James.P.Lux at jpl.nasa.gov
Fri Nov 1 16:33:36 EST 2002
At 10:13 AM 11/1/2002 -0800, David Mathog wrote:
> > >
>I just don't get the manufacturers' choices for surge suppressor
>lines. For some odd reason the joule ratings seem to go down
>on most lines of surge strips as the current limits go up. Both
>the APC and Tripplite rack mount models, for instance, don't
>have particularly large joule ratings. Max joules also tends to be
>lower, in general, in units with metal cases than in units with
>plastic cases - see, for instance, the Intermatic product line.
>On paper, at least Surgex, Brickwall, etc.seem to have all the
>oomph one would ever require. But their prices are very high.
>Are my selection criteria unreasonable? All I want in a strip is:
>big MOVs (high joule rating - longer expected lifetime)
>metal case with some holes in it (for zeroU mounting,
> I'd accept a plastic case if there was some way to
> mount it in the rack that didn't involve glue or duct tape)
>disconnect sockets on MOV failure
>15/20A breaker (fuse would be ok).
There's a lot more to surge suppression than MOVs and Joule ratings...
There was an article in IEEE Spectrum a few years back on this (at least 5
You're also concerned about things like the clamping voltage and the source
Bear in mind that MOVs are a "consumable" component. Each "hit" they take
wears them out a bit. However, some surge suppressors use a vacuum spark
gap, which doesn't wear out, but, which works a bit differently than a MOV.
An MOV acts like a voltage limiter, and the energy that would normally go
into making the voltage rise higher than the limit is dissipated as heat
inside the MOV, and in any other circuit capacity.
A spark gap, on the other hand is like a triggered switch. When it fires,
the voltage across it goes very low, and some other part of the circuit has
to absorb the energy (a resistor somewhere, typically). A spark gap would
be combined with an inductor and capacitor in a typical configuration. The
spark gap fires (very quickly) and the energy is dumped into the L or C.
After the transient ends, that energy is dissipated, either by dumping it
into the load's resistance, or in the parasitic resistance in the
components themselves. The key is, suck up the energy quick, and release
it slow (in a relative sense.. this all happens in microseconds)
There's also a bit of specsmanship going on, particularly in the consumer
world. If you're down at Best Buy looking at surge suppressors, you have
to have something to compare them on, since functionally, they all do the
same thing. The commercially advertised figure of merit is "Joule rating",
which is measured in a standardized way. But, the performance of a surge
suppressor is characterized by a lot more than just one number (let through
energy, clamp voltage, life, etc.), and most people don't have any idea
what size transients they're trying to deal with (if you transients are all
10 Joules, then whether you have a 100 J suppressor or a 1200J suppressor,
they'll both work).
There is an incentive for a mfr of consumer surge strips to optimize their
design for performance on the single test that they can advertise the
results of. This doesn't necessarily make for good design overall. They
figure, correctly, that as long as it works reasonbly ok, for the vast
majority of transients (which are a lot smaller than the worst plug strip
can handle), the consumer will be happy. For those it can't handle, they
gamble that the consumer won't try to collect on the warranty, and even
then, if they get a few hundred claims a year at $5K/claim, it's still
cheaper than spending $.50 more on each of the million or more suppressors
A surge suppressor intended for the commercial, industrial, professional
market, on the other hand, is being sold to someone who probably a) has a
good idea of what kind of transients they're trying to deal with; b)
understands a lot more about how the suppressors are rated, and the various
design alternatives; and c)is very aware of the cost of having the system
down (over and above the actual hardware cost). Very different market,
very different real (as opposed to perceived) requriements. Consider, for
instance the number of plug/unplug cycles for a consumer surge suppressor
vs an industrial one...
There also might be some regulatory anomalies.. In order to get UL listing,
for instance, the requirements in a plastic (i.e. flammable/meltable)
enclosure are very different than those in a metal enclosure (which can be
relied on to contain melted/destroyed parts inside the box). Likewise, the
"design duty" (heavy, light) is different for units intended for consumer
and industrial use.
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