Big saw on a long extension lead keeps tripping. How do I uprate the supply?

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  • Big saw on a long extension lead keeps tripping. How do I uprate the supply?
  • Wiksey

    To add to what Slogo said above, fitting a Consumer Unit isn’t a DIY job, it’s a notifiable job, but…

    To make it easier to understand your wiring later, as you connect to the neutral bar count in the number of slots – on the LH RCD you’re using the 4th MCB so connect the black wire to the 4th connection point on the corresponding neutral bar (4of6 counting from the left). Doesn’t seem important now but as the board gets filled up if makes life easier.
    As mentioned above you’ve got no main earth connection to the CCU, but there does seem to be a small earth from the incoming supply? Sounds like you’re looking into your earth connection though.
    To meet regs all earth wires need to be marked, so you’ll need the sleeving even though it seems a waste of money (should be able to get a smaller roll than 100m though)
    As also mentioned before all an RCD does is measure the difference between incoming and outgoing supply. If it’s more than the stated 30mA it will trip as this indicates a fault to earth path. It’s your MCB that trip if it’s overloaded – this can either trip instantly or after a couple of minutes depending on the fault.
    The bakelite fuse you mention is the property of the electricity supply company and you shouldn’t be pulling it – like me telling you is going to stop you!
    If the incoming supply has a 30A fuse there’s not much point in anything further down the line having a higher rating than this as you are then relying on that 30A fuse as your protection and if it blows you can’t just pick one up at Toolstation. You need to limit the onward supply at the consumer unit instead so no one circuit is higher than the 30A.



    Whatever you do it will be safer than the mess you’ve got there!

    But . . .I’d take a step back first and have a look at the incoming supply from the power company. Western power will check it for free. If you’ve got the old style two/three cables coming into the property they will more than likely want to replace it with a single insulated cable.



    I’m always pleased to help people with electrical stuff, but this isn’t a DIY job that you can learn on the fly.

    The “earth” you connected incorrectly, isn’t an earth. It’s neutral. This is basic.

    TN-C-S doesn’t require an earth stake. The installation looks like it’s TN-S. This doesn’t require an earth stake either. This is basic too.

    There is a requirement to size the earthing conductor correctly to be able to carry the PEFC. These can be found in the Wiring Regulations (BS7671)or calculated using the adiabatic equation. This is important because if you don’t, the main earthing conductor will not be able to carry the fault current in the time between the fault occurring and the main fuse blowing – thus setting fire to that nice piece of flammable wallpaper it runs underneath.

    Are your water/gas/other conductive parts that bring earth potential into the property bonded? This will ensure you don’t get a shock if a fault occurs and you’re touching the sink at the time.

    You can fit all the RCDs you want. There’s still an earth fault on the saw that’s causing it to trip. It was there 3 weeks ago when I first told you and it’s still there now.

    An electrician will have that done in half a day. Safe and certificated. He can also test the lead out to the saw. This will show that the saw has a problem.

    I respect you for wanting to give it a go, but it’s not just a case of connecting 3 wires up. The skill is not making it work, it’s making it fail gracefully when a fault occurs. This requires knowledge, experience and suitable test equipment.

    There is a time and place for DIY and a time when a tradesman is worth the money. This is that time.

    Sorry, but it has to be said.



    HAHAHA Well when the house burns down and everyone inside dies, or someone accidentally gets a shock and dies. Don’t come crying to STW. *ive killed everyone in the house with my shoddy wiring what should i do?* post

    Don’t forget to tell them you did it yourself when the shit hits the fan and they ask for the installation and test certificate! I wouldn’t bother with house insurance either. its void. You sound like you don’t have it anyway.

    Enjoy the new year it could be your last!

    Premier Icon gofasterstripes


    On so many levels, regarding so many posts!

    But yeah, I’d get it looked-at by a pro. Maybe you can get a STW-rate off someone here?


    Just to add to the confusion, last year I was wiring a motor and inverter up (not something I’m familiar with), but looking through the specs of inverter it had a leakage current to earth of around 20ma. Odds are that it would trip an RCD.
    If this saw was originally 3 phase, and has been converted to run off a single phase via an inverter, there’s a reasonable chance that it is ‘earth leaky’ by design and no amount of faffing with RCDs will make any difference.

    Slogo, why are you pretending you know about electrics when you don’t appear to know any more than I do?
    I’ve posted pictures of what I’ve got above, so explain to me which bit is going to catch fire.

    Thanks to rwamartin and others for the knowledgeable replies.
    To cover some of the points raised;

    I know a 63A RCD downstream of a 30A bakelite fuse doesn’t make sense, but I’ve got no way of isolating the bakelite fuse, so that’s what I’m stuck with. I’ve got a 16A MCB downstream of that anyway.

    What I’m starting with has probably been there way over 50 years. tacking a modern consumer unit on has got to make it safer.

    I’ve got no mains water, so there’s no earth connection there.

    The earth connections from the existing fuse boxes to the incoming overhead cables look very thin. I’d guess one is 1mm and the other 6mm.
    There’s no main earth to the CU because there’s nothing I could connect it to at the moment.

    As far as I can tell, the incoming cables haven’t got armouring. I’m not going to start scraping insulation off to check and the box where the earth cables connect to the incoming cable is sealed.
    Without knowing that, I can’t be sure if it’s TN-S or TN-C-S
    Is it normal for individual overhead cables to be armoured?

    I’m doing some major repairs to the bungalow and the wall that it’s all fixed to will be coming down, that’s why I’m just running temporary sockets on spurs at the moment.
    You’ve all convinced me that I should make this a priority now, so I’ll fix a suitable board to the new wall and get the MEB or a qualified contractor to move the meter and get everything to comply with modern standards.

    The saw works fine now. Maybe the old 13A plug in RCD was faulty, but it hasn’t tripped the new 16A MCB or 63A 30mA RCD yet.
    NVR switch and belt and blade guards are next on my To Do list.


    “I wouldn’t bother with house insurance either. its void”



    MTG if you want all the regs, I can send you a link to to download some PDFs, just drop me an email.

    I’ve fitted my own consumer units / pulled apart the incoming supply – it’s all pretty straightforward.

    If you want a pro who is very good and charges fair rates, try alanl (on here), he has done a load of work on the wife’s rental property for us. He’s Leicester based IIRC.

    Thanks Footflaps.
    Once I start getting a bit further on with the bungalow, I’ll be doing as much of the wiring as I can myself, so it would be helpful to have all the information.
    I’m in Worcestershire, so I don’t suppose it would be cost effective to pay someone to travel from Leicestershire. Thanks for the recommendation anyway.

    Going back the earlier warnings, the only thing that I can see that is actually dangerous or a fire risk, is that 1mm earth cable from the grey fuse box, top left, to the black incoming neutral connection, centre right.
    It’s probably been there since before I was born and hasn’t melted yet.


    Keep plugging away MTG if you feel confident in your work, I personally would get it checked over as is just for piece of mind. However, I personally think the most dangerous thing you’ve got with the whole set up is that **** saw, it’ll have your fingers off in a jiffy!

    Premier Icon gofasterstripes

    Split the problem in half.

    Try hanging another large appliance that you know DOESN’T cause an issue off the extension. If it still trips, it’s the wiring, if it doesn’t it’s the device.

    I guess it’s the device, with slightly degraded insulation, myself. If it sits around in the damp, it could well be a circa 30mA leakage. Heck, the RCD may well be more sensitive than you think.

    Have you tried taking a multimeter [M?] to the earth>live end of the cabling, and the same with the thing plugged in? And for the neutral>earth connections?

    Premier Icon molgrips

    It’s probably been there since before I was born and hasn’t melted yet

    Er.. You know how it’s meant to work, don’t you? You won’t find out if it’s too small until you have a fault, and then it might be too late… Doesn’t make a difference how long it’s been there does it?

    I did have a fault when the mice chewed through the original wiring. That’s why I cut back to just a double socket mounted on the fuseboard itself and extension leads from there.
    The earth cable is too small by current standards, but obviously did it’s job at the time.

    And gofasterstripes, keep up at the back. 😛
    The saw is working now.


    I studied Marine electrics and have been a fully qualified marine electrical/engineer for 10 years. I don’t touch houses.

    I’m the Chief Engineer on a 108ft super yacht and a few smaller boats with in the company i work for.

    Enjoy fixing your shitty electrics!

    Premier Icon gofasterstripes

    keep up at the back

    I pride myself in not knowing what’s going on.

    Slogo – it’s the weekend, mate. Put your feet up and set fire to the end of a 9-skinner. You clearly need to chill out.

    Well, I’m Chief Engineer and Captain on a 9ft tandem.


    Going back the earlier warnings, the only thing that I can see that is actually dangerous or a fire risk, is that 1mm earth cable from the grey fuse box, top left, to the black incoming neutral connection, centre right.
    It’s probably been there since before I was born and hasn’t melted yet

    Pretty standard, the original earth wire on our distribution board probably couldn’t take the >100A load required to blow the service fuse (see photo). I added an additional wire and compression clamp (not really suitable as it can crush the paper insulation if done up too tight). I recently got a proper Earth clamp from alanl which I’ll fit. NB With the addition of RCDs the earth no longer needs to blow the main fuse as hopefully the 30mA RCD will trip long before a 100A load is put through the earth.

    [url=]TN-S Earth Bonding[/url] by brf, on Flickr


    Ive seen worse consumer units. But I’ve seen better ones burn houses/boats down!

    Wouldn’t kill you to rewire it with the cables cut to the right lengths and with 90 degree bends. It does the job now but in 5 years time when you have more stuff hanging off it you’ll deeply regret not starting with a tidy job.

    Premier Icon gofasterstripes

    Inside Flaps’ Fusebox 😉

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