Sparky, Sparky, Zap!

Neither spark-gap devices nor silicon avalanche diodes are a good fit for protecting process-plant electronics.

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By Dan Hebert, senior technical editor

Providing surge suppression for protection against high voltage incidents is not as easy as it sounds. Most available devices have operational issues, and many don’t meet current UL standards.

Surge suppression devices (SSDs) and transient-voltage surge suppression devices have had to comply, in the U.S. at least, with UL 1449. UL 1449 was revised in February and is now substantially more stringent.

End users need to make sure that any surge suppression device they buy now complies with the new UL standard. They also need to evaluate existing SSDs that do comply.

There are basically three choices in terms of surge suppression technology—spark--gap, silicon avalanche diodes (SADs) and metal oxide varistors (MOVs).

The first is spark-gap. Spark-gap devices handle huge voltages and currents, but as many control engineers hav found, they have horrible let-through power. SADs also have bad let-through. Neither is a good fit for protecting process plant electronics.

For applications where let-through power must be quickly clamped and securely limited, this leaves the third technology—MOVs. “Devices built with parallel MOVs rely on what is often referred to as varistor math. The theory is that if one varistor will handle n kA, then 20 varistors in parallel will handle 20n kA. What actually happens in practice is that the failure rate from aging and stressing is enough of a variable that one or more of the parallel varistors will fail before the others,” says Dr. Kostas Samaras, president of SSD supplier Raycap.

As evidence of the propensity of a single varistor to fail in a parallel varistor installation, one will often see MOV devices protected by low-rated fuses. Dr. Samaras showed us at a recent presentation a sample parallel varistor assembly rated for 100 kA and protected by a 30A fast- blow fuse. The fuse is expected to blow faster than the MOV. Obviously, something is wrong with that picture.

Raycap grew from being the local distributor in Greece for Raychem products to buying the Raychem surge suppression technology in 2002. It now boasts  $50-million in revenue per year, with electrical protection systems accounting for $10 million of that total. The new electrical protection requirements of UL and the IEC spurred them to some remarkable new product development efforts, Samaras told us in our meeting.

“We decided to develop a new kind of SSD,” says Samaras. “This device would not produce fire or flame, would never expose the load to the transient, and would have a life expectancy and reliability greater than the equipment it was designed to protect. We wanted a device that would protect the load at all times.”

Ted Arbuckle, Raycap’s sales director for North America, adds, “We wanted to exceed UL1449, 2nd edition, and IEC 61643-1 as well. We wanted to make an MOV as good as gas discharge tubes and SADs. We wanted a device that would absorb 1000A for 7 hours at a maximum temperature of 45 ºC to exceed the requirements of UL1449. We developed such a device, and we call it Strikesorb.”

Instead of ganging multiple varistors, Raycap designed single, huge MOVs that are thick and wide (80 mm in diameter and approximately 5 mm thick) and have a straight through path to ground or neutral.

“Lightning likes straight paths,” Samaras explains, “and a typical MOV has three 90-degree bends. We wanted a straight path. Furthermore, we designed for an MTBF (mean time between failures) of 25 years in a harsh electrical environment.”

After developing data from the more than 250,000 units installed to date, Samaras said, “We are pleased to find that our design mean time between failure has stood the test.” Samaras said that the module can even be installed directly behind a main breaker.

So what does this mean to the end user? First, you better check to see how many spare MOVs of the old type you have in stock, because when they’re gone, you may not be able to buy new ones. Second, compare the  equivalent specs of the SSDs you currently use and the requirements of UL1449 2nd Edition. You may find that you are having some trouble finding SSDs that meet or exceed the new requirements of UL and the IEC.

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