I work in a laboratory/pilot plant facility that uses both 110 and 220 volt for some of the various lab equipment, heaters, and motors. Normal electrical service was restored later in the morning.

We found it strange that the voltage could be cut in half. How exactly does that happen? I realize 220V is two 110's. Don't see how the voltage splits downward that far.

Ken

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.

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.

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.

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

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