I have a problem with a 480 volt 3 phase 3 wire ungrounded electrical system. ?

If there is a short on one of the phases the voltage on all three phases changes. One phase is 74 volts second is 240 volts and the third is 490 volts. I would like to know how I can protect each machine individualy so when a short does happen it will not affect the whole factory.

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  • 1 decade ago
    Best Answer

    A standard overload with single phase protection will work if the voltage imbalance is as bad as you say

  • 1 decade ago

    A short circuit will either heat up until the wire burns through and the resistance then becomes infinite, or it will take all the current without leaving any voltage. Yet your question specifies 3 voltages, one for each phase as well as the supply voltage of the whole system.

    I think if one phase is shorted out the others will not normally change since the power generator is separately wound for each phase too. In this case they don't need protection. However if protection was necessary and is possible it would require a really large store of power on a capacitor or coil, to provide sufficient to allow the system to slow down in a gradual manner.

    The alternative is a voltage sensitive switch (sprung solinoid)so that once there is a significant voltage loss or (current surge) in any one phase, then the whole of the power is cut off. This kind of fuse is commonly used in domestic supplies.

  • 1 decade ago

    You have a phase-to-ground fault (through impedance) on the first of the three phases.

    There is no simple way to provide automatic phase-to-ground fault protection on an ungrounded system, especially if you would also like to have 'selective protection' such that only the faulted portion of the system is shut down as a result of the fault.

    You could ground the neutral of the system, but that might then require that you rework the protective system to be able to detect ground faults.

    A better solution might be to apply 'high resistance grounding' between neutral and ground. When applied with a pulsing contactor, this scheme provides both an automatic detection of the ground fault and an ability to localize the fault (using a clamp-on ammeter).

  • 1 decade ago

    You are apparently measuring each phase to ground. With an ungrounded system this has no particular value. To protect the system each circuit needs the protection of a circuit breaker. The breaker protects the wire that feeds the circuit. If you are concerned about a phase short to ground then you need some kind of ground fault sensing device.

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  • Anonymous
    1 decade ago

    Your system must be 480 volt Delta connected system. To avoid a grounded voltage which is unbalanced, connect a 480volt/240 volt grounding transformer which is wye connected at the secondary. This transformer may be used for industrial plant lighting and should trip the primary supply each a grounded phase occurs.

  • Elaine
    Lv 4
    4 years ago

    Three phase 3 wire is often called a 'delta' connection since the load(s) are distributed between each pair of phases. The 4 wire systems are also called 'Y' systems since the loads are distributed between each phase and a 4'th 'common' phase. The 4 wire system is intrinsically a bit safer since the common phase can also be made electrically neutral (ground). Doug

  • Are you sure that your system is ungrounded because it looks like you have one phase to ground fault. Normally these faults are not detected by conventional methods on ungrounded systems.

    However you can very well protect your system by installing separate breaking & protection devices for each load.

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