Find faults fast(er) with
faulted circuit indicators
One of the best ways to locate a fault in the shortest time.
F
inding faults quickly is the key to
decreasing customer outage time
after a fault has occurred. In the past,
often it took longer to find the fault
than repair the fault. Faulted circuit
indicators (FCIs) have emerged over
the past couple of decades as one of
the best ways to locate a fault in the
shortest possible time. Unlike
thumping or radar-locating equipment, a faulted-circuit indicator will
not tell you the exact location of the
fault, but will tell you the section of
line where the fault has occurred.
The early designs and applications of
fault indicators provided a great deal
of education for utility engineers,
operating personnel and faultedcircuit indicator manufacturers. While
history is the best teacher, it is also
the most expensive and when
specifying you should be sure not to
make the same mistakes people
have made in the past.
Following are three items of interest
that should be understood and
considered when making recommendations for the installation and
operation of faulted-circuit indicators.
False Tripping
The FCI knows only that the current
in the cable to which it is attached
has surpassed its trip level. The FCI
indicates a faulted circuit by displaying a red target. In the case where
active components of the electrical
distribution system contribute to the
fault current flowing into the faulted
portion of line, FCIs that are not
between the source and the fault
may be tripped. This tripping is
caused by a discharge of active
components downstream of the
location of the faulted spot on the
line.
Some of the items that can contribute to this discharging situation are:
• Capacitor banks
• Three-phase delta connected
circuits
• Long lengths of URD cable
• Rotating machinery loads
URD cables are capacitors by their
construction and are probably the
most overlooked part of active
components in the URD arena.
Computer simulations show that
URD cable discharge can be several
hundred amperes with its time to
decay depending upon circuit
parameters. As you would suspect,
higher voltage URD cables (i.e.
25kV, 35kV) deliver higher peakcurrent amperes. The length of the
cable is a determining factor in both
the magnitude of the current and the
time for it to decay.
Figure 1 ( next page) shows a
simple URD circuit illustrating the
current flow into the fault that may
cause the indicators to trip.
It has been the recommendation of
FCI manufacturers to use the highest
level of trip current possible so that
the trip level is above the active
component discharge current, but
below the fault current at that location on the line. This is shown in
Figure 2 (next page). Depending
upon the available fault current at the
application point of the FCI, this
recommendation may be easier said
than done. It was with this motive in
mind that a project was initiated to
develop a faulted-circuit indicator that
would not only indicate the presence
of a fault on that section, but point to
the direction of the current flow.
The directional fault indicators
provide a road map to finding the
faulted section of line. Figure 3
shows that a directional fault indicator will lead the operating personnel
to the faulted section of line regardless of whether the additional fault
current downstream from the fault is
caused by an additional source or by
active components on the distribution
system. When using non-directional
FCI’s, the recommendation of
selecting a trip current as high as
possible to avoid false tripping by
active components still applies, but if
problems persist in these locations a
directional fault indicator is the
answer.
NOTE: Because Hubbell has a policy of continuous product improvement, we reserve the right to change design and specifications without notice.
®
POWER
SYSTEMS, INC.
573-682-5521
Fax 573-682-8714 http://www.hubbellpowersystems.com
ANDERSON
®
®
®
UNITED STATES • 210 N. Allen • Centralia, Mo 65240 • Phone: 573-682-5521 • Fax: 573-682-8714 • e-mail: hpscontact@hps.hubbell.com
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©Copyright
2000 Hubbell • 210 North Allen Street • Centralia, MO 65240
®
1
Bulletin 16-9202
Figure 1
False Resets on Secondary
Reset FCIs
Probably the best fault indicator to
be applied on padmount transformers is a device that has the sensor
connected to the primary cable and
then a connection from the sensor
through an EPDM cable to the
secondary spade of the transformer.
Since the insulating material EPDM
is used in the connection, leakage
current of microamps is used to both
power the FCI and indicate the
presence of a normal circuit by
sensing the secondary voltage.
Normally, the reset level or the
secondary voltage that the indicator
considers to be “normal” is 35-40%
of nominal (e.g., approximately 45
volts on a 120 volt secondary).
Figure 2
2
Now consider a situation where a
delta-connected load is being served
by a grounded wye-grounded wye
transformer protected by single
Figure 3
phase devices as shown in Figure 4
(next page). If one of the phases of
the transformer is not energized as a
result of the opening of a singlephase protective device, the secondary spades of the transformer will
indicate some voltage on all three
phases. Let’s assume in our situation
that B phase is the circuit that is not
energized by the source as a result
of a single-phase open protective
device; the A & C phases would be
at full line potential, while the B
phase would have voltage present on
the secondary spade (b) as a result
of backfeed. It is not uncommon for
this voltage to be high enough to
reset the fault indicator on this B
phase. You should now see the
problem. There is a fault on B phase,
but the FCI has reset to the normal
condition as a result of backfeed, and
this incorrect reading is no help to
operating personnel trying to locate
the fault.
The answer is to raise the level of
secondary voltage that the fault
indicator considers “normal” to a
point much closer to full secondary
voltage. This feature that many
utilities specify for their three-phase
applications is known as reset
restraint. Reset restraint is a percentage range (e.g. 70-95%) of the
full secondary voltage before the
faulted-circuit indicator considers the
system to be truly energized and
“normal” taking into account any
fluctuations normally seen on the
system. Current reset devices are
also susceptible to false reset in this
scenario, since they will see
backfeed current in the circuit shown
in Figure 4 on the next page.
Calibrating for a Specific
Diameter Cable
When applying a secondary reset
FCI to three-phase transformers
that could serve delta loads,
always consider the use of reset
restraint.
What if you want to put an FCI on
a conductor other than the diameter for which it is calibrated? Ask
for a trip current vs. cable diameter
chart. A sample chart is shown in
On many fault indicators the user's
cable diameter of application is
requested for factory calibration.
During production, the unit is installed on a cable (the same size as
the user specifies) and is calibrated
for proper current, + 10%.
The diameter of cable for which the
FCI has been factory calibrated for is
permanently stamped on the FCI
units. The permanent hot stamp
marking appears as a “C” followed by
a number. For example, a unit
stamped C1.1 is calibrated for 1.1
inch diameter cable to trip at the
specified trip current + 10%.
3
Figure 5. In this case, the unit is a
200 amp trip, factory calibrated for
0.475” conductor. The range of
conductors is 0.28” through 0.57”.
Not all faulted-circuit indicators
require the cable diameter for
calibration, but when it’s required,
ask yourself, “what is the range of
conductors on which the unit may
be applied?”
To request a chart, the following
should be provided:
1) Type of unit (e.g. Current
reset, Secondary reset,
Electrostatic reset)
2) Desired trip current in amps
3) Diameter range of conductor
- min., max.
4) Calibration point within the
range
In the example shown in Figure 5,
items are:
1) Type of unit: ERL (Electrostatic Reset)
2) Desired trip current: 200
amps
3) Diameter range: min. = 0.28",
max. = 0.57"
4) Calibration point: 0.457"
Figure 4
Trip Rating vs.
Cable Diameter
for ERL200
Calibrated at 0.475"
A helpful hint: if you know the range
of cable you have, calibrate for the
average diameter and the chart will
give you the exact trip currents.
Conclusion
There are many application aspects
of faulted-circuit indicators. This
article addresses three of the more
frequently asked questions.
The proper application and operation of existing designs along with
the introduction of the new Chance
LCD and Directional designs have
brought faulted-circuit indicators a
long way and your customers have
spent much less time in the dark. ❐
&
TiPS NEWS
Reprinted from
October 1992
Figure 5
©Copyright
4
2000 Hubbell • 210 North Allen Street • Centralia, MO 65240
Bulletin 16-9202