Surge protectors normally contain circuitry that responds to a rate of change of a current or voltage to prevent a rise above a predetermined value of the current or voltage and typically includes resistors, capacitors, coils, tubes and semiconductor devices. Surge protection devices are often implemented on printed circuit boards in order to minimize the size of the device, to automate production, and to reduce the cost of production. Two factors that are important in the design and performance of a protector circuit are the turn-on voltage and limiting voltage. The turn-on voltage is the voltage at which the protector begins shunting a surge to ground. The limiting voltage is the voltage that is passed through to the power line when the protector is shunting a surge to ground. Surge suppressor circuits typically present a high impedance (open circuit) to circuit ground under normal signal conditions and present a low impedance (short circuit) to circuit ground when a voltage or a current exceeds a predetermined threshold. Primary surge protector circuits usually allow the voltage on a transmission line conductor to rise very rapidly when a lightning strike or other surge occurs on the line, until the breakdown voltage of the gas tube, triac, or other crowbar device goes conductive, and the impedance from the data conductor to ground or to some other reference potential reduces very rapidly. Surge protection components manufactured by thick film technology include often several protection resistors on one substrate, either adjacent to each other on the same side of the substrate or as printed on the both sides of the substrate. It is also known to provide surge protectors with a failsafe mode of operation. A surge protector provided with a failsafe mode of operation continues to protect personnel and equipment in the event that the primary protection element overheats, or both the primary protection element and the secondary protection element overheat. Some surge protectors are also provided with a fail-short mechanism, which is a device that protects against longer-duration power surges. When connected between an electrical conductor and ground, a fail-short mechanism conducts electrical current only in response to a power surge of a relatively long duration. Current sensitive devices are also employed in conjunction with the voltage sensitive devices. Such devices may include a spring loaded element which is moved by the spring to connect the transmission line to ground when excess current is encountered.

Surge protectors utilize various types of protection elements to divert unacceptable levels of voltage to ground. There are generally three types of surge suppression devices or regulator devices commonly employed in low cost power line surge suppression units which include metal oxide varistors (MOV), silicon avalanche diodes, and gas discharge tubes (GDT). These devices have different operating characteristics and ratings and different failure. These devices are often aggregated together to take advantage of the desirable characteristics of each device. For example, an MOV may be used in conjunction with a gas tube as a back-up protection device to provide continued protection to personnel and equipment in the event that the gas tube fails. An MOV may also be used in conjunction with a gas tube as a hybrid protection device to reduce the reaction time of the surge protector or to reduce the impulse breakdown voltage of the gas tube without permitting the MOV to burn out. A typical surge protection device may include diodes connected in parallel that conduct in the event that high energy signals occur. Surge protection filters have been constructed utilizing Zener diodes selected to shunt current across a signal source to ground when the voltage across the diode exceeds an arbitrary design threshold. A voltage in excess of a selected threshold is termed an overvoltage. When the diode senses a high-energy electrical signal and if the voltage of the signal is above the threshold voltage, the diode starts conducting current to ground. The magnitude of voltage at which the signal causes the diode to conduct is called the clamp voltage and is equal to the sum of the threshold voltage of the diode and any voltage drop caused by impedance in any electrical connection of the diode. Transient surge protectors, also known as transient suppressors, are electronic elements similar to Zener diodes and are used as the main component in a number of state of the art surge suppressors. Transient surge protectors are small, inexpensive, rugged, and can handle repetitive high currents (up to 6000 amps), and react fairly quickly to power spikes. However, these devices do not react quickly enough to reliably protect a communications port.

Varistors have also been used as protective elements to shunt current in the event of an overvoltage. Metal oxide varistors are two-terminal electrical devices that have a nonlinear voltage-current relationship. Metal oxide varistors (MOVs) are typically utilized as voltage limiting elements. If the voltage at the MOV is below its clamping or switching voltage, the MOV exhibits a high resistance. If the voltage at the MOV is above its clamping or switching voltage, the MOV exhibits a low resistance. At low voltages, a MOV exhibits a high impedance between its two terminals, but at voltages higher than a predetermined limit voltage the impedance rapidly changes to a low impedance. Varistors have symmetrical current-voltage characteristics for positive and negative voltages, resistance values which decrease rapidly with increasing voltage once a voltage threshold is reached, and greater load capacity than typical Zener diodes. Thus, varistors are often preferred to Zener diodes in surge protection filter applications. Gas tubes contain hermetically sealed electrodes, which ionize gas during use. When the gas is ionized, the gas tube becomes conductive and the breakdown voltage is lowered. A surge protector usually includes a parallel combination of a gas pressurized discharge tube and an air gap connected between an electrical conductor and ground. The gas discharge tube conducts in the presence of an electrical surge to direct the electrical surge to ground. The air gap operates as a backup protection element for the gas discharge tube in case the gas discharge tube is vented.

Various types of surge protectors are used to protect electrical equipment from electrical power surges. Roughly speaking, surge protectors are divided into primary type surge protectors and secondary type surge protectors. Primary surge protectors are generally located where electrical lines enter a building. These surge protectors are designed with a relatively large "surge capability" so that they can protect against relatively large surges. Primary surge protectors are located where lines enter a building, therefore they can protect the portions of the lines which run inside the building as well as devices which are located on the lines. Primary surge protectors can be grounded by building ground wires. Building ground wires have good connection to the Earth itself, and therefore can provide excellent grounding for large surges. Secondary surge protectors are generally located on the inside of a building. Because they are generally located indoors, secondary surge protectors are not generally required to be as rugged as primary surge protectors. Secondary surge protectors typically have a lower surge capability than primary surge protectors. Primary and secondary surge protectors are each further sub-divided into surge protectors for each type of electrical line. For instance, there are AC power line primary surge protectors, co-axial cable primary surge protectors, and telephone line primary surge protectors. The various types of electrical lines have different constructions, different kinds of interfacing hardware and are subject to different types of surges. Furthermore, the various types of electrical lines typically follow different paths within a building.

Isolator surge protectors (ISPs) are electrical devices that are designed to block the flow of unwanted DC current in electrical systems while simultaneously allowing the flow of normal AC current to ground. Isolator surge protectors also allow the flow of transient and fault currents to ground. Such fault currents may be several orders of magnitude higher than the normal operating currents of the system in which the ISP is used. ISPs are used in various high power electrical system applications such as high power transformers used for the transmission and distribution of electrical power. ISPs may also be used in association with systems that protect metal structures against corrosion by the application of a DC bias voltage. Many metallic structures and systems are protected against corrosion by cathodic methods. Telecommunications systems employ very large numbers of surge protectors to connect voltage sensitive switching equipment and other equipment to outside phone lines. Phone lines are subject to current surges caused by lightning and other extrinsic phenomena associated with the location of the phone lines. A variety of different protector circuits have been utilized to limit damage to data transmission lines, particularly telephone lines, due to lightning strikes and other voltage surges on the lines. Building entrance surge protectors (BEP) are permanently wired to power lines where they enter a building and are designed with a relatively large surge capacity. They are located where power lines enter a building so that they can protect the portions of the power lines that run inside the building as well as devices which are connected to the power lines. A building entrance protector is used in telephone equipment where telephone lines from outside plant wiring are joined to customer premises equipment. Within the BEP there is an input wire termination device that receives the telephone lines contained within the outside plant wiring. Also contained within the BEP is an output wire termination device that receives the telephone lines required for the customer premises equipment. Located in between the input wire termination device and the output wire termination device are fusible links.

Surge protectors are employed to protect a wide variety of electrical devices. The majority of electronic equipment such as computers, communication systems, automobiles, military surveillance equipment, stereo and home entertainment equipment, televisions and other appliances are very susceptible to stray electrical energy created by electromagnetic interference or voltage transients occurring on electrical lines. Voltage or power fluctuations in the line connecting the power supply to the device being powered can push voltage or power beyond these normal operating ranges, and risks damaging the equipment. Surge protection is necessary to protect valuable installations utilizing high frequency data communication lines, without adversely affecting data communications. Typical installations requiring protection include satellite receivers, local area networks (LAN), wide area networks (WAN), radio communication equipment. Generally, surge protectors are adapted to be plugged into a standard electrical outlet and to be positioned between the source of electrical power and the sensitive electronic or electrical device to be protected from electrical surges. The protectors are situated near the end of the transmission line to which the equipment is connected and serve to ground the transmission line in the event of voltage or current excesses of sufficient magnitude to damage the equipment.