Custom Search
|
|
|
||
 MIL-HDBK-419A
Turn-on time. Turn-on time (response time) is the time required for an arrester to turn on and
b.
clamp a transient after turn-on voltage is impressed across device terminals. All basic suppressor devices used
in manufacture of surge arresters are voltage dependent for ionization, breakdown, and other phenomena
associated with breakdown. Therefore, a low turn-on voltage enhances a faster turn-on time. Turn-on time
requirements for a surge arrester must be directly related to the withstand level for equipment and components
being protected. For instance, if only heavy duty electrical equipment, such as motors, contactors, and
switches are being protected, relatively slow turn-on of 1 to 5 microseconds can be tolerated. However, if
solid-state electronic equipment, or a combination of electrical and electronic solid-state equipment is being
protected, turn-on time becomes much more critical. In general, the most rapid response time available is
desirable. However, cost and current dissipation capability normally place constraints on such selection
criteria. Four types of arresters are currently manufactured as noted below. Additional data for each type is
provided in 1.3.3.5.15.
(1)
Gas-filled spark gap with series-connected nonlinear resistance.
(2)
Zinc oxide nonlinear resistor (ZNR) or metal oxide varistor (MOV).
(3)
Solid-state.
(4)
Hybrid of above components (development stage).
Important turn-on time characteristics. Generalized characteristics for the three basic types of
c.
surge arresters are listed in Table 1-11. Turn-on time of 50 nanoseconds is sufficiently fast to protect all
except very critical components that would directly receive transient energy prior to turn-on and clamp of the
surge arrester. Solid-state units may be used for protection of very critical equipment components, and the
gas-filled spark gap type will provide adequate protection for heavy duty electrical equipment such as motors,
contactors and switches. However, arresters with slow turn-on time and high turn-on voltage should not be
used to protect electronic equipment that has low-voltage, fast turn-on transient suppression devices or circuits
included as an integral part of the equipment. Otherwise, the transient suppression in the equipment will turn
on and attempt to dissipate transient energy before the surge arrester installed at the main service disconnect
means turns on. In most cases, this will rapidly destroy equipment-level transient suppression. The impedance
and inductance of power distribution panels and power distribution wiring within the facility will tend to slow
down transient rise time and also dissipate some transient energy both before and after the surge arrester turns
on. The resistance and inductance works in conjunction with the surge arrester at the main service disconnect
means to provide additional protection. However, the true degree of protection thus provided varies widely due
to varying transient waveforms, and size and length of distribution wiring within the facility. In summary, the
most important characteristics for turn-on time are:
(1) Turn-on time must be rapid enough to preclude damage to equipment resulting from over-
voltage before the surge arrester turns on and clamps the incoming transient.
1-64
|
 
|
|
|
||
