In article , Phil Allison wrote:

"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.

Why would that be, in your words, "totally irrelevant" to whether or not
the O.P. had any electrical/electronic failures elsewhere in his home
attributable to voltage surges?

As opposed to 3 transformers blowing in the same appliance with nothing
else anywhere in the home running into trouble from voltage surges?

While I have experienced roughly 8 transformers surviving repeated
abusive pulse-in-reverse-direction developing about 2 KV across the
primary without any degradation against ability to do so?

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.

Why would the solution be deploying a capacitor, rather than repairing
the poor connection or deploying a voltage-dependent spike-absorbing
device such as an MOV?

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.

So, why should we hear about problems about that from only one customer
blowing 3 of them?

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.

Why should 1 uF protect the trannies in question while 22-47 uF fails to
protect a compact fluorescent lamp from a line voltage surge that blows
even other electronics in 2 houses but did not blow any in-home trannies?

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.

Certainly protects against applying for even a microsecond more than
roughly 250-300 volts across transformer primaries, where it appears to me
that we agree that transformer primaries usually survive 2,000 volts pulse
voltage?

I seem to think that the trannies are probably blowing from either
improper wiring (connecting only 1 of the 2 primary winding sections
possibly noted for 120V usage), or from secondary load malfunction
including bridge rectifier failure in manner of a diode "failing open".

Or, extreme-oddball trouble such as sticking a magnet to the tranny.
But that's grasping-at-straws, like line voltage irregularities that blow
3 trannies in 1 piece of equipment but draw no other complaints such as
blowing of electronics downstream of the tranny in question, or elsewhere
in the house where one appliance blew 3 trannies.
--
- Don )