I've been doing a bit of browsing
Most of the whole house protectors are shunt mode (which, by the way,
can actually cause some problems, making things worse)
Siemens various models
In any case, a shunt mode suppressor will also suppress transients
originating within the house.
There is a case where they will be of less effectiveness.. if you have a
transient induced on the branch circuit between the panel and your load,
then the transient propagates both directions, and, depending on the
relative lengths of the wire, it will get to the load before it gets to
the clamp. Once the clamp goes into effect, an inverted transient gets
reflected back, so you can calculate the maximum width of the
unsuppressed pulse. (you could use something like 2 ns/ft as a
propagation speed... so for a 100 ft run, with the transient induced
next to the load (or downstream from the load), you get half a
microsecond or so before the voltage is clamped.
If the transient is induced on a branch circuit other than the one your
load is on, the suppressor is between the transient source and your
load, so it would clamp the transient before it arrives at the load.
If you put a point of use transient suppressor *with a higher voltage*
than the whole house protector, it will take care of the half
microsecond impulse before the whole house protector kicks in, and won't
have to absorb as much energy.
FWIW, statistics show that most transient damage occurs from transients
originating outside the house, typically from a lightning strike or MV
line / LV line fault somewhere (MV = 10-30kV, LV = 120,240,480V). That
makes the rise time of the transient much slower (it's low pass filtered
by the power line), and also makes the whole house protector more
In the event of a MV/LV short (the only kind of line transient I've had
personal experience with in Southern California.. we don't have much
lightning here), you've probably got a significant overvoltage that
lasts 8-10 milliseconds or longer (until the MV breaker trips or fuse
blows). You'd have to hope that the surge suppressor can hold on that
long without blowing its internal fuses. Or, ideally, it would short,
and trip your main breaker. Since most of these whole house protectors
have energy absorptions in the few kilojoule range, I'm not very
sanguine about their ability to protect against this kind of fault.
Figure the case of a 14.4kV feeder shorting to the 240V drop into your
house (this has actually happened to me). If the series impedance of
the feeder, through the drop, into the grounding system, is on the order
of an ohm or so, the fault current is around 10-20kA.
If the surge protector is based on an MOV, the 400V MOV is going to
dissipate about 4-8 MW, or, 4-8 kJ/millisecond. If it takes a half
cycle for the breaker to trip/fuse blow, that's 8 ms, and about 50kJ
(which is why things literally explode when this happens).
A surge protector based on a spark gap, which, once it fires, has a much
lower clamping voltage, will dissipate less energy in the protector, so
has better survivability.
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