Underfloor Bus Bar Systems

Hmm, I suppose it could be either way depending on how you define it.

A socket is usually a point (point of utilisation) so a normal socket gets a plug(top) in it and is wired via a flex to something you wish to use.

However a plug and socket extension just moves the actual point further down the road so do you now calssify the initial socket as a point or just the subsequent socket on the end of the extension lead?


Does it matter? obviously it probably does or you would not have asked

Humm, interesting question.


In the case where the underfloor busbar system feed floor boxes that containing (usually) BS 1363 sockets, I think I would have automatically have thought as the sockets in the floorboxes as being the end of the "fixed wiring" and so "sockets" in the usual sense, so the tap off points on the busbar system as just some proprietary connector system. As some types of floor boxes contain local overcurrent - or more to the point RCD - protection (e.g. to provide the additional protection that most sockets require these days) it would undermine that approach if the tap-off points were treated in the same way as ordinary sockets and so had to be provided with 30mA RCD protection for instance (especially in situations where loads were large, discrimination important or high protective conductor currents present).


I do see your point that the exact wording of BS 7671's definitions don't obviously support that position though. A lot of these definitions go back many many years and it's not uncommon to find that they've been overtaken by changes in technology. I am tempted to think that they've had this problem before - someone's pointed out that lighting track systems would be caught in the ordinary definitions and so they've had to add an exception.to take them out again - so maybe the same reasoning should be applied to underfloor systems. There's probably a similar issue with prefabricated wiring systems used in some modular buildings - as they have a kind of "plug & socket" connections between wall panels or other large units of the building that are bolted together on site.


Remember that most of the regulations' wording is made up by committees and then later amended by more committees - which isn't always the best way to produce clear and succinct prose. So sometimes you need to think beyond the actual wording to understand what was really intended - and then apply that reasoning to your particular application (which is almost certainly at least slightly different to anything those committees had in mind all those years ago). Often precise adherence to the letter of the regulations isn't required - just as long as you produce a solution that's no less safe.


  - Andy.

There is also the Electrak system, which is basically a 63A busbar system, allowing connection of floor-boxes using a 32A plug and socket arrangement. 


Regards,


Alan.

An understanding of the difference between the terms socket-outlet, connector, and tap-off is essential to the application of BS 7671.

For example, if a 63 A busbar trunking system has 32 A tap-off points, does it need RCD protection in accordance with Regulation 411.3.3?

Answer:



The tap-off may be considered to be part of the busbar system or other system components - see definition of busbar trunking system on page 25 of BS 7671:2018 - and this means they are neither connectors (page 26) nor socket-outlets (page 37).

Nice try Graham!


I certainly agree with the sentiments.


I'm still not entirely convinced that BS 7671's working is clear though - especially when the likes of MK describe the tap off points in their system as "sockets" and the tap offs typically supply flex (if contained within flexible conduit). http://www.mkelectric.com/en-gb/Products/mkpowerdistributionsystem/Documents/Interact%20UnderFloor%20Power.pdf

Seemingly some of the "tap offs" can contain 13A fuses - so could in theory be used to supply (portable) appliances directly - making the fundamental differences between them an say a 13A plug seem even less clear (maybe the arrangement should be considered more like a FCU serving an appliance rather than a plug & socket).


 - Andy.

Sorry Andy,


The ability of an electrical accessory to supply an appliance (or flex) doesn't make it a plug either.


An FCU or SFCU can do that.


A tap-off unit can be more than one thing ... just like an accessory on a circuit such as a socket-outlet or FCU.


But that doesn't make the tap-off itself a socket-outlet ... it might contain one, though.

What's the actual issue here? (other than semantics) RCD protection presumably?


I would argue that the connection points on a busbar are not socket-outlets as we would normally refer to them as they are not designed to accept a BS 1363 or BS EN 60309 plug. They may well still be described as a socket but in my view it is the much broader definition which also encompasses the attachment you put on a wrench to make it a 'socket wrench' or half a shoulder joint.


P

The ability of an electrical accessory to supply an appliance (or flex) doesn't make it a plug either.

Indeed - I was just making the point that the approach of thinking "the difference being that the plug is intended for both electrical connection and mechanical retention of a flexible cable, whereas a tap-off unit need not directly supply a flexible cable" doesn't really help given BS 7671's rather outdated definitions, as many types of tap-offs are supplied pre-wired with flex. The same argument applies the other way around too - if I plug a "wall wart" kind of appliance into a BS 1363 outlet, does the outlet cease to be a socket? (of course not I would have said, since it's still capable of accepting a plug that supplies a flex... but that then brings up back to the OP's point of being able to distinguish them using BS 7671's definitions).


I'm not arguing against your conclusions - quite the reverse! But I do still sympathise with the OPs situation and think that BS 7671's definitions could do with a bit of an update (or complete overhaul). As things stand we're still not even quite sure what's meant by "fixed wiring" - is a dangling flex permanently connected at one end to the rigid building wiring part of the fixed wiring or not?


  - Andy.

So, if I have a kitchen appliance (say a fridge) plugged into a socket in the kitchen the socket must comply with the socket requirements, but if it is hard-wired it does not need to comply with the socket requirements?

I think a pragmatic approach is appropriate here. It is important to consider the intent of the regulations and ensure that the installation is no less safe than it should be.

Having been involved in the development of Standards for years I can confirm the wording "made up by committees and later amended by more committees", but it is even worse than that. Standards undergo a review process and it can happen that a change in technology may make a change in the requirements desirable but it is impossible to get the change past the review process as the reviewers don't understand the background and see the change as a watering down of the requirement and hence of safety (I know of examples, though not in BS7671). It is also common when new requirements are introduced that the full implications on other sections are not initially recognised.

Alasdair