Many years ago, Transient Voltage Surge Suppressors (TVSS) were ‘invented’ and implemented without a full understanding of surges or safety issues. Since those early days, TVSS have been renamed to Surge Protective Devices (SPDs). The knowledge and regulatory base has grown substantially.
As a generalization, surge protectors ‘activate’ upon sensing overvoltage. Overvoltage is different than overcurrent. For example, overcurrent protectors are typically fuses or circuit breakers that detect an excess amount of current, which might overheat wiring or devices, and then open the circuit to disconnect power. The most common overvoltages are extremely short duration and termed ‘transient overvoltages’, lasting millionths of a second. These are generally associated with lightning, circuit switching, cycling, etc. (The collective technical community is now embracing the term ‘surge’, but ‘transient overvoltage’ is probably more descriptive terminology. Note that layman understanding of ‘surge’ may be far different than technical intent, thus causing semantics challenges.) During a transient overvoltage, a surge protector ‘activates’ and redirects harmful surge energy to neutral or ground, then resets itself automatically.
Keep in mind that surge protective devices (SPDs) are purposely placed in harm’s way and are intended to sacrifice themselves to protect the more valuable load. There are relatively rare abnormal situations where overvoltages are not transient-related, but can last portions of a second, seconds or longer. SPDs may ‘activate’ and chase these overvoltages in an attempt to protect the load. Unfortunately, SPDs were intended for short duration surges, not ‘long-term’ or sustained overvoltages. Surge protectors attempting to control ‘long-duration’ overvoltages tend to overheat. Therein lays the safety issues. If the SPD overheats slowly, it might eventually overheat and smolder or catch fire. If that process is accelerated dramatically, the surge protector might explode. It is now common for SPDs to include automatic overtemperature and overcurrent safety disconnectors.
Most surge protective devices (SPDs) include internal suppression elements that operate by varying impedance. These are electrically connected in parallel with the load. During normal operation, the SPD sits at high impedance and awaits an overvoltage. The high impedance prevents current flow. Upon sensing an overvoltage, the SPD greatly reduces its impedance, thus allowing the overvoltage to pass through the SPD as current; thus diverting the surge and protecting the load. Most SPD failure scenarios involve an ‘in-between’ state where the SPD has lowered impedance some, but not enough to become a short circuit. There become multiple scenarios of ‘reduced impedance’ that can be safety-tested.
Surge Protectors rarely fail from surges, rather from system-level overvoltages. Common causes of SPD failures are sustained system-level overvoltages caused by improper application, improper installation, power systems losing their reference to ground, etc. Compounded power system problems can also affect SPDs.
Following are significant regulatory milestones relating to Surge Protectors:
· In August 1998, UL 1449 Second Edition added safety tests that simulated various power system problems believed to damage SPDs. This included controlled SPD failures at 5 amperes and below, and a higher-current test to failure, up to 25,000 amperes.
· In 2002, the National Electrical Code (NEC) introduced Article 285 requiring that SPDs include Short Circuit Current Ratings (SCCRs). This triggered additional safety testing above the previous 25,000 ampere rating; sometimes up to 200,000 amperes.
· In February 2007, UL 1449 Second Edition Revision added additional controlled-failure tests from 10 amperes to 1,000 amperes. These tests proved difficult, prompting product changes, defections away from UL, with some manufacturers literally leaving the industry.
· The 2008 NEC Article 285 included changes as a precursor to upcoming UL changes.
· In September 2009, UL 1449 Third Edition became effective. This effectively obsoleted old-school secondary arrestors, created Type ratings, changed performance testing and added Nominal Discharge Current testing. As with previous UL changes, this turned the surge industry upside down.
Advanced Protection Technologies remains an industry leader with an unparalleled history of Surge Protective Device (SPD) safety. APT products meet and exceed applicable UL testing, proudly wearing the UL Mark. Many products include robust, large-block MOVs for better performance and safety, automatic thermal and overcurrent disconnectors, patented TranSafe circuitry, robust NEMA and UL rated enclosures, etc. We intentionally drive SPDs into failure simulating compounded power-system problems to ensure the safest possible surge protector. Customers are always welcome at our facility and test labs to witness testing.
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