TVSS.(YOUR LETTERS: OUR READERS SPEAK OUT)(Letter to the editor)

Dear Editor:

Matt Hemphill's article "What is TVSS Technology?" (July, p. 16) issue is, at the very least, misleading and stands for correction. Mr. Hemphill infers, though, that designs with multiple components (MOVs and the like) in parallel are inferior and should not be trusted. There are TVSS manufacturers with parallel component designs that do what they claim and pass the rigorous UL 1449 testing, all with in-house test facilities and third-party confirmation.

In addition, those of us who are serious about TVSS performance know that there are two main criteria for protecting critical equipment from voltage spikes: minimum let-through voltage and maximum impulse current handling. Mr. Hemphill fails to mention that the UL 1449 Standard also requires performance testing. While performance testing may arguably be inappropriate for a safety standard, performance ratings for let-through voltage are registered for all listed TVSS products as suppressed voltage ratings. Quality TVSS products made with parallel strings of MOVs or SADs can meet the lowest suppressed voltage rating (SVR) at 330 V (UL 1449),, while single MOV devices usually do not. Well-matched current paths in multiple component designs usually limit let-through voltage better than a single disk.

Mr. Hemphill's discussion of fusing is misleading as well. It is true that fuses or breakers (or other overcurrent protective devices) are incorporated into all TVSS designs (during UL 1449 testing and in real-world application) to protect equipment and personnel from violent electrical phenomena. Regardless of where they are installed in the circuit, they are necessary for all TVSS designs to disconnect safely from the surrounding system. Successful TVSS designs include internal, external, or both overcurrent components, depending on the need. The same is true for monolithic, single-disk solutions, as they require breakers or fuses up-line to pass the UL 1449 testing and for application in the field.

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In the article, the "main circuit breaker" is recommended to provide the overcurrent protection for the TVSS. The issue here: Mr. Hemphill states, "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?' It would seem to indicate that Mr. Hemphill recommends the facility be shut down completely when the TVSS device conducts during an over-voltage "swell" event. Voltage regulation in this country is exceptional and most critical equipment with switch-mode power supplies can handle wide voltage swings. In my experience, most facilities or operations directors will prefer not to have their equipment turned off by a failing surge suppressor and wait until someone can get an electrician to the scene to remove the fault and replace the breaker/fuse.

Jim McDonald, PE, BSEE, MBA