Lawnmower trips MCB

Postby Chris12 » Fri May 11, 2012 9:19 am

I have just bought myself a new lawnmower, it's rated at 1800W, the problem I have is that it keeps tripping the MCB, when it does start up it runs fine but if I stop and then restart the breaker trips, I have only tried it plugged in the garage and haven't actually tried to cut my overgrown grass yet, from what I can see the socket in the garage is a spur taken from the CU and is on it's own 16A breaker, I called Bosch who could only suggest changing the MCB with one with a time delay, so is this the best solution or does anyone have any other advice? Any advice would be much appreciated.
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Simply Build It

Postby Chris12 » Fri May 11, 2012 5:31 pm

As a follow up having took another look at the socket I realised how easy it would be to make it into a ring instead of the spur, so my thinking was if I did this I could then put in a 20A MCB to see if this solves the problem?
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Postby ericmark » Fri May 11, 2012 6:32 pm

It would seem you have an inrush problem where the mower uses a lot of power just to start it going. This is all down to the letter in front of the amps on the MCB. Old was type 1, 2, 3, and 4 now it's B, C, and D.
A B type will take 5 times the rated amps, C = 10 and D = 20. So a B16 will take up to 80A and a C16 will take 160A for those few seconds it takes to start.

However changing the MCB is not that simple. The idea is that a short circuit will trip the MCB in a split second but to do that enough current must flow. So one has to measure the loop impedance. The meters are expensive and the job of changing very easy so just not worth DIY best option is to get an electrician to test and if possible change. Do make sure you get the paperwork from him.
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Postby sparx » Sat May 12, 2012 5:16 pm

all valid comments from ericmark again.
Looking at the rating of 1800w thats roughly 2.4HP, quite large and should draw 7.8 amps on load.
However as ericmark says inrush starting current will be 5-6 times that for a short time.
To protect the motor/circuit from overload don't put in higher current mcb, it is probably a type 'B16'
and whilst a type 'C' may stop the problem the earth loop impedance of the circuit will need to be
below 1.16 Ohms whereas a type 'B' may be up to 2.32 Ohms.
This will need confirming before any changes made,
regards Sparx
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Postby Chris12 » Sat May 12, 2012 7:51 pm

Thanks for all the info, I can now understand the problem but I will have to hand it over to an electrician who can do the necessary checks, should have bought a petrol mower:)
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