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:
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
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.
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