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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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  • 0
    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 ...


     


Reply
  • 0
    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 ...


     


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