On May 8, 1:07 pm, wrote:
Is the Joule rating of an SPD important?
While conceptually an surge protection device (SPD) with a larger
energy rating will be better, comparing SPD energy (Joule) ratings can
be misleading. More reputable manufactures no longer provide energy
ratings. The energy rating is the sum of surge current, surge
duration, and SPD clamping voltage.
In comparing two products, the lower rated device would be better if
this was as a result of a lower clamping voltage, while the large
energy device would be preferable if this was as a result of a larger
surge current being used. There is no clear standard for SPD energy
measurement, and manufacturers have been known to use long tail pulses
to provide larger results.

MOV manufacturers do not play the 'joules' games that some plug-in
protector manufacturers play. Plug-in protector typically uses as
little or less than 1/3rd and never more than 2/3rds of rated joules
during protection. During some surges, a plug-in protector may use 0%
of its joules because the massive surge voltage is same on all wires -
as surge seeks earth ground destructively via electronics. No voltage
between wires means the protector never sees any of the destructive
surge - does nothing for protection. So how many joules does it
really use?

An effective 'whole house' protector uses 100% of its joules for all
types of surges which is why 'whole house' protectors can routinely
earth direct lightning strikes without damage - why these protectors
suvive and absorb less energy due to an exponentially longer life
expectancy.

Joules that actually get used during each surge provide a ballpark
measurement for a protector's life expectancy. Further numbers are in
an above reply to VWWall on 7 May 2008.

SVR, typically 330 or 400 volts, printed on the box, required by the
UL, and more often called "let-through voltage". A vague number so
that consumers can make ball park comparisons. No useful for making
engineering decisions.

A plug-in protector rated at 330 volts will start conducting at
maybe 200 volts. When a larger surge occurs, it conducts at 900
volts. Protector rated at 330 volts conducts between 200 and 900
volts. What happens when conducting at or above 900 volts? MOV self
destructs - vaporizes. Also called those 'scary pictures' - what every
MOV manufacturers defines as unacceptable operation.

So what does that SVR (threshold or let-through) voltage really
measure?

Discussed is a 70 SVR difference. Irrelevant since the difference
between ineffective and proper earthing is thousands of volts. If not
properly earthed, then even a tiny 100 amp surge puts that protector
at something approaching 12,000 volts. 70 volts or even 330 volts is
completely irrelevant.

Properly routed ground wire (no sharp bends, etc) can make
thousands of volts difference as described by so many professional
citations. What defines protection? Quality of and connection to
earth ground can make thousands of volts difference.

Page 42 Figure 8 from Bud's IEEE citation. Will a 330 or 400 volt
protector make any difference? Of course not. With either protector,
that surge is still 8000 volts destructively finding earth ground
through an adjacent TV. How to eliminate up to 12,000 volts?
Shorten the 50 feet AC electric wire between protector and earth
ground to zero feet.

Bud posts that electronics contain internal protection of 600 or 800
volts. Intel ATX specs demand that internal protection exceed 1000
volts. Just another reason why 330 or 400 let-through volts is
irrelevant. Relevant is 900 volts during a typically destructive
surge on a 330 or 400 volt protector.

"My surge protector sacrificed itself to save my computer".
Reality. A protector was so grossly undersized that voltage exceeded
900 volts. MOV did what no MOV must do - vaporize. What protected
that computer? Computer's internal protection protected the
computer. But a naive computer assembler *knows* the protector
provided protection. A myth promoted by grossly undersizing plug-in
protectors. To be effective, a protector must earth a direct
lightning strike and remain functional.

Why argue over which jelly bean is prettier when the room will be
engulfed by a flood. 70 volts difference in SVR is trivial when
improper earthing can mean another 8000 or 12,000 volts during the
typically destructive type of surge.