Why wouldn't the RCD reset?

[Roo]

Had a job on Thursday to change the meter (bog standard 5min job).
I tried to isolate the load by hitting the test button on the RCD (this board had no main switch) but as the test button didn't work had to flick the switch on the RCD manually.

Completed job checked polarity did all my test's etc and tried to turn the load back on, but the RCD wouldn't reset (switch wouldn't stay up).

Turned off all MCB's and went round unpluging all equipment I could see but still couldn't get it back on.

Pulled the cutout fuse again cutting all power and tried to reset it but still nothing.

In the end I ended up speaking to my new big honcho at work who told me to fit an isolator, leave it off and tell the customer to get a spark in and send us the bill.

I have no doubt the RCD (MK 80A, about 10 years old) was at fault, and after going back to the property to check the customer was back on supply was told that the spark had said it was the RCD.

But I just wondered if you knowledgable people could tell me what might have possibly happened to the RCD?

 

[flameport]

The RCD mechanism had jammed, so the test button didn't work. There is also a fault on the installation, meaning the RCD should have tripped long ago but didn't due to the jammed machanism.

Operating the switch manually released the jammed mechanism, and now it can't be reset due to the fault which existed all along.

 

[JohnD]

not quite right

"Pulled the cutout fuse again cutting all power and tried to reset it but still nothing."

when there is no supply across the RCD, it can't trip as there can be no fault current (even if there are circuit faults).

But I agree it is broke.
usually when an old RCD fails, it will fail to trip, often because it has seized due to not being regularly tested. sady this is a "fail-dangerous" and not a "fail-safe"

The old MK does not have a "half-way" position (some, e.g. the MEM RCD) will put the switch into an intermediate position on trip. You have to push it all the way to "Off" before you can return it to "On" position

I don't know if that old MK RCD would have had a fault-detect mechanism that prevented it from resetting if it had an internal jam. This might have been accidental, the way damaged parts wouldn't engage, or it might even have been designed in. MK used to have some really good product designs and were formerly top quality.

 

[ebee]

"MK used to have some really good product designs and were formerly top quality."

Well maybe 50 years ago ut they lived on their name since.

The original logic (not logic plus) were crap, switched sockets were only single pole switching and of all distributors selling MK and other leading brands it was always MK that had high returns for faulty items.
Now the logic plus might be a better animal I don`t know.