In general, the capacitor bank fuse should be selected based on the highest anticipated capacitor bank current. Specifically, the fuse selected should have a maximum continuous current-carrying capability, as differentiated from its nominal ampere rating, greater than this highest anticipated current level. It follows, then, that the capacitor bank current must be accurately known. However, this current is not simply derived from the capacitor bank voltage and kVAR ratings. The maximum system operating voltage can be as much as 6% higher than the nominal voltage rating of the capacitor bank. The capacitor units themselves are permitted by the Standards to have manufacturing tolerances of +15% in capacitance, and the presence of harmonics can add as much as 10% to the RMS value of the current. These factors, taken together, would require that the nominal capacitor bank current, calculated based on rated voltage and kVAR, be increased by an allowance as high as 34% (1.06 X 1.15 X 1.1 = 1.34), although an allowance of 35% is more commonly used.

In practice, however, the operating variables described above rarely attain the maximum values listed, and it is likely that they will not all be at their maximum values at the same time. For example, the effect of capacitor-unit tolerance, is usually accepted to be 8% or less. In addition, harmonic currents are generally less than the value of 10% cited above — particularly for ungrounded-wye or delta-connected capacitor banks, since there is no path for the flow of third-harmonic currents, or currents at multiples of the third harmonic. Accordingly, somewhat lower allowances can generally be used. The values shown in Table 1 are based on a reasonable mix of these variables and will be used for all capacitor bank current calculations in this article.

Table 1 — Practical Allowances for Operating Variables
Operating Variable	Allowance, Based on Capacitor Bank Connection
Operating Variable	Grounded-Wye	Ungrounded-Wye or Delta
System Voltage Level	6%	6%
Capacitor-Unit Tolerance	8	8
Harmonic Currents	7	2
Total Allowances	22%	17%

ANSI Standard C84.1, Voltage Ratings for Electric Power Systems and Equipment.

Operate as Promptly as Possible in Response to an Evolving Capacitor-Unit Failure

To ensure the earliest possible operation in response to an evolving capacitor-unit failure, the capacitor bank fuse should have a low ratio of 300-second maximum clearing current to normal capacitor bank current because, as was shown in previous articles, the increases in capacitor-unit current and phase current are relatively small in the initial steps of the capacitor-unit failure process. Additionally, the capacitor bank fuse should have a steep time-current characteristic in the low current region. A fuse having such a time-current characteristic will operate faster for a given value of phase current than will a fuse having a slower (less steep) time-current characteristic.

To illustrate the importance of evaluating fuse link time-current characteristics when selecting a capacitor bank fuse, consider, for example, a 1200-kVAR grounded-wye connected capacitor bank rated 13.8 kV three-phase, with two 200-kVAR capacitor units per phase. Table 2 lists a number of fuse links having different ampere ratings and speed characteristics that could be considered for application as the capacitor bank fuse. In each case, the ampere ratings represent the fuse link manufacturer’s recommendation. For purposes of comparison, Table 2 also lists, for each ampere rating and speed characteristic, the fusing ratio, along with the more significant parameters of maximum continuous current-carrying capability and maximum clearing current at 300 seconds. Total clearing time-current characteristic curves for the fuse links listed in Table 2 are shown in Figure 1. The curves in Figure 1 illustrate graphically the wide variations in clearing times for the various fuse links in the low current region.

TABLE 2 — Characteristics of Various Capacitor Bank Fuse Links — 1200-kVAR Grounded-Wye Connected Capacitor Bank Rated 13.8 kV Three-Phase, with Two 7.97-kV, 200-kVAR Capacitor Units Per Phase
Fuse Link Rating and Element Material	Fusing Ratio	Maximum Continuous Current-Carrying Capability, Amperes	Maximum Clearing Current at 300 Sec., Amperes
50K-Sn	1.0	75	125
50T-Sn	1.0	75	122
50K-Ag	1.0	66	117
50T-Ag-Cu	1.0	63	120
60QR-Ag-Cu	1.2	66	101
65N-Ag	1.3	66	106
65 Std.-Ag	1.3	80	143
75H-Cu	1.5	75	132

In order to determine which fuse link ampere rating and speed characteristic listed in Table 2 provides the best protection for the capacitor bank, it is necessary to consider each fuse link’s response for an evolving series-group failure of an individual capacitor unit. Clearly, the fuse link that operates with the smallest number of series groups of packs shorted will provide better overall protection for the capacitor units in the bank. If more than one of the fuse links listed operates with the same number of series groups shorted, the fuse link operating in the shortest time and thus having the lowest let-through I²t would be preferred.

Figure 1. Total clearing time-current characteristic curves
for fuse links recommended for a 1200-kVAR grounded-
wye connected capacitor bank rated 13.8 kV three-phase,
with two 7.97 kV, 200-kVAR capacitors units per phase.

The relative effectiveness of the various fuse links is illustrated in Tables 3A-C. In Table 3A, the column headed “4 Series Groups of Packs,” note that all of the fuse links under consideration operate when the third series group (of four) is shorted. The 65N-Ag fuse link clearly provides better protection for the capacitor units in the bank since its operating time is the shortest: only 0.94 second. In Table 3B, the column headed “5 Series Groups of Packs,” note that the 65N-Ag fuse link operates when only three out of five series groups are shorted, while the remaining fuse links do not operate until four series groups are shorted. Finally, in Table 3C, the column headed “6 Series Groups of Packs,” although a number of fuse links operate when four series groups are shorted, the 65N-Ag fuse link, once again, provides a higher degree of protection for the capacitor units in the bank since its operating time of 3.5 seconds is considerably less than that of the other fuse links.

Another conclusion that can be drawn from Tables 3A-C is that silver or silver-copper-eutectic-element fuse links will operate faster at a given value of current than will tin-element fuse links of the same ampere rating and speed characteristic. This is due, in part, to the smaller tolerances in melting current that can be achieved with silver or silver-copper-eutectic-element fuse links (10% in terms of current) as compared with the tolerances in melting current normally associated with tin-element fuse links (20% in terms of current).

The reason for the effectiveness of the silver-element "N" Speed fuse link, as compared with the other fuse link speeds evaluated, can be seen by studying their total clearing time-current characteristic curves in the relevant range of currents. Figure 2 shows the fuse link total clearing curves and the currents associated with various numbers of series groups of packs shorted for the example from Table 3C involving six series groups of packs. As can be seen from Figure 2, the 65N-Ag fuse link’s lower 300-second maximum clearing current and relatively steep time-current characteristic result in its having the shortest response time for an evolving series-group failure. A similar analysis of other capacitor bank ratings and configurations showed that of the fuse links recommended, the “N” Speed fuse link provides superior protection in about 70% of the cases. Accordingly, standardization on the “N” Speed fuse link for distribution pole-top capacitor bank protection will result in excellent protection against case rupture for the capacitor units in the bank.

TABLE 3A — Characteristics of Various Capacitor Bank Fuse Links — 1200-kVAR Grounded-Wye Connected Capacitor Bank Rated 13.8 kV Three-Phase, with Two 7.97-kV, 200-kVAR Capacitor Units Per Phase
Number of Series Groups of Packs Shorted	Fuse Link Rating and Element Material	4 Series Groups of Packs
Number of Series Groups of Packs Shorted	Fuse Link Rating and Element Material	Phase Current, Amperes	Max Clearing Time, Seconds	Let- Through A²s X10³
1	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu	71.6
2	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu	91.8
3	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu	153.0	20 27 2.9 17 1.6 0.94 22 4.0	300 405 43 255 24 14 330 60
4	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu
5	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu

TABLE 3B — Characteristics of Various Capacitor Bank Fuse Links — 1200-kVAR Grounded-Wye Connected Capacitor Bank Rated 13.8 kV Three-Phase, with Two 7.97-kV, 200-kVAR Capacitor Units Per Phase
Number of Series Groups of Packs Shorted	Fuse Link Rating and Element Material	5 Series Groups of Packs
Number of Series Groups of Packs Shorted	Fuse Link Rating and Element Material	Phase Current, Amperes	Max Clearing Time, Seconds	Let- Through A²s X10³
1	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu	69.2
2	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu	81.4
3	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu	107.1	125	732
4	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu	183.6	3.1 7.2 0.79 4.3 0.59 3.9 0.83	73 169 18 101 14 91 19
5	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu

TABLE 3C — Characteristics of Various Capacitor Bank Fuse Links — 1200-kVAR Grounded-Wye Connected Capacitor Bank Rated 13.8 kV Three-Phase, with Two 7.97-kV, 200-kVAR Capacitor Units Per Phase
Number of Series Groups of Packs Shorted	Fuse Link Rating and Element Material	6 Series Groups of Packs
Number of Series Groups of Packs Shorted	Fuse Link Rating and Element Material	Phase Current, Amperes	Max Clearing Time, Seconds	Let- Through A²s X10³
1	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu	67.3
2	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu	76.5
3	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu	91.8
4	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu	122.4	137 145 25 140 5.8 3.5	1154 1221 211 1179 49 30
5	50K-Sn 50T-Sn 50K-Ag 50T-Ag-Cu 60QR-Ag-Cu 65N-Ag 65 Std.-Ag 75H-Cu	214.2	1.4 0.35	47 12

Nominal capacitor bank current is 50.2 amperes. With an allowance factor of 22%, capacitor bank current is assumed to be as high as 50.2 x 1.22, or 61.2 amperes.
Fuse link element materials are identified by means of chemical symbols: Sn for tin elements; Ag for silver elements; Ag-Cu for silver-copper eutectic elements; and Cu for copper elements.
Fuse link time-current characteristics have been adjusted to reflect preloading by the prior-step escalated current. While such preloading reduces the total clearing time (and consequently the I²t in the faulted capacitor unit) for a given number of series groups shorted, it does not cause the fuse link to respond at an earlier step in the series-group failure process.
Let-through A²s in faulted capacitor, beginning with the particular series-group failure which results in enough current to operate the fuse link.
Fuse link does not operate.
Fuse link operated one step earlier in the series-group failure process.

Figure 2. Total clearing time-current characteristic curves for fuse links shown in Table 3.
Because of its low 300-second maximum clearing current and relatively steep time-current
characteristic, the 65 N-Ag fuse link has the shortest response time for an evolving series
group failure of an individual capacitor unit having six series groups of packs.

The next article will describe how to select the capacitor bank fuse to protect capacitor units against case rupture. Go to Unit 4.