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Twin immersion heaters

Former Community Member
Former Community Member
I am in the process of going unvented.  I have to use immersion heaters, and the new cylinder has two.  I have one feed to the existing immersion.  It is on 2.5mm2 T&E and connects to a 32A MCB on the CU.  I'm adding a DP isolating switch in the cylinder cupboard.  Will that be complaint for one of the immersion heaters?

To fully install the cylinder (ie the second immersion), I plan ask an electrician to either
(1) run an additional 2.5mm2 T&E cable back to the CU, connect it to the existing 32A immersion heater MCB at the CU. So the MCB protects two separate cables and the 2.5 T&E is within its capacities. 

Or
(2) run a new 6mm2 T&E cable back to the CU, connect to the existing MCB and in the cylinder cupboard, split the feed to two DP isolators, one for each immersion and its timer.


I'd be grateful for advice - are both approaches compliant?  Is one better than the other?  The amount of work will be similar.
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    I've seen an immersion fail and  melt the wiring supplying it, where it changed from 3kW to about half the expected resistance, to earth, so had there been an RCD it would have tripped, but this was in the dark ages when an RCD was not called that and I had not yet used one in earnest.

    The hot wire 15A fuse didn't melt but the PVC on the wires did.

    Just the one mind, in the mid 1980s.

    Looked undamaged when it came out, so maybe a manufacturing defect.

    Mike

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    I presume they no longer heated the water though, probably because the element failed due to overheating in a steam bubble?
  • 0
    Customers never cease to be amazed at how big some of the holes became in their immersion heaters before finally packing up, often without taking a fuse or MCB out.
  • 0
    Grahams fault sounds very unlikely to me as the element is very tightly packed around with aluminium oxide or similar like pyro. I suppose if one split and the packing washed out there could be a contact to the case but it sounds very unlikely. In principle, the same could happen to cooker and oven elements, but I have never seen one which failed any way except open circuit. The position of such a fault in an element would have to be just right too, to take a dangerously high current and yet not trip the OPD. With a 2.5mm cable in the usual household installation methods, the current would have to be about 40A, and even after a very long time the cable might get to 100C, unless completely surrounded by insulation, Any more current would trip the OPD at some point (all using a B32). The risk is very minimal. The two thermostats should still keep the water from boiling, and such a connection would probably not last long if wet. With a B16, there is only any risk from 1.5mm cable, again very small.
  • 0
    gkenyon:
    davezawadi (David Stone):

    What is wrong with the B32 and two 2.5s Sparkingchip? Overload is impossible as I'm sure you realise?


    A concentric heating element is one device for which protection against overload should seriously be considered unless RCD protection is provided, especially where the normal operating current is close to the tabulated current rating of the cable (considering all correction factors). It's one of the very few components, where the fault of negligible impedance doesn't always happen.


    A possible fault on a concentric heating element is a short to the element casing, with a connection to either Line or Neutral broken.


    If this occurs on the Neutral side, then the element will run at overload, dumping its current back down the protective conductor. If there is an RCD, great, it will operate. Otherwise, if you are lucky, the element will be stressed and "blow" again before anything overheats. But if you're unlucky, conductors can overheat and protective devices still not operate.


    In the case we are discussing, assuming the heating element is no more than 13 A, it's probably only an issue for reference method 103, possible 101 at a push but at that current I'd expect the element to blow itself again  - so David's point is probably correct for this application. If the heating element is rated higher than 13 A, however, we start to run into the potential for a problem.


    I thought it worth sharing the benefit of experience, just in case anyone has a situation with a marginal CSA on a heating element circuit (without RCD protection) in future ...


     




    Over several decades I have replaced many immersion heaters. Most were 3kW domestic types, both copper sheathed and the harder "Incoloy" sheathed types. The most usual point of failure in a split occurring in the outer metal sheath. With a B.S. 3036 fused circuit  the element may continue to work even with water ingress. With an R.C.D. the R.C.D,. will trip off. 


    I have found two basic failure types. 1. An open element when the resistance wire has broken but the outer sheath is intact. And 2. A split outer metal sheath where the element is intact but wet.


    Also terminal failure can occur but that normally causes no danger.


    I have never found an element that has developed a lower resistance due to an internal short between element and sheath, although I suppose that that may occur, but will be a very rare event.


    I would be interested to hear of others' cases in this matter.


    Z.


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    Sparkingchip:

    Don’t fit plugs and use sockets, they will melt.


    Not even for a 2kW load?


    Z.


  • 0
    gkenyon:
    Am I the only one using a BS4343 'ceeform' on his immersion heater? ?

    Hopefully not in a house or similar? Reg 553.1.201 ... "shuttered type"?


    (Sorry for being pedantic)




    Yes it is in (my) house.


    I know that 553.1.201 forbids it ... which is a pain, but still....


    We have a few.... a 16A for the immersion heater, a 16A & 32A in the garage for 'general purpose' and a 32A on the side of the conservatory together with a tap for the "dog washing shower" ?


    In my defence, they are all of the switch-interlock variety.


    In our 'old' house we also had a 63A TPN ceeform in the garage!


  • 0
    I would expect the time for two immersions to heat the full 250 litre cylinder would be more like 206 minutes than a 100 minutes, as the top immersion will switch off when the top section of the tank is hot.
  • 0
    And bear in mind the top immersion is way up the cylinder.
  • 0
    I will correct myself, you can connect both immersions on a Megaflow and on page 6 it gives the heat up times for the various combinations Megaflo-Eco-installation-manual.pdf (heatraesadia.com)


    Presumably it is saying that with both immersions you will heat the whole tank in a couple of minutes more than the just the top section, but assuming the water stratifies in the tank I'm not quite seeing it, as hot water rises!


    If you are connecting both immersions you will need to double the circuit up.

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