In contrast, a three-phase sectionalizing device will open on all phases, regardless of which phase is faulted. A three-phase FCI will show a “FAULT” indication; however, it does not indicate which phase. For this type of application, it is better to use three single-phase FCIs. Here, only the FCI on the faulted cable will show a “FAULT” indication. The use of three single-phase FCIs also works well on underground circuit exits from a distribu- tion substation. In many cases, these circuit exits are protected by a three-phase sectionalizing de- vice. If the sectionalizing device has indicators to show the faulted phase, a set of FCIs is needed on the load end of the underground segment only. However, if the protective device does not have phase indicators, a set of FCIs must be placed at each end of the underground segment.
Some areas may have very long segments of underground cable. These segments may contain above-ground sectionalizing points or grounding points. Placing an FCI at these locations will lo- cate the exact faulted cable section.
FIGURE 3.13: FCI Placement on Three-Phase Underground Feeder. FIGURE 3.12: FCI Placement on Overhead Feeder with Underground
Segment.
Recloser FCIs Underground Line Segment FCIs
Three-Phase Underground Feeders
The most extensive type of underground feeder connects two substations. During normal opera- tion, this feeder has an open point with each side being fed by a different substation. In this application, the FCIs are placed on the circuit exits and on either the incoming or outgoing ca- bles in each sectionalizing cabinet. Figure 3.13 shows this arrangement.
Another consideration for this type of system is the choice of a trip rating. To select a proper trip rating, the cooperative engineer must con- sider the load and fault currents during normal and alternate feeds. If possible, a trip rating should be selected that will respond to the fault current available during normal and alternate feeds. Another option is to use an adaptive-trip FCI. As this FCI adapts to different line current levels, it responds properly during normal and alternate feeds.
A third consideration is the use of a three- phase FCI or three single-phase FCIs. As covered in the preceding subsection, a three-phase FCI is suitable only when the feeder is protected by single-phase sectionalizing devices. If the de- vices are three-phase, the only way to identify the faulted phase is to use a single-phase FCI on each cable, unless the three-phase protective de- vice has an individual target for each phase.
Substation A FCIs FCIs Substation B Switchgear 1 Switchgear 2 Switchgear 3
Underground Residential Subdivisions
An underground residential subdivision usually consists of single-phase transformers and cable operated as an open-loop system. Figure 3.14 shows this system with one FCI for each trans- former. This arrangement should work properly regardless of the location of the loop open point.
Large subdivisions can be more complicated. These subdivisions often contain multiple single- phase loops and may contain a three-phase under- ground sub-feeder. In addition to being placed at each transformer, FCIs must also be placed in each switching, sectionalizing, or junction cabinet. Fig- ure 3.15 shows FCI placement in a large subdivi- sion. If SW1 and SW2 were three-phase junction cabinets without fused taps, then FCIs must also be placed on each load-side cable. This arrange- ment lets field personnel open the cabinet and determine which phase has the faulted cable.
FIGURE 3.14: FCI Placement for Single-Phase Open Loop.
FIGURE 3.15: FCI Placement for Underground Subdivision with Three-Phase Source.
Switching Cabinet Switching Cabinet Riser Pole Riser Pole Riser Pole N.O. Riser Pole
Single-Phase, Pad-Mounted Transformer
N.O.Normally Open Point FCI
LEGEND
Three-Phase, Pad-Mounted Transformer Single-Phase, Pad-Mounted Transformer
N.O.Normally Open Point FCI LEGEND N.O. N.O. N.O. N.O.
SELECTING A RESET METHOD Manual Reset
The manual-reset type is the simplest and least expensive FCI. It typically costs half that of the automatic-resetting types. As expected, there are trade-offs for this reduction in cost. First, service personnel must reset this FCI in the field. Any tripped indicators that service personnel miss will continue to show a “fault” indication. Dur- ing a future outage, these indicators will confuse crews and probably increase the time required to locate the faulted cable section. If this be- comes a common occurrence, crews will soon ignore the fault indicators.
Failure to reset an FCI is more likely on an underground than on an overhead system. On an underground system, the FCIs are usually lo- cated inside pad-mounted enclosures. After a crew locates the faulted line section, they must open all enclosures located before the faulted cable section and reset each FCI. During after- hours power restoration or during inclement weather, this step may be neglected.
This device has two other limitations:
• No coordination with current-limiting fuses, and
• No remote indicator.
Because it operates more slowly, this FCI can- not be used on underground systems protected by current-limiting fuses. Without remote indica- tion, crews cannot determine the indicator status without opening each enclosure. FCIs are, thus, less desirable when used on an underground system placed along the front property lines. For these reasons, the use of manual-reset FCIs is not recommended.
Automatic Reset
FCIs are also available with automatic reset. After tripping, these devices can sense when the cable is re-energized and will then reset to a “NORMAL” indication. Because the reset is auto- matic, these devices are more likely to show cor- rect indication than is the manual-reset type. As a result, the automatic-reset FCIs can be a more reliable fault-locating tool.
Manufacturers offer many types of automatic reset. The costs of these different types are very similar. These types have different appli- cations based on their limitations. Each type of automatic reset and how it is best used is de- scribed below.
Current Reset
Current reset is the most common type of auto- matic reset. The device uses the same sensor to detect fault and load current (see Figure 3.16). After tripping, this device resets to “NORMAL” when it detects the return of load current in the cable. The load current must be higher than the reset current level. The standard reset current levels are three amperes, 1.5 amperes, and 0.1 ampere.
Before selecting a current-reset FCI, determine the normal load current. On 35- and 25-kV sys- tems, the normal load current in a single-phase residential subdivision may be less than three amperes. For example, a load of 30 kW on a 24.9/14.4-kV system has a current of about two amperes. An FCI with a three-ampere reset level would never reset.
The lower reset levels, 1.5 amperes and less, are very sensitive and can be susceptible to the magnetic fields of nearby cables. These stray fields can lead to false tripping and resetting in the following applications:
FIGURE 3.16: Current-Reset FCI. The unit has
a flag display housed inside a clear viewing window. Courtesy of Fisher Pierce Division
primary to the secondary side of the transformer. As a safety feature, this sensor has a lumped re-
sistance probe and 30-kV insu- lated cable. The resistance probe will limit the fault current if there is a primary- to-secondary insulation system failure.
The low-voltage-reset FCI is ideal for lightly loaded circuits where the load current is not high enough to reset a current- reset FCI. This FCI is not affected by the magnetic fields of nearby cables during reset; therefore, this device would be suitable for a lightly loaded three-phase circuit. The current sensor to detect fault current would not have to be as sensitive as a sensor that must also detect load currents of less
than three amperes to reset. The more sensitive sensors re- quire magnetic shielding to minimize the effect of nearby cables. This is described in more detail in theCurrent Resetsubsection on page 113.
For three-phase use, it is im- portant to know the minimum reset voltage. This value should be high enough to prevent a false reset caused by a feedback voltage. This effect is described in theBackfeed Currentssubsection earlier in this section.
• Single-phase junction cabinets,
• Single-phase fuse cabinets,
• Three-phase junction cabinets, and
• Three-phase switchgear. Some of these FCIs can be equipped with magnetic shield- ing to prevent this problem.
The current-reset FCIs re- quire only a current source to reset. Therefore, these devices
can be placed in all types of pad-mounted equipment and enclosures.
Low-Voltage Reset
The low-voltage-reset FCI is equipped with a probe that connects to the secondary voltage terminal of a transformer (see
Figure 3.17). The current sen- sor has contact with the pri- mary circuit neutral. When the FCI senses the proper amount of voltage between the sec- ondary terminal and the circuit neutral, it will reset. Most units have reset voltages of 120 volts
or 277 volts nominal and can be used in single- phase or grounded-wye, grounded-wye three- phase transformers.
The voltage sensor will likely cross from the
Figure 3.17: Low-Voltage-Reset FCI. Courtesy of E.O. Schweitzer
Manufacturing Division of SEL.
FIGURE 3.18: High-Voltage-Reset FCI. Courtesy
of Fisher Pierce Division of Thomas and Betts.