Arc Voltage:
The arc is a self-sustained discharge that has a low voltage drop (typically 20-40 volts) and is capable of supporting currents of thousands of amperes. In a spark gap, the arc is established by transition from the dc breakdown or glow voltage to the arc mode. When an arc occurs in a spark gap between two refractory electrodes such as Tungsten, very high temperatures are created at the surface of the electrodes and rapid vaporization (erosion) of the electrode occurs.

Capacitance (C):
The electrical characteristic which permits the storage of electrical energy in the electrostatic field between two conductors. The amount of charge that can be stored by a capacitor is measured in farads.

Cutoff Voltage:
The voltage level below which the application of a trigger pulse fails to achieve gap breakdown. The normally recommended range of operation of the applied voltage of a triggered spark gap is between 50 – 80% of the main static breakdown (MSB).  Operation at lower applied voltages may be obtained; however the probability of a misfire increases at lower voltage levels.

DC Breakdown Voltage:
Voltage at which the spark discharge occurs when the voltage across the gap is increased slowly. A ramp rate is usually specified with a typical value of 1000 volts per second.

Electromagnetic Pulse (EMP):
A short duration pulse of electromagnetic energy from an unspecified source. This event is primarily associated with a nuclear explosion.

Follow-On Current:
The follow-on current may occur in either a DC or AC circuit.  In an AC Circuit, after occurrence of a transient voltage at the end of each half cycle, there is a period in which the current is nearly zero. In this period, there is a very rapid deionization and the current will extinguish. As the voltage reverses, if the current continues to flow it is known as follow-on current.  In DC applications, it is essential to insure that the protector will be extinguished after the transient has passed in order to be able to protect against reoccurring transients.

Insulation Resistance (IR):
The resistance between the electrodes of a spark gap which is not ionized. The measurement is commonly made at 100 Volts.

Nuclear Electromagnetic Pulse (NEMP):
The electromagnetic pulse originated from the explosion of a nuclear bomb.

Peak Discharge Energy:
The maximum stored energy that a gap can dissipate per firing, during intermittent duty, without permanently changing its breakdown rating by more than 10%. The principal effect of excessive peak energy is electrode erosion and glass rupture.

Pulse Breakdown Voltage:
Voltage at which the surge arrester ionizes when subjected to a fast rising voltage such as (dv/dt = 10kV/µs).

Rise-time (tr):
The time required for the initial edge of a voltage or current waveform to go from 10% to 90% of the peak value.

Transient:
A sudden change or pulse that is not intended in a circuit.

Transition Time:
The time interval between the instant when the spark gap breaks down and the establishment of the arc voltage.