Rich,
Sorry, but there appears to be no intelligent answer to this question.
However, I do have a personal interpretation. In fact there are three possible
interpretations:
1. If what you are trying to avoid is energy hazard, and if indeed delivering
20 J quickly
is enough to cause an injury, then any time greater than 20J/240VA (=83 ms)
is
hazardous. By this reasoning, anything over 83 msec is "continuous."
If the power source is higher than 240 VA, then the hazardous time is
proportionately lower. So, by this definition, "continuous" is very short
indeed.
If anybody says that "continuous" is longer than 20/P, then they are also
saying that
20 J instantaneous is not a hazard.
2. Another way to look at this question is to relate it to human reaction time.
The
principle of energy hazard is that we don't want to have somebody lose his
finger
because his ring shorts out a high energy source. But as soon as this
begins to
happen, it is reasonable to presume that the victim will remove his finger
as fast
as possible. Once the finger is removed, there is no longer a hazard.
Normal
human reaction time is in the order of 500 to 800 msec. So I suppose we
could
define this as "continuous."
3. Yet another way to look at this question is by reference to IEC 950, Table
9. No, it
is not clearly stated, but the 3rd note implies that "continuous" is
anything greater
than 100 ms.
Human reaction time is longer (more generous?) than the other two times derived
above. But in any case, it seems to me that safety can not be achieved by
fuses. For a fuse to open in less than 1 sec, it would have to be impractically
small. The alternative implies that the 20 J limit is meningless.
Disclaimer: This is definitely not an "official" interpretation.
Regards,
Egon Varju
