Twin immersion heaters

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.

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.