Following are frequently asked questions about surge suppression, and definitions of terminology used when specifying surge suppressors.


Q: What is TVSS or SPD?
A: TVSS is an abbreviation for "transient voltage surge suppressor". This term is giving way to “surge protective device” or SPD. A TVSS or SPD is a device that attenuates (reduces in magnitude) random, high energy, short duration overvoltages caused by lightning, utilities, switching, etc. Such anomalies occur in the form of voltage and current spikes with a duration of less than half an ac voltage cycle. These high energy power spikes can damage sensitive electronic equipment, such as computers, instrumentation, and process controllers.

Q: How do surge suppressors work?
A: Surge Suppressors divert high energy power away from a load by providing a lower impedance path to common point earth ground. This is similar in concept to pressure relief valves that protect water heaters from overpressure. Surge suppressors used most often for panelboard protection have metal oxide varistors (MOVs) connected in parallel.

Q: What types of components make up a surge suppressor?
A: The device most commonly used in AC voltage surge suppressors are MOVs, a solid-state device made of zinc oxide materials.

MOVs are voltage sensitive semiconductors, which change from high impedance to low impedance when sensing an overvoltage condition. MOVs are packaged for specific voltages and current handling capacities.

Other devices (more typically found in dc applications) include single junction diodes and gas tubes that ionize at preset voltages.

Q: Where are surge suppressors installed?
A: AC voltage surge suppressors are typically installed in these three areas: at a utility service entrance for protection of an entire facility. in distribution panelboards and switchboards for protection of sensitive downstream loads; connected to a wall outlet for individual protection of a specific piece of equipment, such as a computer or solid-state controller.

Q: What is surge current capacity?
A: Surge current capacity, as defined by NEMA standards, is the maximum amount of surge current that a surge suppressor can pass for a single transient event. This level is used to indicate the protection capacity of a particular surge suppressor design, and when specifying surge suppressors. For example, in a high exposure application with very large transients present from lightning, a higher level surge current capacity might be desired. Be aware that surges have natural limitations and that larger surge current capacity tends to add redundancy rather than the implied ability to handle an extremely large surge. For example, an entire lightning strike cannot go through wire; much like a fire hose has difficulty shooting through a soda straw. Consequently, suppressors do not need to be sized for entire lightning strikes. There are valid reasons for adding excess surge current capacity for redundancy reasons.

Q: What is clamping voltage?
A: Clamping voltage, also referred to as peak let through or suppressed voltage rating, is the amount of voltage a surge suppressor permits to pass through it to the attached load during a transient event. Clamping voltage is a performance measurement of a surge suppressor's ability to attenuate a transient. For example, a surge suppressor might limit a 6,000V surge so that only 400V is ‘visible’ to the load. The clamping voltage is 400V. This performance value is confirmed by Underwriters Laboratories during tests conducted while evaluating a surge suppressor for listing.

Q: What features should be considered when selecting a surge suppressor?
A: Two important areas to consider during the selection of an surge suppressor are performance and safety, and include the following criteria:

Performance: 1) surge current capacity; and 2) clamping voltage.

Safety: 1) the individual suppression circuit should be fused to clear an inoperative MOV during an extreme transient event, and 2) provide overcurrent protection for the surge suppressor during a fault condition.

Q: What surge current capacity is required?
A: Surge current capacity is dependent on the application and the amount of required protection. The selection of the proper surge suppressor is not an exact science and cannot be scientifically calculated from a standard algorithm.

Questions to consider when specifying the proper surge current capacity for surge a surge suppressor include:

• What is the geographic location of the facility and its susceptibility to lightning? (For example, Florida is a high-lightning area; California is a low-lightning area.)
• Is the facility in a rural or urban setting?
• Is the facility the tallest building around?
• Is the facility at the end of the utility grid?
• If it is an existing facility, what is its power quality history?

The SurgeCalc® Form in the Applications section is a good tool for determining the surge current capacity. Based on the above information, and taking into account the cost of protection, the following is a good rule of thumb: a surge suppressor with a surge current capacity in the range of 100kA to 240kA would be used in conjunction with a service entrance panelboard or switchboard. A surge suppressor with a surge current capacity in the range of 80kA to 160kA would be used in conjunction with a downstream panelboard.

Q: What is important when specifying TVSS?
A: When specifying TVSS, submit a clear, concise specification detailing the required performance and design features. A minimum specification should include:

• UL suppression ratings
• Peak surge current per mode (L-N, L-G, and N-G)
• Diagnostics
• Voltage service

Q: What is the difference between UL 1449 Listed and UL 1449 Component Recognized?
A: UL 1449 Component Recognized products are required to pass the same performance tests as UL 1449 Listed products.

Q: What are C62.41 and C62.45?
A: C62.41 and C62.45 are IEEE standards used to describe the characteristics of a transient and how a surge suppressor is tested to evaluate performance.

C62.41 defines a transient and describes the transient environment at three separate facility locations. These locations are a service entrance (Category C-the most severe), a distribution panelboard or switchboard (Category B), and a wall outlet (Category A). C62.41 is also a reference document that specifying engineers refer to for application information for defining a transient environment in a particular facility location.

C62.45 describes in detail how a surge suppressor performance test is to be conducted.


The following definitions apply specifically to surge protective devices (SPD). They are provided for further clarification of the performance specifications in the data sheets.

Crest Value (peak):
The maximum value that a wave, surge, or impulse attains. It is generally associated with the front of a wave.

Clamping Voltage:
The peak voltage across the surge protective device (SPD) measured under the conditions of a specified surge current and waveform. Peak voltage and peak current are not necessarily coincident in time.

Impulse:
A wave (surge) of unidirectional polarity. In testing, the risetime and duration of the impulse are specified, e.g., an 8/20μs impulse, a 10/1000μs impulse.

Maximum Continuous Operating Voltage (MCOV):
The maximum rms value of the power frequency voltage that may be applied continuously between the terminals of the surge protective device.

Nominal System Voltage:
A nominal value assigned to designate a system of a given voltage class, e.g., 120/240 Volt single phase. Note: see ANSI C84.1

Operating Duty Cycle:
One or more operations per unit of time as specified.

Pulse Life:
The number of surges of a specified voltage and current amplitude and waveform that may be applied to a SPD without damaging or changing it. The time interval between surges must be specified.

Maximum Single Impulse Current:
The maximum amplitude of current which may be applied for a single 8/20us impulse.

Power Dissipation:
The power dissipated by a protective device while connected to an AC line of the rated voltage and frequency while no overvoltage condition or surge exists. Steady state power dissipation.

Response Time:
The time domain response of a surge protective device to the front of a voltage waveform depends on the rate-of-rise of the incident wave, the impedance of the surge source and connecting wiring, the effects of protective device reactance, and the response behavior of conducting mechanisms within active suppression elements. In other words, response to the front of a wave can be affected more by the test circuit conditions, including lead inductance, than by the response time of the active suppression element.

Surge:
A transient wave of current, potential, or power in an electric circuit.

Surge Let-Through:
The voltage seen by the protected load, includes the SPD clamp voltage plus the voltage drop in the connecting wires. The part of the surge impulse which passes through the protective device.

Surge Protective Device:
A device for limiting the surge voltage on equipment by discharging or diverting surge current. A SPD should be able to repeatedly perform these functions are specified.

Response Time:
The time required for a device to make the transition from ‘off’ to ‘on or vice versa. Be aware that almost all suppressors using traditional surge suppression technologies are fast enough. For example, the response time of most MOVs is in the nanosecond range, whereas the rise time of a surge is in the microsecond range. This makes the suppressor about a thousand times faster than the surge! Response time is a common specification item, despite having more ‘gee-whiz factor’ than genuine usefulness. Be aware that the speed of electricity is about 6”-8” per nanosecond and that propagation delays through wire are substantially longer than SPD response times.

Voltage-Current (V-I) Characteristics:
The relationship between the suppressed voltage and the magnitude of the surge current which induces this voltage.