A question came in recently to ArresterWorks regarding ArresterFacts 033.
The basic question was:
The test procedures indicate that one or more surges of square wave or ½ sin waves with 2-4 ms in duration can be used to raise the arrester temperature during the Operating Duty Test to its maximum stable limit. However the formulae offered in the ArresterFacts 033 to calculate the energy requirement from a systems perspective assumes a single switching surge.
Is it acceptable to compare a multi-surge thermal energy rating to a single surge requirement?
After considering the question my response was:
Indeed, the thermal energy test can be a one or more surge test and the thermal rating is the sum of the thermal energy of all the surges it took to raise the temperature of the arrester to its point of instability.
Also as you point out, the quick formula for calculating the thermal energy requirement on a system is a single surge analysis.
It is the generally accepted understanding that the number of surges it takes to raise the temperature of an arrester to its thermal limit is not relevant to the test, therefore multiple impulses are allowed. When running a system analysis either by transient software or the simple formulae in 60099-5 it is the total energy requirement and could be from either one or multiple surges.
As a result of this question I have revised
ArresterFACTS 033. Additionally it reflects the recent modifications to include sub classes of station and distribution arresters. Improved protective level definitions were also added.
Read ArresterFacts 033 Understanding the New IEC Energy Tests