Questions on: Immersion switch replace with timer

[SimonH2]

I will get a double pole RCD to replace the single pole MCB in the consumer unit.

Noooooooooooo
An RCD and MCB do different things and are not interchangeable. An MCB is there for over-current protection, and RCD does not have any over-current protection and would not protect the circuit against overloads and/or faults - translation, could burn the house down in extreme cases.
Besides which, it's unlikely that you'll find a DP device that would fit in one of the outgoing ways of most CUs.

But, I think something's been missed here ...
You have an existing DP switch with flex outlet - just keep that and add the time switch. Without some "oddball" wiring, it means the switch doesn't isolate the timer, but is does still leave you with local isolation for the immersion element. Don't discount this - my late father in law called one day to say the power was off, the RCD had tripped, and he couldn't reset it because the immersion heater is faulty. Mind you, he didn't call for something like 5 days - so we had to throw all his no-longer-frozen food out.

 

[Taylortwocities]

Nicknock
You are making this much more complicated than it needs to be.
All you have to do is add the timer either the before or after the existing isolation switch. Thats all.

 

[SimonH2]

Exactly, just move the supply cable into the timer, short bit of cable from the timer to the existing switch/flex outlet, job done.
That's probably the best way round to do it anyway - if you need to work on the timer then it can be switched off at the CU, but otherwise you can leave the timer powered up (so it keeps time & settings) and just switch off the immersion heater.

 

[EFLImpudence]

You're not supposed to do the same with a boiler/central heating programmer/timer.


Why would you want to switch off the immersion (by using the switch and not the timer) in a situation where you wanted the timer to keep running?
(Apart from that the timer in question has a month back-up battery)

 

[NickNock]

Noooooooooooo
An RCD and MCB do different things and are not interchangeable.

Sooorrry I meant RCDO, like that which is protecting the outdoor sockets. My understanding is they give the best of both functions.

Switch AND timer?
The reason I'm uncomfortable about this is the 'idiot' factor. Switch & Timer in series makes it easy for 'someone' to deliberately or accidentally switch the rocker switch when the immersion is required and expected, leaving no hot water for those occasional very early mornings when I have to be on the road at 4 or 5am. Boiler comes on at 6am - it's very noisy and would wake GF up. Fair to say that most folks don't understand ANY electrics at all, and so would be puzzled by the timer and switch in series.

Anyway, I had to make an executive decision last night and finish the job, as airing cupboard contents were spread around the landing and plaster dust had to be cleared up. Have gone for my original proposal in the OP sketch, but only because you kind people have given me the confidence that it is both compliant and 'good' (as oposed to shoddy!).

THank you SO MUCH for some superb ideas, comments and feedback, and for the time everyone's taken to help. Gobsmacked by everyone's input and helpfulness. I hope I can reciprocate on others' projects.

cheers, Nick

 

[Taylortwocities]

It looks like you have ignored the advice about having a DP isolation switch. If the immersion element fails, you may have problems as you will only be able to partly isolate the circuit at the consumer unit (line only).
A DP switch allows you to isolate the line and the neutral. Instead of the flex outlet plate, you should have used one of these
https://www.tlc-direct.co.uk/Products/CM1799.html. It has a switch and a flex outlet.

EDIT: In fact most immersion heater instructions state:
This immersion heater must be wired through a double pole isolating switch with a contact separation of at least 3mm in both poles.
ALso, most plumbers will refuse to work on the immersion unless the necessary isolation is in place.

 

[NickNock]

It looks like you have ignored the advice about having a DP isolation switch. If the immersion element fails, you may have problems as you will only be able to partly isolate the circuit at the consumer unit (line only).
A DP switch allows you to isolate the line and the neutral. Instead of the flex outlet plate, you should have used one of these
https://www.tlc-direct.co.uk/Products/CM1799.html. It has a switch and a flex outlet.

EDIT: In fact most immersion heater instructions state:
This immersion heater must be wired through a double pole isolating switch with a contact separation of at least 3mm in both poles.
ALso, most plumbers will refuse to work on the immersion unless the necessary isolation is in place.

Taylortwocities,
thanks for your concern but no!, I didn't ignore the advice just slightly arthritic fingers aching and didn't want to retype: i'll have this covered at the consumer unit as I explained earlier in the thread. If I cannot source a DP RCBO, MCB/RCD, then I will install a DP switch in line in the garage. It's the only place that the cabling is accessible and where there is space to mount the switch, and where it will not 'confuse' other occupants now or in the future.
Obviously I'll be labelling that switch clearly and will add a label to that effect on the flex plate in the airing cupboard.
I also have an O&M file for the house that will stay with it when the time comes to move. Any alterations we've done are documented in there, including schematics, pipework alterations, CAD sketches, instruction documents, receipts etc.

Having said all that, I worked on this project by switching off the existing SP MCB and then testing all wires with multimeter (L-N, L-E, E-N).
I am also awaiting a reply from TimeGuard as to the relay contacts gap dimension and the device failure modes.

Cheers

 

[EFLImpudence]

I'm sorry Nick but you have made a mountain out of a mole hill and a hash-up out of a simple job.

 

[NickNock]

I'm sorry Nick but you have made a mountain out of a mole hill and a hash-up out of a simple job.

How so?

 

[EFLImpudence]

Well, for the sake of some cable restraint and imagined lack of room next to the towels, you are substituting DP RCBOs or a switch in the garage.

 

[kai]

Another advantage of having the clock on show - you now have an extra Time Piece to use, without buying a separate clock - all in one unit.

 

[Iggifer]

Another advantage of having the clock on show - you now have an extra Time Piece to use, without buying a separate clock - all in one unit.

Really? I wouldn't want that on show in my house. Once you've got the set up with times right, you need to change it twice a year - if it doesn't adjust to and from BST on it's own. That's it. A cupboard is where it should live

 

[NickNock]

Well, for the sake of some cable restraint and imagined lack of room next to the towels, you are substituting DP RCBOs or a switch in the garage.

And that qualifies as a mountain out of a mole hill, and a hash up? I'm truly puzzled why you should suddenly jump to that unconstructive comment when I've been nothing but polite and grateful for everyone's input.

I explained my reasoning about not having a switch in series with a (timer-)switch - inadvertent operation of the switch when that is what the timer is there to do.

I also feel that covering electrical devices with bone dry tinder adds (probably tiny) risk and thus surely cannot by definition be 'best practice', same going for locating electrical devices in a position that is highly likely to become so covered, which rules out any area further back beside the towels (after all most folk just bundle piles of laundry into their airing cupboard, and I always try to think about the next occupants).

In design failure mode and effect analysis (DFMEA), once a risk is identified, there's an obligation to evaluate it and if possible reduce the risk, if cost of doing so is acceptable and appropriate to the mitigated resultant risk. I reckon any easy win is worth it, as I enjoy DIY anyway so the labour element is free.

In the spirit of a previous comment about compliance and best practice (the flex through the plaster board bit), the flex outlet plate seems a very neat solution.

Also the suggestion made of putting a switch downstairs: I see no difference between that and doing the self same thing but in the garage.

However, I've a better idea: if the single throw DP switch in the cupboard really is the best idea instead of a DP MCB, a secret key 20A DP switch with flex outlet is the answer for installing next to the timer. I'll get one of those and swap it for the flex outlet plate.

End of!

 

[NickNock]

Another advantage of having the clock on show - you now have an extra Time Piece to use, without buying a separate clock - all in one unit.

Viz Top Tip

 

[EFLImpudence]

And that qualifies as a mountain out of a mole hill, and a hash up? I'm truly puzzled why you should suddenly jump to that unconstructive comment when I've been nothing but polite and grateful for everyone's input.

Well, yes - three pages on fitting an immersion timer.
I meant a design hash-up; not the installation work.
I was not impolite.

I explained my reasoning about not having a switch in series with a (timer-)switch - inadvertent operation of the switch when that is what the timer is there to do.

Nonsense.
The switch is for isolation.
The timer is for convenience.

I also feel that covering electrical devices with bone dry tinder adds (probably tiny) risk and thus surely cannot by definition be 'best practice', same going for locating electrical devices in a position that is highly likely to become so covered, which rules out any area further back beside the towels (after all most folk just bundle piles of laundry into their airing cupboard, and I always try to think about the next occupants).

Not applicable. You just need a switch instead of the flex outlet.

In design failure mode and effect analysis (DFMEA), once a risk is identified, there's an obligation to evaluate it and if possible reduce the risk, if cost of doing so is acceptable and appropriate to the mitigated resultant risk. I reckon any easy win is worth it, as I enjoy DIY anyway so the labour element is free.

That explains a lot of the problem.

In the spirit of a previous comment about compliance and best practice (the flex through the plaster board bit), the flex outlet plate seems a very neat solution.

It is - and so would be a switch.

Also the suggestion made of putting a switch downstairs: I see no difference between that and doing the self same thing but in the garage.

Neither do I. Both silly.

However, I've a better idea: if the single throw DP switch in the cupboard really is the best idea instead of a DP MCB, a secret key 20A DP switch with flex outlet is the answer for installing next to the timer. I'll get one of those and swap it for the flex outlet plate.

No - it may need isolating in a hurry.

End of!

OK.