Table Saw trips RCBO

[Bedders]

I have a ring cct (2.5mm ) in my cabin which is protected by a 20amp RCBO. This has been in existence for over 3 years with no issues. I recently bought a table saw and when turned on, on its own all is OK. However the other day I had a heater and a dust extractor on, then turned on the table saw, the RCBO immediately tripped. It only trips when other appliances are on and you turn on the table saw. My question, is there a way to dampen the start up of the tablesaw motor or would I have to upgrade the RCBO to 32amp. The table saw is a Dewalt with a 2000watt motor. It is a bit aggressive on star up. Ideally is there something it can be plugged into to reduce the initial surge it creates.

 

[plugsandsparks]

Try starting saw first, dust extractor second and heater last. If the heater is a 2KW fan heater you will be close to the limit without the extra load of starting the motor anyway. Saw , hopefully, when its running will take a much lower current than what its rated at.

 

[Bob Geldoff1234]

What ever you do,do not change the RCBO to a 32amp.The 20amp is there to protect the cable.You are basically overloading the circuit.As Plugsandsparks says use the saw first or unplug other items while using the saw.

 

[timhoward]

The simplest solution is probably turn the heater off while sawing!

As the others have said, you are likely pushing the circuit design as heating generally consumes a lot of power. I'd expect if you look at the ratings on the extractor and the heater and add up the power in watts, and then add 2000 for the saw you'll be at over 4600 watts which in simple terms is the peak limit of a 20A circuit, and it shouldn't be running with even this load for long periods.

If you will want these three things on together regularly then you need someone to look at the supply to the cabin (which is the limiting factor) and see what can be done. There may well be simple solutions but a lot more info is needed to say something is safe.

When you say ring circuit, is it actually a complete loop or a more common daisy-chained (radial) socket circuit?

 

[Bedders]

The simplest solution is probably turn the heater off while sawing!

As the others have said, you are likely pushing the circuit design as heating generally consumes a lot of power. I'd expect if you look at the ratings on the extractor and the heater and add up the power in watts, and then add 2000 for the saw you'll be at over 4600 watts which in simple terms is the peak limit of a 20A circuit, and it shouldn't be running with even this load for long periods.

If you will want these three things on together regularly then you need someone to look at the supply to the cabin (which is the limiting factor) and see what can be done. There may well be simple solutions but a lot more info is needed to say something is safe.

When you say ring circuit, is it actually a complete loop or a more common daisy-chained (radial) socket circuit?

It's a sub board in a cabin fed by a 10mm swa 3 core, at about 20meters from the main board feed. The cct is a true ring cct wired with 2.5mm cable. Not a radial.

 

[mainline]

What ever you do,do not change the RCBO to a 32amp.The 20amp is there to protect the cable.You are basically overloading the circuit.As Plugsandsparks says use the saw first or unplug other items while using the saw.

32amp is the normal rating for a 2.5mm ring final ?

 

[mainline]

It's a sub board in a cabin fed by a 10mm swa 3 core, at about 20meters from the main board feed. The cct is a true ring cct wired with 2.5mm cable. Not a radial.

Just up the 20amp to a 32

 

[Bedders]

32amp is the normal rating for a 2.5mm ring final ?

Yes it is, but at the time of install they had a 20amp spare and we did not have anything heavy in there. So I could easily upgrade the RCBO.

 

[Bedders]

Just up the 20amp to a 32

Think that's the best solution. Thanks all for your input.

 

[pc1966]

Is it a B-curve or C-curve? If it is just the peak start-up that is tripping it then changing to 20A C has less concern about the specifics of the cable feeding it.

 

[Bedders]

Yes it is a B curve

 

[HappyHippyDad]

I would not be putting a 32a rcbo just because its been suggested by someone I don't know on an Internet site.
A 32a rcbo may be fine or it may be dangerous. You wouldn't know without further tests. I always think there must be a reason why someone has chosen to put a 20a on what seems to be a ring.
The 20a type C suggestion is good, assuming you can get an EFLI result low enough.

 

[Bedders]

I would not be putting a 32a rcbo just because its been suggested by someone I don't know on an Internet site.
A 32a rcbo may be fine or it may be dangerous. You wouldn't know without further tests. I always think there must be a reason why someone has chosen to put a 20a on what seems to be a ring.
The 20a type C suggestion is good, assuming you can get an EFLI result low enough.

The 20amp was first used as it was spare and we did not need any heavy loads so it would have been fine. The correct fuse rating for the ring would have been a 32amp RCBO. So fitting one would be OK as it is a true Final Ring cct. However I like the idea of the C curve as I had forgotten about that characteristic.

( For those who may be reading this B Curve RCBOs trip between 3-5 times full load current and are mainly used in residential applications where loads are resistive e.g. lighting fixtures, domestic appliances with low surge levels.
C Curve RCBOs trip between 5-10 times full load current and are intended for high-level overcurrents. They are the normal choice for commercial and industrial applications where fluorescent lighting, motors etc. are in use.)

However I have come across this, and will be installing as it aids the saw as well. A Soft Start Module.

In addition as we may put further power tools in now I think a C curve RCBO will be in order.

Many Thanks for your reply as this has prompted many solutions and my further understanding. It puts me in a much better position when inviting quotes from electricians.

 

[ferg]

I would not be putting a 32a rcbo just because its been suggested by someone I don't know on an Internet site.
A 32a rcbo may be fine or it may be dangerous. You wouldn't know without further tests. I always think there must be a reason why someone has chosen to put a 20a on what seems to be a ring.
The 20a type C suggestion is good, assuming you can get an EFLI result low enough.

Working on the principle that the information the OP has given us is correct then a 32a would be a good solution.
They said that it is a ring and the 20a was installed at the time as that was what they had to hand.
If the installation paperwork verifies this then I see no issue.
My personal advice would be to get a local sparky to do the swap so they can carry out required tests etc. and ensure the correct part is installed.
Obviously there is no such thing as a 10 minute job but this is pretty close.

 

[ferg]

Working on the principle that the information the OP has given us is correct then a 32a would be a good solution.
They said that it is a ring and the 20a was installed at the time as that was what they had to hand.
If the installation paperwork verifies this then I see no issue.
My personal advice would be to get a local sparky to do the swap so they can carry out required tests etc. and ensure the correct part is installed.
Obviously there is no such thing as a 10 minute job but this is pretty close.

OP replied before I posted mine. >.<

 

[timhoward]

It's been an interesting thread. It's slowly emerged that there seems to be a well engineered distribution circuit and the fact it was a "ring" got slightly overlooked at first as the very logical reaction was 'that's more than 20 amps so of course it will trip'.

Those commenting at the beginning didn't know there was a 10 sq mm SWA feeding it and it initially sounded like a cheap and nasty job.

To my thinking there are two definite things to check before changing that device:
-Is the Zs low enough for a B32? If not it would rely on RCD technology to turn off in time under fault conditions, and a new certificate needs to state that RCDs are being employed for fault protection. (If moving to a type C then even more reason to check this).
-Do end-to-end tests confirm it's a healthy ring final circuit and is eligible to be protected by a 32 amp device in the first place?

I'd also want to know the OCPD of the distribution circuit, it might be 25 amp MCB for all we know and the problem just moves. Unlikely, but possible!

It comes down to a simple choice
-stick a B32 in and hope all is ok
-get 5 minutes of testing done and be sure all is ok.

 

[mainline]

I would not be putting a 32a rcbo just because its been suggested by someone I don't know on an Internet site

The same could be said about not changing it.

A 32a rcbo may be fine or it may be dangerous.

Can you explain how it could be dangerous ?
Even with a break in the ring ?

There must be a reason why someone has chosen to put a 20a on what seems to be a ring.

Yes because as already said they didn't have a 32.

The 20a type C suggestion is good, assuming you can get an EFLI result low enough.

How can putting an underrated 20amp rcbo on a circuit that requires around 5kw be a good suggestion.

 

[Bedders]

It's been an interesting thread. It's slowly emerged that there seems to be a well engineered distribution circuit and the fact it was a "ring" got slightly overlooked at first as the very logical reaction was 'that's more than 20 amps so of course it will trip'.

Those commenting at the beginning didn't know there was a 10 sq mm SWA feeding it and it initially sounded like a cheap and nasty job.

To my thinking there are two definite things to check before changing that device:
-Is the Zs low enough for a B32? If not it would rely on RCD technology to turn off in time under fault conditions, and a new certificate needs to state that RCDs are being employed for fault protection. (If moving to a type C then even more reason to check this).
-Do end-to-end tests confirm it's a healthy ring final circuit and is eligible to be protected by a 32 amp device in the first place?

I'd also want to know the OCPD of the distribution circuit, it might be 25 amp MCB for all we know and the problem just moves. Unlikely, but possible!

It comes down to a simple choice
-stick a B32 in and hope all is ok
-get 5 minutes of testing done and be sure all is ok.

All the work was carried out by an electrician and certified. FYI THE DISTRIBUTION IS from a Henley block off the main CSU fed into and additional CSU for distribution. A 62amp breaker is fitted in that CSU which then has the 40amp MCB feeding the cabin.

 

[HappyHippyDad]

I'm sure it's 95% likely that a 32A breaker would be fine. Not only fine, but more suitable than the 20A. However, the only times I have ever come across a 20A breaker on a 'ring', is when there has been a fault found on the ring, or a DIYer has added spurs off of spurs and someone has downsized the breaker.

The OP has clearly stated that the 20A breaker was used simply because it was the only one available. If this is correct, then again, 95% likely all will be fine. No offense is meant to anyone (@mainline , or the Op), I would just need to be 100% certain a 32A is suitable rather than 95% and this could only be done by being there and seeing/testing it for yourself.

I would still advise @Bedders to get an electrician in if he intends to start changing the design of the circuit (ie 32A breaker).

He sounds experienced and knowledgeable from his posts, so I expect all will be well.