On Fri, 9 Sep 2016 00:48:54 +1000, Chris Jones
wrote:

It is possible that the solder has dissolved too much of the fine wire,
making it too thin. I have had a lot of trouble soldering very fine
wire, especially with lead-free solder - the wire will get thinner as it
dissolves in the molten solder, leaving it even less robust than its
original fragile state. Leaded solder that was deliberately
pre-saturated with copper ("Savbit") was supposed to be good for
preventing that, but I found it generally unpleasant to use.

As your coils develop their faults over time, I also wonder if the
manufaturer left some fairly active flux inside the encapsulation that
might be slowly eating the wire near the solder joint when it is warm.

Really don't know. The Pentair/Tyco/Goyen engineer that I discussed
this with asked for a failed one to be returned for evaluation - so I
sent two (as I have a pile of them otherwise just gathering dust).
When I followed up a few weeks later, he had departed and his
replacement was unaware of the whole matter, and could find no record
of their return despite supplying him the RMA number.

He also declined my offer to send more, really being disinterested in
finding out the reason for the failure.

Perhaps you can tell whether the break is at the solder joint, by
measuring the low-frequency capacitance of each terminal of the damaged
coil with respect to everything else. I doubt that knowing where the
break happens would be much use to you, but it is something the
manufacturer should be looking into.

The failure mode is going open cct when hot. As they cool continuity
returns.

I have made up a test lead with pea lamps that I connect up so there
is a (6V) lamp in series with each coil. When there is a failure I
connect this into circuit and re-energise the oven. (Doesn't take
long, I'm getting pretty polished at that now!) It typically takes
about ten minutes to get back up to failure temp. Then I can see
which coil is going O/C and replace it from my stock of new spares.

To be able to test it would require a fair bit of setting up.

See above re manufacturer "interest". They sell quite a number of
these during our winters as they are used in gas wall furnaces. I
guess they are figuring that as long as they keep selling them as
replacements, why investigate.