John Gilmer wrote:
MOVs normally fail in thermal runaway and low resistance. All surge
suppressors (US) should be listed under UL1449. UL1449 requires
overheating MOVs be disconnected by a thermal disconnect.

That means that you regular old circuit breaker in you main panel will
"protect" the MOV from thermal runaway. Trouble is that once the CB trips
you lose protection until to notice the open breaker.

The UL required "thermal disconnect" must be close proximity to the MOV
and responds when the MOV gets hot at end of life (thermal runaway). It
is inside the suppressor.

It operates at end of life - the MOV conducts at "normal" voltages.


One would think that "someone" would make a MOV equiped device with a
self-resetting thermal breaker as part of the design. ALL of them seem to
have either fuxes or the MOV self-destructs in a way that doesn't set the
device on fire or create a short.

A well designed suppressor matches internal protection to the MOVs. They
disconnect the MOVs when they are at end of life. For overvoltage (much
longer duration than surge and will rapidly destroy MOVs) a few plug-in
suppressors will disconnect and save MOVs and protected equipment.


Carefully follow the manufacturer’s instructions. Keeping connecting wires
short is very important.

Well, it takes te impulse a little more than a nano-second to travel a foot.
If the response time of the MOV is measured in micro or even mili seconds,
the length of the wiring just doesn't make any difference.

The response time of MOVs is negligible. And surge rise times are over a
microsecond.

The problem is voltage drop. Surges are short duration events and thus
basically high frequency. The inductance of wire dominates over resistance.
The IEEE guide on surges and surge protection:
http://www.mikeholt.com/files/PDF/Li...ion_May051.pdf
has information on lead length and voltage drop (pdf page 32). At 3,000A
surge current, 6 inches of lead adds 70 volts to the clamp voltage.


Equipment most likely to be damaged has connection to both power and
signal (phone, cable).

Amen!

If a strong surge produces a 1000A current to earth, and the resistance to
earth is a very good 10 ohms, the voltage at the service ‘ground’ will
rise to 10,000V above ‘absolute ground’. The way to protect equipment is
to keep the ground reference for power and phone and cable at the same
potential.

Yep!

That requires a *short* ‘ground’ wire from phone and cable entry protectors
to the ‘ground’ at the power service.

That's why all utility wires are supposed to come into the house at the same
general location so that the grounds can be bonded together. The "cable
folks" often don't bother. Ditto for the "dish" folks.

It seems like a fairly common problem for signal services to be at
distant points. Cable installers are notorious for not correctly bonding
to power service grounds. Dish is probably worse.

The IEEE guide has an example of a cable service with too long a
‘ground’ wire causing a high voltage between cable and power wires
(starting pdf pabe 40). The guide says that if a short interconnect
cannot be made "the only effective way of protecting the equipment is to
use a multiport [plug-in] protector." (You could also run the cable to
near the power service, install a second ground block, and distribute
from there.)



With adequate ratings, using service panel suppressors, and plug-in
suppressors on “sensitive” electronics with power and signal connections,
you can protect against almost all lightning (not including a direct
strike to the house - very uncommon).

Well, you can get "local" break out boxes that will protect AC plugs and 1
or 2 coax and/or 1 or 2 phone lines. If the grounds are bonded well at
the electric service grounds I suspect that your local suppressor may have a
shorter than expected life.

"Local suppressor" is plug-in suppressor? Should have easier life if
interconnection at services is short - would have less use of voltage
limiting device from signal to plug-in suppressor ground.

If using a plug-in suppressor, all wiring (power, phone, cable, ...) going
to a set of protected equipment must go through the suppressor.

AMEN!

--
bud--