(3) This circuit operation may generate the requirement for a properly sized (2-microhenry
minimum) inductor to be installed in series with applicable ac conductors. If its need has been ascertained, it
must be installed between the surge arrester and the integral equipment-level transient suppressor. It may also
be designed as an integral part of the surge arrester or the equipment-level transient suppressor.
(4) The equipment-level suppressor will immediately start toward its clamp voltage as transient
current is conducted. Because of resistance and inductance in the power distribution lines and panels, the surge
arrester will turn on very soon (nanoseconds) after the equipment-level suppressor(s), and will dissipate most of
the remaining transient energy. After the surge arrester turns on, the equipment level suppressor(s) are
required to dissipate only the transient energy resulting from the clamp voltage of the surge arrester.
(5) Thus, the surge arrester dissipates most of the transient surge, and the equipment-level
suppressor(s) provide equipment protection against fast rise time transients and reduce the surge arrester clamp
voltage to levels that can be safely tolerated by protected equipment. In summary, the clamp voltage for the
surge arrester must be low enough while dissipating a high-energy transient to provide adequate equipment
protection taking into consideration:
Protection provided by transient suppression that is an integral part of the facility
Reverse standoff voltage. Reverse standoff voltage is specified in various ways by surge arrester
manufacturers such as maximum allowable voltage, voltage rating, and reverse standoff voltage. For usage
herein, reverse standoff voltage is defined as the maximum voltage that can be applied across the surge
arrester and still permit the surge arrester to remain in an off state (current leakage through arrester to ground
100 microamperes or less). Good engineering practice dictates that the surge arrester remains off during
(1) Design of effective lightning transient protection requires that the surge arrester turn on very
rapidly at the lowest voltage possible when a transient occurs. In addition, it is desirable that a low clamp
voltage be maintained across the surge arrester while conducting surge current to ground. Turn-on voltage and
associated turn-on time as well as clamp voltage are proportional to reverse standoff voltage. That is, an
arrester with a low reverse standoff voltage has a lower turn-on voltage (and thus a faster turn-on time) and a
lower clamp voltage than an arrester with a higher reverse standoff voltage. Therefore, it is important that the
surge arrester has the lowest possible reverse standoff voltage.
For effective protection, the reverse standoff voltage should be between 200 to 300 percent of
nominal line-to-ground voltage of the appropriate ac service lines for a spark gap type surge arrester that is to
be installed line to ground. The reverse standoff voltage should also be between 200 to 300 percent of nominal
line-to-line voltage of appropriate ac service lines for a spark gap type surge arrester that is to be installed
line to line. The reverse standoff voltage for MOV and ZNR type arresters should be 175 25 percent of the
nominal line-to-ground or line-to-line voltages of the appropriate ac service lines.