New table saw keeps tripping the MCB

miller745

miller745

miller745

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Got a Dewalt DW745 for general woodworking/carpentry use to replace the cheapy screwfix special that lasted a year and on startup this new saw instantly trips the B16 breaker most of the time. If the saw starts then it'll keep running.

The shed is on a 40A MCB from the house CU, running to a 2 way CU inside the shed and the sockets are on the B16. Saw is rated at 1850 watts which would be ~8A, but even with nothing else on it still trips the 16A breaker most of the time.

Must be the inrush current causing the breaker to trip, so how can I get around it?

Options I can think of are;
Try a C16 or D16 breaker in the shed CU
Swap to a 4 way CU and have the saw on a dedicated circuit with appropriate breaker

Any others?

Out of my comfort zone doing either of those but I'd like to know what options there are to have a think as which is best.
 
Just stick a c type breaker in as like you say it's the inrush of current which is tripping the mcb. C types are designed to ignore this inrush.

EDIT: is there a 30ma rcd installed?
 
What is tripping the main switch or the mcb?
If it is the mcb change it to a type c or d as suggested previously as the only elements that change are the in rush characteristics and max Zs.
 
Do not change it to a D curve, thats just bonkers. Also before changing to a C curve you should, technically, check that you can meet the maximum zs. I only say as I would assume your shed is a reasonable distance from the main CU.
 
It's the MCB tripping. I'd guess theres <10m of cable from the house CU to the shed CU, got the phone number of a leccy that my plumber uses a lot so I'll get him to have a look at it.
 
There could potentially be a leak to earth, this can also cause the RCD to trip. Unlikely on a new bit of kit but can't rule it out. Unfortunately you will need an insulation/megger or PAT tester to test this out. Test live/neutral to earth. If it's a real bad short then you may pick continuity up with a good multimeter.
 
Without a data sheet for the motor it's hard to be definitive but I'd honestly say it's just an inrush issue. Not out of the realms of possibility that the motor is pulling 4-6x rated current at startup that will easily trip the mcb on overload.

I'd put money on it not being an earth fault.
 
Lumeycom said:
Do not change it to a D curve, thats just bonkers. Also before changing to a C curve you should, technically, check that you can meet the maximum zs. I only say as I would assume your shed is a reasonable distance from the main CU.
Click to expand...

Why not a type d? Do you know what the inrush current is?

Rough guide

Type C 5-10 times

Type D 10-20 times

Also the duration of the current is a factor.

Obviously check with the manufacturer of the said mcb
 
because it makes it far more unlikely to come within the permissible maximum zs, they also tend to be expensive comparatively and given that its a 2 way db in the shed I doubt there would be a type approved breaker for it as its likely to be a wylex nsb type db.

also given that its not pulling out the 40a serving the shed then "10-20" times is more than likely to be unnecessary.
 
There is very little cost difference, we can go around in circles all day but without knowing the inrush current none of us know. All we know at the minute is that it is not a discrimination issue :)
 
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