Peter,
The metrics we use to specify the more complex surge waveshapes are
only a modest improvement over the answer "a red one" to the "what car
do you drive?" question.
The ring and possibly 8/20 (part of the combination) waveshapes
have current reversals, which complicates matters. Without details of
what your different results were I can only make generalised comments.
IEC 62475 defines the stand alone 8/20 waveshape. There was
considerable committee discussion on how you can't get a perfect
8.00/20.0 waveshape with the simple RLC circuits that many standards
show. For example they worked out for 20 µs ±20% the nominal 8 µs is
constrained to 6.9 µs ±0.5 µs or ±7.2 % not the allowed ±20%.
Under-swing or current reversal is allowed up to 30 %. It seemed to me
that the people in the IEEE PES SPDC C62 formulation group where adamant
that no under swing occurs. By enforcing this design constraint the
resultant waveshape moves further away from the 8/20 goal.
In life testing, 8/20 current reversal is bad news for GDT surge
protective components (SPCs) as it shortens the number of cycles that
can be endured.
Several years ago at an ATIS PEG conference there where two
interesting presentations that mentioned ring waves. The Telcordia one
used a standard C62 generator to track down the part of the distributed
system which was vulnerable to lighting. The HydroQuebec approach was to
use a fast rising (1.4 µs) surge to shock excite the system to produce
its natural ring wave (nothing like the C62 one of course).
As John correctly noted the generator is a network and its apparent
reduction to a Thévenin source equivalent by quoting an effective or
fictive source impedance of 2 ohm can form a mental trap.
These surge generators are then applied to further networks and
EUTs that may have non-linear elements like MOV SPCs or rectifiers. All
these things can exaggerate differences between generators.
On the theoretical side there was a recent ITU-T contribution on
recommended Spice models for 1.2/50-8/20 combination generators.
Surprisingly some of the models didn't fully meet the 1.2/50 and 8/20
requirements.
We have to work with the tools we are given and expect variances
because not everything is exactly the same.
Mick
On 21/08/2015 19:50, Peter Tarver wrote:
Good morning.
I'm wondering if others have experienced cases where different
manufacturers' surge test equipment (ANSI/IEEE C62.41 ring and
combination waves) with nearly identical open-circuit voltage and
short-circuit current calibrations have led to very different results. In
these cases, other than addressing the issue by using the surge generator
that produces the worst-case result, what were thought to be the causes
for the different results (ignoring the real possibility of a marginal
design).
Regards,
Peter L. Tarver
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