On 6 Aug 2009 at 20:04, Jinx wrote:

> The element in our hot water cylinder failed yesterday (didn't quite
> make its 19th birthday) and an electrician came today to replace it
> 
> Looked OK on the outside. Clean, shiny. But, oh, the horrors on
> the inside. A little corroded. Just a little
> 
> http://home.clear.net.nz/pages/joecolquitt/element1.jpg
> 
> http://home.clear.net.nz/pages/joecolquitt/element2.jpg
> 
> Looks like a nasty surgery thing I saw on Discovery
> 
> This must be long-standing corrosion and it absolutely amazes me
> that it has taken until now to pop the circuit breaker. Surely the
> winding must have been wet before yesterday. After it dried, most
> of the filler just fell out. My nephew up north is doing an electrician's
> apprenticeship and says he sees that sort of breakdown regularly
> 
> So I was chatting with the sparkie, as you do, and asked him why
> the damage was closer to the Active/Live connection. (you can see
> in the first picture that the damage pretty much stops where the
> element turns around, ie halfway between A and N). He couldn't
> say. He said that he asked once at college why, if the current is
> AC, that Active and Neutral aren't regarded as interchangeable.
> But the answer he got was "they just aren't"
> 
> Anyway, I've wondered about it too. I've thrown out IEC leads,
> like on the kettle for example, because the Active connection has
> suffered heat and corrosion damage. I'd have thought that elements
> are more or less just resistive, nothing complicated wrt power for
> example
> 
> Why does the corrosion happen that way ?

Hmm, I haven't thought this through too far but... Remember that in NZ neutral and 
earth are bonded together at the switchboard. Your cylinder is earthed, and water 
conducts. So the potential difference between Phase and water (230V) is greater 
than Neutral and water (0V). That doesn't explain the mechanisim of corrosion, but 
it might be a clue as to why it's worse at one end than the other?

Without an earth leakage breaker your dodgy element was probably quite happy  
heating the water... the usual amount by power in the element, plus a bit extra 
through conduction!

PS. My hot water cylinder is leaking so needs replacing very soon.. I'll check out the 
element when it comes out.

-- 
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brown@clear.net.nz


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