3phase 480

Discussion in 'General Electronics Chat' started by eels1212, Feb 21, 2008.

  1. eels1212

    Thread Starter New Member

    Feb 21, 2008
    7
    0
    Hi,
    Can somebody help me? I have a 480 3phase heater that is not working, when I measure phase to ground I get 277 each leg. When I measure phase to phase I get 480 on L3 to L2 but not L1 to L3, and that is what the 24v control transformer is attached to and there is no voltage on the secondary of that transfomer. The transformer is good.
    Thanks Much.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I'll bet you won't measure any voltage between L1 to L2 either.

    You have a problem with your L1 somewhere.

    You measure 277V to ground everywhere because of the primary of the 24V control transformer. If you disconnected it from L2, you would find 0V to ground on L1.
     
  3. Electra Guy

    Member

    Feb 4, 2008
    17
    0
    Sgt Wookie is correct. What I don't understand is did you check phase at the heaters, the load, or the source? If the source bad fuse or breaker. If at the load bad fuse or breaker, if the heaters bad heater.:) Let us know...
     
  4. eels1212

    Thread Starter New Member

    Feb 21, 2008
    7
    0
    I checked for voltage on the load side of the fuses inside the heater. I will go back to the switch panel and check the fuses there.
    Thanks a lot you guys.
     
  5. eels1212

    Thread Starter New Member

    Feb 21, 2008
    7
    0
    I disconnected the transformer and yes it showed no voltage on 1 leg. I found a blown fuse in the switch panel. This is a 50kw heater at 480v 3phase the amp draw is listed at 60.2 amps the fuses are 70 amp. This heater is a 2 stage that is jumped out to 1 stage the manufacturer says this is ok but with that configuration at start up it is a big load. Could that blow a 70amp fuse?
    Thanks again.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Why is it jumped out to 1 stage?

    Not a big puzzle as to why the fuse blew.

    Get it put back to 2-stage configuration. Have the 1st stage heat up for perhaps a minute or two, and then let the 2nd stage kick in.

    Heaters have a resistive response very much like lightbulbs. When you first switch on a lightbulb, it's like a dead short. As current rushes through the filament, warming it up, the resistance of the filament rises. Eventually, if the bulb (or heater element) is within it's design limits, the resistance of the filament rises enough to cause a decrease in current, just enough to maintain the filament at the desired temperature to produce light - or in the case of a heating element, heat.

    Going back to a multi-stage configuration will lessen the instantaneous load demands on your panel considerably, and make it unlikely that you will blow a fuse unless there is a real problem.

    There must be a reason why the 2-stage configuration was eliminated; probably burned relay contacts.

    If you are not absolutely certain of what you're doing, you should have a licensed electrician repair the circuit.

    If you repair the circuit yourself and get it wrong, or if there is a fire caused by your actions, the company handling the insurance for the building could righteously refuse to pay to repair the damages.

    Get a pro to come in and fix it right. That way you have the documentation to back you up in case anything goes wrong.
     
  7. eels1212

    Thread Starter New Member

    Feb 21, 2008
    7
    0
    It was an option from the manufacturer to use a jumper to make it single stage. This heater was installed recently (not by me),and replacing an older one that had a 2 stage internal thermostat, in the same location same size and specs as the older one. The guy that installed it connected it to a single stage thermostat. I have already ordered a 2 stage thermostat. I am taking off the jumper and will only use the first stage until I have installed the new thermostat.
    Thanks, you guys have been a great help.
     
  8. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    A couple more comments: 70A fuses are a bit light; the national electrical code requires wiring, fuses, etc., to be rated 125% of the load so the fuses should be 75A. Also, use dual element/time-delay type fuses like Buss FRS-75 or Gould-Shamut TRS-75 which are more current inrush-tolerant.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    Adding to wireaddict's statement - EVERYTHING up to the load must be rated for handling the current rating of the fuses. Connectors, wiring, etc. If you simply start using a larger fuse, you may be exceeding the rating of the wiring or connectors, which would be unsafe.

    If you have any doubts, consult a licensed electrician.
     
  10. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    The reason for having the rating requirement is to allow the fuses to protect the equipment (instead of the equipment protecting the fuses.) The chain breaks at the weakest link. If the weakest link is not the fuse, things get expensive. And dangerous.
     
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