Mitre saw tripping main breaker - Help!

[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.

 

[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.

Can you also find out what size of armoured cable it is? For example 4mm or 6mm CAS (Cross Sectional Area).

 

[AllanW]

Can you also find out what size of armoured cable it is? For example 4mm or 6mm CAS (Cross Sectional Area).

Hi pc1966
Below are images of the supply unit in the house(63amp rated) and also how it connects in. I'm not sure how to check the case ( no labels) but the outer diameter is 16mm.
[automerge]1590055637[/automerge]

Hi pc1966
Below are images of the supply unit in the house(63amp rated) and also how it connects in. I'm not sure how to check the case ( no labels) but the outer diameter is 16mm.

That may help, the large grey cable runs from here and runs approx 40ft till it junctions with the armour cable.

 

[pc1966]

Looking at the picture the obvious sore thumb is the grommet not properly protecting the grey cable at the bottom of the switch!

That is an isolator switch and a 50A MCB, there is no RCD protection at that point unfortunately. Had the SWA cable gone all the way that is not an issue (if punctured the earthed armour shorts out and makes it safe), but care is obviously needed in case the cable is damaged (so should be at least 50mm back from any wall surface if hidden, etc).

Has the garage CU got an RCD? A photo of it might help.

From the 16mm estimate of the SWA cable it is probably 6mm, as the T&E is also probably 6mm to match, but doing remote investigation is obviously not an exact science. If so the current limit for 13m T&E + 30m SWA run is probably around 34A based on voltage drop.

However, a 50A B-curve MCB for short circuit protection for the cable might not disconnect fast enough to meet the wiring regulations, it would need checked (the Zs measurement mentioned before).

So probably you could use a higher trip level breaker, such as a 16A C-curve, 20A C-curve, or 32A B-curve, but it would mean:

• Measure the total earth fault impedance Zs at the garage CU
• Check feed cable specifications
• From Zs check the cable feed breaker is acceptable
• From Zs check what final breaker is acceptable
• Checking the wiring of the garage sockets are rated above 16A (in case they are in 1.5mm T&E for some odd reason)

Your best plan would be to get a professionl electrician in to check the details before changing anything.

 

[AllanW]

Looking at the picture the obvious sore thumb is the grommet not properly protecting the grey cable at the bottom of the switch!

That is an isolator switch and a 50A MCB, there is no RCD protection at that point unfortunately. Had the SWA cable gone all the way that is not an issue (if punctured the earthed armour shorts out and makes it safe), but care is obviously needed in case the cable is damaged (so should be at least 50mm back from any wall surface if hidden, etc).

Has the garage CU got an RCD? A photo of it might help.

From the 16mm estimate of the SWA cable it is probably 6mm, as the T&E is also probably 6mm to match, but doing remote investigation is obviously not an exact science. If so the current limit for 13m T&E + 30m SWA run is probably around 34A based on voltage drop.

However, a 50A B-curve MCB for short circuit protection for the cable might not disconnect fast enough to meet the wiring regulations, it would need checked (the Zs measurement mentioned before).

So probably you could use a higher trip level breaker, such as a 16A C-curve, 20A C-curve, or 32A B-curve, but it would mean:

• Measure the total earth fault impedance Zs at the garage CU
• Check feed cable specifications
• From Zs check the cable feed breaker is acceptable
• From Zs check what final breaker is acceptable
• Checking the wiring of the garage sockets are rated above 16A (in case they are in 1.5mm T&E for some odd reason)

Your best plan would be to get a professionl electrician in to check the details before changing anything.

Pc1966
Thanks for that, very sound advice, I will send an image of the workshop unit. To my knowledge all socket cabling 2.5 t and e and 1.5 for the lights.
Al'
[automerge]1590062135[/automerge]

Looking at the picture the obvious sore thumb is the grommet not properly protecting the grey cable at the bottom of the switch!

That is an isolator switch and a 50A MCB, there is no RCD protection at that point unfortunately. Had the SWA cable gone all the way that is not an issue (if punctured the earthed armour shorts out and makes it safe), but care is obviously needed in case the cable is damaged (so should be at least 50mm back from any wall surface if hidden, etc).

Has the garage CU got an RCD? A photo of it might help.

From the 16mm estimate of the SWA cable it is probably 6mm, as the T&E is also probably 6mm to match, but doing remote investigation is obviously not an exact science. If so the current limit for 13m T&E + 30m SWA run is probably around 34A based on voltage drop.

However, a 50A B-curve MCB for short circuit protection for the cable might not disconnect fast enough to meet the wiring regulations, it would need checked (the Zs measurement mentioned before).

So probably you could use a higher trip level breaker, such as a 16A C-curve, 20A C-curve, or 32A B-curve, but it would mean:

• Measure the total earth fault impedance Zs at the garage CU
• Check feed cable specifications
• From Zs check the cable feed breaker is acceptable
• From Zs check what final breaker is acceptable
• Checking the wiring of the garage sockets are rated above 16A (in case they are in 1.5mm T&E for some odd reason)

Your best plan would be to get a professionl electrician in to check the details before changing anything.

This is the workshop unit. And the 16amp is the sockets the two 32amp are unused and the 6amp does the lights.

 

[pc1966]

Well the garage CU is obviously of the same era so that kind of makes sense.

But no RCD protection - not good.

Can you see how the SWA cable is terminated, has it got a proper gland? (brass thing, usually sheathed with rubber condom-like cover, grips the steel wires and usually has a brass ring tag "banjo" that earth is connected to)

I see 3 cables off the socket circuit, so it is not (all) a ring and that might be why they are on 16A and not 32A as 2.5m radials are not rated for above 20-25A depending on the thermal conditions.

Is there a ring of a few sockets and another odd socket added somewhere? Or is it 3 radial circuits?