New power saw trips garage mcb - what options do I have?

[droc]

Help please, I've just purchased a new chop saw from Screwfix, 2000W, for use in the garage. When I press the start trigger, the local mcb immediately trips.

Garage is fed from main house, 15m ish run. House has Volex split CB (100/63) feeding dbl garage via 16A type B mcb, on RCD side using 1.5mm 3C SWA (original fix by house builders). Garage has Hagar CB, with separate lighting circuit (6A), 4 x 5' tubes and ring main, 5 x twin sockets fed from 10A type b mcd, 2.5mm twin & E cables surface clipped. There is a separate circuit for the garage door, but thats it. I fitted it a few years ago. Until now the 10A mcb has been fine for the sockets (small power tools), but not now. Ideally I'd like to use a 32A mcb in the garage for the ring main but the cable from the house I don't think is big enough. Please don't suggest I uprate the cable, it is run under the house/patio, non ducted and would me a major job.

1. Could I replace garage Hagar ring main mcb with 16A type C mcb. If I did this, I think I'd lose discrimination with the house mcb feeding the garage

2. Could I fit a 20A Volex mcb to protect the SWA to the garage and maintain discrimination?

3. I don't believe Volex make Type C mcb's could I use another type to repalce current 16A type B with 16A or 20A type C?

4. As I believe the root cause is the saw motor inrush current is there another option?

thanks for any advice offered.

 

[davy_owen_88]

1. Could I replace garage Hagar ring main mcb with 16A type C mcb. If I did this, I think I'd lose discrimination with the house mcb feeding the garage

Not unless you can ensure that your EFLI results are low enough to ensure automatic disconnection within the specified times.

2. Could I fit a 20A Volex mcb to protect the SWA to the garage and maintain discrimination?

I wouldn't, but 1.5mm² SWA is rated at 22A, but whether you can do this safely depends on how the cable is run through your house.

3. I don't believe Volex make Type C mcb's could I use another type to repalce current 16A type B with 16A or 20A type C?

See my answer to your first question. You can't just got swapping MCB's around until you get the quick fix you want. It is very dangerous to alter circuit characteristics when you have no idea what you are going - no offence.

4. As I believe the root cause is the saw motor inrush current is there another option?

Get a better quality saw with a soft-start feature.

 

[Steve]

that saw uses 8.69 amps in normal use. Lets assume startup of two times that (it is probably more) - this is 17 amps for at least a couple of seconds.

This is why its tripping the TEN AMP MCB. You may replace it with a 16 Amp type B MCB but as you say you will loose the discrimination.

Alternatively run a heavy extension lead to a socket in the house on a 32A MCB.

Do you really have a ring main on a 10 amp MCB? What was the point of this?

 

[droc]

Thanks for the feedback. The 1.5mm SWA was as installed by the house builders and unless I wish to run a new cable, which would involve considerable work, thats what I am stuck with.

I'm asking questions as I do not wish to change any part of the system and impart subsequent risks to any user.

I do have an external socket which I could run to the garage but I'd prefer not to have trailing 13A extension cables.

The ring main was installed for power around the garage and sized for small tools, given the limitations of the 16A and 1.5mm SWA garage feed that I inherited.

Is there an external soft start box that could prevent the high inrush?

 

[davy_owen_88]

Have you tried powering up the saw in the house using a 32A circuit to see if the same thing happens? I wouldn't have thought the relatively small universal motors fitted to most mitre saws would draw the 3-5 x MCB rating needed for immediate tripping.

Give the saw a try in the house and get back to us.

 

[droc]

Davy,

I've just plugged it inside and it runs ok. The start is a bit violent, a definite jolt. The motor is physically small, circa 4" diameter x 6" long. The saw in question is as the link below.

http://www.screwfix.com/app/sfd/cat/pro.jsp?id=30997&ts=99520

Perhaps this is down to the saw being a cheapie (relatively speaking)

If a solution can't be found I can just return the saw, but I'd rather not.

Its not ideal I know, any ideas please?

 

[davy_owen_88]

I wouldn't have thought the relatively small universal motors fitted to most mitre saws would draw the 3-5 x MCB rating needed for immediate tripping.

I've just plugged it inside and it runs ok.

Oh well...

If you have the test gear to ensure that a type C MCB will meet the disconnection times then that is one option.

You could do as crafty says and install a larger breaker (16A type B) for the ring circuit. Discrimination is going to be pretty screwed up anyway if you have RCD's etc. This might not work though if your start up surge is still too high - but I really wouldn't expect it to draw more than 40A at startup unless something was wrong with it.

Finally you could just bite the bullet and start digging a trench for a new cable.

 

[droc]

Thanks for the response Davy. I don't have the test gear unfortunately to test disconnection times.

I'll give the 16A type B mcb a go, if that still trips, I'll put the 10A back and return the saw. Digging a trench under a patio etc is not really an option!

 

[Reynardine]

that saw uses 8.69 amps in normal use. Lets assume startup of two times that (it is probably more) - this is 17 amps for at least a couple of seconds.

To measure the start up current accurately, perhaps you can use a multimeter or maybe buy a special plug that gives digital reading of watts drawn or current drawn by your saw ?

 

[Reynardine]

Here is an example of the plug that may help you diagnose the initial current surge of your saw...

http://www.maplin.co.uk/images/Full/l61aq_withplug.jpg

 

[wiggles]

To measure the start up current accurately, perhaps you can use a multimeter

Could I just caveat that with EXTREME DANGER don’t do it!!!. If your sick of life, this would be the way to go. You would have to connect the multimeter in series to do this (tricky). Most multi-meters also bypass the fuse to measure a maximum current of 20amps. An amateur attempt at this sort of trick could easily prove fatal with the high currents being talked about. For example, having the multi-meter set to measure voltage instead of current would result in a direct short! Flash, bang, dead.

or maybe buy a special plug that gives digital reading of watts drawn or current drawn by your saw ?

Much better idea or even better a cheap digital clamp. Some of the plugin devices don't like induction devices like motors.

 

[droc]

thanks for the concern wiggles, i'm not a domestic spark (more digital controls) but know enough not to put my dmm in line as suggested. I think for £4 or so, it'd be worth my while trying a 16A mcb, if it doesn't work then I'll use either an extension cable or return the saw.

What I'm surprised about is the lack of single phase soft start devices, I accept my garage supply is a bit lacking but surely other people have come across this problem also.

 

[OLD]

I have a sip table saw 3hp it runs fine on a dedicated cct on the garage distribution but it use to blow 13a fuses eventually on the ring.So would it be possible to have a seperate cct for the saw with a mcb able to handle the start up current.You could have a 16a industrial plug & socket so no other tools could use it.

 

[streetlighter]

now 1.5mm is really too small for a submain cable wouldnt have cost the builder much more to install 2.5 or even 4.0 mm cable.

 

[droc]

I agree streetlighter, but they ran the minimum for a light and a garage door opener.

OLD, the problem is that I only have a 1.5mm cable feeding the garage, which limits my supply options.

In reality I'm looking for a quart from a pint pot.