Mitre saw tripping main breaker - Help!

[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.
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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'
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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?

 

[AllanW]

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?

The 3 cables, one leaves clockwise, runs around 4 double sockets and returns is that a ring circuit? Then one runs as a spur heading anti clockwise to a double socket where I plug in my compressor and extractor when I'm spraying stuff. Yesterday I installed another spur through a 20amp switching box to a 16amp blue socket and plug for the chop saw but obviously it still trips. The only reason I use the 16amp circuit is because the cabeling is all 2.5 and I thot it would have to be 4 or 6 mm for the 32 amp circuit.
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Jeez you have given me so much valuable information, I really appreciate it. Wish you were local I'd give a good few jobs for it and the others in the house. I will double check the ring circuit by the advise and any doubts at all I will contact a proper electrician to change unit and make the necessary adjustments . Thank so much pc1966 wish everyone was as helpful as you and hope I could help others with knowledge I have too.

 

[pc1966]

Best plan is to get a local spark in. They can look at all aspects and hopefully get it safe and reliable by checking the supply arrangement and addressing the lack of any RCD protection, and lack of replacement parts.

While they are in you can also look at emergency lights as it seems it is used as a workshop.

 

[marconi]

Re #1: The OP reported:

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.

For the OP: What work have you done in your home and where during the week you mention? The electrician you call in will need to know.