Fan isolators

[JohnW2]

Am I missing something in the regs (or something else), or is this an example of a commercially-led wiring practice? ....

Since extractor fans with timers (and some other devices - thermostats, PIRs etc.) have both a 'permanent L' and 'switched L' feed, it is clearly desirable to be able to isolate both of these with a single 'isolator', to allow safe working on the device without having to remember two separate necessary 'isolations'.

At some point in the last couple of decades or so, most accessory manufacturers have added 3-pole 'fan isolators' to their ranges, and these are now used extensively.

However, this begs the question as to 'why 3-pole'? As discussed in another thread, it is possible to read the regs as requiring isolation of all live conductors (including neutral) in TT installations for all 'items' - but most people don't universally do that and, in any event, most installations are not TT.

So, is there some good reason (other than the fact that manufacturers started making 3-pole 'fan isolators') why we are isolating N as well as L for extractor fans, but not for other things? The only explanations I can think nof are:

• 1. There is obviously a need to 'do something' with the third conductor, and switching it is obvioulsy one option. However, it would often be possible to run a single cable through the isolator with the insulation on this third conductor intact. In any event, since that is not always going to be possible, convenient or desirable, the manufacturers could have made a 2-pole fan isolator with an extra terminal for terminating the neutral conductors.
  
  2...Most (maybe all) DP switches/isolators have 'L' and 'N' markings on their terminals. I don't really see that as much of an issue with proper cable identification (insulation/sleeving colour) but, in any event, they obviously could have manufactured dedicated 2-pole 'fan isolators', which not only had provision for termination of neutral conductors but also had its isolator terminals labelled L1/L2 or somesuch, rather than L/N.

So, was the inclusion of neutral switching just a unilateral decision on the part of manufacturers, or was it done to satisfy a regulatory, or other, need?

I have to say that, quite apart from the fact that I have a TT installation, I'm a great fan of isolating neutrals (not the least because it prevents all those RCD trips when one touches a neutral conductor against an earthed back box!), but that's basically a personal thing.

Kind Regards, John

 

[electronicsuk]

So, is there some good reason (other than the fact that manufacturers started making 3-pole 'fan isolators') why we are isolating N as well as L for extractor fans, but not for other things? The only explanations I can think nof are:

Aren't we? What about 20 and 45A DP isolators? 13A SFCUs?

 

[Taylortwocities]

Now RCDs and RCBOs spreading over every circuit like ivy, the more oportunities there are to isolate lives and neutrals the better!

 

[JohnW2]

So, is there some good reason (other than the fact that manufacturers started making 3-pole 'fan isolators') why we are isolating N as well as L for extractor fans, but not for other things? The only explanations I can think nof are:

Aren't we? What about 20 and 45A DP isolators? 13A SFCUs?

I didn't say all other things. Sure, there are a good few situations in which we are using DP isolation, but there are a good few situations in which we rarely do. In particular, and in context, in lighting circuits (which is what fans with timers will most commonly be connected to), it must be pretty rare to see DP switching/isolation of anything (apart from the fans).

Don't get me wrong. A I said, I'm a great believer in DP switching and isolation - but I was interested to discover whether I was missing something which specifically made it desirable, necessary or required that fans should have neutral isolation - or whether this was merely a decision on the part of the manufacturers.

Kind Regards, John.

 

[JohnW2]

Now RCDs and RCBOs spreading over every circuit like ivy, the more oportunities there are to isolate lives and neutrals the better!

I couldn't agree more - hence there is an awful lot of DP (and, where appropraite, 3-pole) switching/isolation in my house, including a fair bit of use of 'fan isolators' for purposes unrelated to fans!

However, as I've just written to electronicsuk, the existence of these 3-pole fan isolators (and non-existance, as far as I am aware, of dedicated DP ones) made me wonder whether I was missing some regulation, or practical/engineering reason, making this a specific issue in relation to fans. I presume not.

Kind Regards, John

 

[electronicsuk]

I didn't say all other things. Sure, there are a good few situations in which we are using DP isolation, but there are a good few situations in which we rarely do. In particular, and in context, in lighting circuits (which is what fans with timers will most commonly be connected to), it must be pretty rare to see DP switching/isolation of anything (apart from the fans).

Surely that's because light switches are there to provide functional switching, rather than for the purposes of isolation/servicing?

 

[Paul_C]

There has long been inconsistency in the I.E.E. Regs. regarding this, and you can start right at the supply intake and main switch. Why is it acceptable to use an isolating link on the neutral for a 3-phase 4-wire or 1-phase 3-wire supply but a 2-wire supply with one side earthed requires a D.P. switch?

 

[JohnW2]

I didn't say all other things. Sure, there are a good few situations in which we are using DP isolation, but there are a good few situations in which we rarely do. In particular, and in context, in lighting circuits (which is what fans with timers will most commonly be connected to), it must be pretty rare to see DP switching/isolation of anything (apart from the fans).

Surely that's because light switches are there to provide functional switching, rather than for the purposes of isolation/servicing?

I suppose that was really my point - that light fittings etc. only have functional switching, but fans seem to much more commonly 'enjoy' isolators. I guess that fans may require more frequent maintenance (and possibly 'tinkering') as well as fans, and I realise that (as, for example, strongly argued by BAS in an April 2009 thread here) there is an argument for not bothering to have isolators at all for fans.

Whatever, I don't think this is really relevant to my question - which, assuming that one does choose to have the facility to isolate a fan (or, indeed, a lighting fitting if one so chose), is whether there is a fan-specific reason why it is desirable to include neutral in the isolation - and I continue to presume that the answer is 'no' (even though I personally always would, for reasons I've described).

Kind Regards, John.

 

[flameport]

or whether this was merely a decision on the part of the manufacturers.

That is the most likely situation. Given that there will be 3 wires, 3 sets of terminals would be an expected design.
It is likely that the cost of producing a 2 pole switch with a separate terminal is equal to (or even higher than) a switch with 3 poles.

While it is possible to use a 2 pole switch and a separate neutral connector, this would just be using existing parts. Manufacturers can charge more for a 3 pole switch with a fan symbol on it, thereby increasing their profits.

Single pole switching for lights is the lowest cost and simplest option, same as with socket outlets that have single pole switches. DP switching increases cost and in most situations provides no benefit. In some instances it can cause problems - http://www.beama.org.uk/en/publications/guidance-documents.cfm/rcd-handbook section 9.3

 

[EFLImpudence]

Isn't it just for the often mentioned and unusually considerate reason that 'we' can work on the fan with the light on should there be no window.

Also, if the fan does go wrong, probably on a Friday evening, the occupants can totally isolate it and carry on as normal - if only they knew.

 

[JohnW2]

or whether this was merely a decision on the part of the manufacturers.

That is the most likely situation. Given that there will be 3 wires, 3 sets of terminals would be an expected design.
It is likely that the cost of producing a 2 pole switch with a separate terminal is equal to (or even higher than) a switch with 3 poles.
While it is possible to use a 2 pole switch and a separate neutral connector, this would just be using existing parts. Manufacturers can charge more for a 3 pole switch with a fan symbol on it, thereby increasing their profits.

Thanks - so more-or-less as I suspected.

Single pole switching for lights is the lowest cost and simplest option, same as with socket outlets that have single pole switches. DP switching increases cost and in most situations provides no benefit.

Indeed. To change to DP switching for lighting circuits, in particular, would require some major changes in wiring practices and a whole new set of accessories to be designed and produced (mind you, as above, maybe that should cause manufacturers to favour such a change ) As for benefits, the only real one I could think of would be avoidance of the 'nuisance' I've already mentioned - RCD trips due to neutral and earth touching during work on a circuit isolated only by a SP MCB! There is, of course, the ongoing uncertainty (at lest in my mind) as to what the regs and OSG between them mean about the requirement for DP isolation of 'every item' in an TT installation - although I'm sure not many people would take much notice, even if that reg were interpreted as indicating such a requirement!

In some instances it can cause problems - http://www.beama.org.uk/en/publications/guidance-documents.cfm/rcd-handbook section 9.3

Yes, I've heard of that problem, although have never personally knowingly experienced it or known of anyone who has. I presume that the part of the publication to you refer is:

Double-pole switching within the fixed wiring is known to produce a strange phenomenon whereby switching OFF a double-pole switch supplied through an RCD can cause the RCD to trip. ......The phenomenon is explained by the fact that while capacitance between neutral and earth will exist in all installations, the earth leakage through this capacitance will be negligible due to the low (almost zero) potential between neutral and earth. When the neutral pole of a double-pole switch is opened, the voltage across this capacitance will suddenly increase, with a subsequent increase in neutral-to-earth capacitive earth leakage currents.

At first sight, I would have thought that such a mechanism would only be likely to cause an RCD to trip if the neutral switching in question was a long way from load (a relatively unusual situation). If the switching were local to the load, the amount of neutral-earth capacitance left on the load side of the switch would be extremely small and, I would have thought, hence very unlikley to result in enough current to trip an RCD - but maybe I'm wrong, or am missing something?

Kind Regards, John.

 

[JohnW2]

Isn't it just for the often mentioned and unusually considerate reason that 'we' can work on the fan with the light on should there be no window.

Indeed, and I would subscribe to that - but at least in terms of safety, that's a reason for having isolation of both L conductors, not for also isolating neutral.

Also, if the fan does go wrong, probably on a Friday evening, the occupants can totally isolate it and carry on as normal - if only they knew.

Except for the (pretty unlikely) problem being of an N-E fault arising within the fan on that Friday night, the above comment again applies. Mind you, I suppose that you could have said the same about a fault in a light fitting arising on a Friday night - unless it was due to a N-E fault, it could be temporarily solved with the (SP) functional switch.

Kind Regards, John.

 

[RF Lighting]

IT's quite simple, and you've already been told the answer.

Isolation - switch all live conductors.

Functional switching - switch line conductor(s)

 

[JohnW2]

IT's quite simple, and you've already been told the answer.
Isolation - switch all live conductors.
Functional switching - switch line conductor(s)

How do you square that view with 537.2.1.1 which we keep discussing?

537.2.1.1 appears to say that for TN-S or TN-C-S, isolation of the neutral conductor is not required - i.e. in practice, that isolation of the neutral conductor is only required with a TT system.

Edit: I should probably have added that my understanding of the main difference between isolation and functional switching is that an isolator is not required to be able to break the circuit under full load (although it often/usually probably does have such a capability, in practice), whereas a functional switch obviously must have that capability.

Kind Regards, John