UFC 3 -520-01
June 10, 2002
Table D -2. Minimum Recommended MCOV Ratings
AC Power
Minimum
System Voltage
MCOV Rating
Single-Phase
120
150
120/240
300/150
Three Phase Wye
208Y/120
300/150
480Y/277
600/320
D-2.4
Surge Current Rating.
D-2.4.1 The surge current rating provides a relative measure of the surge protector's
ability to withstand surge currents and is an indicator of the peak surge current that the
device subassemblies and modules are designed to ha ndle on a one -shot basis without
failure. The surge current rating is usually based upon the total summation (or near-
total) of internal subassembly or individual MOV ratings on each phase or protection
mode.
D-2.4.2 For large surge current ratings (over 100,000 amperes), it is unlikely that the
entire surge protector was actually tested to such a high level. Test laboratories are
capable of generating an IEEE C62.41 8x20 sec impulse up to about the 100,000
ampere level. What this means is that surge protectors with surge current ratings in the
hundreds of thousands of amperes have most likely not been tested to this level.
Furthermore, it is possible that a surge protector with such a high rating would fail if
subjected to such a high level; circuit board traces, internal fusing, or connecting wire
would probably open under the high transient current. Another point to consider with
regard to surge current ratings is that the ratings usually assume equal parallel
operation of the MOVs, but the nonlinear behavior of MOVs virtually assures that such
precise parallel matching is unachievable.
D-2.4.3 For MOV-based designs, the surge current rating provides a relative measure
of the amount of MOV surface area that is available for surge protection. As MOVs are
subjected to surge voltages, the MOV grain boundaries degrade with time. By including
additional MOVs to obtain a higher surge current rating, the surge protector effectively
provides an operating and aging margin. In other words, a higher surge current rating
can provide a longer operating life. Surge protectors that are rated for hundreds of
thousands of amperes of surge current do not actually expect to experience such a high
current level. Instead, the higher rating provides assurance that the surge protector can
withstand a number of smaller surges, each of which damages MOV grain boundaries
by some amount, without experiencing complete failure of the surge protector. By this
approach, the surge protector should have a longer life.
D-4
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