SURGE PROTECTION AND GROUNDING
9-1. Voltage Surges and Potential Gradients.
(1) Impulse sparkover voltage. Impulse spark-
over voltage is the highest value of voltage at-
Even the best designed electric system is subject to
tained by an impulse of a designated wave shape
overvoltages resulting from physical conditions not
and polarity applied across the terminals of an
subject to the owners control. Dangerous potential
arrester prior to the flow of discharge current.
gradients can result also from improper design.
This voltage plus the lead length voltage contribu-
a. Causes. Lightning imposes voltage surges
tion is the highest that can be impressed on
aerial lines either by direct strokes or by induc-
protected equipment because, at this level, the
tion. Such surges can be transmitted to under-
arrester will sparkover and discharge the surge to
ground lines. Opening and closing circuits in large
ground. Arrester front-of-wave sparkover voltage
generating plants or switching stations can raise
will be compared to the insulation lightning im-
voltages to two or three times normal for a brief
pulse (chopped-wave) crest value that the protected
period of time. In addition, excessive voltages and
equipment is required to withstand for purposes of
currents can result from short-circuit conditions
determining the protective margin.
when line-to-line or line-to-ground faults occur,
(2) Discharge voltage. Discharge voltage is the
because of inductive/capacitive characteristics of
voltage that appears across the terminals of an
the line between the electric power source and the
arrester during passage of discharge current. Ar-
rester maximum discharge voltage will be com-
ate overvoltages also as discussed in chapter 5.
pared to the BIL value that the protected equip-
b. Elimination. Since voltage surges can result
ment is required to withstand for purposes of
in personnel injuries from electrical shock, insula-
determining the protective margin.
tion damage to equipment, and possibly fire, surge
(3) Impulse protective level. For a defined wave-
shape, the impulse protective level is the higher of
these surges. Grounding systems will be designed
the maximum sparkover value or the correspond-
to limit potential gradients to values established
ing discharge voltage value.
by IEEE Std 80. Proper relaying will ensure
(4) Maximum continuous operation voltage
isolation and disconnection of faulty equipment
(MCOV). the MCOV is the maximum designated
and lines when a short-circuit occurs.
root-mean-square (rms) value of power frequency
voltage that may be applied continuously between
9-2. Methods of Controlling Voltage Surges
the terminals of a gapless arrester.
and Potential Gradients.
(5) Voltage rating. The nameplate voltage rat-
a. Surge arresters. Surge arresters will be of the
ing of an arrester is the maximum permissible
valve-type or the metal-oxide-varistor type. Gap-
operating voltage at which the arrester can oper-
less, metal-oxide arresters are preferred because of
(a) Operation. An arrester has a maximum
their better operating characteristics. Surge arrest-
ers are used to safeguard apparatus against haz-
voltage level above which the arrester cannot seal
ards caused by abnormally high voltage surges.
off the 60 Hz line (follow) current, after sparkover
Such overvoltage can cause serious damage if
on surge voltage. If the correct nameplate rating is
arresters are not correctly coordinated with the
used, the arrester can interrupt 60 Hz line current
even though there is a line-to-ground fault on
insulation strength of the protected equipment,
another phase. If the 60 Hz follow current is not
and are unable to discharge the energy properly.
immediately extinguished, the arrester may fail.
To function correctly, arrester protective levels
(b) Sizing. On a modern overhead, grounded
must be lower than the insulation withstand
strength of equipment to be protected. Surge ar-
wye primary distribution system (effectively
rester protective margins will comply with IEEE
grounded system), the arrester is able to reseal at
Std C62.1, IEEE Std C62.2, and IEEE Std C62.11.
a voltage level that does not exceed 1.25 times the
b. Characteristics. Arrester characteristics will
nominal line-to-ground voltage. For a main electric
supply station with a 13,2OOY/7,620 V secondary,
have protective margins coordinated with the ap-
the minimum arrester rating would be 1.25 x 7.62
propriate equipment insulation characteristics in
= 9.53 kV. A 9-kV arrester might not reseal so a
accordance with IEEE Std C62.2. Lead lengths
10-kV arrester, which is the next higher standard
must also be taken into account.