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Topic Title: Immersion Heater Supply Question.
Topic Summary: Opinions Invited Please.
Created On: 04 August 2017 04:56 PM
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 04 August 2017 04:56 PM
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Zoomup

Posts: 3266
Joined: 20 February 2014

So there you are at the customer's house. The immersion heater does not work. Its switched fused connection unit has burned out. The fuse draw holder has fallen out and the whole face is blackened by heat damage. So you say to the customer, "O.K. no problem, I will fit a new switch. I will fit a 20 Amp double pole switch with a neon indicator." Then you discover that the immersion heater does not have its own dedicated circuit. It is fed from a ring final circuit. What is the best and most cost effective solution to get the immersion heater working again?

You views on this mater will be greatly appreciated.

Bye,

Z.
 04 August 2017 05:16 PM
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richardcs

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If you use a 20A DP switch and wire it in 2.5mm or a sufficiently short length of 1.5mm then it's protected against short circuit by the ring final B32 and against overload by the nature of the load. The ring loading is no worse than before and the reliability is greatly increased.

I can't see a problem with this course of action. Yes the long-hour 3 kW load on the ring is not ideal but the risk of overload is minimal and you've not made it any worse. I feel it's a more reasonable course of action than pushing the customer towards a new circuit from the origin.
 04 August 2017 05:43 PM
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Zoomup

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Hello richardcs,
thanks for the reply. I pondered over this matter for some time. I do not like cables supplied and not protected by a suitably rated protective device. I did argue that the immersion heater could not become faulty and cause a sustained overload by virtue or its construction. If the sheath corroded and allowed water into the heating element then the R.C.D. would trip off. The supply 1.5mm2 flex is unlikely to overload even if connected directly to the ring final, and the load is a fixed load. Despite my considering all of those things I still installed a 16 Amp. M.C.B. by the 20 Amp. switch in its own small enclosure. Belt and braces. I just do not like cables being protected by a protective device rated at much more that the cable's current carrying capacity. There are two ring final circuits in the small house so overloading should not be a problem.

Bye,

Z.
 04 August 2017 05:44 PM
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daveparry1

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Depends on size of feed cable though, many older installations were done in 1.5mm / 3.029, if this is the case just replace the fcu.
 04 August 2017 05:51 PM
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geoffsd

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I agree with Richard.
 04 August 2017 08:27 PM
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Alcomax

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A 16 amp circuit breaker has been added to the mix and this is likely to be in the cylinder cupboard?
 05 August 2017 08:45 AM
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Zoomup

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Originally posted by: daveparry1

Depends on size of feed cable though, many older installations were done in 1.5mm / 3.029, if this is the case just replace the fcu.


Yes Dave, I could have just replaced the switched fused connection unit but it is difficult to determine which makes of such are reliable these days for a 3kW load. I did not want to install new one and then find it has failed shortly after replacement.

Z.
 05 August 2017 08:46 AM
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Zoomup

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Originally posted by: Alcomax

A 16 amp circuit breaker has been added to the mix and this is likely to be in the cylinder cupboard?


Yes that is correct as I said above.

Z.
 05 August 2017 01:18 PM
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leckie

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Well I think Zoom has done a good job.

Although there is an argument that the nature of the load does not allow an overload to occur. Also that the mcb or rcd will provide short circuit protection, but this assumes a fault of zero impedance. So just fitting a 20A switch may indeed comply with the regs. However, I am sure many of you will have observed connections to immersion heaters that have become overheated - it doesn't take much to increase the resistance of a connection that is subject to long periods of constant load on a bit of flex to a crappy immersion heater thermostat connection. People also tend to dump all sorts of stuff on the cylinder, dangle clothes on the flex, etc. Or maybe the stat will begin arcing, etc. So the fault my not be of zero impedance or the overload restricted by the design of the load. Now I know we are not required to consider this by the regs, but if it was me I think I would have done exactly what Zoom did or replace the 13A fused connection unit with an equivalent accessory of good quality.
 05 August 2017 02:20 PM
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geoffsd

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Originally posted by: leckieHowever, I am sure many of you will have observed connections to immersion heaters that have become overheated

They have usually done this with a 16A MCB in place.

If the original FCU was on the ring and not a spur, then increasing the flex to 4 sq.mm. would have been another solution.
 05 August 2017 02:40 PM
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Alcomax

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In a parallel reality somewhere this may be a good job. What was the original point of failure... a FCU that would cope more easily with 3kW if it was not in an above normal ambient temp environment? So additional points of failure in a potentially fairly warm cylinder cupboard have been introduce. What was initially distress maintenance has morphed into a distressing solution.
 05 August 2017 03:11 PM
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alancapon

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It is likely that the original FCU failed due to the high ambient temperature found in airing cupboards. What is the ambient temperature rating of the MCB/enclosure you have installed in its place?

Regards,

Alan.
 05 August 2017 03:29 PM
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Fm

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Hire a competent electrician
 05 August 2017 03:42 PM
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weirdbeard

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Originally posted by: geoffsd

I agree with Richard.


Agreed!

-------------------------
:beer)
 05 August 2017 03:47 PM
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dustydazzler

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Get a BADger in on the case, 2 flicks of their neon screwdriver and I am sure they will have it badgered up
 05 August 2017 04:46 PM
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leckie

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So it seems that there is concern about the ambient temperature. What would it be do you think? MK reckon their accessories are good for 40 degrees C. So I would think that should be OK. I think MCB's are rated at about 50-60 degrees C. So that should also be OK. So if the devices are operating within specification we can ignore ambient temperature - but then the fused Spurs are rated at 13A and we know they fail on a regular basis on a 3kW load that is on for a few hours at a time!

The 4.00mm flex is a good idea, but I think it might be a struggle to secure the heater cable clamp on and make a good connection with many element types.

So if they won't pay for a dedicated circuit what's the best option?
 05 August 2017 04:53 PM
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leckie

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As I said earlier, I think Zooms solution is pretty decent. The MCB and any accessory should be fine from an ambient temperature perspective according to MK anyway. I have already given my reasons why I think overload protection would be advantageous, although maybe not strictly required by the regs. And I think if a connection did still overheat, a 16A device is likely to offer greater protection than a 32A device.

And I didn't think much of the competent electrician jibe either!
 05 August 2017 05:01 PM
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dustydazzler

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What is a competent electrician ?
I thought we fall into 2 simple categories
1 Badger
2 electrician
 05 August 2017 05:20 PM
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geoffsd

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Originally posted by: leckie

I have already given my reasons why I think overload protection would be advantageous, although maybe not strictly required by the regs. And I think if a connection did still overheat, a 16A device is likely to offer greater protection than a 32A device.

It's not really an overload, as such, is it?

Any overheating is likely caused by a loose connection where the 12.5A current of the 3kW immersion is drawn through a few strands of the conductor.
It is not caused by a sustained 9.6 ohms contact (ignoring the element, which you can't) drawing 25A causing a 16A MCB to trip in just under an hour - or 5.1 ohms for 32A MCB.
Any respective higher resistance contact will not cause the MCB to trip.

Including the 19 ohms of the element makes the supposed worries not possible.
A reduction in resistance is necessary to increase the current enough to trip the MCB.
 05 August 2017 05:20 PM
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geoffsd

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duplicate
IET » Wiring and the regulations » Immersion Heater Supply Question.

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