Discrimination

oo7 · 8 Mar 2014

Hi guys.

I've not had to look at discrimination since college so could do with a little refresher.

The job is to provide power to a domestic shed (30m away from CU) supplying a 32a ring (possible mig welding, milling machine etc..) and a 6a lighting circuit, all protected by an RCD.
My plan is to take from a Henley block to a new 2way CU (sub 1). A 40a MCB protects power to the shed via 30m of 6mm SWA (have yet to check cable calcs, may need 10mm on a 50a MCB) to the above 2 way CU in the shed (sub 2).

So back to discrimination. I understand how to check when you have two different types of breaker (main cut out BS3036 and MCB BS60898) but what I'm unsure about is where we have two of the same breakers in series like the 40a at sub1 and the 32a at sub2. Now according to page 301 of the regs there shouldn't be a problem but I've read before about doubling the circuit breaker to "ensure" discrimination. Is this just how lazy people work out discrimination from two different types of breaker or is this something I need to be considering even with two types of the same breakers?

Cheers,
Chris

JohnW2 · 8 Mar 2014

oo7 said:
....So back to discrimination. I understand how to check when you have two different types of breaker (main cut out BS3036 and MCB BS60898) but what I'm unsure about is where we have two of the same breakers in series like the 40a at sub1 and the 32a at sub2. Now according to page 301 of the regs there shouldn't be a problem but ....

Discrimination in relation to what? As I understand it, in relation to a very modest overload or a slowly increasing overload, you might see some discrimination between a B32 and a B40, but in terms of fault conditions (i.e. very high fault currents), I doubt whether you'd get any reliable discrimination at all. It's common to hear people talking in terms of at least a 3:1 ratio in MCB ratings (e.g. a B6 vs. B20, or B20 vs B63) to get reasonable discrimination. As I think you're implying, I think that, in general, you're much more likely to get discrimination if you have a fuse upstream of an MCB.

Kind Regards, John

oo7 · 8 Mar 2014

Hi John.

That's my understanding of discrimination too.

I guess all I'm asking is my installation design ok with regards to discrimination of the MCBs or have I made a fundamental error?

Cheers,
Chris.

JohnW2 · 8 Mar 2014

oo7 said:
Hi John. That's my understanding of discrimination too. I guess all I'm asking is my installation design ok with regards to discrimination of the MCBs or have I made a fundamental error

Well, as I said, in terms of faults I don't think you'd get any useful discrimination at all with what you're currently proposing. I suppose it's partially a question of how large a cable you're prepared to use for the sub-main. If that were not a major concern, rather than have your 'sub 1' in the house (with a B40, or maybe B50), you could have a switch-fuse with a 60A or 80A fuse, but the sub-main cable would obviously then have to have an adequate CSA to be adequately protected by the fuse.

Kind Regards, John

oo7 · 8 Mar 2014

Just so I'm 100% sure we're on the same wave length, you're talking about one of these bad boys?

http://www.tlc-direct.co.uk/Products/WY108M.html?source=adwords&kw=&gclid=CMyZ65jsg70CFWvkwgod26cAWA

We're then talking 16mm for 60a fuse or 25mm for 80a. This is starting to top reach £400 just to get into the shed. Not a problem as I haven't quoted yet but I hadn't thought that powering a shed was this expensive.

JohnW2 · 8 Mar 2014

oo7 said:
Just so I'm 100% sure we're on the same wave length, you're talking about one of these bad boys?
http://www.tlc-direct.co.uk/Product...&kw=&gclid=CMyZ65jsg70CFWvkwgod26cAWA[/QUOTE]
That's certainly a possibility - although many, like that one, are enormous. The KMF800 / 800KMF is far more compact.

oo7 said:

We're then talking 16mm for 60a fuse or 25mm for 80a. This is starting to top reach £400 just to get into the shed.

Click to expand...

I don't have tables to hand bit, if you used 90°C SWA, I think you might get away with 10 mm² for 60A and 16 mm² for 80A. The length of the cable run (hence voltage drop), and whether or not there are any bonding requirements in the shed might, of course, impose requirements on the CSA of the SWA.

oo7 said:

Not a problem as I haven't quoted yet but I hadn't thought that powering a shed was this expensive.

Click to expand...

Well, it's at least partially the price one pays for fussing about discrimination. If it were my house and shed, I probably would not give discrimination a second thought, so would probably be going with your Plan A

Kind Regards, John

Taylortwocities · 8 Mar 2014

Does this help?
http://www2.schneider-electric.com/...abted206168en/abted206168en/pdfs/page_164.pdf
and the first few pages of

this

Adam_151 · 9 Mar 2014

Two MCBs will very rarely discriminate.

I've seen many times a fault on a circuit protected by a 16A breaker take out the 63A C type supplying the board.

You get better luck if you have a fuse or MCCB as the supplying device.

To prove discrimination requires getting the manufacturers data out. In the industrial and commericial world the designer will often have access to computer software pre-loaded with this data which does it all for him.

A B32 and B40 have no discrimination for sure

oo7 · 9 Mar 2014

Thanks guys.

Taylortwocities, thanks for the reading.

bernardgreen · 9 Mar 2014

oo7 said:
Not a problem as I haven't quoted yet but I hadn't thought that powering a shed was this expensive.

Well, it's at least partially the price one pays for fussing about discrimination. If it were my house and shed, I probably would not give discrimination a second thought, so would probably be going with your Plan A

It depends on what the shed is being used for. If loss of lighting would create a hazard then I would ensure that a trip of the MCB protecting power sockets in the shed would not also trip the supply to the shed.

If a risk assement suggests protection of the lighting is necessary then I would use a wire fuse in the house to protect the cable to the shed with significantly lower rated MCBs in the shed. That could mean the cable to the shed has to be larger than is necessary to supply the shed's normal requirement to ensure the cable can carry the fault current needed to blow the wire fuse in the event of a short circuit on the cable.

JohnW2 · 9 Mar 2014

bernardgreen said:
Well, it's at least partially the price one pays for fussing about discrimination. If it were my house and shed, I probably would not give discrimination a second thought, so would probably be going with your Plan A

Click to expand...

It depends on what the shed is being used for. If loss of lighting would create a hazard then I would ensure that a trip of the MCB protecting power sockets in the shed would not also trip the supply to the shed. ...

Yes, we've been over this many times before and, as you go on to say (and as oo7 said) taking steps to achieve that can add considerably to the cost of the job.

Firstly, I suspect that in very many sheds, loss of lighting would not create any more hazard than would loss of lighting anywhere else - it's really 'workshops' (places where powered tools are used) which may present an issue (and, even then, very many sheds with power tools have some natural lighting, and many are never used during hours of darkness).

However, IF, for whatever reason, one does assess that loss of lighting would represent an unacceptable hazard, then provision of 'emergency'/standby lighting is nearly always going to be the cheapest (and, as below the only satisfactory) solution, rather than 'unnecessarily' up-rating the supply simply so as to achieve OPD discrimination.

Indeed, if one assessed that sudden loss of lighting in the shed would present a hazard, then merely attempting to achieve OPD discrimination would not be adequate/safe. Such a loss could be due to failure within the lighting circuit itself. Also, we have not discussed RCD protection in this shed - but, very commonly, lighting and sockets will be protected by a common RCD in a 'mini CU' in the shed. In all those cases one needs 'emergency lighting' if loss of light is deemed to be a hazrad - and, once one has it, potentially costly measures to achieve OPD discrimination are not, IMO, necessary.

Kind Regards, John

oo7 · 9 Mar 2014

Just thought of another way to save expense and concern about diversity and that would be to (almost) do away with it altogether.

If I were to take a 32a RCD protected MCB from the house CU, run 6mm SWA to the shed, then just run the sockets in 6mm (it's a small shed, three 2g and one 1g) with a 6a SFCU for the lighting.

As for the lighting there's a street light just above the roof window which provides enough lighting to put tools down and get out of the shed safely.

JohnW2 · 9 Mar 2014

oo7 said:
Just thought of another way to save expense and concern about diversity and that would be to (almost) do away with it altogether. If I were to take a 32a RCD protected MCB from the house CU, run 6mm SWA to the shed, then just run the sockets in 6mm (it's a small shed, three 2g and one 1g) with a 6a SFCU for the lighting.

I suppose that's an option. You might struggle to wire the sockets in 6mm², but there's no reason why you couldn't go down to 4mm² once you're in the shed, even if there are reasons why the SWA has to be 6mm². I think most people would be unhappy if there was not a 'means of isolation' within the shed - but installing a suitable switch/isolator would be no problem, and that could even be the means of transitioning from 6mm² SWA to 4mm² cable.

However, that would not really make the discrimination issue go away - as Bernard has said, the main possibility that worries some people is that a fault on socket circuits could take out the lighting - and that would still happen with what you are now suggesting. However, given ....

oo7 said:
As for the lighting there's a street light just above the roof window which provides enough lighting to put tools down and get out of the shed safely.

...that really removes most of Bernard's concern - and, in any event, as I said, if protection against loss of lighting were really crucial, the only real way of achieving it is with 'emergency lighting' (i.e. with battery backup).

Kind Regards, John

oo7 · 9 Mar 2014

6mm cable required because it needs to be buried, there's no structure to support the cable run from one end of the garden to the other.

Thanks for pointing out the isolation in the shed, great way to bring in the 4mm.

I might just go for the emergency lighting anyway as I've never fitted one and this is as good a reason as any.

Thanks again guys.

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Discrimination

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