> For usage, Standler's book:"Protection of Electronic Circuits from
> Overvoltages" is the hot ticket.
I just purchased Standler's book and it offers some interesting insight
concerning branch circuit surge protection. On p. 289 of the 2002 Dover
Publishing edition, Standler recommends the popular "all three modes of
protection," typically hyped by manufacturers of branch circuit protection
devices. This form of protection employs three MOVs:
1) Line to Neutral;
2) Neutral to Ground; and
3) Line to Ground.
>From p. 289:
"...the three varistors shown in Fig 19-3 produced the lowest clamping
voltages for Vhn, Vhg, and Vng for an 8/20 us waveform...some low-cost
protection modules contain only a single varistor, which is usually
connected between the hot and neutral conductors. Although a single
varistor is better than none at all, this is a marginal practice. Martzloff
and Gauper (1986) showed that the use of only V3 [W9AC: Line to Neutral] in
Fig. 19-3 could produce a large value of voltage between neutral and
grounding conductors when surge currents passed through the inductance of
the neutral wire."
Clearly, Standler is advocating the use of the "all three modes of
protection" type of varistor arrangement on branch circuits. Note that
Standler does not discuss the series-mode type of suppressor advocated by
K9YC and other professionals in the audio and instrumentation industries
(i.e., SurgeX, Zero Surge, and BrickWall). That form of suppressor uses
only a single MOV device between line and neutral. However, according to
those manufacturers, the difference is that the surge is first stored in a
large value of C and slowly dissipated to neutral to prevent the very issue
described by Standler where a surge on the neutral "could produce a large
value of voltage between neutral and grounding conductors when surge
currents passed through the inductance of the neutral wire."
Here are my conclusions, not that they're correct by any means:
1) Use of "all three modes of protection" using MOVs is fine at the service
entrance panel or utility meter. There's little risk of independent
potential rise on any conductor as the result of a diverted surge from a
MOV;
2) Avoid "all three modes of protection" MOV devices anywhere on a branch
circuit, despite recommendations by Standler, Martzloff, and Gauper; and
3) If a branch circuit MOV device is used, use a series-mode device between
line and neutral whereby the surge potential is stored and slowly dissipated
through the neutral conductor. These are expensive devices but I suppose
that's relative only to the amount of potential damage caused to equipment
by using incorrect surge suppression devices for the job -- or no surge
suppression at all.
Paul, W9AC
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