Bud-- wrote:
Pop` wrote:

Typing with my palms again there; ignore my preceding post.


wrote:

We're moving into a house that has older two-wire ungrounded wiring.
....

There is also a product sold by Tytewadd, which clamps at 130V,
maximum current 10,000 amps, and 1.5 ns response. It is
specifically advertised as protection for "sensitive equipment". But...
it has a total energy dissipation of only 70 joules, far far
less than the previous class of units.

If one can find the info anywhere, this is actually intended to be used in
conjunction with other equipment, which makes the 70 joules a min, not best,
but usable, energy limit.



....
Equipment can withstand perhaps 800V. Selecting too low a clamp
voltage results in the suppressor clamping more surges than is
required to protect equipment.

Of course, but you're neglecting the time/power curve of current as it
reaches and passes the "knee" of the conduction cycle.

Clamping a surge results in power
dissipated in MOVs (the primary protection element), and MOVs
deteriorate and can eventually fail as cumulative dissipation goes
up.

True, but what you're missing is that, nowdays, even the varistor ckts, and
all the solid state ckts, also include a fusing arrangement that completely
opens the ckt. In good equipment, changing the fuse is all that's required.
Cheaper equipment may not allow for the fuse to be changed, but, the fuse
"fixes" the old problem of the MOVs if you insist, going bad without knowing
it.

In addition, low clamp voltage can cause rapid failure if there
is utility overvoltage.

No, not in a properly designed protection ckt. In a misdesign, yes, but few
of those exist on the market due to UL/CSA/EC/NOMs, etc.. If you don't meet
their specs, you don't sell it (or aren't supposed to be able tog).

The author of the NIST guide on surges says
that overvoltage is the most frequent cause of failure of surge
protectors.

1. MOVs are for the cheapie units, and do actually a pretty fair job of
protection. There is some accuracy in your statement, but ... 800V (peaks =
800 x 2.828) is going to be pretty disastrous to most equipment that sees
it. It looks like you're missing the point of application points for this
equipment.
2. Your complaint about 130V clamping voltage is incorrect. The "knee" of
the operating impedance waveform initiates at 130V. It begins passing
measurable current, usually still in milliamps, at about 150V. That will
NOT burn out the varistors and other solid state devices used. Actually,
varistors have given way to better components these days but are still used
in some designs, especially inside the protected equipments.
As the energy content passes the "knee" voltage, current pass increases
exponentially until it is essentially a short circuit.


A major reason manufacturers have low clamp voltage ratings on their
surge suppressors is that it "sounds better". Higher voltages provide
better overall protection.

No, it's because it's good design these days, and/or the not so often
mentioned idea is to sacrifice the protector rather than the protected
equipment. Such vague terms as you are using are misleading and
misinformation at best.


Joules are the important number; the higher the better. 70 joules
is next to useless for a whole-house arrangement. I'm surprised the
numbers don't go higher than 1,000 too, but haven't fiddled in that
market for some time. Joules are a measure of the total power it can
consume while "protecting" the house, so get as high as you can.

The IEEE guide on surges at:
http://www.mikeholt.com/files/PDF/Li...ion_May051.pdf
says Joule rating is not reliable for _comparison_ because there is no
standard for how the rating is determined, and comparisons are thus
apples and oranges. The guide recommends current ratings, IIRC, and
has guidelines on adequate ratings.

The above is not what the IEEE guide says; you need to go read it again. It
points out the misuse of the term, and possible problems with it, but does
not recommend current ratings; it recommends well beyond just the current
ratings. When you read such documents, you have to take the entire document
into consideration, not just the paragraphs that seem to support your own
stance, because there is often a lot more to it than the one paragraph.

But energy ratings are still important. MOVs deteriorate with surge
hits.

Yes, that's true. Also time is extremely important. As is total energy.
The unfortunate part of all this is that such information as in the IEEE
goes well beyond the use of typical lay people; it's not targetted at, nor
meant for, them. When the discussions go in this direction, the layperson
is left out completely.

With a higher Joule rating a suppressor can withstand larger
hits. And the total cumulative energy rating of a MOV will be much
higher than the stated energy rating

No, it would be much LESS!! It becomes less effective also under such
circumstances. This is one of the main reasons fusing technology came into
being for surge protection. A fuse has its own time/heat/melt/etc ratings
which works in conjunction with the specs on the device doing the energy
detouring.

if the stated energy rating is
much higher that the single event hits. Higher energy ratings do not
just raise the simple sum of energy hits that can be absorbed.

I don't know what the rest of that means.


Also very important is a single point ground - the phone, CATV, ...
protector earthing wire connecting with a short wire to the earthing
conductor from the panel, close to the panel. Surge currents on the
earthing wires can produce large voltage drops which can appear
between power and phone wires.

No, the difference in potentials is between the ground references for the
stated systems, not all the wires. All grounds are not equal.

Single point ground minimizes the
difference between power and phone (CATV, ...) wires.

Not sure what that's about; guess you're still reading the IEEE specs maybe.

I'm not trying to fly in your face here; simply to keep things straight and
nothing more. I'm about out of time now so you can say pretty much what
you'd like.

If your'e interested in some good links on grounding techniques with respect
to sensitive equipment, let me know and I can look a few of them up for you.
IEEE is pretty good, and so are some of the folks who interpret it, but
there are some better links for the layperson that will make things easier
to understand. I'd simply have to relocate them again; I don't see them in
my Favorites - damn that last rebuildG.

Regards,

Pop`