Cable Type (20 chars)

[hightower]

So according to one book I have it states that achieving discrimination can be a very complicated matter, and when dealing with protective devices of the same type in series it's usually acceptable to double the rating at each step.

With that in mind, If I install a 40A Type B at the supply, and the highest rating at the remote board is going to be 20A Type B, can I safely assume I've taken appropriate steps to ensure discrimination?

 

[davesparks]

It's no different to a multitude of other installs I've come across where a distribution circuit is protected by a 32A breaker and then feeds a 20A socket circuit and a 6A lighting circuit.

EDIT: Sorry, thought this statement needed clarification as didn't mean it to sound like "X done it so why can't I?"

Discrimination between MCBs is going to be hard to achieve. So what's the way out, fit a larger cable to be able to fit a larger MCB? Fit a different type of protection?

Exactly, someone else failing to comply is not an excuse for non compliance.

The answer is feeding a submain from further back in the installation where there is a suitable switchboard/panelboard to feed a distribution circuit from.

The arguments against installing a final circuit don’t make sense to me.

 

[hightower]

Okay, so it will be Eaton equipment, can somebody confirm if I am reading their data correctly:

http://www.eaton.uk.com/ecm/idcplg?...aveAs=0&Rendition=Primary&dDocName=PCT_356894

Second page, I think I've correct in looking at first column which only shows 63A + MCBs upstream. With a 63A MCB I will achieve discrimination with any downstream MCB up to 40A ONLY if the Pfc at the fault is 630A or less. Am I reading the right data, and reading it correctly?

This is all very interesting, I must say - not the type of stuff that gets covered at college in any great detail.

 

[hightower]

The arguments against installing a final circuit don’t make sense to me.

Yes, I'm starting to think that myself now...

 

[Adam W]

There's a very famous open air history museum in my area - last time I visited I noticed the outside lights, the ones running all round the pathways fixed to fencing etc, were wired in SY. I think it looked like SWA in to an enclosure and then SY from the enclosure to the fitting (which was quite long in some cases).

This has been one of the problems - it's been viewed as 'outdoor cable' where in reality it isn't suitable for use outdoors.

 

[Risteard]

So according to one book I have it states that achieving discrimination can be a very complicated matter, and when dealing with protective devices of the same type in series it's usually acceptable to double the rating at each step.

With that in mind, If I install a 40A Type B at the supply, and the highest rating at the remote board is going to be 20A Type B, can I safely assume I've taken appropriate steps to ensure discrimination?

No.

 

[telectrix]

my thoughts would be to use a fuse upstream. discrimination could be achieved that way.

 

[hightower]

my thoughts would be to use a fuse upstream. discrimination could be achieved that way.

The Eaton data does allow for that in more cases (if I’m reading it correctly), but then that’s going to require a new box at supply with the fuse isn’t it. The more I consider this, the more I’m thinking I’ve overthought it - a couple of final circuits will suffice I think.

 

[westward10]

It is being overthought. It is a couple of insignificant circuits, yes you may not achieve discrimination by using mcbs but what is the worst that will happen.

 

[hightower]

I appreciate that, but discrimination is explicit in the regs

 

[Richard Burns]

Okay, so it will be Eaton equipment, can somebody confirm if I am reading their data correctly:

http://www.eaton.uk.com/ecm/idcplg?...aveAs=0&Rendition=Primary&dDocName=PCT_356894

Second page, I think I've correct in looking at first column which only shows 63A + MCBs upstream. With a 63A MCB I will achieve discrimination with any downstream MCB up to 40A ONLY if the Pfc at the fault is 630A or less. Am I reading the right data, and reading it correctly?

This is all very interesting, I must say - not the type of stuff that gets covered at college in any great detail.

Table A does show discrimination at or below 630A for the two different type of circuit breakers.

The requirement for discrimination is to minimise nuisance and avoid danger, whereas in this case unless you have some critical kit running on the first board then the loss of power is unlikely to be a major problem, a mild inconvenience at best so there is no specific requirement to ensure discrimination at all times only for more critical applications.

 

[westward10]

I appreciate that, but discrimination is explicit in the regs

It does HT and I admire you for wanting to do the correct thing but sometimes life is too short

 

[westward10]

I have this week replaced a 36 way board in a care home. This board supplies two other insignificant boards, one in a shed the other a laundry. The laundry board is now supplied by way of an rcbo due to installation methods with the shed on an mcb. I know these are existing circuits but sometimes it really is not an issue.

 

[7029 dave]

I'd never even heard of this "NYY-J" before today. It sounds like the old "pre-wired conduit" that was around before my time.
I don't see that it has any benefit over any other existing wiring system, such as flex in conduit. It sounds to me like an answer to a question which nobody has asked.

Basically it;s High Tuff Adam, nice to work with.