power saw causing mcb to trip

[JDLDIY]

Just bought a brand new mitre saw, but when plugged into my garage CU, it often trips the mcb, meaning I have to let go of whatever i am doing and reset it, then start again - all a real pain!!

I can't be overloading the circuit as I have nothing else plugged in. The mcb's never tripped before and I've run all sorts of tools from the same circuit, so does it point to to a fault in the saw? What's likely to be the cause of the problem?

Cheers

JD

 

[Lectrician]

Start up current.

What rating is the MCB and the saw?

 

[JDLDIY]

Start up current.

What rating is the MCB and the saw?

I'll have to get back to you on the numbers as I'm at work now, I think the sa is rated at 1800W

 

[Steve]

but when plugged into my garage CU

how did you acomplish this?

is it the MCB or an RCD that's tripping?

 

[JDLDIY]

OK, the saw is rated at 1800W, the mcb is at 16A. It's the mcb which is tripping, never the rcd so far.

but when plugged into my garage CU

how did you acomplish this?

It's just plugged in via it's plug to a double socket, wired up to the 16A mcb.

Thanks all.

JD

 

[plugwash]

yeah a B16 is a little on the oversensitive side for running some portable equipment.

how exactly is this garage CU fed?
how is the socket fed off it? (single socket? ring? multi socket radial?)
what size cables are used?
is the 16A breaker the only one in the garage CU?
is the saw the only appliance you are using in the garage at the time?

 

[JohnD]

Its very cold at the moment, lots of garages benefit from a big electric heater...


edited to say: My mistake, says nothing else plugged in.

 

[JDLDIY]

yeah a B16 is a little on the oversensitive side for running some portable equipment.

The mcb says NSB16 on it if that helps.

how is the socket fed off it? (single socket? ring? multi socket radial?)

It's a multi socket radial.

what size cables are used?

2.5mm T&E

is the 16A breaker the only one in the garage CU?

Nope, there is a 6A one as well, off which is run a double fluorescent light.

is the saw the only appliance you are using in the garage at the time?

The mcb trips both when I run the saw on it's own and when I use it in conjuction with my workshop vac.

Thanks, what do you think?

JD

 

[JDLDIY]

Anyone got any more ideas? I've been in touch with the retailer of the saw and am waiting their reply.

 

[Adam_151]

if you can meet the 0.4 disconnect time with a C16 breaker, then you could swap it, but its unlikely that you'd have the equipment to measure Zs, though if you knew what earthing arrangemnts you had, you could take the worst case value for Ze, and calculate R1+R2 if you knew the lengths and types and sizes of cables used in the garage submain and the socket radial

 

[JDLDIY]

if you can meet the

I was OK up to this point but then I lost you! What is causing the mcb to trip then, not a problem with the saw? Why would the CU be OK with all of my other power tools including circular saws, bench tools etc?
JD

 

[Adam_151]

motors take a startup surge when they start, I guess becuase of the big blade and that, the startup surge of the saw is quite big, a B type breaker will trip in 0.1 at somewhere between 3 and 5 times its rated value, and usually tends towards the lower end of the scale, so if your saw is pulling 55A onstartup for example, that'd most likely trip the breaker, type C breakers are motor rated and they are designed to handle the startup surges of motors, and trip between 5-10 times the rated value in 0.1.

the problem is that you can't blindly change the breaker, if a live wire touches the case of a piece of plugged in equipement, and flows down the earth, you have to be sure that the current will be large enough to open the breaker and disconnect the power in 0.4 seconds, a C16 will need a bigger current to do this than a B16, the current that flows is determined by the earth loop impediance (Zs), which is made up of the external part (Ze), plus the cables in your installation (R1 +R2; (the ristanace of the phase conductor + the cpc))

 

[JDLDIY]

Many thanks Adam_151 for your replies. So, basically then is there nothing I can do to correct this problem?

JD

 

[teachergeezer]

what you need to do is change the mcb from a b type to a c or even d type. your local wholesaler should be able to order the correct one. in response to the Ze issue, this is only relevant where you are not using an rcd for protection against whats termed 'indirect contact'. as this is a garage you should have an rcd fitted as a amatter of course, either in the 2 way cu in the garage or back at the house's main incomer. this must be a 30 milli-amp unit and is not to be fitted by a non-registered electrician

 

[kai]

How did the plug fuse in the saw hold up?
What rating was the plug fuse?
Probably a case of discrimination failure has struck again!