Wiring Horstmann E15 Immersion Timer

27 Jun 2007
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United Kingdom
Just need some advice on wiring this particular E15 timer into my existing wiring, as regards regulations. (I'm in Rep. Of Ireland but AFAIK they're very similar to the UK - so hence the posting here).

I have an immersion running from a switch with on/off & sink/bath. The cable to the immersion is 4 core - (Switched Live to Sink/Switched Live to Bath/Neutral/Earth).

The supply to this immersion switch runs from a dedicated 20A MCB in the consumer unit.

The Horstmann instructions say - "Means of disconnection from the supply having at least 3mm contact separation in both poles must be incorporated in the fixed wiring"

So my questions are;

1. I'm assuming the above means a DP switch must be put in before the timer that isolates both the neutral and live? I take it the MCB is only isolating the live. But why the requirement to isolate neutral as well? This would mean adding an additional switch to the fixed wiring. Also this DP switch will *always* be left on anyhow, so what purpose does it have, being as the MCB can isolate the supply to repair/replace the immersion element?

2. I originally meant to just replace the existing switch with the E15 planning not to wire the sink feed as this would be unused and just wire the Live bath feed (for the large immersion element). But what if I did? What's the UK regulations say on this? Is it law to have a DP switch on an immersion? The timer is acting as a DP one surely anyway.

I will probably put in the DP switch before the timer but I just wondered is it actually required in this setup? Any advice is appreciated.

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Firstly, in the UK, heating elements in larger water tanks have to have their own dedicated supply so the immersion feed should not be added to. its there for the immersion, nothing else.
To do it properly you should be looking to run a spur from a ring circuit.

Secondly. The neutral is considered a live conductor as well as the pahase conductor. In some installations there will be a potential difference (voltage) between earth (the pipes in your house) and the neutral - this could give your plumber a surprise if he comes looking for a fault. the DP switch ensures full isolation from the supply and any service engineer will be looking for that - many will not work on an installation is its not there.

In any event, the supply to the immersion is 20A. The supply to your programmer and boiler should be fused at 5A (it should say this on the Horstmann instructions.)

I have seen several instances where a 13A fuse has been left in a fuse for a heating circuit. A fault then usually kills either the programmer or the boiler control board before the 13A fuse blows.

So... to do it properly:
run a spur from a ring circuit
put on a switched fuse connection unit (FCU)
change fuse to 5A
Thanks for the reply! Great now I understand the reasoning behind the DP switch.

However you mention running a spur and fitting a new FCU with a 5A fuse. But the circuit already exists. Separate cable direct to immersion switch (not fused) from CU through a 20A MCB.

The boiler timer is completely separate. This timer is only for the immersion and will close the circuit for 15/30/60 mins.

The immersion is dual element 2.8KW and 2KW and this timer switches max load 3KW. I only plan to wire the 2.8KW - I'd need more than a 5A fuse on that if I added a FCU...!

So all I'd need do in this instance is add a DP switch before the timer? No FCU required as the MCB would cover this right?
Ahh - I misunderstood. thought this was a central heating programmer.

OK. Turn off power first!

Remove sink bath switch that exists
replace with an FCU with 13A fuse. (the E15 timer has a max 13A rating)
Connect the live feed from yr consumer unit to the feed terminals on FCU.
connect the FCU load terminals to the timer input (at least 1.5mm cable)
Connect the cable to the BATH immersion to the output terminals on the timer
Make sure earth connections are made and that all terminations are tight

You can remove the cable for the sink immersion inside the immersion cover.

Thats it.

edit: added timer connx
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Taylortwocities said:
Remove sink bath switch that exists
replace with an FCU with 13A fuse. (the E15 timer has a max 13A rating)

Or change the MCB from a B20 to a B16? An immersion timer is usually rated at 15A or more. otherwise you will be protecting a single appliance with both an MCB and a fuse, which will probably not discriminate.

Edited: You are right, this timer says rated at 13A :( this is very tiresome as it goes on a radial circut and you can't get a 13A MCB. "Some people" might use a 16A MCB and forget it. Or find a 20A timer.

If you have a dual-element immersion heater (these are quite expensive) then it is an advantage to retain the option, and keep the sink/bath switch.

The sink/bath switches I have seen usually have a 20A DP on/off switch then a sink/bath switch on the same faceplate
Taylortwocities - Thanks!

That was the way I was going to do it except that I wasn't going to use a FCU just a DP switch. I will now though.

The immersion tank and element was installed by the plumber when the house was built. The switch is a combined DP on/off sink/bath unit. However I can't use the timer after that switch as it only can switch one live feed. If I want to continue to use sink/bath switch I'd have to install a FCU -> Timer -> Immersion Switch.... starts to get messy and I have only one socket flush with the wall (where the exsiting immersion switch sits).

Edit... Wait a sec - I wonder if I can use the DP from the immersion switch to the timer then back to the bank/sink switch. I'd have to check the connections on that but I think it's a sealed unit without the linking wire across from switched live to the bath/sink input.... so no and besides it won't be fused....


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