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SparkyChick

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Hi all,

I've been looking at a number of threads about supply cables (from meter/isolator to consumer unit) that are longer than 3m (as I have a few customers where this is the case) and that this requires protection at the origin of the supply in the form of a 'switch fuse'.

Obviously switch fuse is self explanatory and they are readily available from wholesalers, but then you have the RCD requirement if the cable is buried in the wall without mechanical protection etc.

So my question is this... does the use of the term 'switch fuse' preclude the use of other devices such as an RCD/MCB combination for the purpose of protecting the supply cable?

I would say the requirements are 'an ability to isolate' and 'an ability to protect against overcurrent' and therefore an RCD/MCB combination is ideal because the RCD will provide isolation (with the added benefit of earth fault protection) and the MCB will provide the required overcurrent protection. If it has to be a switch fuse, what is the reasoning behind that?
 
Hi SC - like you say, the cable needs protection and we are not permitted to rely upon the DNO fuse beyond 3m as I understand it. I'll give my thoughts, but I'm very happy to learn more :)
I think a fuse has been used due to cost, availability and reliability for that rating (say 60A or 80A). If there's an appropriate mcb available nowadays then why not? Cables in walls should be protected but the use of a 30mA RCD for this can lead to discrimination issues with those in CU. So I'd go for mechanical protection or SWA say. And the best for last ... the switch fuse unit ... metal box or no?
 
This is a can of worms created by the I'll thought out regs.

If the existing tails are buried in the wall you should give then 30 ma RCD protection, but this means a single up front rcd, which is frowned upon!'

In existing set ups it's your call
 
Part of this has been bought about by the hotel/restaurant I posted about the other day. My initial thoughts on the main supply cable were 'completely inadequate' (it's 10mm/sq or 16mm/sq T+E - plan on measuring it) but it's been there for many years and there are no signs of heat or physical damage in the parts I can see, but then they are in the open. I clamped the supply when I was there last and the whole place was pulling less than 5A, but it's very quiet. I wouldn't like to do the same on a busy day.

Based on the requirement in the regs to provide 30mA RCD protection to buried cables, I immediately thought RCD/MCB combination rather than switch fuse, but as Wilko points out, this raises the issue of discrimination so I was kind of secretly hoping that 'switch fuse' means 'SWITCH and FUSE' with no wriggle room then it becomes a no brainer and the discrimination issue goes away. But as you've suggested Murdoch, having done quite a bit of reading it's just not clear.

Unfortunately, because I can't establish where the cable runs, I am heading towards some cast iron knickers for my butt and the 30mA RCD/MCB combination route when I quote for the work. But ideally I would run some nice fat SWA on a steel cable tray around the outside of the building and then use a traditional switch fuse, option 2 for that quote item I think :)
 
as the cable has been installed for a number of years, there's no requirement in BS7671 to fit RCD protection.
 
as the cable has been installed for a number of years, there's no requirement in BS7671 to fit RCD protection.

But as soon as I look to improve it I'm obliged to comply with BS7671 which would require me to use RCD protection. Or are you saying I can improve it by installing a switch fuse at the origin rather than the distribution end and skip the RCD protection because it's been in situ for many years?

If it's the later, surely thats open to abuse because taking that stance to it's logical conclusion... "it don't need RCD protection when I upgrade your consumer unit mate because the wiring's been here for years" ;) or am I missing something?
 
What do your schemes tech support say?

When I spoke to NAPIT about it to confirm my suspicions, they just said install a switch fuse at the origin and cover it with a minor works certificate, which I can do.

But I think quite a bit about stuff and I got thinking about the switch fuse and what's acceptable for that purpose because based on the intended requirements of isolation and overcurrent protection there are lots of options and having read a bunch of stuff, I just could not find any clear guidance on what's acceptable to use where a 'switch fuse' is called for, hence my original question.

The hotel is just an example because it fitted nicely and encompassed the issue about discrimination in RCDs.
 
The use of a fuse at the origin rather than an MCB may assist with overcurrent protection co-ordination, depending on the size of company fuse and the largest MCBs in the CU. I wouldn't personally go for the single overall RCD, so if I felt it needed upgrading I would be looking at a cable not requiring RCD protection. MICC gives two possible advantages - no need for protection and easy concealment indoors due to its small diameter and ease of dressing into position.
 
I would personally not give it rcd protection. I would carry out a risk assessment with the outcome being the risk of someone suffering injury by drilling through the cable is considerably lower rhan the risk to persons in the event of lighting loss due to nuisance tripping. In my eyes an acceptable risk.
 
my take on this is that although anything you install needs to comply with latest regs., existing components of the installation only need to be safe for continued use. otherwise, we'd be rewiring the whole installation for the sake of fitting a new socket.
 
As a sensible ish compromise you could protect the sub-main with a 100ma 'S' type RCD but this would be a non compliance....
 

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