PGA-016A Littelfuse Inc, PGA-016A Datasheet - Page 46

• Manufacturer Part Number: PGA-016A
• Description: BULK / WATERTIGHT COVER FOR PGR-6200 / PGR-7200
• Manufacturer: Littelfuse Inc

Specifications of PGA-016A

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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There are two methods used to detect ground faults
in ungrounded systems. One method is to monitor the
voltages between the phases and ground. As a ground fault
develops, the faulted phase will collapse to ground potential,
causing an indicator light to dim. The indicator lights on the
unfaulted phases become brighter.
A better method to detect a ground fault is to measure
the insulation resistance. As the insulation deteriorates, a
relay continuously monitoring the insulation resistance can
alarm at different levels for predictive maintenance. A visual
indication or meter can also be used.
Solidly Grounded Systems
Due to the problem of ungrounded systems, a shift in
philosophy occurred and designs moved from ungrounded
to grounded systems. In most cases, the type of grounding
system chosen was solidly grounded. A solidly grounded
system is a system of conductors in which at least one
conductor or point is intentionally grounded (usually the
neutral point of transformer or generator windings). The
problem with the direct connection is that ground fault current
can be excessive, causing Arc-Flash hazards, extensive
equipment damage, and possible injury to personnel.
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Figure 6 illustrates an example of the dangers associated
with solidly grounded systems. In this example, a ground-
fault occurs and the overcurrent protection is set at 600 A.
GROUND
In a solidly grounded system, the wye point (or neutral)
of the power source is connected solidly to ground and
offers a very stable system that maintains a fixed phase-
to-ground voltage.
The high ground-fault current is easy to detect with fuses,
circuit breakers, or protection relays, allowing for selective
tripping (tripping the faulted feeder and not the main feeder).
When a ground fault occurs, high point-of-fault damage
can quickly result since the energy available to the ground
fault is only limited by the system impedance (which is
typically very low).
Due to excessive ground-fault current and Arc-Flash
Hazards, the faulted feeder must be removed from
service. This does not allow for continuous operation
during a ground fault.
NEUTRAL
PHASE C
PHASE A
PHASE B
FIGURE 5
POWR-GARD
Ground Fault Protection
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Aside from converting this solidly grounded system to
resistance grounding, the best way to prevent damage is
to detect low-level ground leakage prior to it becoming a
ground fault. In order to accomplish this, the protection relay
must be able to sense a low-level ground leakage without
nuisance tripping.
In modern facilities, equipment often generates noise or
harmonics that may interfere with a protection relay’s ability
to function properly. For example, the noise or harmonics
may be higher than the desired ground-fault relay settings,
causing the relay to falsely operate when there is no fault on
the system. The protection relay must be able to filter out
noise or harmonics to provide reliable protection.
Resistance Grounded Systems
Resistance grounding is the only method of grounding
that solves the problems commonly associated with both
ungrounded systems and solidly grounded systems. The
name is derived from the addition of a resistor between the
system neutral and ground. The specifications of the resistor
are user determined to achieve a desired ground fault
current, which must be greater than the system capacitive
charging current (explained later in this section).
®
Assume that this ground-fault is not a bolted fault, but an
arcing fault due to an insulation breakdown or a partial
reduction of clearances between the line and ground.
Because of the arc resistance, the fault current may be
reduced to as low as 38% of the bolted-fault level. The
fault current can be in the range of a normal load or a
slight overload.
The fault current may be low enough that the overcurrent
device (600-A circuit breaker) does not sense the fault
current, or may pick it up but not trip for a long time.
The energy being supplied by the source is concentrated
at the arc and could cause severe equipment damage
very quickly. This energy release could cause a fire that in
turn, could damage the premises and present an extreme
hazard to personnel.
Protection Relays
© 2009 Littelfuse
600 A / 3 P
FIGURE 6
FAULT
EQUIPMENT FRAME OR BARE COPPER
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POWR-GARD
ARCING
FAULT
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Protection Relay Catalog