RCCB failure mode??

Hi All,

Split load board..

Well here i was at about midnight, when suddenly half the house electrics trip off..

All MCB's on, [as they would be] but both RCCBS also on too... Hmmm

Try the test buttons.. one works, the other does not.. Hmm some more..

If one RCCB has a feed, then for obvious reasons they both do.. [as the inputs are linked]

Ok, so i soon discover that the one RCCB has a feed, but no output..

Mess about with the button a bit and the output comes back on, sometimes! [and goes off too without a sound or the thing "tripping"]

Ok, so i found a possible earth fault [immersion heater] so this explains the original trip and fault cleared.

More messing reveals that the RCCB trips if it sees an earth fault ok, BUT, resetting the thing does not always work [as in an output from the thing] AND sometimes, despite the thing obviously has a live feed, the test button does not work..

Is this a usual type of thing to see??

john..

have you turned it all the way off first? some devices trip to a half way position, not all the way off

the position of the lever is also irrelevant if it trips, i.e if you hold it on it can still trip. as such, if its a tight fit next to others then sometimes the spring force pushing the lever down isnt strong enough. usually obvious when you push it down as itll move easily

It's been noted elsewhere on this site that a neutral to earth circuit fault can prevent an RCD test button from working by "shunting away" the test current.

Was it raining? Outside lights? I’d start by isolating any outside lights then see what happens? 

Hi All,

Well, 48 odd hours later the thing has not tripped again!!

Seems the earth fault i found was the cause of it, but as to the funny behavior of the lever, no idea, must be sticky like Andy said.

Geoff1946 said:

It's been noted elsewhere on this site that a neutral to earth circuit fault can prevent an RCD test button from working by "shunting away" the test current.

Click to expand...

Hmm, i would have to sit down and draw out the current path to get my head round that one.. i do know that on TNCS, a neutral/earth fault on any particular circuit, can trip an RCD on a completely separate circuit. [the neutrals all being connected together at the neutral bar..]

Thanks all..

john..

I would test the RCD to make sure it is working properly, and if the operation is not "normal" just replace it.

apprentice87 said:

Hi All,

Well, 48 odd hours later the thing has not tripped again!!

Seems the earth fault i found was the cause of it, but as to the funny behavior of the lever, no idea, must be sticky like Andy said.

Hmm, i would have to sit down and draw out the current path to get my head round that one.. i do know that on TNCS, a neutral/earth fault on any particular circuit, can trip an RCD on a completely separate circuit. [the neutrals all being connected together at the neutral bar..] 

Thanks all..

john..

Click to expand...

I tried to paste in a circuit here, but can't for some reason, so perhaps I can describe so that you can visualise this:

The test circuit switches in a resistor between the live input and neutral output of the RCD. The current it sends hence passes through the neutral side of the balance transformer, but has bypassed the live side of it. Hence this imbalance trips the breaker. All is well.

NOW consider if that neutral output of the RCD is earthed by a fault.  The test current will flow to earth rather than back through the transformer.

Ahhhh.. Yes, see what you mean..

Meanwhile i just cam home to find the thing has tripped again and i cannot get it to reset.. Just bridged it out until i can get one tomorrow.. Far from ideal i know, but no worse than before there was an RCD there in the first place!!

Just a thought.. i will go and disconnect the output side and see if it will reset then... i will report back in a bit!!

john..

Hi All,

I just disconnected the outgoing neutral from the RCD and switched off all the MCB's it feeds thus isolating the output side of the thing. It still will not reset or the test button work either.. It did the once, but that was all, so it is bridged again now..

Must be a dead RCD is all i can think.. It is one of them Denmans "steeple/curve" ones..

I will get another tomorrow..

john..

does sound like the RCD is dead

Andy™ said:

does sound like the RCD is dead

Click to expand...

I think it is... Still bypassed, will get a new one though..

In the past i have had to replace loads of immersion elements. When i took them out, they would be all split open along much of their length with the internal heater wire in full contact with the water. I presume the only reason i am here typing this is the low conductivity of clean water and equipotentional bonding.. Now as soon as the element does anything the RCD trips straight off..

john..