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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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 05 August 2017 05:31 PM
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daveparry1

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It's always seems to me that it's the fuse holder contacts in fcu's and 13amp plugs that are the weak point, it's almost always those that burn up, apart from occasionally loose terminations.
 05 August 2017 06:33 PM
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Alcomax

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I did think that the OP was a wind up, perhaps a bit of fun. I do hope that the eager accomplices are just playing along. The alternative does not bare thinking about.

Twice in the last 12 months have seen the result of overheating switch gear in cylinder cupboards : one an RCD in enclosure for a shower, the other a 16 amp mcb in enclosure bodged off a RFC, then feeding a 2kW bathroom fan heater and some other socket outlets upstairs. Think of what is protecting against what. The ambient temp outside an enclosure will determine how gained heat inside the enclosure is lost and how quickly, if at all. Especially if conductor inside is above 50 degrees . So the 16 amp MCB cannot protect itself from overheated terminals, that is left to the 32 amp device. Luckily the cylinder cupboards did contain the fires and then quickly extinguished themselves. Both were not rammed full of towels etc, just room for the cylinder, however if they were, the consequences would have been far worse.
 05 August 2017 07:04 PM
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geoffsd

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Originally posted by: Alcomax
Especially if conductor inside is above 50 degrees . So the 16 amp MCB cannot protect itself from overheated terminals, that is left to the 32 amp device.

How can either do that?
 05 August 2017 07:17 PM
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Alcomax

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Originally posted by: geoffsd

Originally posted by: Alcomax

Especially if conductor inside is above 50 degrees . So the 16 amp MCB cannot protect itself from overheated terminals, that is left to the 32 amp device.


How can either do that?


Never said they could. They will not, just some think the MCB will for some reason.
 05 August 2017 07:41 PM
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geoffsd

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I beg your pardon.
 05 August 2017 07:51 PM
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Zoomup

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Originally posted by: geoffsd

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.


Hello Geoff,
thanks for your observation. I do not normally carry 4.0mm2 heat resisting flex onboard my vehicle, and I would find it difficult to connect it to the immersion heater and stat terminals due to its physical size if I did.

Z.
 05 August 2017 07:54 PM
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Zoomup

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Originally posted by: Alcomax

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.


Hello Alcomax,
it is commonly known that a fused connection unit is a point of failure for immersion heaters. The contacts between the 13 Amp fuse and its contacts often fail due to I2R heating. I do not like using fused connection units for big continuous loads like immersion heaters. A 20 Amp switch with no internal fuse is far more robust for this application.

Z.

Edit. Typo. I2t changed to I2R. I made a mistake there. The heating effect is due to current squared times resistance, answer is in Watts. I must go to Specsavers.

Edited: 06 August 2017 at 08:05 AM by Zoomup
 05 August 2017 07:58 PM
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Alcomax

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Originally posted by: Zoomup

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 07:58 PM
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Zoomup

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I must have had Reg. 434.2 in mind when I undertook the job. The original post was no wind up.

Bye,

Z.
 05 August 2017 08:10 PM
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Zoomup

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Originally posted by: leckie

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!


Hello leckie,
someone on this site once said this:"Just a minor correction to M's post above. Terminals on 60898 devices are limited to 60K rise over ambient". So we must be right in assuming that the 40 deg. C airing cupboard temperature will be no big problem. Of course at ceiling height it will be warmer than at floor level I suppose. My new devices are situated about half was up the wall.

Bye,

Z.
 05 August 2017 08:12 PM
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Alcomax

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Originally posted by: geoffsd

I beg your pardon.


No worries, the context could be either way looking back at it.


So it is not a wind up. When you are faced with a job like that the only option is to use a 20 amp DP switch installed as "distress maintenance". It is not installation work it is maintenance. Inform that it does require a new dedicated circuit but that is client choice. Leave it at that.

Not a good idea to start putting another point of failure in when you have just claimed to have removed another [ the FCU ]. We have not even considered what is on the rest of the RFC, however, given an immersion heater is in use 24/7 , is it not likely that there is on demand electric heating [ convector / fan heaters ] on that circuit, even if there is night storage heating? So it is not unreasonable to consider that the connections of the ring at the original FCU could be subject to hefty loading now and again, therefore being fairly warm and that is not even factoring in the 3kW load of the immersion.
 05 August 2017 08:17 PM
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geoffsd

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Originally posted by: Zoomup

I must have had Reg. 434.2 in mind.


433.3.1(ii) comes before that.
 05 August 2017 08:36 PM
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Zoomup

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Originally posted by: geoffsd

Originally posted by: Zoomup



I must have had Reg. 434.2 in mind.




433.3.1(ii) comes before that.


433.3.1 (ii) "...provided that the conductor is protected against FAULT CURRENT in accordance with the requirements of Section 434".

434.2 applies at the point where a reduction in conductor size occurs.

Z.
 05 August 2017 09:07 PM
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leckie

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Before I post this reply I should just perhaps mention that I have just had five pints at my local, so if I am misunderstanding things please take that into account.......

So, it seems to me that people are sniping at Zooms method. That in some way adding a 16A MCB that is probably rated to perform at 50-60 degrees C is in some way inferior to adding a 20A switch that is probably rated to perform at 40 degrees C.

so why would that be then?
 05 August 2017 09:16 PM
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geoffsd

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Originally posted by: Zoomup
433.3.1 (ii) "...provided that the conductor is protected against FAULT CURRENT in accordance with the requirements of Section 434".

Yes, are you saying it wasn't?
What was the Zs and is the CPC of sufficient CSA?

The CPC of 2.5 flex is 2.5; the ring will only be 1.5.

434.2 applies at the point where a reduction in conductor size occurs.

Only if not protected against fault current.
 05 August 2017 09:19 PM
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geoffsd

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Originally posted by: leckie

So, it seems to me that people are sniping at Zooms method. That in some way adding a 16A MCB that is probably rated to perform at 50-60 degrees C is in some way inferior to adding a 20A switch that is probably rated to perform at 40 degrees C.

Not sniping; just trying to explain that it - the MCB - is not necessary.

It's no different than an oven on a 32A circuit.
 05 August 2017 09:33 PM
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KFH

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A couple of years ago I replaced an overheating 13A SFCU on an immersion heater which was on its own circuit with a 20A DP switch. Last week the customer complained of the immersion heater occasionally turning off by itself, the symptoms indicating the 20A switch was faulty. After replacing the 20A switch with another one I opened up the faulty switch, one of the little plastic rockers was showing signs of heat damage and was slightly shorter than the undamaged one.

You can't win with the poor quality of modern equipment. As an aside the HW tank and switch are in a quite large room which does not get hot.
 05 August 2017 09:47 PM
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Alcomax

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Originally posted by: leckie

Before I post this reply I should just perhaps mention that I have just had five pints at my local, so if I am misunderstanding things please take that into account.......



So, it seems to me that people are sniping at Zooms method. That in some way adding a 16A MCB that is probably rated to perform at 50-60 degrees C is in some way inferior to adding a 20A switch that is probably rated to perform at 40 degrees C.



so why would that be then?


Well the 20 amp DP switch is a vast improvement on the 13 amp FCU in a cylinder cupboard.

The MCB is not required and it is introducing a point of failure into an environment [ cylinder cupboard ] where there is an identifiable risk of conductors in switch gear operating at a higher temperature than would be the case outside the cylinder cupboard and so premature failure.

I have only had two pints of beer, but that is to be sorted soon

But before that and the inevitable escalation of goggle on goggle debate on how far we as electricians can dance along the cutting edge of worst practice, please consider that regs are over engineered to allow headroom for the inevitable bodging and badgering. That slack needs to be there, call it basic rules of thumb. When it's gone there is no room for error and the stakes are high.

Improver's, apprentices, DIYer's look at this forum, so there should not be encouragement to be cavalier with basic circuit configurations and provision. What is lost here is that the immersion should be on its own circuit. But it is not. Why turn distress maintenance into electrical installation works by further adding to the circuit?
 05 August 2017 09:53 PM
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Alcomax

Posts: 182
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Originally posted by: geoffsd

Originally posted by: leckie



So, it seems to me that people are sniping at Zooms method. That in some way adding a 16A MCB that is probably rated to perform at 50-60 degrees C is in some way inferior to adding a 20A switch that is probably rated to perform at 40 degrees C.


Not sniping; just trying to explain that it - the MCB - is not necessary.



It's no different than an oven on a 32A circuit.


Or a 2.2kW night storage heater 2.5 twin/e on a 16 or 20 amp MCB.
 06 August 2017 08:20 AM
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Zoomup

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Mornin' All,

Please consider this scenario.

A non R.C.D. ring final supplies an 3kW immersion heater via a 20 Amp double pole switch. The ring final is protected by a 32 Amp. M.C.B. The supply is TN-C-S. The immersion heater has a 1.5mm2 heat resisting flex at the 20 Amp. switch supplying it. The heater becomes faulty and the heating element comes into contact with the metal sheath causing a live to earth short circuit in the tank. A fault current of 32 Amps flows for at least an hour as the tank water is cold. No other loads are connected at the time. What are the possible results?

Bye,

Z.

Edited: 06 August 2017 at 08:26 AM by Zoomup
IET » Wiring and the regulations » Immersion Heater Supply Question.

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