How does this work?

[mattsilkwood]

I was at a plant a few days ago to replace a bunch of thermistats with a programable type.
They were all supposed to be 24vac controls. I had to pull new wire from the units because I needed the common for my new thermistats and they had two wire cable pulled.
So I open up the first unit and I find a 480/120 control xformer. What they had done was take 120v through the thermistat and back to the starter for this unit.

This paticular thermistat ran two units so I go open the other unit and find the exact same thing.
After tracing down the old cable I find that they have spliced both hots together then through the thermistat and back out to the starter on each unit.

Niether of the control xformers were bonded.

My question is how was it possible to tie both of these xformers together like this and not pop the fuse on the secondary on one or the other?

This is a drawing of what I found.

View attachment 4129

 

[Buck Parrish]

Would being on the same phase have any thing to do with it?

(I didn't see the drawing untill I quoted the post)

 

[realolman]

I'm not exactly sure what you're stating here, or what the question is.... but I'd say it's probably precisely because the transformers are NOT tied together in more than one place.

The two transformers together would act similarly to a single center tapped transformer

 

[singledad234]

i have done this before on package units, as long as u keep the phases the same it will work, i have tied in as many as four five ton single phase units and kept the phases the same it will work

 

[K2500]

With the transformers not being bonded, i would think the secondarys could be tied together anyway you wanted without blowing fuses.

Might have gotten a little confusing if you where shutting down a single unit for service, though.

 

[mattsilkwood]

Thanks guys It just clicked. The reason it doesn't blow is because they only have one reference to each other. No different than bonding one leg.

A classic case of me over analizing things.

 

[realolman]

......Might have gotten a little confusing if you where shutting down a single unit for service, though.

I'm not sure confusing is a strong enough word...

Seems to me that the common connection of the control transformers at the thermostat could energize the low voltage side of the control transformer in whatever other of the "units" the OP is talking about... even though that other unit is "disconnected", locked, and tagged at the unit's disconnect.

That will in turn energize two phases of the entire high voltage motor wiring distribution of that "disconnected" unit "

The presence and magnitude of the voltage may depend upon whether the thermostat is open or closed at the time the voltage check of the LOTO is accomplished, and what the impedance of the coils or whatever else is hooked to the the circuits.

It just seems to me to be a bad idea... I think use a seperate thermostat or a relay.

 

[K2500]

I'm not sure confusing is a strong enough word...

...
It just seems to me to be a bad idea... I think use a seperate thermostat or a relay.

I agree, I said confusing while thinking about finding voltage where none should be. I did'nt take into account the position of the t-stat while verifying absence of energy. I should have said Dangerous.

 

[mattsilkwood]

I haven't been back on site to check it but I don't think it would backfeed because would only have voltage in reference to the other transformer.
Being as how niether transformer is bonded, the only reference to the other transformer is the one leg that is tied together, so, no way to complete the circuit.

 

[RICK NAPIER]

It use to be very common to tie one leg of the transformers together. I saw a lot of this on heating systems when the control valves took more power than they do now.