A damaged surge protection device

There are more than 75 transient voltage surge suppression (TVSS) device manufacturers in the United States, and they share a global market of $1 billion. The overwhelming majority of these surge protection manufacturers utilize the same technology. The most common approach to surge suppression relies on utilizing a multitude of commercial quality, low current-rated metal oxide varistors (MOVs) or silicon avalanche diodes (SADs), manufactured in bulk, and positioned in parallel arrays on electronic printed circuit boards (PCBs). Many such TVSS devices place the equipment they are designed to protect in peril, leaving them ultimately unprotected due to improper suppression circuit design, inadequate surge current capacity, and inappropriate testing procedures. More distressing is the fact that poorly designed TVSS equipment has been responsible for starting facility fires and for compromising human safety conditions.

The UL 1449 Safety Standard for Transient Voltage Surge Suppressors, 2nd edition, comes as a result of the industry’s awareness of catastrophic TVSS failures. As of February 9, 2007, suppliers of UL listed TVSS devices must comply with the revisions of the UL 1449 standard to maintain their UL Listing. Many TVSS manufacturers are unable to meet the revised UL requirements. Products that were UL Listed prior to February 9, 2007 are now in danger of losing that listing. Article 285 of the National Code (NEC) has required that TVSS devices be listed and labeled with a short circuit current rating (SCCR) since 2005. Therefore, TVSS equipment that loses its UL Listing and those products that can’t acquire it should no longer be used. The UL 1449 changes ensure that TVSS devices will not create fire or shock hazards as they are subjected to prolonged and abnormal alternating current overvoltage conditions that cause them to conduct continuous overcurrent values. It is this abnormal current conduction that causes a TVSS to overheat and ignite fires. To assure TVSS safety, UL testing requires that surge protection equipment safely withstand available short circuit current conditions at different intermediate current levels ranging from 100 to 1000 amps for up to seven hours. These tests are destructive trials that necessitate the TVSS to either safely conduct the short circuit current values for seven hours or to disconnect themselves from the test circuit before fire and shock hazards develop.

Design

Many TVSSs are poorly designed and claim performance ratings that are deceiving. Conventional surge protection design parameters call for placing multiple components in parallel arrays to enable the TVSS to suppress greater surge current values. These components are typically soldered into PCBs with wire leads that are angled with 90 degree bends to accommodate populating the board. It is common practice for TVSS manufacturers to multiply the surge current capacity of each individual suppression component by the number of parallel arrays to report the finished product’s total surge current capacity.

Often referred to as “MOV Mathematics,” this arithmetic is misleading, and it can lead to an extremely dangerous situation. While it is possible to electrically match MOV operating characteristics, it would have to be done across the TVSS’s full operating spectrum to mean anything, which would be cost prohibitive. Poor mechanical design factors preclude parallel, attached MOV components from equally sharing the current associated with a surge event. In these situations, one individual MOV will be called upon to handle more energy during a surge event than its neighboring components. This often causes the stressed component to fail prematurely, and sometimes catastrophically.

To provide uniform distribution of surge current over the total area of a MOV protection element, a monolithic TVSS design that utilizes a single, large distribution-grade metal oxide varistor (MOV) becomes appropriate. This unique design parameter eliminates “MOV Mathematics” and protects sensitive electronic equipment with a high-energy handling suppressor product. Combined with low contact resistance and low let through voltage characteristics, the resulting surge suppressor provides the utmost protection against threatening surge events. In addition, the absence of 90-degree lead wires provides a straight electrical path to disperse surge current across the entire surface area of its large single disc distribution-grade varistor material, enabling it to withstand repeated surges.

A TVSS installed in an environmentally sealed housing also minimizes the effects of MOV “aging” and eliminates the risk of catastrophic failure modes that result in explosion or fire. In a hermetic housing, the varistor can efficiently dissipate very high surge current values as well as the generated heat.

A damaged varistor array

Fuses

Conventional TVSS technologies continue to rely on thermal and overcurrent fusing to comply with the revised UL 1449 requirements. Existing surge protectors that met the earlier (1998) revision of UL 1449 already utilized fuses and thermal disconnects. They were called upon to withstand testing parameters that subjected TVSSs to five amps of current conduction for seven hours. It was intended for these devices to disconnect from the circuit within a few seconds upon being exposed to that low level current value. While they may have been able to meet the requirements of the five-amp test, they are challenged by the revised test parameters that call for intermediate current testing levels at 100, 500, and 1000 amps. The new testing parameters seriously complicate fusing or thermal disconnection design considerations. The engineering approach many TVSS manufacturers continue to take is to design a disconnect mechanism to react to these currents quickly, disallowing the full impact of the 100, 500, or 1000 amps of test current, to prevent the TVSS from failing in a violent manner.

Many manufacturers are plagued with fuse coordination problems that prevent their TVSSs from disconnecting themselves from the circuit as they are called upon to conduct sustained overcurrent values while continuing to divert large values of short duration surge current. They may have to combine more than one fuse type in series to accommodate the short circuit current testing up to 1000 amps that the revised UL 1449 standard demands. The utilization of faster and more sensitive disconnects exposes these surge protectors to failures resulting from the conduction of high value surge currents. The TVSS’s performance capabilities may be seriously compromised by sacrificing surge current protection levels to meet the demands of UL’s enhanced safety requirements as it utilizes internal fuses to protect itself from sustained overcurrent values.

The electronics industry has accepted the end-of-life for a surge protection component to be an open circuit. Therefore, the failure mode of existing TVSS components has been designed towards open circuit conditions. Typically, a fuse is utilized to disconnect the device from the circuit under adverse current conducting situations. In other words ,there are times when a TVSS is designed to disconnect itself from the power supply in order to prevent catastrophic modes of failure. It is alarming that the critical load, which is supposed to be secured by the TVSS, is now left to deal with the full force of the power surge without protection.

It is preferable for the TVSS to meet the stringent requirements of the UL-1449, 2nd edition, safety standard without the use of internal fuses. A fuseless TVSS will not disconnect from the circuit under adverse conditions. The absence of fuses allows the TVSS to be directly connected on the power bus bars after the main circuit breaker. This configuration enhances overall suppressor performance by eliminating the counterproductive impedances that are associated with the wiring leads that are required to install conventional TVSS equipment. Should the properly designed fuseless TVSS fail, it should do so by going to a direct short. At such time the TVSS must be designed to withstand the full available fault current until the main circuit breaker clears to disrupt power to equipment loads. The clearing of the main circuit breaker precludes equipment loads from being damaged from whatever the catastrophic event was to cause the problem at hand.

Testing

The majority of TVSS manufacturers have neither the testing equipment nor the facilities to adequately test their TVSS products to verify their performance claims. If any testing is performed, it is generally conducted at very low levels. The results from those tests are often manipulated to support the calculated performance characteristics. Furthermore, the testing can be rendered useless by failing to test for the actual protection that is provided to a load in a typical installation. Tests are conducted with short cables and let through voltages are measured at the protector and not at the load. The practice of incomplete and inappropriate testing procedures produces dangerous situations when inadequate protection devices are utilized. These practices mislead the suppressor user. Therefore, it is important that reputable TVSS manufacturers submit their products to independent laboratories that specialize in high current surge testing to determine their product’s true performance capabilities. TVSS performance characteristics should be documented by reputable third parties, and not solely by the TVSS manufacturer.

 “MOV Mathematics” does not provide accurate and reliable testing data. Independent laboratory testing and field testing are appropriate when collecting device specifications and characteristics. Independent lab tests provide third-party verification to surge rating claims while true load testing provides additional information, such as installation parameters, that are not obtainable in a lab. These testing methods provide reliable data as to the effectiveness of the TVSS in protecting the load.

A critical load should never be exposed to intense surge anomalies. The TVSS should protect that load at all times and especially during abnormal operating conditions. Improper suppressor designs, misleading surge rating quotations, and inappropriate testing procedures all contribute to exposing critical electronic equipment to damaging surge events. In these situations, the equipment, for all practical purposes, remains unprotected. The UL 1449, 2nd edition, safety standard has increased the requirements for TVSS systems to ensure that protection devices are able to absorb and dissipate surge energy without sacrificing human safety concerns. By eliminating inherent design flaws, internal fuses, and “MOV Mathematics,” TVSS devices will be better equipped to protect the critical equipment loads, as opposed to themselves, during a surge event.