An FCI has a sensor to detect the current magni- tude present in a cable. A current that exceeds the trip rating of an FCI causes the display to show a faulted condition. Unfortunately, the sensor cannot distinguish between fault current,
inrush current, and backfeed current. The indicator simply responds to any current that exceeds its trip rating. As a re- sult, inrush and backfeed cur- rents that exceed the trip rating cause false tripping.
Inrush Currents
Inrush current is a higher than normal current that occurs when a distribution circuit is energized. The inrush current decays to the normal current value after some time. The types of inrush cur- rents and their decay times are explained above in the subsectionEffect of Inrush Current on Sectionalizing Devices.
When power is restored to a de-energized line, an inrush current will flow through the cable. If this inrush current exceeds the trip rating of an FCI, the FCI will show a fault condi- tion. Manual reset units will continue to show a fault condition until they are reset by hand. However, automatic resetting units will change back to a “NORMAL” indication when the inrush current decays to the normal load current level. In this situation, only the manual reset units continue to show a false trip condition.
The FCI can be
a valuable
fault-locating tool.
Inrush and backfeed
currents that exceed
the trip rating cause
Two other situations produce false tripping and obscure a fault location. The first is when a three-phase recloser or breaker protects the un- derground cable. For example, a fault on phase A trips the FCIs on phase A. The recloser or breaker opens and interrupts power to all three phases. When the recloser recloses, phases B and C experience inrush current. If this current exceeds the FCI trip ratings, then those FCIs will show a “FAULT” condition. Usually the recloser locks open before the FCIs can reset. The out- age crew now finds FCIs tripped on all three phases. Figure 3.7 illustrates this phenomenon.
The second situation is when a single-phase recloser protects a main line with one or more laterals. A fault on the main line trips the FCIs along the main line. During reclosing, some of the laterals may experience inrush that exceeds Three–Phase
Recloser A-Phase Fault
B-Phase C-Phase FCI 1 Inrush Current FCI 2 FCI 3 FCI 4 Load 1
FIGURE 3.7: Inrush Current Resulting from Operation of Three-Phase Recloser.
FCI, normal indication FCI, fault indication
LEGEND Single-Phase Recloser Fault FCI 1 Inrush Current Inrush Current FCI 2 FCI 3 FCI 4 FCI 5 Load 1 Load 2
FIGURE 3.8: Inrush Current Resulting from Operation of Single-Phase Recloser.
FCI, normal indication FCI, fault indication
LEGEND
the FCI trip rating. Again, the falsely tripped FCIs remain in “FAULT” indication following recloser lockout. Figure 3.8 illustrates this situation.
It is difficult to predict the magnitude of in- rush current. Therefore, it is difficult to choose an FCI trip rating that is greater than the un- known inrush value. For this reason, most man- ufacturers offer an inrush restraint feature on their FCIs. Typically, this feature disables the trip response for 15 to 60 cycles following the ener- gization of cable. The 15- to 60-cycle delay al- lows the inrush current to decay to its normal load value. The inrush restraint feature increases the cost of the FCI by about 35 to 40 percent. This additional cost is easily justified on under- ground systems that “see” the cycling action of a source-side recloser.
Backfeed Currents
Backfeed currents continue to produce false trips and resets of FCIs. However, unlike inrush currents, backfeed currents can remain on the system for long durations. Therefore, a time- delay feature will not alleviate the problem. To address this situation, the cooperative engineer needs to be aware of situations that likely pro- duce backfeed currents.
Backfeed currents can occur on three-phase circuits when a single-phase fault is cleared by a single-phase protective device. For example, a fuse will clear a cable fault on one phase while the other two phases remain energized. Any load-side capacitors connected to the faulted phase may discharge into the fault. If the circuit impedance is low enough, this discharge current could be large enough to trip FCIs located be- tween the fault and the capacitor bank.
More common backfeed currents result from a delta-connected motor load on a grounded-wye, grounded-wye transformer. For example, con- sider an underground system that serves several three-phase transformers. A cable fault in the first cable section is cleared by a fuse. The other two phases remain energized and continue to supply partial power to any delta-connected motor loads. The motors produce backfeed currents along the underground cable to the fault loca- tion. If the current level is high enough, it will falsely trip the FCIs between the cable fault and
the delta-connected motor load. All FCIs on the faulted phase may show a “FAULT” indication.
These same backfeed cur- rents and voltages can also produce false resets. Because the FCI trip level is usually hundreds of amperes and reset current level is usually less
than three amperes, false reset is a more likely problem than is false tripping. A feedback volt- age can also exist on the faulted phase. These voltage levels can reach 50 percent of the nor- mal line-to-ground voltage for a grounded-wye, grounded-wye transformer. For grounded-wye delta transformers, this voltage can reach 86 per- cent of the normal line-to-ground voltage. Most low-voltage reset units have a minimum reset voltage that is lower than 86 percent of the nominal voltage. Therefore, these units would not be suitable for grounded-wye, delta trans- formers with delta-connected loads. Because grounded-wye, delta-connected transformers should not be installed on a distribution system, this situation should not occur frequently.
SELECTING A TRIP RATING
Load and Fault Current Magnitudes
The trip rating of an FCI is the current magnitude that causes the FCI to display a fault condition. An ideal trip rating is low enough to sense the minimum available fault current and high enough to ignore load, inrush, and backfeed currents. To meet this criteria, the FCI trip rating should be close to the available minimum fault current level. If the available fault current level is unknown, manufacturers suggest a trip rating of two-and- one-half to three times the expected load current. At long distances from the substation, the available fault current drops substantially. As a result, the available fault current may get close to the mag- nitude of the load current. Again, the trip rating should be close to the fault current magnitude. However, the margin between the trip rating and the inrush and backfeed currents is decreased. Thus, the FCI is more susceptible to false tripping.
The accuracy of the trip rating also affects se- lection. Most FCIs have an accuracy of ±10 per- cent. For example, an FCI with a trip rating of
FIGURE 3.9: Trip Response for Peak-Current- Sensitive Units.
800 amperes could trip for any current in the range of 720 to 880 amperes. Therefore, it is important to select an FCI that remains sensitive to the mini- mum fault current throughout its range of trip ratings.
Conductor size also affects trip ratings. The FCI sensor mounts around an underground cable and senses the magnetic field produced by the flow of current. This magnetic field is a function of the radial distance from the conductor. The larger the radial distance, the weaker the mag- netic field. FCIs are typically calibrated at a spe- cific cable diameter. If the actual cable diameter is less, then the trip rating is reduced. Likewise, a large cable diameter increases the trip rating. The manufacturer should be asked to supply the