Immersion heater - timer wiring

[JohnW2]

However, (as I said "If new") I would consider it poor practice to 'future proof' the cable but have a smaller capacity accessory fitted in the cable run with a 50A mcb - or not to fit a 32A mcb.

Fair enough.

You're splitting hairs, there are no smaller mcbs or t&e.

I was't really meaning to split hairs - merely pointing out that there are situations in which (because of non-availability of lower-rated cable and devices) one happily accepts degrees of 'overkill' of cable and MCB size which are, proportionately, far greater than you were describing as 'poor design'.

Perhaps I've never come across a sustained 13A load.

There are plenty of 3kW fan heaters, and other types of heaters, around, and they have been known to be operated almost 'continuously'.

Kind Regards, John.

 

[EFLImpudence]

but, these immersion timers are particularly prone to overheating (the diagnosis usually being that the installer did not tighten the neutral sufficiently) I don't think the digital ones are very well made.

Yes, I gather that overheating of these things is quite common. However, if, as you say, it's usually due to insufficient tightening of the terminals, one can hardly blame the timer for that - the same could happen with any component in an installation, no matter how adequately rated and how well made! I'm also intrigued by your saying that it's usually the neutral that isn't tightened sufficiently - does this imply some sort of 'colour bias' on the part of installers?

I was pointing out that it is the installer who is blamed.

I have never seen one where the Live has overheated - and when I say overheated, I mean the whole of the Neutral side of the timer has melted, including back up inside the mechanism.

I have read that some seem to think that, subconsciously, the Neutral is deemed 'less important' so that the same care is not taken with it.
However, once the thing has melted, the cause is difficult to determine.

Greenbrook recently had a 'bad batch' of which many failed in this way.
This leads me to believe that most of the others aren't quite so bad.

 

[JohnW2]

I have never seen one where the Live has overheated - and when I say overheated, I mean the whole of the Neutral side of the timer has melted, including back up inside the mechanism. .... I have read that some seem to think that, subconsciously, the Neutral is deemed 'less important' so that the same care is not taken with it.

Fascinating, but that sort of subconscious effect (which is essentially what I was calling 'colour bias') certainly has many precedents in a wide range of fields.

Kind Regards, John.

 

[pretzelz]

Hi all,

Finally I have a chance to write. Life has been very hectic!

Riveralt asked:

what do the manufacturers instruction say regarding providing a means of isolation.
Does it say you have to utilise a 13Amp Fused Connection unit?
A 20Amp Dual Pole Switch or nothing?
What does it say about cable size and MCB rating?
And is the immersion on its own circuit?


manufactures instruction says:
- contact rating 16A, 240V, mains frequency 50hz, connections 2.5mm.
- "check that the mains voltage and frequency correspond to the rating plate (on back of timer)" -- 13A, 240V, 50hz, 3120W
- Do not overload. Refer to the Ampere rating on the rating plate.

These are the only installation instructions given. Nothing given about the wiring or terminals.

The immersion is on its own circuit in my house. It has a 16amp switch in the electric box.

Can I take it that this timer does not require connecting to a fuse before hand? And, the instructions kindly given from Riveralt detail what to do with the existing wires which are connected into the original switch's fuse:

Riveralt wrote:
There will be a supply cable that comes to the switch/fcu from the consumer unit - this will contain a red/black and earth cable.
There will be a load cable that goes from the switch to the immersion heater - this will contain a red/black and earth cable.
So the Red supply goes into L in.
The red load goes into the L out.
The supply and load black cables together go into the N.
The earths need to be sleeved and connected together.

Sunday I will finally get an opportunity to look at the setup in detail.

Many thanks to all who have given thoughts to this.

original switch:

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• pretzelz
• 14 Mar 2012
• 1

timer switch:

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• pretzelz
• 14 Mar 2012
• 1

 

[EFLImpudence]

manufactures instruction says:
- contact rating 16A, 240V, mains frequency 50hz, connections 2.5mm.
- "check that the mains voltage and frequency correspond to the rating plate (on back of timer)" -- 13A, 240V, 50hz, 3120W
- Do not overload. Refer to the Ampere rating on the rating plate.

It would appear that all is well as the instructions state the contacts are rated at 16A.

It is confusing to have a different figure printed on the casing.

However you need to leave the switch in place and wire the timer between the switch and immersion.
The switch will be useful to switch off should anything go wrong or when you go on holiday.

 

[pretzelz]

Are you suggesting to leave the switch in place because this contains a fuse, to come before the timer? Or just to turn on and off all the power to the timer if necessary?

I ask because the timer has an on/off override button. Also it would be a messy install compared with a straight swap. I had presumed the old switch could come off the switch box external to the heater cupboard and be replaced with the timer switch. I think riveralt's instructions were for this too.

 

[EFLImpudence]

Are you suggesting to leave the switch in place because this contains a fuse, to come before the timer?

Not because it contains a fuse - just use a 20A DP switch (unfused).

Or just to turn on and off all the power to the timer if necessary?

Yes, to isolate the timer and the immersion.

I feel confident in saying that if you look at any other house you will not find one which does not have a switch.

I ask because the timer has an on/off override button.

That is to turn it on/off without altering the time settings.
It is not good enough to isolate in the event of a fault.
Even the MCB in the CU will not disconnect the neutral.

Also it would be a messy install compared with a straight swap. I had presumed the old switch could come off the switch box external to the heater cupboard and be replaced with the timer switch. I think riveralt's instructions were for this too.

As you have said yourself - this is such a simple circuit I am surprised anyone could disagree.

The circuit should be - all in 2.5mm² cable.

16A mcb >>> 20A DP switch >>> 16A timer >>> immersion.

 

[kai]

16A mcb >>> 20A DP switch >>> 16A timer >>> immersion.

The quoted setup seems acceptable for your immersion, provided the manufacturer's instructions do not demand a 13amp fuse.

The manual off on the timer is useful for Summer holidays, when the hot water can be de-activated (unless you are running it for ten minutes every day in Winter for frost prevention measure).

 

[EFLImpudence]

The quoted setup seems acceptable for your immersion, provided the manufacturer's instructions do not demand a 13amp fuse.

I don't want to keep banging on but -

If the manufacturer of the immersion does specify a 13A fuse then IT IS WRONG.

There is NOTHING in the immersion heater which requires a 13A fuse.

I am aware of 134.1.1 and the need to comply with it but if the m.i. said fit a 3A fuse (misprint, mistake or whatever) what would you do?