Aplliance or MCB?

[pagster]

Last night the socket MCB went off in my house.Its happened before so i knew to switch off all appliances and try to trace the source of the problem.We thought we found it when we plugged in the TV and it tripped again but we left it unplugged switched the MCB (and RCD) back on and plugged in a diffrent TV set (in another room) and that tripped it.We came to the conclusion it was the tv's but we turned it all on again and plugged in the vacuum the next day and that tripped it again.

Could it be the appliances?
Have we found the offendeding aplliance?
Or is it perhaps a problem with the RCD/MCB?

 

[ban-all-sheds]

If it was the socket MCB that tripped, why did you have to turn the RCD back on?

 

[pagster]

Sorry made a bit of a mistake in the 1st post.The RCD tripped and we found it was the socket MCB causing it.We then did all the aplliance removing/testing part but the socket MCB keeps causing it to trip when a TV or Vac is plugged in

 

[HDRW]

Sorry made a bit of a mistake in the 1st post.The RCD tripped and we found it was the socket MCB causing it.We then did all the aplliance removing/testing part but the socket MCB keeps causing it to trip when a TV or Vac is plugged in

OK, let me confirm this:

The thing that's tripping is the RCD - the MCB isn't?
The RCD trips when you connect something to any socket?

If so, then the problem is a wiring fault on the socket ring - possibly a disconnected Neutral, or a short between Neutral and Earth.

The reasoning behind this is that an RCD trips when there's a current imbalance - not all the current flowing down the L is coming back up the N. That's why it trips only when you connect a load - until then there's nothing flowing in the L. So when you connect something the current flows down the L and then goes somewhere other then back up the N - the problem now is to find out why!

The fact that it happens with different appliances pretty-much rules them out as culprits.

Cheers,

Howard

 

[ban-all-sheds]

A N/E fault will trip the RCD with no load connected. In fact, a N/E fault will trip the RCD even if the MCB is turned off.

 

[HDRW]

A N/E fault will trip the RCD with no load connected. In fact, a N/E fault will trip the RCD even if the MCB is turned off.

How? There has to be a current imbalance - if there's no current flowing there's nothing to cause the trip. (Obviously if there's current in other circuits it could, so the MCB for the fault doesn't have to be on, but another one does, and has to be supplying a load).

Cheers,

Howard

 

[mapj1]

Indeed. Check for N-E shorts - a few ohms in shunt with one side of the RCD can cause it to trip only when the total load exceeds a few tens of amps, giving the very confusing symptom of varying culprits. This could be in either the wiring or an unsuspected low wattage applience.
A test box with some resistors to check the RCD headroom can be useful with faults of this type - as 'it seems to work now' may not be the end of the story. I'd suggest at least 10mA of no trip headroom when unloaded is needed for reliable operation in service.

 

[ban-all-sheds]

How? There has to be a current imbalance - if there's no current flowing there's nothing to cause the trip. (Obviously if there's current in other circuits it could, so the MCB for the fault doesn't have to be on, but another one does, and has to be supplying a load).

There's always something, somewhere - an appliance on standby, a light, the fridge - anything at all. All the circuit neutrals are common, so if one has a path to earth they all do.

 

[plugwash]

unless your supply is TN-C-S there can still be a voltage difference to drive current down a N-E fault even if nothing in your property is powerd.

 

[mapj1]

Yes, but it does rather depend on the NE fault resistance - if its a heating element in an applience on a ratty old flex at the end of a 50m long radial feed, then the current sharing between it and with the 1 turn of 6mm sq wire round a toroidal core in the RCD will be such that the imbalance at the RCD does not reach the 15 to 30mA needed to trip it, until the L_N loop current is tens of amps. I agree it depends - a dead short will trip every time.

But, for example, on a PME supply or TNCS near the substation, a fault resistance of 1 ohm, shunting an RCD pick-up coil of 3 milliohms, say, will only divert 30mA when the main current reaches 10A...

 

[ban-all-sheds]

But none of this is helping our poor supplicant....

Pagster - do you just have the one socket circuit?

Do you see this problem no matter what appliances you plug in?

Can you trigger the trip by using anything in any socket?

Does the trip happen if you plug something in but don't turn it on? What if you wiggle the plug in the socket?

Do you see it if you turn other circuits off (e.g. lighting, shower, cooker, immersion)?

Have you tried unplugging items that normally remain untouched, e.g. fridge, washing machine, microwave?

How long have you lived here? Is it a recent problem? Have you done/had done any work? (and I mean any work, even if apparently nothing to do with the electrics)

At the end of the day it may be a duff RCD, or you may need to get a sparky in, but the more you can do to narrow down exactly what makes it trip the quicker he'll find the problem?