RCD tripping with mains saw

[Jack Lawson]

Earlier this week I bought from Homebase a mitre saw:

I had read reviews and seemed to fit my needs perfectly. Got it home - and tested it. I found that when I started the saw, it tripped my RCD on the supply. Tested it further - sometimes it tripped - sometimes not. This was on initial start-up with no load. Took it back - got a replacement. The new one is just the same. About 1 in 10 of start-ups the RCD will trip. The saw seems perfect for my needs - very accurate mitres and 90 degree cuts are spot on - I think I will live with what for me is a minor problem - but thought I'd just share.

[The Natural Philosopher]

check for earth neutral sorts.

In whatever its plugged into

Staring current on electric motors can be very high, and if some of that isn't going back down the neutral...

[Bob Minchin]

Test it with the earth disconnected and the saw on an insulating mat.

If is still trips then your RCD is a bit suspect.

DO NOT Run it like this. It is only a test method.

[Tim+]

Is it definitely a RCD tripping and not just a MCB?

Tim

[Tim Lamb]

I have a 9" angle grinder which consistently trips a 20 amp MCB......

[simon mitchelmore]

You could try it on a different circuit to see if it does the same, if it doesn't then there is a mains circuit fault. I had an RCD trip only on heavy loads due to a neutral to earth fault elsewhere on the ring main. Hindsight and correct diagnosis would have saved me an afternoon f&%^ing about!

[ARW]

You mean it trips a B type 20A MCB?

[Tim Lamb]

Yes. Fine elsewhere on re-wireables. Makita.

My workshop socket outlets are end fed so limited to 20 amps.

>

[John Rumm]

My previous workshop initially had a 20A MCB protected socket circuit, and things like my 3kVA site transformer would trip that on switch on about one time in five. Its fine on a B32 or C20 though.

[John Rumm]

Is it even earthed, or is it double insulated?

This sounds more like a "sensitised RCD" nuisance trip as a result of transients caused by the large inrush current. It probably only happens when you manage to time the power on to coincide with the peak of the mains waveform.

See

[Brian Gaff]

Some rcds seem to be a bit sensitive. I have a plug in one on the lawn mower and its fine but the strimmer trips it for no obvious reason. Not being sure quite how they detect things when you can get double insulated, no earth and earthed things to plug in and its still supposed to work. Brian

[Jack Lawson]

Excellent - it is in fact a MK (ELECTRIC) 5916S Thermal Magnetic Circuit Breaker, SENTRY Series, 240 VAC, 16 A, 1 Pole, DIN Rail

I now realise that there is just one RCD in the box - and it was in fact not that which was tripping - I should have looked properly. It was the fact that it tripped sometimes - not others - which confused me. The start-up current of the motor must be round about the tripping current of the device - and nothing to do with RCD nuisance tripping!!! (Since I made my post I had been reading up about RCDs and have seen that they *can* trip when there is no earth-leakage

- known as nuisance tripping.)

Many thanks - problem solved.

[Andy Burns]

Well, mystery solved but problem remains. What else is on the MCB that trips? Is the circuit wired with 2.5mm^2 cable? It may be possible to replace it with a 20A one, or a "type C" 16A one.

[Jack Lawson]

Yes - thanks - that is just what I had come to think it was as I read up after making my post - and found about nuisance tripping.

However - as Tim+ questioned - was it the RCD? - and of course it wasn't.

I assume that I can put a larger MCB in - the cabling is a single spur from the distribution box to the double socket in the garage - not on a ring.

What MCB would you suggest please?

[Andy Burns]

If it's 2.5mm^2 cable you should be fine with a 20A type B (you already have type B) but it's possible the saw might still trip it.

What's the total length of cable from CU to garage? Someone might do the calcs to make sure a type C would be safe ...

[Jack Lawson]

It is a radial with a double 13A socket on it with 2.5mm^2

I will try a type B 20A unless anyone suggests otherwise.

[Jack Lawson]

6ft radial

[The Natural Philosopher]

Others are more expert, but I'd stay with the 16A, but try and get a type that is 'slow blow' that can take a second of overload.

[Andy Burns]

For a 20A type C MCB, the maximum Zs is 1.15 ohms to ensure 0.4s trip time, I think it's fair to say 6' of 2.5mm^s cable meets that comfortably, 60 metres would be getting marginal!

[Andy Burns]

a 20A radial is a "standard circuit"

a type C MCB is just that, and with only 6' of cable there's no problem using either a 16A or 20A.