wiring. Balanced two-wire models are available that allow ionization from the first discharge to cause
immediate conduction of both halves of the tube so that circuit imbalance is minimized. Coaxial models are
also available for use on coaxial lines such as antenna feed cables. Gas tubes have small capacitances and
virtually no loss in the nonconducting states. The glow state occurs in circuits whose impedance limits the
discharge current to less than about 100 mA; the voltage across the tube in this state is about 100 V. The arc
state occurs when large currents are caused to flow; the voltage across the tube in the arc state is usually 10 to
20 V. Gas tubes should not be used on energized lines that can sustain the arc or glow discharge.
Spark gaps and gas tubes display a negative dynamic resistance at the firing point, where a decrease in voltage
across the device is accompanied by an increase in current through it. This property of spark gaps and gas tubes
sometimes leads to unpredicted instabilities in the protected circuits. In addition, the discharge is a sudden
change in voltage and current that may shock-excite the protected circuit. It is usually recommended that a
linear filter be placed between the device and the protected circuits to minimize the effects of the negative
dynamic resistance and shock excitation.
10.4.2.3.2 Metal-Oxide Varistors. MOVs are capable of diverting currents up to tens of kiloamperes and, when
packaged and installed to minimize terminal and lead inductance, they are effective for large rate-of-rise
transients. Although they are nonlinear, MOVs do not display the negative dynamic resistance and shock
excitation characteristics of the spark gaps and gas tubes. Their nonlinearity may produce intermodulation
effects in RF circuits. The MOV stops conducting when the applied voltage decreases below the "knee" of the
V-I curve. It is ideal for protecting energized lines, since it has no current-extinguishing problems. The MOV
typically adds nanofarads of shunt capacitance and megohms of shunt resistance to the protected circuit. It
should be used with caution on high-frequency circuits and high-impedance circuits. The maximum energy
dissipation capability for large MOVs is tens of kilojoules. Just above the failure threshold, they usually fail as
a short circuit or low resistance. However, for energies well above the failure threshold, the devices may be
physically destroyed, sometimes explosively.
10.4.2.3.3 Semiconductors. A number of avalanche devices are available for use as surge limiters. The
semiconductor devices limit at lower voltages (1 to 100 V) than the MOVs and gas tubes, but they are less
tolerant of large peak currents and large energies than the other devices. Peak current ratings up to about
100 A are available. Because the devices themselves may be damaged by transients arriving on external wires
and cables, they are not recommended for facility-level use. They may be used to protect equipment inside the
facility and circuits that are entirely inside the shielded facility. The semiconductor devices add nanofarads of
shunt capacitance to the protected circuit and may aggravate intermodulation problems.
10.4.2.3.4 Filters. Linear filters may also be used as barrier elements on penetrating wires, but at the outer
(facility-level) barrier, filters are always used in combination with surge arresters. On power lines, for
example, the line filter usually cannot tolerate the peak voltages, so a spark-gap surge arrester is used to limit
the voltage, and the filter isolates the interior circuits from the negative dynamic resistance and shock
excitation of the spark-gap discharge. The shunt input capacitance of the filter may also be used to reduce the
rate-of-rise of the voltage, so that the firing voltage of the surge arrester will be lower. A variety of low-pass,
bandpass, and high-pass filters is available for power and signal line protection.