Mitre saw tripping main breaker - Help!

[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.

 

[alasdairp]

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.

I hope the boiler and the outside security light are on fused outlets. They are both possible candidates for the short. And, no, switching them off will make no difference as the Neutral will not be switched.

 

[westward10]

I hope the boiler and the outside security light are on fused outlets. They are both possible candidates for the short. And, no, switching them off will make no difference as the Neutral will not be switched.

Most switched spurs are double pole.

 

[alasdairp]

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.

I hope the boiler and the outside security light are on fused outlets. They are both possible candidates for the short. And, no, switching them off will make no difference as the Neutral will not be switched.

Most switched spurs are double pole.

True, most are, if they are wired correctly - assuming they are both proper fused spur outlets - the boiler probably is - and not just a light switch for the security light put in by some previous DIY person. I have seen that before for outside lights. Anyway, I appreciate the correction and reminder. It is most likely to be a N-E short elsewhere on the circuit then. It doesn't change the fact that he needs to get expert practical help on site. It is passed the guessing stage.

 

[AllanW]

I hope the boiler and the outside security light are on fused outlets. They are both possible candidates for the short. And, no, switching them off will make no difference as the Neutral will not be switched.

Hi all

I've just signed up to the forum as I'm at a loss with what the problem is - I've asked the (sort of) electrical guy at work and he is also out of ideas.

I myself have next to no electrical knowledge, so please bare with me..

My SIP 12" Mitre saw is being problematic. 2000 w, 13amp fuse in plug.
Manual here: http://www.sipindustrial.com/manuals/01504.pdf

I've recently moved into a new home where, the first time I used the saw there were no problems at all. A week later I take the saw out and pull the trigger. The blade spins for an instant, stops immediately and the main breaker is tripped.

I then repeatedly try flipping it back on and testing it out in different sockets (upstairs, downstairs, oven socket etc.), different/no extension leads, checking the fuse, all ending up with the same result - initial spin up then trip.

I take the saw to work (carpentry workshop) - works absolutely fine. Wiggle the cable about while running it to see if the problem is there, continues to run perfectly.

I take the saw to another house just round the corner, with presumable similar wiring - absolutely fine there as well.

So I suppose:
What's wrong/different with my house that's causing the saw to trip the main breaker?
Is the saw itself problematic, but other properties seem to be able to just deal with it?
Why would it work properly on initial use but fail to work a week later?

Thank you very very much in advance!

Joe

View attachment 37244 View attachment 37245

Funnily enough I have the same saw as Joe and as do many others we all have the same or similar problem. Mine is a lumberjack make but identical and I mean identical in every respect except the colour. I have a 32 amp consumer unit in the workshop it has a 6amp mcb a 16amp mcb and two 32 amp mcbs only the 6amp and 16amp are being used. The 16amp is a complete ring around the workshop when I put my saw in and try to use it , it trips the mcb only the mcb 16 amp but if I plug it into my 25meter extension it works no problem and never trips the mcb ....why and how do I fix it. The saw is only a week old out of the box?

 

[telectrix]

Funnily enough I have the same saw as Joe and as do many others we all have the same or similar problem. Mine is a lumberjack make but identical and I mean identical in every respect except the colour. I have a 32 amp consumer unit in the workshop it has a 6amp mcb a 16amp mcb and two 32 amp mcbs only the 6amp and 16amp are being used. The 16amp is a complete ring around the workshop when I put my saw in and try to use it , it trips the mcb only the mcb 16 amp but if I plug it into my 25meter extension it works no problem and never trips the mcb ....why and how do I fix it. The saw is only a week old out of the box?

this looks likea classic case of start up current tripping a type B MCB. the solution is usually to fit a type C, either 16A or even 20A, providing that the circuit parameters comply with a type C. also, ant replacement must be the same make as the CU assembly. (can't fit a MK in a WulexCU.).

 

[davesparks]

The 16amp is a complete ring around the workshop when I put my saw in and try to use it , it trips the mcb only the mcb 16 amp but if I plug it into my 25meter extension it works no problem and never trips the mcb ....why and how do I fix it.

That's a very weird arrangement, it's not normal to have a ring circuit fed by a 16A MCB.
Do you have the installation certificate from when it was installed? This will give us vital information as to whether or not the MCB could be safely replaced with one more resistant to this tripping problem. Specifically we need to know the earth fault loop impedance (Zs on the certificate) to be able to advise.

 

[pc1966]

Having a 16A MCB on the ring might have been an attempt for selectivity with a 32A or 40A MCB in the house CU.

More info on your system (e.g. what feeds the garage CU in the house) and ideally a test result on the garage CU to know what impedance there is would be useful.

 

[AllanW]

That's a very weird arrangement, it's not normal to have a ring circuit fed by a 16A MCB.
Do you have the installation certificate from when it was installed? This will give us vital information as to whether or not the MCB could be safely replaced with one more resistant to this tripping problem. Specifically we need to know the earth fault loop impedance (Zs on the certificate) to be able to advise.

Hi Dave,
Thanks for replying.
No I dont have a certificate, the wiring was installed, I believe back in 97 or 98. The 16 amp circuit exits the consumer unit goes through 4 double sockets then returns to the consumer unit. A single spur from the consumer unit leads off and terminates at a double socket from the compressor runs (2hp) when needed.
I also have a 160amp welder which is used on the "ring" circuit (obviously not at the same time) and neither it or the compressor ever trip the mcb.

 

[pc1966]

if I plug it into my 25meter extension it works no problem and never trips the mcb ....why and how do I fix it.

The "why" part comes down to the resistance of the extension lead limiting the motor's start-up current to a level that is below the MCB trip point.

A 16A B-curve MCB has an "instantaneous" magnetic trip at 3-5 times its rating, so 48-80A in this case. If your total resistance of cable and motor windings was, say, 5 ohms then your current is automatically limited to 230V / 5 Ohm = 46A so the MCB would not trip instantly, and then after a fraction of a second the motor is up to speed and drawing only the 4-8A typical running current so the slow thermal part of the MCB can ignore the short overload.

 

[AllanW]

Having a 16A MCB on the ring might have been an attempt for selectivity with a 32A or 40A MCB in the house CU.

More info on your system (e.g. what feeds the garage CU in the house) and ideally a test result on the garage CU to know what impedance there is would be useful.

Hi pc1966
The feed from the house, is taken from after the meter but before the domestic consumer unit into an rcd with a single mcb, its connected to the workshop via 30mtr of armour cable. The house bound mcb has never tripped . I'll check what the rating is.