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

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?

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?