Ungrounded Neutral Systems:
§ In ungrounded system there is no internal
connection between the conductors and earth. However, as system, a capacitive
coupling exists between the system conductors and the adjacent grounded
surfaces. Consequently, the “ungrounded system” is, in reality, a “capacitive
grounded system” by virtue of the distributed capacitance.
§ Under normal operating conditions, this
distributed capacitance causes no problems. In fact, it is beneficial because
it establishes, in effect, a neutral point for the system; As a result, the
phase conductors are stressed at only line-to-neutral voltage above ground.
§ But problems can rise in ground fault
conditions. A ground fault on one line results in full line-to-line voltage
appearing throughout the system. Thus, a voltage 1.73 times the normal voltage
is present on all insulation in the system. This situation can often cause
failures in older motors and transformers, due to insulation breakdown.
§ Advantage:
1.
After the first ground
fault, assuming it remains as a single fault, the circuit may continue in
operation, permitting continued production until a convenient shut down for
maintenance can be scheduled.
§ Disadvantages:
1.
The interaction between
the faulted system and its distributed capacitance may cause transient
over-voltages (several times normal) to appear from line to ground during normal
switching of a circuit having a line-to ground fault (short). These over
voltages may cause insulation failures at points other than the original fault.
2.
A second fault on
another phase may occur before the first fault can be cleared. This can result
in very high line-to-line fault currents, equipment damage and disruption of
both circuits.
3.
The cost of equipment
damage.
4.
Complicate for locating
fault(s), involving a tedious process of trial and error: first isolating the
correct feeder, then the branch, and finally, the equipment at fault. The
result is unnecessarily lengthy and expensive down downtime.
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