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 MIL-HDBK-419A
a.
Gas-filled spark gap with series-connected silicon carbide block. The gas-filled spark gap arrester is
capable of conducting very high currents. Some units have an extreme duty discharge capacity of 150,000
amperes peak for one transient with a 10-by-20 microsecond waveform. Minimum life of such units is
dissipation of 2500 surges of 10,000 amperes peak surge current with a 10-by-20 microsecond waveform.
Impulse sparkover (turn-on) voltage is 1400 volts peak for a transient with a 10 kV/ s waveform for two types
of arresters. Some typical discharge (clamp) voltages are listed in Table 1-12 for 10-by-20 microsecond
waveforms of the transient amplitudes listed:
Table 1-12. Typical Maximum Clamp Voltage for Spark Gap Arresters
Maximum
Peak Surge
Clamp Voltage
Amplitude
2,000 Volts
10,000 Amperes
40,000 Amperes
3,000 Volts
150,000 Amperes
5,500 Volts
(1) Follow current. The typical discharge (arc) voltage across a spark gap is 20 to 30 volts while it
is in full conduction. Because of the low arc voltage, the voltage and current available from the ac power
supply would maintain the spark gap in an on state after a transient was dissipated until the first zero crossing
of the power supply or until a supply line fuse opened, a line burned open, the spark gap burned open, or the
service transformer burned open. For this reason, a silicon carbide block (nonlinear resistor) is connected in
series with a spark gap to ground to ensure that the spark gap extinguishes on the first zero crossing of the
connected line, and, more importantly, to limit follow current through the spark gap after a transient is
dissipated until the first zero crossing of the powerline (8.3 milliseconds maximum). The silicon carbide block is
a nonlinear resistance, and resistance decreases as applied voltage increases. Thus, the resistance is relatively
high at powerline voltages to limit follow current, but decreases to a fraction of an ohm when high-level
transient voltage is applied. However, the resistance remains high enough to generate a relatively high clamp
voltage when discharging high-amplitude transient currents.
(2) Sparkover (turn-on) voltage. Sparkover time for the spark gap arrester is directly related to
transient risetime since a finite amount of time is required for the spark gap to ionize and transition from the
off mode through the glow region and into the are mode of operation. Also, ionization time is to some extent
related to the risetime of the transient. Transition time from off to arc mode of operation is typically 150 to
200 nanoseconds after sparkover voltage appears across arrester terminals.
1-68
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