A Little Workshop Electrical Advice

Hoping that not everyone is in London enjoying themselves and spending all their pocket money on essential goodies and beer.

I have decided that the workshop electrics need updating now that with the Bridgeport the machines are taking over. At the moment the supply comes off a 30 amp fuse in the house consumer unit. As the main isolation switch is in the workshop anyway I thought I might take a separate feed from that to a second consumer unit for the workshop. I had though of a cheap Crabtree 100A unit but have no idea what sort of MCBs that I would need or if this is a reasonable way to go. At the moment apart form the usual small machines; I have three smallish lathes, two mills and a 150 amp Mig welder to feed.

Is this a reasonable approach or am I looking to over engineer the problem? Would it be best left on the normal house unit? Any advice on the consumer unit and MCBs that I should use if necessary would be very much appreciated. I am aware of the electrical wiring regulations and will get it checked by a competent electrician before I throw the switch.

Best regards

Keith

Reply to
jontom_1uk
Loading thread data ...

You can get small RCD/MCB distribution units with isolator switch included from Legrand and others like Moeller, they are rather more industrial looking but better for that and more robust.

Having the workshop isolatable from the rest of the house is a good idea from the safety point of view.

If you have any motors that are directly on the MCB rather than through an inverter then you'll need to go to a time-delay type, C or D rating.

I'd suggest a 40A power

Reply to
Peter A Forbes

Keith,

It would be worth adding a consumer unit so you can have separate MCBs for lights and machines in there; then you won't be plunged into darkness when something overloads.

From the sounds of it 30A is enough, but try adding up the currents for all the machines you might use at once, then repeat with the largest machine drawing starting current. I believe the 3 types of MCB are: type B (safest; trips at 3-5x rated current, for loads such as lights, domestic sockets, little motors), C (5-10x, more suitable for big motors) and D (10-20x and rather more specialist). See

formatting link
I have an unused 125A 3-pole crabtree disconnector switch if you're interested.

cheers, Guy

Reply to
Guy Griffin

Peter/Guy

Thanks very much for the advice I'll clear the decks and get on with it. At least it is only raining outside.

Peter, thanks, I'm in south Wales so I think a little too far away to take advantage of your very kind offer but I do very much appreciate you making it.

Guy, again thankyou for a very kind offer which I will keep in my (back pocket) if I may while I do a little more investigation. For some reason known only to the previous house owner (and builder) I currently have a huge MEM isolator switch in the garage which appears to control only the outside light, all 500W of it! I'm sure that it will handle more than that and will check the input supply to it. Perhaps he had grand ideas for the garden?

Best regards

Keith

Reply to
jontom_1uk

Make sure you buy a consumer unit from a manufacturer who also produce C and D types of MCB, not all do. Also if you are not using an RCD as additional protection your Earth fault Loop (Zs) may prohibit the use of C or D type MCB's, (the higher the amperage of the MCB the lower the Earth Loop Impedance needs to be).

Reply to
John Woodison

John

Thanks for that I'll get a unit that has C and D MCBs available. Not sure if I understand your second piece of advice, do I need to protect this circuit with an RCD? The house currently has an 80Amp 80mA earth leakage trip? do I need something similar for the garage circuit? I have a feeling I'll soon get out of my depth here.

Regards

Keith

Reply to
jontom_1uk

If you have an 80mA earth leakage trip I would guess you have a TT system (earth spike in the back garden). As a safety precation (and according to the wiring regs), you should really, also have a 30mA RCD in the workshop, or on the supply to the workshop. Whilst the 80mA trip should allow any type of MCB, 50mA of current can be fatal, hence the requirment for a 30mA RCD.

Reply to
John Woodison

As long as the RCD rating is whatever is required (30mA or 50mA) then there should be no problems with any type of MCB.

I don't understand John's comment about the earth loop for the C and D rated MCB's as the total trip current is the same, it is just the speed of tripping which is slower on the C and D rating units, usually because they have a thermal content as well as Magnetic.

If the cable sizes are too small for the current, then I could see a possible fault condition problem, but otherwise and given suitable cable sizes there should be no problems.

Hager, Legrand and MK all do compatible units, we use Hager most of the time.

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

formatting link

Reply to
Peter A Forbes

Another complication may be that Inverter/VFDs don't make good bedfellows with sensitive RCDs, so the equipment you use might conspire against you having an ideally protected system. I think the only real way to find out is to try it, & if your inverter trips your RCD on startup then think again.

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

Reply to
Tim Leech

Reply to
John Woodison

I've had that problem. When I originally installed a VFD to operate a car lift (a couple years before I discovered rotary phase converters), I wired it through a double contact switch, and each time on initial power-up it would trip the RCD. So thinking it was maybe a faulty switch, I wired it direct into the MCB and it powered up fine. A new larger double contact switch was bought and installed, thinking that would solve the problem, but it started tripping the RCD again. At this point the penny dropped, that the VFD was only tripping the RCD when both live and neutral were switched at the same time, so the negative wires were joined directly at the switch, and it's been fine since.

Reply to
moray

I can confirm your problem with the RCD tripping out as well: I recently acquired a milling m/c off a friend, I installed it next to my Lathe, switch it on and plunged the workshop into darkness! I have tried all sorts of things, even replaced the mechanical on/off switch with a Siemens motor contactor, done an earth leakage test, but it would still trip the RCD on start up. Having read what you have done, and connected the neutral wires together and only switching the phase has completly solved the problem! Thanks for your help: I have been banging my head against the wall for some time because of that little device in the consumer unit!!!

Reply to
Jasengine

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.