Pub discussion over consumer units

[Stevo]

In the pub last night with a few friends, one of whom has just had a new alarm fitted to his house. He said the fitter switched off his power by flicking the RCD and MCB switches on the consumer unit. He then wired up the alarm, power must have been off about 5 minutes max. Now when he went to switch the RCD back on, he had to flick it a few times before it stayed on and said that sometimes they need to cool down as they can be temperamental. Is this right? Reason I'm asking is I used to have one of those plug in breakers you use with a lawn mower, on it it stated that you should test it each time you use it by pressing the button on it, each time I pressed it, it would take a few flicks before it would stay on, I thought it was dodgy so threw it out lol. So can RCD's be temperamental?

Steve

[John McLean]

The RCD may be doing the task it was designed for; or you have a faulty unit, they do go faulty!. If you test it by pressing the button and it repeatedly trips until final latching, I would suggest that you either have an electrician do a complete range of tests on the RCD with the correct equipment; or take a chance and replace the unit if it is troublesome, (having adopted the correct isolation procedures). Functional tests should be carried out on RCD's every 3 months i..e. by pressing the button to ensure that it trips. There should be a notice on the DB or CU spelling this out. This test introduces a deliberate fault condition on the unit. Jaymack

[meow2222]

no

more likely your mower is dodgy.

oh yes

NT

[john2]

In this case I think the mower cable might be the problem. I store a rubber extension cable in a dampish container over winter and it usually throws the plug-in 13A RCD I use with it for the first couple of days. The RCD works fine, it's the cable that's a bit leaky.

john2

[John McLean]

Where do you store the mower, it's more likely to be a source of condensation on the motor electrics. Assuming of course, that this is the cause of the RCD tripping.

Jaymack

[Stevo]

Sorry, I think I may have confused people with my description. The alarm wasnt fitted in my house, it was in a friends, when the fitter flicked the RCD switch back on, it tripped immediately back off a few times then stayed on. He doesnt have random trips on his consumer unit, only time anything ever trips is when a light bulb blows. Hence the can RCD's be temperamental when switching them back on? As for the plug in circuit breaker I had, that was just an afterthought, it was bought for a lawnmower but the lawnmower was never plugged in when I pressed the test button, this would need a few flicks of the switch before it would settle. From this I'd gather that RCD's can in fact be temperamental. Steve

[john2]

well the mower was actually a strimmer, and I use the same extension lead for an SDS drill as well. The RCD flipped out even without anything plugged in at the far end as I remember. The ext. lead came from Lidl some years ago, which might be relevant.

john2

[Dave Liquorice]

Cooling down, spherical objects.

"Temperamental", well was the electrician trying to switch on couple of rings with kit connected and switched on. Particularly things with SMPSU's, (computers, TVs, monitors, digi boxes, DVD/VCRs etc etc), these all have lots of suppression components connected across the mains and until the C's have charged up will "leak" like buggerey making an RCD thing there is too much earth leakage and trip. I can well see it taking a few attempts to get an RCD to stay in in those circumstances but once in (and the C's charged) all will be well.

Probably wise, at least for a normal 30mA RCD. I have a plug in 10mA jobbie and that is a right pain if you have a decent length extension attached, the cable capacitance is enough to trip it.

[Andy Wade]

Err, the supply is AC and the filter capacitors charge, discharge, charge with the opposite polarity and discharge again 50 times a second. There is no initial leakage or settling period extending beyond the first mains cycle. Moreover in most cases there will be bleeder resistors to discharge the capacitors when the power is removed, to stop you getting a belt off the pins of the mains plug just after unplugging

- thus each start-up after closing the RCD contacts will be similar.

There will be an initial transient current if the RCD contacts are closed other than at a voltage zero crossing, but that will be on a time scale of microseconds and shouldn't bother an RCD which is looking for current imbalance lasting for several milliseconds.

[Guy King]

The message from Andy Wade contains these words:

Not in a switchmode PSU they don't. The supply is rectified and fed to the input storage capacitors, from where it's tapped off as and when the voltage in the output storage capacitors falls below the set level.

[Derek ^]

I think you and Andy are talking at cross purposes.

IE filter capacity as in filter capacitor/smoothing capacitor

and Filter capacitor as in mains input filter.

BICBW.

DG

[Andy Wade]

No, you're quite right; Guy introduced a red herring. I, and I presume Dave L, was talking about 'earth leakage' currents through the Class Y capacitors in RFI filters. Inrush currents, whether related to charging reservoir capacitors, or to motors or transformers, are a different matter entirely - these (should) flow between phase and neutral, so should not be seen by the RCD unless they're going astray due to a fault, in which case the RCD will do its job and trip.

[Guy King]

The message from Andy Wade contains these words:

Yes, sorry, I didn't read Dave Liquorice's bit properly.