AC Isolation

The physical disconnection of the AC utility power from electronic systems to protect against power transients and lightning ground potential rise. Typically an automatic process based on detection sensor(s).

All Mode

An AC surge protector that provides all modes of power protection, typically referenced to 240V split phase and 3 phase power systems. The modes are: Line-Neutral (L-N), Line-Line (L-L), Line-Ground (L-G) and Neutral-Ground (N-G)


The property of parallel conductive surfaces to store energy in an electrical field between the surfaces. The degree of capacitance varies directly with the area of the parallel plates and inversely to the distance of separation between the plates. A property of a capacitor is to pass AC signals and block DC signals. The AC impedance will vary inversely with frequency.


A relay that is designed to control power in AC power systems. A contactor is frequently used in electric motor starters and electrical power panels.


Pertaining to equal electrical potential; where all points on the grounding system (sometimes referred to as a plane) strive to be at the same potential to mitigate ground current loops.


A current arc from one conductor to another when struck by lightning. A typical example is a lightning strike to the ground protection wire on an aerial power line that arcs to the phase or neutral line below. In the first several uSec of lightning contact the immediate voltage can exceed 100KV while the impedance to ground can be very high. The phase lines are at a moderate voltage with respect to ground and provide another path to ground, as does the neutral line.

GPR (Ground Potential Rise).

The increase of electrical potential in the earth with respect to another location. This may result from a downed power line or a power distribution fault, referred to as 60Hz GPR; or a lightning ground strike (L-GPR). GPR is typically a momentary condition and magnitudes can vary between mV and KV.


A direct physical connection to the earth, or a reference point in an electrical circuit from which other voltages are measured.

Ground Resistance

Ground resistance is the resistance of the grounding system to current flow. However, ground impedance is a more complete characterization of grounding conductivity that includes capacitive and inductive reactance. Grounding impedance is primarily a function of soil resistivity, surface area contact with grounding electrodes and the physical dimensions of the conductors.

Ground Ring

A series of buried grounding electrodes connected by a conductor such as a wire or cable, in direct contact with the earth encircling the grounded structure.

Grounding Electrode

More commonly called a ground rod which is driven into the earth for the purpose of providing an electrical connection to the earth.


The total opposition to current including resistance, inductive reactance and capacitive reactance. In DC circuits there is no reactance and the impedance is just the resistance. (Refer to Reactance)


The property of a conductor that provides a direct opposition to AC current from a self-induced magnetic field caused by a change in current.

L-GPR (Lightning Ground Potential Rise)

Ground potential rise created by a lightning ground strike. L-GPR radiates in all directions and is typically more severe than 60Hz GPR which returns to the distribution source. L-GPR creates extreme potential differentials across the earth surface. The differential voltages decrease with distance based on soil conditions.

MGB (Master Ground Bar)

A metal bar used as a common grounding point in communication systems. The MGB contributes to providing a single point ground.

Neutral-Ground Bond

The point where the power neutral line and ground are bonded together at the power transformer and the power service disconnect. NEC requires an N-G bond at the service disconnect location; no other N-G bonds are permitted after that point.

Neutral-Ground Mode

The protection mode in a surge protector that limits voltages between the neutral and ground lines. Other modes are line to neutral and line to line. Not all surge protectors have a N-G mode.

Optical Isolation

Use of a di-electric plastic fiber to transmit signals and concurrently isolate the electronics on either side of the fiber from each other electrically.


(Electric Potential or Voltage). A description of the difference in potential energy (per unit charge) between points in an electric field. The concept is exemplified by the voltage between two battery terminals. The magnitude of potential is an indicator of the ability to move charge. In Ohms Law the voltage determines how much current will flow through a given resistor.


The opposition to alternating current due to capacitance (capacitive reactance) or inductance (inductive reactance). Reactance plus resistance equals impedance.


The property of a material to oppose the flow of electrical current. Resistance is comparable to friction in mechanical systems. In Ohms Law a given resistance will allow a specific current for a given voltage.

Rise Time

The measure of time for an electrical signal to go from the lowest to the highest level. Typically measured between the 10% and 90% levels.

Service Disconnect

A switch at the AC utility service entrance that allows disconnection of the service from the load circuits. A circuit breaker is typically used as both overload protection and switch. The circuit breaker rating defines the current limit of the service.

Single Point Ground

A grounding system design that ensures all ground conductors from the electronics equipment are connected to a single point on the grounding electrode or ground ring. A single point ground mitigates ground current loops and ground potential differences.

Soil Resistivity

A measure of soil’s ability to oppose conductance of electrical current. Soil resistivity is measured in units of Ohm-meters or Ohm-centimeters. Soil resistivity values can vary significantly from 2 to 10,000 ohm-meters. Moisture content can significantly affect soil resistivity, particularly during prolonged seasonal changes.


A surge protection device is used to mitigate transients and surges on power and signal lines by limiting the voltage between the SPD terminals. An SPD has a high resistance in a normal voltage range and low resistance in higher than normal voltages. The conduction voltage level is referred to as the threshold level. There are various technologies used in SPD’s that offer tradeoffs between speed of operation, protection voltage threshold and energy carrying capacity.


Transient describes short term excess voltage on power or signal lines. The time duration can vary from a few milliseconds to nanoseconds. Excess voltages can exceed several thousand volts. The cause of a voltage transient can be lightning or power switching equipment.