible to reset RCD with MCBs on ?

Thanks for any advice

 

[DESL]

#1 Is it common for RCD to trip "randomly" without MCB tripping ?

Yes

#2 Should it be possible to reset RCD with MCBs on ?

Yes

It all depends on the nature of the fault. RCD is detecting imbalance between phase and neutral.

Your mower could have a fault with a leakage current of ~20mA and water heater plus TV could have some laekage of ~10mA
When all plugged in and powered up 30mA and RCD trips!

 

[iantj]

OK, I can understand that some permanent leakage could cause the problem. Is there a method to check for this ?


Does the fact that I always have to turn off all of the MCBs before resetting the RCD indicate another fault ?

 

[nozspark]

You say that you use a water heater, are you in a hard water area? as the element could be slowly going faulty! It could even be the element in your washing machine.

Best to get someoe in to test it for you!

 

[securespark]

RCD's don't have overload/short circuit protection. Unless it's an RCBO.

 

[JohnD]

...which is by far the best thing to get

 

[iantj]

To give an update.
The original legrand RCD has been replaced by a Volex.
It is now possible to reset the RCD with the MCBs on and so far... RCD is not tripping.

An interesting side effect is that when the outside wiring was checked, the garden RCD tripped before the main house one. Previously only the house tripped.

I can't test the removed RCD, but looks like it was faulty / over-sensitive

 

[jondiy]

Did you replace like for like , ie 30mA trip current for 30mA???

How do you know whether your new RCD is actually working?? OK it has a test button, but it still could be faulty , ie too slow at breaking the circuit! It needs to trip within 200ms at 30mA (or 300ms if a BS EN type) and 40ms at 150mA. Only test equipment can tell you this! I am assuming it IS a 30mA device!!

I would have considered a Legrand RCD to be a lot more reliable than a VOLEX one!

 

[iantj]

Replaced like for like (other than manufacturer)

"Working" in the sense that test button works and also tripped (genuinely) with some rather damp wiring.

Absolutely no idea if it trips within the specified time and I suspect not something most people would ever test when buying new.


For me the biggest benefit of the replacement RCD (since only time will tell if "rogue" tripping has gone for good) is simply that you can reset it by moving from OFF to ON. As opposed to having to switch every MCB OFF, RCD ON, MCBs on.
As my trips were transient I am perhaps blinded to the inherent safety of that approach... An earlier post also suggests that RCDs are not meant to behave that way, also making me think "faulty"

 

[Steve]

Absolutely no idea if it trips within the specified time and I suspect not something most people would ever test when buying new.

MOST PEOPLE in this instance is electricians. they DO test RCDs when new. Its surprising how many dud RCDs make it to installations. Get it tested.

The fact that you had to turn every MCB off to get the old one to stay up would not tell me that the RCD is faulty. It would tell me to investigate the circuits I had to turn off. And the fact that new new RCD will allow a reset without turning off every circuit tells me that it could possibly not be picking up the fault. Good, you may think. No. Someones life may one day depend on that RCD tripping. This is why they are tested when installed.

 

[plugwash]

reputable electricans working to current regs test them. What proportion of RCDs installed that represents is anyones guess.