Paul E. Schoen wrote in message
...

"N_Cook" wrote in message
...
Anyone care to speculate on the timeline of failure?
There was no local hotspot and no lacquer damage more than a mm from the
"spot weld", and a bit of very localised smoke staining travelling a cm
or
so along the affected wires in each direction. Nothing to suggest that
the
initial bridge was between the 2 bifilar wires of the primary, so not
running a 120 volt primary in effect on 240V ac for any time. For 240V
use
the 2 primaries seriesed to give about 2.8 ohm originally , after
failure
then about 0.4 ohm.
The 2 primaries broke into 5 lengths
9.6,14.6,20.1,20.1 and 25.3m long , measured to about 0.2m accuracy.
So originally probably 2 x 45m. Don't know for sure as did not think to
check but the weld was probably 20.1m from one end, but I would suggest
that
bridge occured after an arc to another layer (higher pd) and then
localised
heating to bridge across to the bifilar fellow wire.

In summary , no evidence that bifilar wiring itself was the reason for
failure but more due to the lack of any interlayer insulation. Because
of
the uneven wire spacing between inside and outside faces of the toroid
it
is
too easy for the layers to be jumbled.

It would also be helpful to know how the toroid was mounted, and if the
failure occurred where pressure was applied. Toroids are usually mounted
either flat with washers and rubber gaskets with a single screw through
the
hole, or vertically in an "Omega" bracket, with some rubber cushioning
material around the periphery. But in either case there may be additional
pressure on a "high spot" where two windings may be crossed, and softening
of the insulation from overheating may also contribute to a short.

It is fairly easy to check for dielectric breakdown between the bifilar
windings, but nearly impossible to detect a weakness between adjacent
turns
of a single winding. But it might be possible to use a higher frequency,
or
PWM pulses of higher peak voltage, to produce a higher potential between
turns of the same winding, and observe spikes of current due to breakdown.

Paul




But the primary is buried under the heavy gauge secondaries windings and
then three layers of 0.05mm polyester film strip, each layer of that
probably overlapped by a factor of 3. So I would have thought immune from
any external mechanical effects, exccept vibration perhaps.


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