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Thread: Problem with industrial geyser tank contactor

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    Problem with industrial geyser tank contactor

    I am having a problem with a 36kw Industrial Hot water tank. It is a 3 phase supply with neutral and earth. A thermostat controls a contactor which brings in the element. The contactor has a 220V coil so the circuit is separated into control and main circuits. The main circuit just has 3 circuit breakers which connects to the top of the contactor. The element is connected to the bottom of the contactor with the neutral connected to a neutral bar.The control circuit is just a single pole circuit breaker connected (at L1) to a thermostat and then connected to A1 of the contactor coil. The A2 is then connected to the neutral bar.

    Heres my problem:
    As soon as the thermostat is switched on the contactor should pull in and start the element. Problem is it DOESN'T! When I check the voltage on the coil I get 0V but check between L1 and N I get 0V and L2 and N I get 400V and L3 and N I get 400V? Even if I bypass the thermostat and connect L1 directly to the A1 and neutral to A2 it still doesn't bring in the contactor. The are no phases missing and the neutral is definitely there. I have proved this by switching off the control circuit and then testing between the phases and then line to neutral and everything is correct. It just when I turn on the voltage goes bye bye on L1
    Funny thing is that when I manually push in the contactor is stays in and then when the tank reaches temperature the thermostat kicks out the contactor as it should. Very weird for me as this has never happened to me before.

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    live 1 phase is definitely missing. you probably get feed back somehow,which confuses you into thinking that all 3 phases are fine. or under load L1 drops away.
    take the control circuit put it on L2 . it should work,but 1 bank of elements will be off.

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    Diamond Member AndyD's Avatar
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    It may be your neutral is floating whereby it will assume the same or close to same potential as the phase with the highest load. Test neutral to earth voltage with the contactor in and with the contactor out, if it's more than a few volts then there's a poor connection asomewhere. Beware though, the neutral could be at a dangerous voltage with respect to earth so assume it's live and treat it as live. Either way if it's a floating neutral or missing phase it should be straight forward to find the problem.

    Also pushing contactors in by hand is bad practice, if the load was more delicate than heater elements you could cause considerable damage if the star point neutral is floating or if there's a phase missing.
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    Thanks gents. So I tried the first suggestion of connecting the control circuit to L2 but i still don't get the contractor coming in so tried L3 but no luck. I then tried the second suggestion from AndyD and found that the voltage between earth and neutral with nothing switched on was 0V but then with the control circuit on I get 220v which is obviously NOT a good thing. I then checked and rechecked all all my neutral connections all the way back to the main DB that supplies the block of flats and everything is connected and is tight. Any other possible solutions?

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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by BlueArc View Post
    Any other possible solutions?
    Errm........ find the fault is the ony solution. If you have a neutral that's rising to 220v with respect to earth then the neutral is floating. If the neutral was in tact then it shouldn't ever rise more that a couple of volts above earth potential and even then that would be under heavy load conditions. You'll need to get your tester out and go find the neutral fault. If the neutral is floating at the point of supply back at the main DB then call Eskom or whoever the supply authority is and get them to sort it out on their side. If you're unsure on the testing proceedures then please get someone who is sure to assist you because Eskom will give you hell if you call them out and the fault is on the consumer side.

    *Edit* I doubt the fault is on the supply side because other people in the flats would have complained by now, depending which phase they're on either they'd have zero volts L-N and nothing would be working or they'd have 380V L-N and their lights and appliances would have gone tits-up with a smell of burning by now.
    Last edited by AndyD; 18-Dec-13 at 09:06 PM. Reason: added info
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    Call an electrician .

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    Gold Member Sparks's Avatar
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    Because the contactor stays in when pushed in by hand I suspect the coil is being fed from the wrong side of the contactor. Getting 400V from phase to N also suggests that L1 and N are swopped around. As for 220V between N and earth, I would expect that if there is something switched on somewhere in the building as the N is common. Unless N and L1 are swapped as suspected.

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    Is the neutral bar you refer to connected to the supply neutral?

    What is the supply system?
    The trouble with opportunity is it normally comes dressed up as work.

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