Not sure I fully agree with Wiring Matters > Years > 2025 > 107 - September 2025

I. have come across many heat damaged spurs and the occasional socket over the years supplying immersion heaters so agree with that part. I personally always fit. a 20 amp DP switch. I don't see the need to have to lock it off as you will be working right next to it. If it needs locking off for other reasons do it at the consumer unit as it should be a dedicated circuit although I have seen an odd boiler or heating control tagged onto the same circuit.  Rotary switches are clunky and could be prone to being inadvertently damaged or knocked off the wall in an airing cupboard.

Gary

if you can lock it off at the CU and are feeling cheap you can probably drop the switch rated for isolation part altogether.

Its main use is not for us to work on it once in a blue moon, but for the householder to turn the water heating off when not needed - while on hols or similar . 

Fused spurs only make sense when the supply (dedicated or shared with others), is of a higher rating - e.g a 32A ring. 
To me another obvious fix if you must have one, may be  to put the fused spur unit outside the cupboard. much as one used to put conventional light switches outside bathrooms. being visible  also reduces the risk of it being left in the wrong state for a long time.
Mike

Personally I have found a fair number if burnt out fused switched spurs in immersion circuits, I generally also install 20A double pole switch with Neon. The diagram showing markings on a switch approved for isolation is interesting. I can only see one switch from hager that has this sort of markings. Not having access to BS EN 60669-2-4:2005 I wonder if the markings are actually a requirement? Although as others have said if you actually want to isolate an immersion heater it would probably be done at the the consumer unit.

On the suggestion of a rotary isolator I think it just looks out of place inside a home and I would only use one if essential for safety requirements.

Not like you to post such gibberish Mike presuming this post was meant as a reply to what I wrote.

Gary

Alan B said:

Personally I have found a fair number if burnt out fused switched spurs in immersion circuits, I generally also install 20A double pole switch with Neon. The diagram showing markings on a switch approved for isolation is interesting. I can only see one switch from hager that has this sort of markings. Not having access to BS EN 60669-2-4:2005 I wonder if the markings are actually a requirement?

I can answer this ...

Not all switches to BS EN 60669 series are isolating switches ... as we can see from Table 537.4 of BS 7671, from BS EN 60669 series, only those to BS EN 60669-2-4 are listed as suitable for isolation:

In terms of marking for isolating switches to BS EN 60669-2-4, the marking for isolation should be visible once the product is installed:

'OFF' and 'ON' are suitable alternatives for Ι and Ο.

If the switch conforms to another part of BS EN 60669 in addition to BS EN 60669-2-4, it will be suitable for isolation, BUT it should be marked as above.

Sergio Fernandez said:

Personally I think Rotary Isolator would be far better than 20 amp Double-pole switch

These devices would also likely be suitable if rated AC21A (i.e. suitable for switching resistive load on-load) and certainly in non-domestic premises might be preferable.

mapj1 said:

Its main use is not for us to work on it once in a blue moon, but for the householder to turn the water heating off when not needed - while on hols or similar . 

Or to turn it on once in a blue moon when the boiler has conked out.

Respectfully, I disagree with the use of a rotary isolator in this situation. Although not technically incorrect (so far as I understand) and cost differences are negligible, it feels wrong to be installed in this situation.

Can you expand on why you would be inclined to use the rotary?

In the original article it states the FCU is the wrong item for the job due to its ratings.  This I full agree with but it suggests changing it from a FCU to a dual pole switch which I also agree with as most FCU are single pole.  However the dual pole switch shown in the example lacks a lock off/isolation mechanism.  While this is fine for an Electrician what would a Plumber or Heating engineer make of it?  Are they competent to Lock Off and isolate at the consumer unit?  Should a plumber or heating engineer even touch the customers consumer unit?

Electricity at Work Regulations 1989:
These regulations mandate that precautions must be taken to prevent danger from electricity, and for electrical maintenance, the normal method is to switch off and isolate the power supply

In a reply to my post GK mentions

If the switch conforms to another part of BS EN 60669 in addition to BS EN 60669-2-4, it will be suitable for isolation, BUT it should be marked as above.

Which in hindsight could/should have been included in the original article.  The suggested replacement SHOULD BE NO LESS SAFE THAN THE ORIGINAL.  The original had 3 functions, act as a switch, contained a fuse for safety and was a point of isolation and lock off/out.  I can understand losing one of the 3 functions/features but removing 2 is unreasonable.  

Furthermore it would probably be beneficial if this is added to BS7671 and also the OnSite guide.  Thus helping future electricians who MAY encounter this in NEW installs or in installs that are 30 plus years OLD.  The UK nominal mains voltage was officially changed from 240V to 230V in 1994 to harmonize with European standards, though in practice the supply voltage often remains closer to the old 240V standard.


The purpose of these discussions/debates is to make the industry safer for the benefit of everyone.  In my eyes healthy debate amongst engineers is a way of making things better.  This is akin to having a peer review.

Sergio Fernandez said:

While this is fine for an Electrician what would a Plumber or Heating engineer make of it?

I detest excessive division of labour (carpenters who will not scrape off old paint and painters who will not insert a screw come to mind during my conservatory renovations). However, I am not allowed to, e.g., change the thermocouple on my boiler (surely an electrical part!) 'cos I am not a registered gas fitter. (Screwfix refuse to sell them to me.)

I have no objections to a plumber dealing with the electrical side of an immersion heater, but he (or she) must then acquire the kit and learn how to use it.

Incidentally, how many of you would fill in a MEIWC after changing an immersion heater? And how many would do so when changing the switch?