RCD protected circuits (sockets problem)

[Paul2129]

Hi guys, firstly, I'm not an electrician, I did take advice on this problem from a qualified guy, however until the problem presents itself again, he says that he's nothing to test.

so, here's what I know. I arrived home this afternoon, I notice the heating boiler running, the heating wasn't on. All electrics were fine. I went for a shower (not an electric) used an electric fan heater while I showered, turned this off, and then had a kip for a few hours. There was no one else in the house. I woke to find all the sockets tripped.

the thing that I don't understand, all the trip switches (I know that's the wrong term) we're still up, but the "RCD Protected circuits' switch was off. I attempted to turn this back on, which was accompanied by a large pop (rest of switches remained up, and on)

i called the electrician, he told me to turn everything off, and switch on in sequence, everything came back on, and has been running since

questions : 

1, does the fact that the sockets stopped working mean that the issue is definitely the sockets ?

2, why did the RCD switch trip and not the individual one for the sockets ?

3, why was there a loud pop when I put the RCD switch up, should that only be turned back on when all the circuits are off ?

4, how do I go about troubleshooting the problem ?

many thanks for reading, any advice would be appreciated

 

[Lurch]

1. No. It means the fault is on one of the RCD protected circuits.

2. Well, because the MCB's are for overload and the RCD is for leakage/earth fault protection.

3. Not uncommon, sometimes trying to start everything at once can cause excessive earth leakage.

4. Basically as you have been doing, if it goes again and you can't reset it turn the MCB's off and then reset the RCD. Turn the MCB's on one by one and if it trips the problem is probably on that circuit. If it does happen to be the sockets then unplug everything and reset the RCD and MCB and plug things in one by one until you find the faulty appliance.

Having said all that though, it is probably the fridge. Maybe the shower, but usually the fridge.

 

[Paul2129]

Ok, thanks very much for your help with this. I'm a little bit closer to understanding...... having said that, I don't really understand the difference between overload , and leakage/ earth fault.

i know that the lighting MCD tripped when a bulb blew. So when a bulb pops, that causes an overload.

how about a loose wire ? (I recently discovered a faulty socket in my garage) or if something gets wet, does that trip the MCB or RCD

i will actually going and try and learn a little online, just to further my understanding

quite an interesting subject

 

[binky]

when a tungsten filament lamp pops it vapourises the filament, this temporairly causes a short circuit which trips the MCB

 

[ProDave]

so, here's what I know. I arrived home this afternoon, I notice the heating boiler running, the heating wasn't on. All electrics were fine. I went for a shower (not an electric) used an electric fan heater while I showered, turned this off, and then had a kip for a few hours. There was no one else in the house. I woke to find all the sockets tripped.

That bit worries me. Why was the boiler running if the heating was not on?  This is actually quite a common fault if say the switch in a motorised valve has failed and it will cause the boiler to run with nowhere for the water to go and it will get very hot indeed.  That might even be what tripped the rcd?

Get that looked at.

I don't believe there was "nothing to test" I would bet something is showing a lot of leakage still, just not enough to trip the rcd at the moment.

 

[Lurch]

I kind of see what he is saying, I have sort of said the same thing to people/it's not an uncommon thing to say. It would be worth testing to look for anything obvious but there are no guarantees you would find the fault when it is intermittent is what he should have said rather than just leaving it.

 

[ruston]

Do you have any other problems with the boiler , like having to top it up ?

 

[Paul2129]

The boiler has needed topping up 'on occasions', maybe twice in three and a half years, I think one time was due to a radiator leak.

     The other issue the boiler had was when water was blowing in the exhaust (fitted at the wrong angle) and blowing the circuit board. Fingers crossed, it's not happened since the exhaust was repositioned 18 months ago.

     I don't know why it runs, it was certainly not on the timer to be on. Maybe I'll shove a baby monitor in the room where it is, I'll be alerted to it running unexpectedly

 

[bluetobits]

In summer months when the heating is off, boilers do circulate the heating circuit to keep everything moving nicely while not actually firing the burner. Could this be what you experienced? 

 

[ruston]

It could also be the water leak is dripping into the electrics somewhere . Twice in three years is maybe just enough . 

 

[phil d]

I'm not too far from Blackpool, St Helens, I may be up there later on in the week, I could throw a few tests on it if you like.

 

[Doc Hudson]

Ok, thanks very much for your help with this. I'm a little bit closer to understanding...... having said that, I don't really understand the difference between overload , and leakage/ earth fault.

i know that the lighting MCD tripped when a bulb blew. So when a bulb pops, that causes an overload.

Every piece of cable has a maximum electrical current it can safely carry, (the method of installation also influences this figure). Because electricity always generates heat when passing through a circuit if you have too much current the cable could overheat, melt the insulation and possibly cause a fire. (lose joints and inadequate terminations can also introduce overheating problems to wiring). Your fuse box will have either traditional wire fuses, cartridge fuses or miniature circuit breakers, (MCB's), these should be correctly designed so as to disconnect the power if too much current if flowing.  They are typically rated at 6amps, 10amps, 16amps, 20amps, (25amps), 32amps, 40amps, (45amps) or 50amps. The correct size must be used applicable to the size of the cable installed and the load it is supplying. These devices protect against Overload.

All electricity should flow along the circuit cables through the load and nowhere else, if there is a faulty appliance, a damaged cable, or an idiot putting their finger onto a live part, some electricity will leak from the circuit through this fault path to earth. There are devices called residual current devices, (RCDs), that measure the current flowing up and down the live and neutral wires. If they are not equal the RCD will disconnect the supply. As a healthy adult can be killed with less than half an amp through their heart, a 6amp/10amp lighting circuit MCB, or a 20amp/32amp socket circuit MCB, or a 40amp shower circuit MCB, would not offer much protection to a leakage current while a person is having an electric shock. So RCD's are used to protect against earth leakage currents.

RCD's are more expensive than MCB's, in an ideal world every circuit would have its own dedicated MCB and RCD (or a combined device called and RCBO), in reality a lot of fuse boxes have one or more RCD's protecting groups of MCB's.  So an overload fault will only trip its dedicated MCB, but a leakage fault could trip an RCD protecting 4 or 5 circuits. Where the fault is intermittent in nature, the process of tracking down the cause may need several attempts and/or a bit of isolating or disconnected certain parts of a circuit to remove them from the equation. RCD's have specified tolerance in which they should work. with an appropriate test meter the RCD's could easily be checked to make sure it is not over-sensitive. The cables can be tested to verity their insulation is all above the minimum standard.

Some appliances also have an element of natural leakage which can cause trips if powering up several devices at the same time. Typical suspect for intermittent tripping can be items involving water, (heating controls, kettles, washing machines), or external wiring during poor weather conditions, or compressors on fridges and freezers.

Doc H.

 

[binky]

Not sure, but I think some boilers have an anti-legionaires programme, so they will run up to heat up the system to over 60oC and shut down again.

 

[Lurch]

Loads of options/settings on boilers for random turning on, preheat/anti-legionaries/anti-pump jam etc.

 

[Evans Electric]

Hi Paul 2129  couple of things to keep in mind  :-

As said above , the MCBs trip on overload , thats more current than the cables are designed for . Your lighting is probably 6 Amp , if you attached a couple of 3 KW heaters to your lighting , running at say , 24 Amps , instead of the cables melting , the 6Amp trip would in fact do it's job and "trip" .

The RCD  is a different kind of trip , doesn't react to overloads but trips when a certain amount of current flows to earth .   Your electrician gave good advice when he said turn OFF all MCBs ...turn ON the RCD ..(it should stay on) ...turn on the MCBs one at a time , if one of them trips the RCD  you've narrowed it down.

The first test would be with a meter to see if the RCD is not TOO sensitive , but to be honest it would be going off quite often .

Again as said above , if it went off while you were asleep and the boiler seemed to be running , that might be pointing you to the cause .

 

[Revved Up Sparky]

If the RCD will not reset with all the MCBs off then the next step is to check for a neutral - earth fault. The safest way to do this, in my opinion,  is to isolate the whole consumer unit - preferably using the intermediate isolation switch if your Distribution Network Operator has fitted one. Remove the cover, then with all the MCBs still in the off position disconnect the neutral for circuit 1 from the neutral bar. Replace the cover before restoring power to the consumer unit - then try and reset the RCD. If it resets you may have found the circuit the fault is on - the one which your disconnected neutral corresponds to. If the RCD still does not reset then repeat the sequence until you have found which circuit the neutral - earth fault is on. 

 

[Lurch]

If the RCD will not reset with all the MCBs off then the next step is to check for a neutral - earth fault. call an electrician

 

[Revved Up Sparky]

You're right Lurch. I forgot the thread starter had no electrical skills. 

 

[Murdoch]

If the RCD will not reset with all the MCBs off then the next step is to check for a neutral - earth fault. 

But it could well be a L - E fault....

 

[Revved Up Sparky]

But it could well be a L - E fault....

How ?