DIY Doctor

Outside sockets in my shed and power saw tripping switchboard

Postby Youngdydo » Sat Mar 05, 2016 9:46 am

Ive recently bought a new shed, and kept the power point/plug socket which was in the old shed, which is wired upto the mains. I've followed things properly by correctly wiring another plug socket upto that. I have an internal light which is on a plug, and is plugged into the second socket - I switch it on and there is no problem, it works fine. I used an extension cable, plugged into the original plug socket/power point so I could use my hammer drill outside and it worked fine.
Yet when I plug my table saw in and use it, it knocks the fuse off on the switchboard that controls all the plug sockets.

If anyone has any ideas on how I could stop it from happening I would be appreciative.

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Simply Build It

Postby saki » Sat Mar 05, 2016 2:11 pm


Please note I am not qualified in electrics nor even a keen amateur.

No doubt someone more expert than I can explain the reasons for the fuse being tripped. However I had a similar experience. No outside wiring involved. I used an extension from a mains socket in house to supply power for a workman using a fairly large saw. The fuse on main board continually tripped. Resolved it by plugging saw directly into a different mains socket that was nearer point of use ie taking out the extension solved the problem. Lighter equipment eg drill, mower etc there was no problem using the extension.

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Postby ericmark » Sat Mar 05, 2016 7:19 pm

Not really enough information there are a number of reasons for a trip to open circuit, so I will try to explain.
1) Today most sockets are protected by a RCD either as a stand alone unit or as part of a MCB and I have never seen a RCD without a test button so first question is does the item that has tripped have a test button?
2) The MCB is in fact two devices in one, it has a thermal part which will cause it to trip after some time if the rated current is exceed, because it takes so long to trip there is also a magnetic part and this is variable between three letter designated stages. B will trip between 3 and 5 times rated current, C will trip between 5 and 10 times rated current, and D will trip between 10 and 20 times rated current. So a B16 will trip with a in rush of 80A or a long time at over 16 amp. Some motors and transformers are worse than others, quite common for problems with welding sets.
3) Motors need to build up speed and any excessive volt drop can increase the time taken so it will trip any thermal device.

When we install a socket we should test the loop impedance and prospective short circuit current and according to the protective device used there are different limits. However although the electrician may walk around with a loop impedance meter I would not expect the DIY guy to fork out on one. The Martindale EZ150 plug in tester is about the best one can expect from a DIY man. With a loop impedance tester we can work out the volt drop and we can be sure a short circuit will cause the protective device to work within the laid down time. With out it we can use low ohm meters and calculate but even they are expensive. The low ohm meter normally also works as an insulation tester again a must have for an electrician but expensive for the DIY guy and also we need a RCD tester which tests both the RCD is not too sensitive and that it is sensitive enough and also measures time it takes to trip. Limit is 40 ms which is really a bit too fast for a stop watch.

So you are really trying to do the job with your hands tied behind your back, so start with what is tripping a RCD or MCB or a combined unit called a RCBO? And what does it say on it. For example B16 or 30 mA or 10 mA.

Once I know what is tripping I will try to work out some tests, for that I need to know what you have. It may seem crazy although I like the EZ150 I don't have one as I have a full blown loop impedance meter.
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