On Sat, 9 Apr 2011 06:53:25 -0700, "Bill"
wrote:

"Steve Turner" wrote in message
...
On 4/8/2011 10:16 PM, Tony Miklos wrote:
On 4/8/2011 8:52 PM, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep
blowing
on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram)
he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of
the
wiring diagrams that I also copied to my above flickr site as jpg
images):


http://www.trane.com/webcache/un/fur...7_04012009.pdf



As you can probably see in the pictures, there is visible charring
of
the 115V leads going into the transformer, and of course the 115V
circuit is open (again).

Three times it fried the primary? I'd guess it's getting 240vac at
times. It's
very unusual for the primary to go bad from a short on the
secondary side, and
the picture shows that the secondary wasn't overheated like the
primary was.
Where's all those electricians talking about an open neutrals when
you need
them? If it measures 120vac tie a light bulb into the primary side
and leave it
where it can be seen. Have the family let you know if it ever gets
brighter
than normal. Or better yet, call a pro.

That very thought crossed my mind, but I'm not sure how that could
happen. The unit is on a dedicated 115V circuit tied to a
single-pole breaker. Where would the possibility exist for the
neutral to become hot? In the breaker box?


A neutral in a breaker box could have a loose connection. These
connections can heat up and cool down as more/less electricity is used
on the panel in question. And there may be a point where the
connection fails momentarily or arcs. Then if you lose the neutral
connection, 120 volt circuits become 240 volt circuits via other
circuits turned on in the panel on the other leg.

The solution to that problem is usually to have the electric company
or electrician retorque the main panel lug nuts to the panel
manufacturers tightness specifications in inch lbs. Note POWER IS
ALWAYS HOT TO THESE CONNECTIONS and this is NOT a DIY thing!

A similar problem can be with what is called a MWBC or Multi-Wire
Branch Circuit. These are 2 separate 120 V circuits run to the same
area that share one neutral wire. So 2 hots and 1 neutral coming from
the breaker panel. A loose neutral connection with that one wire and
same problem.


MWBC is not legal for a furnace and never has been (at least in
Canada) and a main panel neutral problem should have manifested
iteself with light bulbs flaring/dimming or other indications of
"wonky" voltage elsewhere in the house.

When the entire panel is having this problem, you would see lights get
brighter as the problem occurs.

If just a problem with one MWBC, then only those circuits would have
the problem.