"Don Klipstein"
Phil Allison wrote:
"Don Klipstein"

Then again, I doubt a voltage spike lasting long enough to burn out a
transformer will be absorbed by a 1 uF cap across a 120V AC line.

** You missed the point entirely.

A voltage spike ( or a series of them) can easily cause insulation failure
in the enamel winding wire of the primary - then the energy to explode
the lead in and lead out wires comes from the 120 volt AC supply.

Is the O.P. having other things in his house blowing from voltage spikes
severe enough to blow transformer primary winding insulation?


** Totally irrelevant.

The furnace unit and the transformer in question are all we are discussing.

You have clearly not bothered to read my first or my other posts in this
thread.

Eg:

" High voltage spikes on the primary could also cause insulation failure
leading to the damage seen in the pics - lightning does this sort of thing.
So also could back emfs from the blower fan if the is a bad connection in
the AC supply feed."

Whenever AC power to that furnace is disconnected, the blower fan will
deliver a back emf spike - meanwhile that poor, little tranny is wired in
parallel with it. Other devices in the house are NOT involved.

Most AC supply transformers can tolerate repeated 2kV spikes on the primary
till the cows come home - but a badly wound one cannot. This is a specific
and fairly recent problem with small transformers made in China and
elsewhere in Asia where the makers are not fully aware of the issue of
insulation failure in the enamel windings.

Once there is a layer to layer insulation failure ( between adjacent wires)
in the primary of that tranny - the AC current draw will jump up to many
amps and may cause the feed in and out wires to explode - as seen in the
pics.

If the spike voltage is suppressed, the tranny will likely survive.

A 1uF cap provides a low impedance path for such a back emf spike, virtually
shorting it out.

OTOH a varistor provides no conduction path until its breakdown voltage is
exceeded, but is also a means of suppressing the spike voltage to a safe
value.


..... Phil