Custom Search
|
|
|
||
 MIL-HDBK-419A
(b) Arc voltage for spark gaps is a nominal 20 to 30 volts. Therefore, when the transient
occurs causing the spark gap to turn on, normal line voltage is interrupted which will usually cause operational
upset of the affected equipment. Also, since the arc voltage is only 20 volts and is across a 120-volt supply, the
spark gap will likely remain in the arc mode of operation and draw current until the supply voltage waveform
crosses zero or until the supply circuit breaker opens. It is likely that the spark gap will be destroyed before
the supply circuit breaker opens. Either condition is very undesirable.
(3) Inductor L2 and capacitor Cl. These two components form an LC network to filter out high
frequency components of transient surges and are required only for equipment susceptible to high frequency,
very short duration (less than 1 nanosecond) transient pulses that might pass across RVl.
Transient suppression grounding. When at all feasible, transient suppressor grounds should be
c.
directly bonded to case ground. When the direct bond is not feasible, the suppressor grounds must be connected
as short and direct as possible to case ground, and the case must have a low bond resistance to earth ground.
Otherwise, the suppressors cannot operate properly.
d.
Functional characteristics. Functional characteristics for transient suppression at the ac input-
equipment interface must be as follows for effective transient suppression.
(1)
Voltage characteristics. The operating (reverse standoff) voltage must be between 200 to 300
percent of the normal line voltage for gas-filled spark gap suppressors. For MOV, ZNR, and SAS type
suppressors, the reverse standoff voltage should be 175 25 percent of the normal line voltage. Turn-on
voltage, discharge (clamp) voltage and the amplitude and time duration of any overshoot voltage must be
sufficiently low to preclude equipment damage or operational upset.
(2) Leakage current. Leakage current. for each suppression component at reverse standoff voltage
must not exceed 100 microamperes.
(3) Self-restoring capability. The surge suppressors must automatically restore to an off state
when transient voltage falls below turn-on voltage for the suppressor.
(4) Operating lifetime. Equipment transient suppression must be capable of safely dissipating the
number and amplitude of surges specified in Table 1-17 or 1-18 as applicable. Clamp voltage shall not change
more than 10 percent over the operating lifetime.
(5) In-line devices. Only inductors designed to have low dc resistance shall be used as in-line
devices for suppression of conducted powerline transients. In-line inductors shall safely pass equipment
operation voltages and line current with 130 percent overvoltage conditions for a period of 50 milliseconds.
Housing. Suppression components should be housed in a separate, shielded, compartmentalized
e.
enclosure as an integral part of equipment design. Bulkhead-mounted, feedthrough capacitors should be used as
necessary to prevent high-frequency transient energy from coupling to equipment circuits. Suppression
components should be directly bonded to equipment case ground when at all feasible. Suppressor connections to
ground must be short, straight, and direct.
1-88
|
 
|
|
|
||
