1 .9 .8 .7 .6 .5 .4 .3 .2 .1 .09 .08 .07 .06 .05 .04 .03 .02 .01 .009 .008 .007 .006 .005 .004 .003 .002 .001 Current (Amperes, RMS) 100 200 200 400 600 800 1,000 1,200 1,400 300 400 500 600 700 800 900 1,000 2,000 3,000 4,000 T ime (Seconds)
cable diameter at which the FCI is calibrated and a correction curve for other cable diameters.
Coordination with Current-Limiting Fuses
Some FCIs are peak-current sensitive and will operate within two milliseconds for any current that exceeds the trip rating. Figure 3.9 shows the response time of peak-sensitive units. The peak- current devices will coordinate with all types of fuses, including current-limiting fuses. Proper coordination means that the FCI will trip before the fuse clears the fault. If the total clear time of the fuse is faster than the FCI response time, the FCI will not show a fault condition.
FIGURE 3.10: Trip Response for 450A and 800A FCIs.
Current (Amperes) T ime (Seconds) 10 100 1,000 0.001 0.01 0.1 1 10
15E 30E 100E
450A
FCI
800A
FCI
10,000
If the FCI is not the peak-current type, its trip response time is a function of the current magni- tude. Figure 3.10 shows the time-current charac- teristics for this type of FCI. Note the difference in the trip response time for the two types. For example, look at the 800-ampere curve of Fig- ures 3.9 and 3.10. The peak-current-sensitive FCI has a response time of two milliseconds. The other FCI has a response time of 0.3 seconds (300 milliseconds).
These slower devices should be compared with the time-current curves for the source-side protective device. For proper coordination with link-type fuses, the FCI curve must be to the left of the total clear curve of the fuse at the mini- mum fault current value. For example, refer to Figure 3.10. For a minimum fault current of 1,000 amperes, a 450-ampere FCI coordinates with a 30E and a 100E fuse. The FCI should also coordinate with a source-side current-limiting fuse. To coordinate, the FCI must trip at the let- through peak-current level before the fuse clears the fault. For most current-limiting fuses, the clear time is approximately three milliseconds. As shown in Figure 3.10, a 450-ampere FCI will coordinate with a current-limiting fuse that has a let-through current of 1,100 amperes or greater.
Adaptive-Trip FCI
The adaptive-trip FCI does not have a specified trip rating. Instead of tripping at a predetermined current magnitude, this device responds to a sudden increase in current followed by a loss of current. Figure 3.11 shows the increase in current magnitude required to set the trip mechanism. For example, consider a sensor type B shown in Figure 3.11. To set the trip mechanism, the FCI must see an increase of 130 amperes within a 50-millisecond time or 100 amperes within an 80-millisecond or greater time. The trip mechanism will release and show a fault indication only if the line current drops to zero. If the line current does not drop to zero within 60 seconds, the trip-set condition will reset to normal. This trip-set and trip-release sequence prevents the FCI from showing a false trip as a result of motor starting load or cold-load pickup. Like the other types of FCIs, the adaptive-trip FCI must be checked for coordination with upstream protective devices. Fuse Minimum Melt Curve
LEGEND
FCI Trip Response Curve Fuse Total Clear Curve
After the circuit is re-ener- gized, this FCI will adjust to the line current within 60 sec- onds. During this 60-second period, the FCI is in trip re- straint. This feature helps pre- vent false trips caused by upstream reclosers. In addi-
tion, the FCI continuously readjusts itself for changes in the nominal line current.
FIGURE 3.11: Trip-Set Characteristics for Adaptive-Trip FCI.
Courtesy of Fisher Pierce Division of Thomas & Betts. 0.01 0.008 0.006 0.004 0.002 0.001 0.1 0.08 0.06 0.04 0.02 1 .8 .6 .4 .2 10 8 6 4 2 100 M L B D Sensor Type 80 60 40 20 Current (Amperes) 10 100 1,000 10,000 T ime (Seconds)
Fisher Pierce Fault Indicator Model 1547 Adaptive Trip
Time Current Curves (5A Base Current)
WHERE TO LOCATE FCIS
For an exact section of faulted cable in an un- derground system to be located, an FCI must be placed at the source end of each cable section. Most cable sections terminate in some type of pad-mounted equipment. Because this equip- ment also provides easy access to the cable, the location is ideal for FCIs. The following subsec- tions show several types of underground sys- tems and the placement of FCIs.
Underground Segments of Overhead Feeders
Overhead feeders may occasionally have segments of underground cable. These underground seg- ments are often installed to avoid overhead line clearance problems. Some applications of under- ground segments are the following:
• Lake or river crossings,
• Highway crossings,
• Transmission line crossings, and
• Airport glide path crossings.
Because these underground segments are part of a main feeder, they are usually not fused. Rather, a set of solid-blade disconnects is placed at each end of an underground cable section.
A set of FCIs at each cable end will enable workers to determine if a fault has occurred on the underground segment. The set of FCIs on the source side will show a “FAULT” indication for a fault on the underground cable or on the outgo- ing overhead feeder. The second set of FCIs on the load side will show a “normal” indication for a fault on the underground cable and a “FAULT” indication for a fault on the overhead feeder. This arrangement is shown in Figure 3.12.
Another consideration for this application is whether to use a three-phase FCI or three single-
phase FCIs. The three-phase FCI has three current sensors and one display. The display shows a “FAULT” indication for a fault on any of the three phases. This indicator is suitable when the underground cable is sectional- ized with single-phase devices. The single-phase sectionalizing device will be open on the faulted phase, thus showing which underground cable is faulted.