184.108.40.206.14 Desirable Operating Characteristics for Transient Suppressors. The transient suppressor character-
istics listed below are required for effective protection at the facility level:
Turn-on (response) time: 50 nanoseconds or less.
Standoff voltage and leakage current: To ensure that the suppressor remains off except during
transient occurrence, the standoff voltage should be between 200 to 300 percent above the nominal line voltage
for spark gap type suppressors and approximately 175 25 percent for MOV and ZNR type suppressors.
Leakage current should not exceed 100 microamperes at standoff voltage.
Polarity: Bipolar or unipolar, depending on line voltage.
Turn-on voltage: 125 percent of standoff voltage maximum at one milliampere for MOV and ZNR
type suppressors. Also, 125 percent of the standoff voltage for gas-filled spark gap suppressors.
Clamp voltage: (Also known as discharge voltage) should not exceed 200 percent of the turn-on
voltage for transients 100 amperes peak or 225 percent of the turn-on voltage for transients 1000 amperes
Operating life: Capable of dissipating number and amplitude of transients projected to occur over a
lo-year period. See Section 220.127.116.11.17.
Self-restoring capability: Essential that suppressor automatically restores to off state when applied
voltage drops below turn-on voltage.
18.104.22.168.15 Characteristics of Different Types of Surge Arresters. Various types of surge arresters are
presently available for purchase as off-the-shelf items from a multitude of manufacturers. Most have desirable
characteristics, and also have undesirable characteristics. Some types have the capability of dissipating
tremendous amounts of current, but turn on relatively slowly (150 to 200 nanoseconds) after turn-on voltage
appears across device terminals. Another type turns on more rapidly (50 nanoseconds or less) but will not
dissipate as much current as the slower devices, unless many devices are connected in parallel which is not
totally desirable. Solid-state arresters are available which have very fast turn-on times but most of them are
limited in current dissipation capability except for expensive units that range in cost from $7,500 to $25,000.
Several hybrid units are currently under development that consist of a solid-state suppressor for dissipation of
low-energy transients, and a separate suppressor section for dissipation of high-energy transients. The two
suppressor sections are normally separated by a choke in series with the protected phase line. The three most
important characteristics of an ac surge arrester are the capability to dissipate the required levels of surge
current, maintain a low discharge (clamp) voltage while dissipating the transient current, and a, fast response
time. The fast response time is important to prevent the appearance of high level transient energy (overshoot
voltage) across protected equipment for an intolerable length of time before the arrester turns on and clamps.
Various types of suppressors are discussed below together with typical operating characteristics.