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 MIL-HDBK-419A
Table 1-11. Generalized Characteristics for Surge Arresters by Type
Firing/Clamp Voltage
Type
Turn-on Time
Current Capacity
Cost
High - 350 to 5500
Extreme duty to 150,000
Moderate - $25
Gas-filled
5-250 nanoseconds
amperes lifetime: 2500
volts (firing)
to $750
spark gap
for 10 kV/s rise
time
surges at 10,000 amperes
Varies - can be equivalent
Moderate - 300 to
Moderate - $50
MOV or ZNR
50 nanoseconds
3000 volts (clamp)
to $1,000
or less, any rise
to spark-gap type
time
Varies - Generally 50 to
Low - 275 to 750
High - $100 to
Solid State
10 nanoseconds
100 amperes except for
volts (clamp)
$25,000
or less, any rise
costly units
time
(2) Turn-on voltage and time for the surge arrester must be compatible with the same
characteristics of transient suppressors/circuits included as an integral part of protected equipment. Otherwise
equipment-level transient suppressors/circuits will attempt to dissipate the transient before the surge arrester
turns on. When this occurs, the equipment level transient suppression will likely be destroyed resulting in
damage or operational upset of protected equipment.
d.
Discharge (clamp) voltage. The clamp voltage, sometimes referred to as the discharge voltage, for
a surge arrester is the voltage that appears across the arrester input terminals and the ground terminal while
conducting a transient surge current to ground. The clamp voltage waveform occurring across the surge
arrester installed at the main service disconnect means appears across the protected equipment after losses
imposed by inductance and resistance of power distribution lines and panels.
(1) In general, a surge arrester with the lowest clamp voltage possible is desirable. An all-solid-
state arrester provides the lowest clamping voltage available (Table 1-11). However, as with turn-on time,
other factors such as current dissipation capability and cost normally place constraints on simply installing a
surge arrester at the main service disconnect means with the lowest clamping voltage available.
(2) In new facilities calling out the latest design equipment, transient surge suppression generally
is included as an integral part of the equipment ac input. Higher clamping voltages can therefore be tolerated
at the main service disconnect means. When good engineering design practices are used, equipment level
suppressors will have a slightly lower turn-on voltage threshold and a slightly faster turn-on time than the surge
arrester at the main service disconnect means. This permits the equipment-level suppressors to maintain a
lower clamping level to provide maximum equipment protection. Therefore, when a transient occurs, the
equipment level suppressor(s) will turn on first.
1-65
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