My nephew the apprentice sent me this in our correspondence about
heater elements

===================

OK, here's the scenario.  I get called to a house with no hot water and
their hot water circuit breaker (16A with the circuit cable size being
1.5mm 2-core plus earth) would trip a minute or two after being reset

So, I took the cover off the cylinder and took a look at the element,
thermostat etc. reset the breaker and tested to make sure I had power
out of the thermostat into the element. test showed I had 230v on the
Phase side of the element, so I put my clamp meter over the Neutral
coming off the element, expecting to have roughly 8 amps due to it
being a 2kW element, but that wasn't the case, I had only a few mA,
if that. I jumped to the conclusion that it was a faulty element as I had
just traced the problem back to it

I drained the cylinder, replaced the element, refilled the cylinder and
wired in the new element.......... only to have the exact same thing
happen!! A little anoyed, and puzzled, I decided to put my voltage
testers on the incoming to see if there were any abnormalities. On
the isolating switch, I tested between phase and earth - 230v, phase
and neutral - 230v, and then...........earth and neutral - 230v dun dun
dunnnnn. I was on to something, as there should be no potential
between earth and neutral as they are connected back at the Earth-
Neutral bars in the switchboard

This also explained why the element wasn't drawing any current.so,
there must be a break in the neutral somewhere. But where?  I
disconnected all the incoming cables at the cylinder and disconected
the hot water circuit's phase, earth, and neutral. I tested each cable
with my multimeter and found there to be a dead short between
phase and neutral at this stage

This told me that the fault was somewhere on the supply cable to the
cylinder. The most likely place to me would be the ceiling, so I jumped
up there with my torch and had a look around, working my way from
the top of the switchboard, towards  the hot water cupboard and
couldn't see any breaks or niks in the cable, but i did notice a bit of
blackness around the hole where the wire went down the wall, so I
pulled the slack up the wall and found the wire to have a big chunk
out of it with some melted copper and mice bite marks on the insulation.
 
So, I pretty much changed the element for no reason.

=============

I'm pleased that he found the problem (before there was a house
fire) and is applying some logic, but see he missed one early step
that would have saved a lot of time

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist