Power loss mystery

Thread Starter


Joined Feb 17, 2007
We have a situation in our printing facility with a new piece of machinery. The unit is supplied with single phase 240v 40a circuit.
The power circuit is taken from a 3 phase supply.

The machine looses power and of course shuts down. This happens 5-10 times an hour. In this condition the normal reading of 240v between L1 and L2 drops to 40v. I also measure 40v from each leg to ground.

The lowered power last anywhere from 1 to 5 minutes then returns to normal.

The three legs at the panel read 240v between each leg but to ground/nuetral
L1 to grd = 240v
L2 to grd = 240v
L3 to grd = 0v

We are using L1 and L2 to supply the machine.

In this condition of loss power I measure the incoming lines to the circiut panel and read 40v theres as well but if I open the breaker we have connected to the machine - the power will return to the normal 240v.

We are looking for any help we can get.

The machine is a heat tunnel and impulse sealer.

Thanks - Dan


Joined Nov 1, 2006
Hi Dan. From what you said it sounds like you get your plant power from a corner-grounded, delta-connected transformer secondary bank system. You have one thing in your favor: the problem lasts long enough to give you a chance to find it; that beats sudden milisecond transients that drop things out, then disappear until the next time.

You apparently have a loose connection ahead of the panel since the voltage on the incoming power lines to the panel read low. Leave the machine and breaker turned on and keep checking all your connections and switchgear back towards your service entrance for low voltage. Don't assume anythig is OK until you test it; if current passes through the power company's KWHr meter, it could be the culprit [though it's rare I've seen it once] or you could have a loose connection at the transformer bank [you'd better get help with that]. Let us know if you need more help or what you find.