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[TowerTalk] whole house surge protectors

To: Tower Talk List <>
Subject: [TowerTalk] whole house surge protectors
From: Jim Lux <>
Date: Wed, 19 Sep 2007 10:34:55 -0700
List-post: <>
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)

Intermatic IG1240RC
Leviton 51120-1
Panamax gpp8005
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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