Mitre saw tripping main breaker - Help!

[JoeCarp]

You're right, I have no prior knowledge. I'm just trying to get my head around what and why, if that's possible.

 

[Pete999]

Joe can't see whether you have answered my question is it the RCD or CB that is tripping? to put into laymans term do you loose ALL power when it trips?

 

[westward10]

Think it is the main rcd switch Pete because he said the far left switch on his picture. Is odd although not impossible a double insulated appliance is causing this.

 

[Pete999]

Joe can't see whether you have answered my question is it the RCD or CB that is tripping? to put into laymans term do you loose ALL power when it trips?

Think it is the main rcd switch Pete because he said the far left switch on his picture. Is odd although not impossible a double insulated appliance is causing this.

Thanks Westy missed that, go on then enlighten me. Not doubting you just wondering how.

 

[JoeCarp]

Yes, all power and lights go. None of the individual switches flip, just the main rcd switch.

 

[Pete999]

Yes, all power and lights go. None of the individual switches flip, just the main rcd switch.

OK cheers

 

[westward10]

No other similarly rated appliance does this like the kettle for example.

 

[alasdairp]

No other similarly rated appliance does this like the kettle for example.

That's the right question to ask - if it is an illegal Neutral - Earth short in the house it should cause the trip when any significant current appliance is used - certainly on the same ciruit - though the saw start-up current surge will be high and could trip the Doepke 30 mA type B trip whereas a simple load (like a kettle) may not. The saw is double insulated - so if it is the RCCB that is tripping - which it is - it almost has to be an improper N-E short in the wiring - OR in an appliance that has been added in the last few weeks. It can't be tripping on surge current (a 63 A MCB) so it must be the imbalance that is causing the tripping. I suggest unplugging all appliances (washing m/c, fridge, cooker, etc) and seeing if the saw still trips the power. If it does - then get a qualified electrician in to sort the problem. Electrical safety issues like this need to be sorted by a qualified and experienced professional.

 

[telectrix]

i would also suspect that the RCD itself may be faulty. it's been sitting there for at least 25 years, and just might be tired of not being used, like an ageing hooker.

edit.a 2nd look at OP's original pic shows garage sockets MCB apparently off. is it this that's tripping or the RCD?

 

[Richard Burns]

The design of the saw itself means that it should not be possible for the saw to trip the RCD unless the RCD were responding to an irregular waveform from the saw motor, which is not common.
The fact that you have tried the saw in different circuits means that the RCD is not responding to a high current flow in the adjacent MCB.
Because the RCD did not trip on the first use of the saw this would tend toward Telectrix's thought of the RCD now having failed or that a fault has developed on another circuit is allowing the saw to use a different path for the current and causing the RCD trip.
It is slightly strange that the trip does not occur instantly as the instant of starting the saw will generate a very short duration but very high level current which should rapidly drop and stabilise as the saw starts to turn. However it may be that the initial start current gets the saw moving and so it is not instantly obvious that the saw has no power.

 

[JoeCarp]

I suggest unplugging all appliances (washing m/c, fridge, cooker, etc) and seeing if the saw still trips the power.

Did this previously (forgot to mention!), still tripped.

 

[The Claw]

I'm an electrician from Australia. I had a very similar problem in the house I'm renting. It turned out to be a neutral to earth fault on a fluoro light fitting in the garage.
The whole house RCD would trip when I used my welder on the outdoor GPO, when I used the vacuum cleaner on the GPO on the garage side of the house (other GPOs were OK) and when I tried to use the garage light or the garage GPO for anything.

Welders, vacuum motors and saw motors can draw a lot of current when starting. This will often cause old-fashioned light bulbs to dim slightly when the other device is started. When a lot of current is drawn, the GPO supplying the device sees a voltage drop. Ideally the GPO active would be 240v and the neutral would be 0v (relative to earth), but a normal house might have say 239v on the active and 1v on the neutral. During starting of your saw the active might momentarily drop to 230v and the neutral rises to 10v. This is not a problem unless there is a fault somewhere in your house where a neutral is connected to earth. Since all the neutrals are connected to one another and normally not disconnected by turning off breakers, the 10v at your GPO neutral will travel through a multitude of paths through all the neutrals. Some voltage will make its way to the neutral/earth fault and thereby flow back through the earth instead of neutral. If this caused total leakage to exceed 30mA, your RCD will trip.

The easiest thing is to call an electrician to find and fix the problem. However if you wish to investigate yourself or can't afford an electrician there are some things you can do:
* disconnect all appliances and retest. It is kinder on your gear not to be switched off and on repeatedly during your testing.
* try your saw again and if the fault persists it is likely in a hardwired light fitting or fan or the wiring itself.
* you could try other devices like a vacuum. If the RCD trips when using certain GPOs this can help you locate the proximity of the fault.
* suspect the ballast of fluoro lights. If you have sufficient skill you could try disconnecting the neutral going in to any suspect fluoro lights and then retest. If you are the sort of person who would buy a replacement fluoro batten and fit yourself then you could tackle this test. Otherwise leave it to an electrician.
* it could be neutral touching earth at the back of a badly wired GPO. Is there a loose GPO that you have recently disturbed?
* it could be neutral touching earth at the back of a badly wired light bulb batten. Replaced any bulbs recently?
* if could be a screw going into the wall and cutting through both neutral and earth in a multicore wire. Put any screws in recently?

Good luck and please tell us what happens.

 

[alasdairp]

Did this previously (forgot to mention!), still tripped.

Another relatively common cause of Neutral - Earth short are immersion heater failures. It may not be a full short but just a low impedance so it needs the high surge current to generate the current required to trip the RCCB. Whatever the problem - a failing RCCB tripping too easily, or a N-E s/c problem - you really need to get a good local electrician in to do the tests and measurements and, as you recently bought the house, ideally a full condition report. Don't stint on electrical safety.

 

[JoeCarp]

OK, I'm pretty sure I've narrowed it down to the wiring itself on my ground floor sockets circuit.

Plugged the saw in in the garage, switched all other circuits off apart from the garage and it worked fine. One by one switched remaining circuits on and it trips on the ground floor sockets.

Unplugged absolutely everything from downstairs sockets and tried again. Tripped.


The only uncertainties I have are the boiler and an outdoor security light (seemingly non-working). They are hard wired, inside a downstairs cupboard so I couldn't physically unplug them, but just switch them off at the wall). I don't know if this is relevant or not?


Having hopefully eliminated all of the appliances, I'm assuming there is no further testing I can do myself?

Thanks again for all the help so far!

 

[alasdairp]

OK, I'm pretty sure I've narrowed it down to the wiring itself on my ground floor sockets circuit.

Plugged the saw in in the garage, switched all other circuits off apart from the garage and it worked fine. One by one switched remaining circuits on and it trips on the ground floor sockets.

Unplugged absolutely everything from downstairs sockets and tried again. Tripped.


The only uncertainties I have are the boiler and an outdoor security light (seemingly non-working). They are hard wired, inside a downstairs cupboard so I couldn't physically unplug them, but just switch them off at the wall). I don't know if this is relevant or not?


Having hopefully eliminated all of the appliances, I'm assuming there is no further testing I can do myself?

Thanks again for all the help so far!

You almost certainly have a Neutral-Earth short on the ground floor ring circuit. If you don't know how to find that, get a qualified electrician in.