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  • All SPD
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  • Modular TVSS
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  • Special Application TVSS

A Joule is a unit of energy defined as a Watt-second. J= W•s J=V•A•s. Joules would appear to be an ideal unit-of-measure for SPD. Unfortunately, there are inherent deficiencies addressed below. While consumer class SPD use Joule ratings for marketing purposes, this is a problematical way to compare suppressors. There are two fundamental problems using Joule ratings for SPD:

1. There is a common misperception that larger surge amplitudes deposit more energy into a SPD. IEEE research shows this is not the case. Energy deposited into MOVs actually decreases as surge intensity reaches certain levels. Therefore, Joule ratings are misleading. The surge industry’s technical community avoids Joule ratings in this context.
   
   
   
   Adjacent is an excerpt from IEEE C62.41.1-2002 page 63 showing that energy deposited into MOVs actually decreases upon reaching certain levels. This is a result of upstream ‘flashover’, which is a physical limitation of all distribution systems. Shaded cells indicate decreased Joules.
   
   
2. Joule determinations are open to interpretation or questionable usage. By definition, Energy is defined as Power x Time. In this context, how are Power and Time defined and where are Watts actually going? For example, are Watts the amount of power that the SPD is exposed to (i.e., impulse size), the power passed through the SPD, or heat absorbed inside the SPD? Each of these is substantially different.
   
   For example, the first calculation below shows Joules when the SPD gets ‘clobbered’ by IEEE C62.41’s most severe surge. The second calculation shows trivial leakage currents for a little over six months. Joule ratings are the same but the stress on the SPD is very different.
   
   
   • Assume IEEE worst case surge: C High - 10,000V, 10,000A, 20µs duration: J= W•s = V•A•s = 10,000V x 10,000A x 20µs = 2,000J
     
     
   • Assume 1µA leakage current through 120V MOV(s) for 193 days: J= W•s = V•A•s = 120V x 1µA x 193 days = 2,001J
     
     
   In another example, can the SPD sustain a “Joule rating” many times, one time, or does it define failure? At least one manufacturer uses a “multiplier”. For example, if an MOV is rated 1J, but can withstand 5,000 hits, the SPD is rated 1J x 5,000 = 5,000 Joules. In summary, Joule ratings are not recognized by surge suppression Standards due to ambiguity.

For further information or an IEEE white paper titled, “No Joules for Surges: Relevant and Realistic Assessment of Surge Stress Threats”, please contact APT Engineering Sales Department at 800-237-4567, or email: info@apttvss.com.