PGA-016A Littelfuse Inc, PGA-016A Datasheet - Page 45

• 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

© 2009 Littelfuse
POWR-GARD
Ground Fault Protection
Table 1 shows the leading initiators of electrical faults.
As as example, in the toaster circuit above, the black or hot
wire is shorted to the metal casing of the toaster. When
the circuit closes, all or part of the current is channeled
through the toaster frame and then the green ground wire.
When sufficient current flows (typically 6 x 15 A = 90 A), the
circuit breaker will open. A protection relay could be installed
to detect currents as low as 5 mA, which would open the
circuit breaker at a significantly lower level, hence, much
quicker than the traditional circuit breaker.
Although the example above shows a solidly grounded single-
phase circuit, the philosophy is the same on three-phase
circuits discussed later. Relays and monitors are specifically
designed to look for the leading initiators shown in Table 1 by
detecting low-level changes in current, voltage, resistance or
temperature.
DC Systems
Direct current (dc) systems have positive and negative buses.
If either bus is grounded, then it is referred to as a grounded
system. If neither bus is grounded, then it is referred to as an
ungrounded dc system. A ground fault on a dc system may
cause damage to the source as well as in the field.
If the system is ungrounded, then it is possible to use a
ground-fault relay by installing a ground-reference module
between the two buses to establish a neutral point. The
ground-fault relay uses this neutral point as a reference to
detect low-level ground faults.
Exposure to moisture
Shorting by tools, rodents, etc.
Exposure to dust
Other mechanical damage
Exposure to chemicals
Normal deterioration from age
LeaDING INItIatOrS OF FaULtS
120 V
•
POWR-GARD
15 A
®
BLACK
WHITE
GREEN
Protection Relay Catalog
®
Protection Relays
RECEPTACLE
FIGURE 2
TABLE 1
l
FAULT
% OF aLL FaULtS
22.5%
18.0%
14.5%
12.1%
9.0%
7 .0%
43
Ungrounded AC Systems
Ungrounded ac systems, shown below, were used where
continuity of power was critical. For example, chemical
plants or refineries involving processes that cannot be
interrupted without extensive dollar or product loss
may have an ungrounded system. However, experience
has proven that these systems are problematic and are
being replaced with resistance grounded systems. Two
major problems with ungrounded systems are transient
overvoltages and difficulty locating ground faults.
•
•
•
•
An ungrounded system has no point in the system that
is intentionally grounded (other than the normal bonding
which is always present to connect the non-current-
carrying metal parts to ground). Grounding occurs only
through system capacitance to ground (as shown in
Figure 4).
Continuity of power occurs because the system can
operate with one phase faulted to ground.
An intermittent or arcing fault can produce high transient
overvoltages to ground. These voltages are impressed
on the phase conductors throughout the system until
the insulation at the weakest point breaks down. This
breakdown can occur at any point in the electrical
system, causing a phase-to-ground-to-phase fault.
Although a ground fault can be detected or alarmed on
the system, there is no way to determine the location of
the fault.
L2
L1
DISTRIBUTE
SYSTEM
CAPACITANCE
PHASE C
PHASE A
PHASE B
FIGURE 3
FIGURE 4
GROUND REFERENCE MODULE
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