MCB trips

[bob]

Hi guys, iv'e got a serious question for you. I have a new consumer unit and the wiring is split into sections: lights up, lights down, sockets up, sockets down etc with a MCB protecting each section(as you do). My problem is that the MCB covering the downstairs sockets trips about once a month. I have noticed a pattern, when I switch on the washing machine, tumble dryer and kettle the MCB trips. They are all plugged into the same double socket outlet. If I am merely overloading this socket outlet then why upon testing it by plugging in everything of heavy load which I could lay my hands on did it not trip? I really put it through it's paces. I have heard about MCB's in B,C and D format. I am using B which seems rather sensitive. Should I try using a less sensitive MCB like D? regards, bob.

[David Hansen]

On 5 Jan 2006 06:19:46 -0800 someone who may be "bob" wrote this:-

Do you ask non-serious questions on the group?

What rating is the MCB?

Assuming that they are all started at the same time and drawing around 13A simultaneously then this might cause a trip, if what I assume is a 30/32A MCB is a bit sensitive. How long does the kettle run for before the trip?

All three appliances are plugged into one double socket outlet at the same time. Fascinating.

You plugged everything you could lay your hands on into this one socket outlet? Fascinating.

Did you? How did you do that?

[bob]

The MCB is rated as 20A. It trips when the kettle is nearly boiled. I double adapted all appliances which had heater elements in them to see if it tripped but it never - basically I tried to overload it but it never tripped. bob

[Dave Plowman (News)]

How have you got all three into a double socket? If using an adaptor, any two together can overload that said adaptor.

[David Hansen]

On 5 Jan 2006 07:07:17 -0800 someone who may be "bob" wrote this:-

Assuming all the appliances are rated at 13A it doesn't surprise me that a 20A MCB operated after a few minutes of 39A. Without looking at the operating characteristics to get precise figures, it is roughly what I would expect. It sounds to me like it is doing precisely what it is supposed to do.

You need to be rather more specific. Were the heating parts of the washing machine and tumble drier actually operating during your tests? Did you blow any fuses in the double adapters? What load did you actually connect? How long was it connected for? How soon was it after the previous load?

20A MCBs are associated with radial socket circuits, which have advantages in some situations but are not designed for feeding multiple large loads. It sounds like whoever did the design did not do a very good job. You may be able to have the radial circuit converted into a ring relatively easily, in which case a 30/32A MCB could be fitted.

[Jim Gregory]

Are your 3 appliances simultaneously sharing 1 double-gang outlet? How? With a plug-in adaptor? If so, naughty! And its local fuse should have blown 1st. Maximum load there is designed @ c25 Amps, but I suspect their total loading far exceeds this, all obviously being high Wattage appliances. Don't forget that a W/M has a 2-3kW element which will heat "hot" water (even if both H & C fill) if not detected hot enough. Can somebone pls remind us briefly how MCBs types A, B, C, D differ? Could not google this info.

[Christian McArdle]

OK. A 20A MCB is woefully inadequete for a socket circuit that includes a kitchen. That is problem 1. Problem 2 is that you must not draw more than

20A from a double socket. Never use an adaptor on a major current using appliance, such as washing machines and kettles. Keep them for audio-visual equipment only. As it happens, the thing is already protected by a 20A MCB, so this has not yet presented itself as a problem.

I would recommend that an entirely new circuit is installed in the kitchen as a 32A ring main. I would probably install two, in fact. I would install one 32A ring (or radial) for fixed appliances without RCD protection and a

32A ring for socket outlets for your kettle, microwave etc.

My kitchen has a 16A circuit for the fridge/freezer, a 32A circuit for dishwasher/washing machine/tumble dryer and a 32A ring for sockets.

20A will simply not cut the mustard.

Christian.

[Christian McArdle]

A doesn't exist. B is general purpose. C allows larger switch on spikes for badly behaved motor systems. (B already allows a 3 times switch on spike and should be used in all domestic socket circuit situations). D is a specialist item and requires special circuit design.

Christian.

[Jim Gregory]

Ta! I had added the "A" as an afterthought, not ever coming across it, since just stringing "B, C, D" looked quaint without it! Just wondering now -why- type A was bypassed! Jim

[bob]

I have run a spur from a double socket* into a 45A switch with neon.

and dryer are plugged. The kettle is plugged into the main double socket* from which the whole spur system runs. The MCB always blows when the washer is on warm up cycle and the tumble dryer has been on maximum heat for about 10 minutes. When we then switch on the kettle it trips just before boiling finishes. bob.

[Jim Gregory]

Now it's clearer, but naughtier! Not surprised about your MCB - it IS behaving OK. A spur off a ring main is usually added to an existing ring for convenience and its load really cannot exceed 13A. The 45A switch (shower/oven?) is made to be used with thicker-gauge T&E cable all the way from CU. You need to expand your existing ring main, inserting a dual-outlet box somewhere so it has two cables, but, given the demand of the appliances, you do need 2 ring ccts or a ring and a direct radial, each with its own breaker. You need to swot up on and appreciate max allowed currents in the ring cables/outlets to be able to sustain intended Wattages - not just any old how. Jim

[bob]

Dear Jim, Thank you for your kind reply. I now understand the problem. By the way the rest of my electrics are fine. I shall sort out the spur, in the mean time I shall not overload it. Thanks to all for your help. bob.

[David Hansen]

On 5 Jan 2006 08:25:25 -0800 someone who may be "bob" wrote this:-

Is this double socket on a ring or radial circuit? I assume it is a radial circuit, given the 20A MCB protecting the circuit.

What size of cable did you use for this spur?

While one could argue that such a setup does comply with the regulations, that is debatable. For a start, for a very long time it has been advised that separate circuits are provided for kitchens and these are designed to cope with the expected loads.

Is this without the kettle?

If the washing machine and tumble drier are heating up then they could be drawing 13A each, the kettle could also be drawing 13A as well. It does sound like the 20A MCB is doing exactly what it should be doing, which is preventing your cables overheating and possibly bursting into flames so burning the house down.

Do the washing machine and drier need to be run at the same time? At the very least drying damp clothes takes more electricity than leaving them to dry for a while.

[bob]

Hi David, The spur runs from a socket within the ring main. I used standard ring main 15A cable for the spur. The trip only occurrs when the washer & tumble (spurred) are switched on and the kettle is boiling (which is pluged into the socket supplying the spur). bob

[Part P...Im luvin it]

'A' was never used to save the confusion between 'A' for Amps and 'A' for a type rating. Obvious really, when you think about it.

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"...Yellow is the new Black..."

[Part P...Im luvin it]

I used standard ring

What is that, 15A ring main cable? Hope it's not what we all think it is!

[bob]

The same cable which forms the ring main for the sockets. God, why? bob

[Christian McArdle]

Too close to 'A' for amps.

Christian.

[David Hansen]

On 5 Jan 2006 09:28:52 -0800 someone who may be "bob" wrote this:-

Leaving aside one type of circuit, you are likely to have one of the following circuits supplying sockets in the house, assuming that it has been wired up correctly:

1) a ring circuit, with a 30/32A protective device 2) a radial circuit with a 20A protective device

both of which are wired with a *minimum* of 2.5 square mm cable.

If the cable can only carry 15A then the 20A protective device is not protecting it.

[bob]

It is a ring circuit with a 20A MCB with a cable of 2.5mm. I understand a ring circuit to be one where the cable starts at the CU, is then wired from socket to socket and then returns to the CU. Am I correct? bob