MCB discrimination and size?

I've just run 6mm SWA from the consumer unit in the house to a workshop some 25m away. The workshop has 4 circuits - 2 lighting and 2 smallish ringmains, run in 2.5mm. The SWA from the house will feed a 4 way CU in the workshop via a separate RCD - my questions are:
1. What size MCB needs to be fitted to the 6mm SWA in the house CU - 32a or 40a? 2. What size MCB needs to be fitted in the workshop's CU - 32a for the ring and 6a for the lights? 3. How do I avoid the lack of discrimination caused by running 2 MCB's in line - ideas include i, using type C's in the workshop or ii, discrimination will be provided by having 40a MCB in the house and 32a in the workshop or iii fitting traditional fuses in the workshop, rather than MCB's
Am I on the right wavelength? Any guidance/advice/tips or suggestions wll be lapped up... thanks everyone for a great NG cheers, Patrick
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What size is the armoured cable ? An RCD only gives protection if an earth fault occurs, so make sure you have a good earth point at the workshop and don't rely on the armoured cable taking the whole brunt. The MCB you use in the house is to protect the supply cable to the workshop from melting and not just to give you enough load for the workshop.

Depends on what you intend to use in the workshop.

This would be the norm' for a standard installation, but it depends on what your supply cable can handle in terms of maximum load at that length.

If you can, try taking the workshop supply from a separate single way metal clad consumer unit in the house. This doesn't have to have any RCD if you have one installed in the workshop with a good earthing point and can be used with a hard wired fuse to reduce any erroneous tripping. It gives more protection from faults in the workshop back to the house.
Get some 16 mm Double Insulted Cable and Two Terminal Blocks from your wholesaler. Take the fuse out from the mains supply unit (the bit before the meter) and then disconnect the supply cables from the meter at the mains switch of the existing consumer unit and put them in the big terminal blocks. Cut the double insulated to the lengths you need to go to both the existing fuse box and your new single way unit. Connect them all up to the terminal blocks and you then have two separate supplies, one for the house and one for the workshop, but all running through the same meter. Believe me, it stops any conflicts.
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On 27 Jul 2003 12:38:25 -0700, Patrick Olner wrote:

You also need and RCD at this point.

That is what I would fit.

Type C are the "slower" ones so that really needs to be at the house end so the faster type B has a (slim) chance to operate before the type C.

TBH there is not that much difference between a 40 and 32A trip. Remember they are designed to carry that current indefinitely and will take a significant amount of time to trip at double their rating.

Fuses are even slower. I was sort of thinking that with an RCD in the house you don't need overload protection at that point but I guess that it still is conceivable that a LN fault could occur causing overload of the SWA. So fuse it at the house end and have MCBs in the workshop CU. It may even be useful to fit another RCD in the workshop CU feeding the rings. Indeed any socket that may have an appliance plugged in that is used outside must have RCD protection. Be aware of the problems of cascading RCDs...
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     snipped-for-privacy@olner.fsnet.co.uk (Patrick Olner) writes:

You can use 40A with 6mm cable. As you also have an RCD at the house end, you can ignore earth fault loop impedance. At 40A design load, your 25m of 6mm cable has used up 80% of the permitted voltage drop -- this probably doesn't matter as the length of circuits in the workshop won't be large, and your design load probably isn't really anywhere near 40A (nearly 10kW).

Yes.
You can't avoid the lack of discrimination caused by running 2 MCB's in line, except by not doing so. You could use a 40A BS1361 cartridge fuse at the house end, or you could ignore the problem -- how often have you had a ring circuit short out live to neutral (which is what will cause this problem) verses having an earth fault in an appliance (which will have much the same effect, tripping the RCD in the house)? I would suggest to you that the RCD represents a much more likely reason for losing all the power to the workshop than a live-neutral short circuit on a ring circuit, so I wouldn't worry about lack of discrimination.
I would ask if you really need two ring circuits -- that's more than 7kW load, and a damn hot workshop;-) I would have thought one would do or if you want more than one circuit, what about two 20A radial ones? Still won't help discrimination issue much though.
You might also want to make the workshop a TT system with its own earth rod, rather than exporting the house earth. If you do this, you still earth the SWA armour at the house end, but you make sure it isn't connected to anything at the workshop end, indeed make sure it's insulated and not exposed. You have to have an RCD if you do this.
If the workshop contains machinary which might cause a danger if all the power is suddenly cut and you lost lighting whilst machinary is still in motion under its momentum, I would look at schemes which didn't compromise the lighting in the event of an earth fault. Please say if this is the case, and I (or someone else) will suggest some suitable alternative schemes.
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- thanks very much to one and all - yet again this fantastic resource and the absolute stars within it have saved the day!
- Andrew, no problem with machines and lighting etc - the workshop will be half darkroom and half craft area, so it does need to supply some pretty hefty items - print processor, water heater, etc, as well as the more usual run o'the mill stuff.
With regard to workshop having it's own earth rod - is this strongly advised? I also should have mentioned that that the fuse supplying the workshop is on the RCD side of a split way consumer unit ... so do I really need RCD protection with the workshop too? Should I decide to do so .. would the 40a cartridge fuse in the house consumer unit be OK being fitted on the RCD side of the house CU?
cheers patrick
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     snipped-for-privacy@olner.fsnet.co.uk (Patrick Olner) writes:

OK, this might be more than I imagined. Can you estimate the power (or better, the VA rating if you know) of all the devices, and estimate the total maximum you might operate at once. For the water heater, please state its type, and in the case of a stored water heater, its water capacity.

It's theoretically safer, but I don't know if many professional electricians would bother. If the workshop building includes metal structural members which can be touched from outside, I personally would certainly do this (metal greenhouse is a good example of such). What type of earthing system is your house supply? (See http://www.diyfaq.org.uk/electrical.html#system for descriptions.)

No. However, it's not a good idea to share indoor and external circuits on the same RCD as it's too easy to trip them (mallicious or water ingress) and lose power in the house. It is however not against the regs to do this. You should have a double pole isolating switch for the external circuit -- a dedicated RCD operating on both poles can double up to perform this function.

I should also have asked what the value of your main fuse is. If it's only 60A for example, that's not enough to provide safe discrimination against a 40A fuse. If it's 100A, then that's OK. Also, you need to do a load calculation to ensure you aren't going to be overloading your house supply.
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snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote in message (Patrick Olner) writes:

Andrew .. thank you so much for your guidance it truly is invaluable .. - yes we only have 60A main fuse, how much of a problem do you think this will and how the hell would I go about changing it? - the main items that will run at once are 1. Water heater, not sure of it's type - a white box about 16"x16"x12", 2kw, it has a 10 written on a small silver label with it's voltage, wattage, etc which may or may not be 10 litres, sorry to be so vague; 2. Print processor, 2.7kw; 3. Ventilation, 70W 4. Enlarger, 250w 5. Film dryer, 500w 6. Depending on the South Wales winter - possibly a heater too, 2-3kw 6. Maybe one or two electric tools such as circular saw, sander, planer etc 7. Outside lighting, 3x150w halogen PIR security lights oh .. and the obligatory kettle!
Also Andrew, how do I do a load calculation? You are the almighty and I am truly humbled - best wishes, Patrick
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