Startco Engineering Ltd.
Page 1
SE-325 Neutral-Grounding-Resistor Monitor
Rev. 8
Pub. SE-325-M, May 6, 2008.
1. G
ENERAL
1.1 M
ODERN
R
ESISTANCE-
G
ROUNDED
S
YSTEMS
A high-resistance-grounded system uses a neutral-
grounding resistor (NGR) with a low let-through current
to limit ground-fault current. High-resistance grounding
is gaining popularity because a ground-fault flash hazard
exists in low-resistance- or solidly grounded systems and
a ground-fault can result in substantial point-of-fault
damage. High-resistance grounding eliminates these
problems and modern ground-fault protection operates
reliably at these levels. Furthermore, the probability of an
arc-flash incident is significantly reduced in a high-
resistance-grounded system.
NGR selection depends on system charging current and
whether the system is an alarm-only or a tripping system.
Alarm-only systems are usually restricted to system
voltages up to 5 kV with NGR let-through currents of 5 A
or less. Occasionally, alarm-only systems up to 15 kV
and up to 10 A are used; however, they are not common
because a ground fault on such a system tends to escalate
to a phase-to-phase fault before the ground fault can be
located and cleared.
System charging current is the capacitive current that
flows to ground when a bolted ground fault occurs. This
current can be calculated or measured. For small systems,
the magnitude of charging current is typically ½ A per
1,000 kVA on low-voltage systems and 1 A per
1,000 kVA on medium-voltage systems.
In an alarm-only system or in a tripping system without
selective coordination, choose an NGR with a let-through
current larger than the system charging current and set
the pick-up current of ground-fault devices at or below
50% of the NGR let-through current.
In a tripping system with selective coordination, use
ground-fault devices with a definite-time characteristic to
achieve time coordination. Use the same pick-up current
for all ground-fault devices—this value must be larger
than the charging current of the largest feeder. Select an
NGR with a let-through current between five and ten
times the pick-up current of the ground-fault devices.
Do not use a grounding transformer with a low-voltage
resistor:
•
The combined cost of a transformer and a low-
voltage resistor is more than the cost of a resistor
rated for line-to-neutral voltage.
•
A transformer saturated by a ground fault through a
rectifier can make ground-fault protection
inoperative.
•
Transformer inrush current up to twelve times rated
current can cause a ground-fault voltage larger than
expected.
•
A parallel transformer winding makes it difficult to
monitor NGR continuity.
•
A transformer can provide the inductance necessary
to cause ferroresonance if the NGR opens.
Following these guidelines will reduce the flash
hazard, reduce point-of-fault damage, achieve reliable
ground-fault protection, and ensure a stable system not
subject to ferroresonance.
1.2 SE-325 NGR M
ONITORING
The SE-325 is a neutral-grounding-resistor monitor for
resistance-grounded systems up to 25 kVac. It measures
current in a transformer or generator neutral, neutral-to-
ground voltage, and continuity of the neutral-grounding
resistor. The SE-325 coordinates these three measurements
to detect a failed NGR or a ground fault and provides one
output contact for shunt or undervoltage operation in a
main-breaker trip circuit. Trips are latched and indicated by
LED’s.
Ground-fault current is sensed by a CT200 window-type
current transformer. Either CT input can be grounded to
meet electrical codes. A trip level of 0.5, 2.0, or 4.0 A is
switch selectable for use with a 5-, 15-, or 25-A grounding
resistor. Trip time is adjustable from 0.1 to 2.0 seconds.
Neutral-to-ground voltage and continuity of the neutral-
grounding resistor are continuously measured through an
ER-series external sensing resistor connected to the neutral.
A resistor fault will be detected if ground-fault current is not
detected and neutral-to-ground voltage exceeds the trip-level
setting, or if NGR resistance exceeds the trip resistance. A
resistor-fault hold-off circuit prevents nuisance trips in
alarm-only systems.
For additional information on neutral-grounding-resistor
monitoring, see “Monitoring Neutral-Grounding Resistors”
at www.startco.ca.
2. O
PERATION
2.1 S
ETTINGS
2.1.1 GF T
RIP
T
IME
Ground-fault trip time is adjustable from 0.1 to 2.0
seconds. Time-coordinated ground-fault protection requires
this setting to be longer than the trip times of downstream
ground-fault devices.
For the extended trip-time option, trip time is adjustable
from 0.1 to 5.0 seconds.
2.1.2 GF
The ground-fault-circuit trip level is 0.5, 2.0, or 4.0 A
when current is sensed with a CT200 current transformer.
Since the ground-fault-circuit trip level should not be greater
than 20% of the grounding resistor let-through current, these
levels are appropriate for use with 5-, 15-, or 25-A
grounding resistors. See Table 1. For other applications, the
trip level of the ground-fault circuit is 0.25, 1.0, or 2.0% of
the primary rating of the 5-A-secondary current transformer.
Содержание SE-325
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