Losing a Phase

Discussion in 'The Projects Forum' started by Megavolt, Jun 2, 2009.

  1. Megavolt

    Thread Starter New Member

    Apr 16, 2009
    I have a couple questions about the possibility of losing a phase because of a thunderstorm.

    I now work at a manufacturing plant that has 480V panels feeding machines. Most of the machines are from overseas, and have transformers beside them that change the voltage to 380V.

    Today I was told that a couple years back there were some power outages during thunderstorms, but only 1 or 2 phases were "knocked out". A couple minutes later, when the machines were restarted, somehow the motors single phased and burned out 13 motors at once. I was also told that the Power company had to come out and replace some parts (fuses?) in an outside transformer. I'm not sure what exactly happened, because I didn't work there at that time, but it's up to me to make sure it doesn't happen again.

    My questions are,
    1. Is it possible for the storm to knock out 1 or 2 phases as I was told?
    2. If so, is there a safety device that can detect a missing leg and perhaps turn off power till it's resolved?

    Like I said, I'm not sure what exactly happened. Any ideas and advice will be appreciated. :rolleyes:
  2. DC_Kid

    Distinguished Member

    Feb 25, 2008
    if the generator is still generating then its very possible that one leg (phase) feed line gets knocked out, either by a switch (fuse) or by wire being melted away due to a lightning strike, etc. this would leave your equipment with only 2 phases in tact with the 3rd one missing. i guess a easy way is to have a LED on each phase leg. if all 3 arent 'on' then you shouldnt power up the equipment, etc.

    i will assume there exists protection gear that senses a missing leg and will cut off the others.

    but i'm no power expert....
  3. rvh002@gmail.com

    Active Member

    May 15, 2009
    Hi, It is most definitely possible to loose one or two phases. In fact in some areas it is quite common. There are various ways to gaurd against this. Depending on the size of the motors, they are nomally started with a contactor(large pupose made relay). The contactor should be fitted with an overload that sits between the contactor and the motor. A proper overload detects the phase failure and trips to protect the motor. Another way is to employ a phase failure relay at the main distribution board. This is a plug in device on a relay base. Normally refered to as 8 or 11 pin bases. This device is wired to control the main circiut breaker via a shunt trip coil. Or if the total load is not high (let's say 80 to 100 amp per phase) you can add a contactor in series between the main circiut and the load. The relay then controls the contactor.
    Hope this helps
  4. Søren

    Senior Member

    Sep 2, 2006
  5. Megavolt

    Thread Starter New Member

    Apr 16, 2009
    Guys, I wasn't aware of these devices. Thanks for pointing me in the right direction.

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