(1) Inductor Ll. The inductor Ll, shown in Figures 1-41 and l-42, is necessary to provide a
voltage increase to cause the secondary ac surge arrester at the main service disconnect means to turn on very
rapidly when suppressor RVl turns on and conducts transient current to ground. The inductor must be capable
of safely passing normal operating voltages and current, and current resulting from 130% overvoltage for a
period of 50 milliseconds. Also, the inductor must:
Have a very low dc resistance.
Present a high impedance to transient surges.
Present a very low impedance to 60 Hz line voltage.
Be capable of safely passing the transient current listed in Table 1-17.
(2) Suppressor RVl. Figure 1-41 shows RVl as a metal oxide varistor (MOV) because the zinc
oxide nonlinear resistor type of MOV is especially well suited for this particular application. Other types of
MOV are constantly being upgraded and are now possibly suitable for use. Other devices are also suitable for
use, and, in some cases will be required. Silicon avalanche diodes are effective for use in protecting very
susceptible equipment. Data for different type suppressors are provided in Section 220.127.116.11. Use of a gas-filled
spark gap for use at the location of RVl is not recommended for two reasons.
(a) Available gas-filled spark gaps with the required current handling capability have a
relatively high sparkover (turn-on) voltage and relatively slow turn-on times. Therefore, if spark gaps are used
for transient suppression at ac inputs, additional suppression including inductors, MOV and/or silicon avalanche
diode suppressors must be added to provide required protection.