Ah sudher... With respect to three phase systems, PHASE LOSS is the total loss of one of the three phases. Often this results in the destruction of any loaded three phase motors in short order as their torque drops significantly, so they slow significantly, which increases the current thru the motors way beyond spec.
VOLTAGE IMBALANCE is when one or more phase voltages are significantly different. For instance 208, 208, 200. This is of course far less serious but will still result in shortened motor life if it is a chronic problem.
As you probably know you can buy Phase Monitors that allow you to set the percentage/voltage imbalance allowed between phases before triggering a motor disconnect. Of course a phase loss will be concidered a bad phase imbalance and trip out also.
Suggestion: The phase loss is a specific case of phase imbalance (the relationship is set versus subset). It could be called very large phase imbalance. Normally, the phase imbalances are relative small for setpoints of relays that have trip, e.g. 3% only. It just depends where it is. Military Type III power supply allows 2% only of voltage unbalance. See:
MIL-STD-1399(NAVY)
SECTION 300A
13 October 1987
SUPERSEDING
DOD-STD-1399(NAVY)
SECTION 300
1 August 1978
(See 6.3)
""MILITARY STANDARD
INTERFACE STANDARD FOR
SHIPBOARD SYSTEMS
SECTION 300A
ELECTRIC POWER, ALTERNATING CURRENT"
It may be appropriate to consider phase loss as an extreme case of voltage imbalance. Further, it may be reasonable to consider phase reversal as even more extreme a case of voltage imbalance.
Loss of connectivity on a phase does not always mean there will be zero volts on the phase. Motors on the system may continue to run and will support some level of voltage on the open phase. So an open phase frequently appears like unbalanced voltage.
This can cause problems with loss of phase detection. Sometimes it is better to sense for loss of current than loss of voltage. Also, voltage imbalance can be detected with a negative sequence overvoltage (ANSI 47) device.
