I know MCBs have two separate trips, a fast one for big faults and a slower thermal trip for moderate over-current.
How quickly should the thermal trip reset itself? Not my problem, but at the livery stables where my horse lives.
TIA
You seem to have an impressive horse. Have you thought of putting it on the stage?
It doesn't reset itself (unlike, e.g. the thermal trips in some model railway power units). Both detection mechanisms trip the same switch.
You manually reset both mechanisms by operating the external switch toggle (hopefully after looking for the cause of the trip).
This video of the insides of a mcb shows the solenoid for short- circuit protection and the bimetallic strip for moderate overloads:
Sometimes (may depend on make?) you have to operate the lever to "off" manually, even though it's pointing at "off" from being tripped before you can operate it back to "on".
the thermal element bends back to enable the mcb to be reset quite quickly. I've had times when it may take over 10 seconds before it'll reset, can't be more precise though, and can't guarantee it's not something else going on.
If you're getting repeating trips with failure to reset, that's 2 problems that need sorting.
NT
On Tue, 28 Nov 2017 14:49:00 +0000, newshound coalesced the vapors of human experience into a viable and meaningful comprehension...
You are correct
Never. A normal MCB is not self-resetting irrispective the magnetic or thermal operation. User intervention is involved, so some sort of alarm would be a good idea.
Slightly confused. Must be missing something.
To go back to the problem, this turns out to be an RCBO of an obsolete design (Square D type QOE).
Typically, it's an intermittent problem. When I arrived it was tripped and would not reset even with all the load turned off. Whipped it out, took it down to Screwfix only to find they don't stock old design.
Took it back, meggered the circuits (all fine), wired it back in and powered up, not tripping with ~ 30 amp load (but didn't try overloading it).
So perhaps a tired device with a worn latch, but behaving after a shake-up? Do they suffer random age failures?
Sorry all, really bad wording in my original post (rushing a bit to get there before dark). I know they need to be manually reset, I was wondering how quickly they would cool down after a trip to permit a reset. Having now looked at the diagram and seeing it is a bimetallic strip carrying the current, the answer is (obviously) fairly fast. I suppose I had in mind that perhaps they had something like a wax thermostat to provide a reasonable delay, so that it might take a few minutes for the accumulated heat to escape to the outside world.
OK, so you've got three trip mechanisms
Was anything unplugged or turned off (maybe via timer/thermostat) between the time it wouldn't and later would reset?
No I did not get the joke either, so don't feel cheated. Brian
Yes, hadn't noticed that when I first posted.
There is a washing machine, so the heater could be a good candidate. I started it running when I finished this evening, it hadn't tripped by the time I had loaded the tools but we will see when I get in tomorrow at 7 am!
I've just remembered that there used to be a small heater in the loo at the far end of the site, which was disconnected last year because it kept tripping one circuit. Will check tomorrow if it is on the same circuit, although from the geometry it probably isn't.
Being a stables, there are plenty of rats and mice, and being old farm buildings means none of the wiring is mechanically protected, and mostly just clipped to wooden beams. So there are *lots* off opportunities for hard-to-locate damage to insulation.
Of course at the moment I don't know if it is an earth leakage or an overcurrent fault. Could be both!
This 32A circuit might not be code compliant, there is a ring main with sockets plus what seems to be an unfused spur running a small lighting load. This starts off in 2.5 mm^2 and ends up away from the main building in 1 mm^2. I suppose there might be a fuse at this point (but I havn't even found the junction yet). I take some comfort in the thought that an unprotected fault on the thin stuff would eventually take out the RCD, but obviously there is a fire risk.
As long as the circuit is not excessively long, even the 1mm^2 section will *probably* be fault protected by the RCBO's fast over current mechanism.
(earth leakage faults will be protected by its RCD functionality anyway)
I think he was suggesting that after a thermal trip, you may have difficulty getting it to manually reset immediately, since you will need to wait a short time for the bimetal strip to cool enough to permit it to reset.
*Some* (though not all) of the wiring looks as though it might have been fitted by someone not over-familiar with the regs and calculations! Apparently a proper "sparks" is coming out to look at it.
Cue a list of 'urgent' problems that aren't
As a followup, the sparks called me *after* doing his investigation, which turned out to be exactly the same as mine: disconnect, megger the wires, find nothing, then say "They might have had too much load on it". Alternatively, a load device might be dying with a fault to earth.
At the moment it is all working fine.
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