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loose hot (not neutral)

Discussion in 'Electrical Engineering' started by [email protected], Sep 3, 2008.

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    Electricians and power engineers know the implications of a disconnected or
    loose neutral conductor. But what about one of the hot conductor phases?

    In three phase, for motors connected as delta, that means single phasing.

    If they are connected wye (how common is this?), it could be different as
    there is some two dimensional aspect of the two remaining phases. So I
    might think that a motor wired this way and losing a phase could still
    start and run, in a derated manner.

    One interesting issue I see with both three phase and single phase is the
    interaction between line-to-line loads (including delta connected three phase
    loads) and line-to-neutral loads. Looking at single phase for simplicity,
    all those L-L loads are not getting their 240 volts. But they are making a
    connection between the live line and the dead line, effectively energizing
    it at a high impedance. Then this voltage gets applied to all the L-N loads
    on the dead side. This results will obviously vary greatly depending on the
    mix and balance of L-L and L-N loads. But I see the potential for limited
    amounts of damage to some equipment attempting to operate on the low voltage
    and higher impedance.

    Another interesting issue is probably only applicable to single phase. This
    is the metering. As I understand it, many, or maybe most, single phase energy
    meters for single phase just use a single transformer for each of current and
    potential. Both hot conductors pass through the current transformer, but in
    opposing directions due to their opposing phase angle. But if the loose hot
    conductor is loose ahead of the meter, the potential transformer will get a
    greatly reduced voltage reading, depending on the balance of loads connected
    to the dead phase and either neutral or the other phase. So whatever current
    does flow on the still live phase, it will be multiplied by the much lower
    (perhaps typically 1/4 as much) voltage the potential transformer reads.
    But I guess that is a blessing in disguise in a bad situation when the power
    company isn't delivering all phases.
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