Hard to venture a guess, but it's possible that a leg of the three phase higher voltage feeder died and this is how it manifested itself on your side of the stepdown transformer.

Probably one of the hot lines was out and you were using a digital meter reading phantom voltage. Likely the voltage was really zero.

Thanks for the replies.

What exactly does it mean when you say a "leg of the three phase"? I've heard this term before. Did we have 2 phase and what is a "phase" and "leg" as it relates to electricity?

I'm a chemist so electricity is out of my area of knowledge, at least anything more than 110V. We did use a cheapie analog test meter showing low voltage. Some of the lights were pretty dim, so I'm sure we really did have 1/2 normal voltage in many circuits.

Many commercial buildings are served by three phase power unlike the single phase power that most residences use. It typically gets passed around at about 13KV and gets dropped down sometimes to 480V before being ultimately dropped to 120V.

All commercial power is generated this way. Your typical residence gets one phase dropped down to 240V with a center tap that is grounded giving two 120V 180-degree out of phase legs.

It's possible your 240 you are using is in fact only the 208 that leg-to-leg three phase would yield. (since they are 120 degrees out of phase, they are off increased by 1/sin(60) not 2.

"What exactly does it mean when you say a "leg of the three phase"?"

A bad mixture of words?

3-phase power has just that, 3 phases.
A Y-system has a neutral that is 0 volts 'betweeen all 3 phases (one end of each phase winding is grounded) while a delta system has no neutral point.
Y systems need 4 wires, delta systems can use only 3.
The penalty for delta systems is circulating current around the delta.

The voltage is a sine wave on each phase running 120 degrees separated from the others.
If you call one zero degrees, the others are at +/- 120 degrees from that one.

Leg is used for single phase power like 120/240 V.
The legs are 180 degrees apart, but actual 2-phase is 90 degrees so 'leg' is used.
The 120/240 V is created from distribution voltage 7.2 kV and up) using a transformer.
The secondary of the transformer is center tapped, and that point is grounded.
This allows 240 V across the entire winding, but 120 V from either end of the winding to the center point.

Pole transformers can be run from line-to-line on the 3-phase distribution system, or line-to-neutral on

If you one phase is missing you can get all sort of strange voltages on the output side of a line-to-line transformer, depending on what loads are present on the 3-phase system.

You did actually have 55 volts if lights were dim. No phantom voltage this time. The power company (POCO) had a problem on one of the lines feeding your building.

OK, when you say the voltage is a sine wave it makes sense to me. Should have remembered that, I suppose.

Our power is actually 208volts coming into the building. The little analog meter was reading around 50 volts or so, more or less. Didn't look close, we were just trying to figure out what the heck was going on. It did fry a board on one of our analytical instruments that's left on all the time.

Thanks for the explanation, guys.

Ken

Dropping a phase can play havoc on any 3 phase motors you have as well.

We've lost single phases in our building at times as well.

I am getting 55 volts across the hot and neutral when the switch(s) are off. (It is two, three-way switches)
IÂve replaced three florescent light fixtures in the past nine years (electronic ballasts bad). On both three-way lighted Decor switches, I notice the illuminated switches flickering constantly. Also, on one of the compact fluorescent bulbs, I notice it flickering (when dark).
Could it be the switches? I donÂt think itÂs "phantom voltage" you mentioned in the previous posted/blogs.
Could it be mutual Induction, or some form of induction, possibly fro wires too close?

"Could it be mutual Induction, or some form of induction, possibly fro wires too close? "

It is very hard to get enough current from coupling to light anything up, including a fluorescent light.

It is more likely the POCO has a 'power quality' problem frnm old infrastructure.

Complain to them and demand a monitor be placed on your service to check for voltages out of spec.

Do a search 'CBEMA curve' and you will find the ITI curve of allowed voltages vs. time.

You can get similar symptoms with an open neutral.

"You can get similar symptoms with an open neutral."

The other leg of the 120 V would be at 185 and any incandescent bulbs would be burning very brightly (if they haven't failed yet).

A dropped neutral on an Edison 120/240 V circuit is usually recognized by the brighter light with changing loads on the circuits.

Depends on the loads and where the neutral has been lost.

"Depends on the loads and where the neutral has been lost."

The loss of a neutral in an Edison circuit removes the center tap tie point from the pole transformer.

The 120 V split across the full 240 V winding is no loner centered, so if one leg is 55 V, the other leg must be at 240 -55 = 195 V.

The 240 V sum does not change, nut the center point now moves around with the load on each 120 V leg.

The loss of a neutral on the high side (7.2 kV+) will result in NO output on the low side.

Most POCOs use 3-phase Y systems and run a neutral.
The circulating currents that can occur in unbalanced 3-phase delta systems cause a lot of trouble in distribution systems.

If the high side neutral voltage varies (or the high voltage phase) the 240 V changes, resulting in proportional changes to both 120 V legs.

It's been a while since I've been on this forum, but I saw this posting and wanted to share information about a similar experience I encountered. I always thought home wiring was either 120v or 0v (240v from two 120v legs). One time, a neighbor of mine was having trouble getting his basement stairway light to work. It wouldn't come on. He had me to look at it. I tried the bulb in another socket and it worked, so I knew there wasn't anything wrong with it. I took out the light switch in the wall and shut off the power to the whole house because I didn't know which breaker went to the light. I used my volt meter and turned the power back on. I got a reading of 50 volts. Thinking that possibly the copper had oxidized too much, I cut the wires back and exposed new copper. I still got only 50 volts. After several hours, the 50 volts still remained. I really believe that something was seriously wrong with the wiring in that house. There's a nearby kitchen light circuit that has really caused problems for him. He's had at least 2 light bulbs explode on that line for no apparent reason at all. He eventually called an electrician who had to order some kind of a special part. I didn't find out what it was. To make sure my volt meter was working right, I tested it out on my home's wiring and got 120 volts. This was probably the most unusual electrical issue I've ever encountered until I read this posting.

Many years ago someone decided that in addition to my regular work I would welcome the challenge of fixing test equipment. As I worked my way through stacks of Tektronix, Lavoie, and Hickcock scopes I came upon one very peculiar fault. It was a solder joint open circuit where several wires met at a connection that appeared perfectly sound on a unit that had worked perfectly ok for several years. The kicker was that this was at a 120vac distribution point not some 1 volt intermittent!

"He's had at least 2 light bulbs explode on that line for no apparent reason at all."

THAT sounds like a missing neutral on the service.

Depending on loading, one leg can approach 240 V as the other heads towards zero.