On Mon, 05 Mar 2007 16:44:59 -0700, Larry Benko wrote:
>I read the article but didn't see where the 3 MOVs is major problem.
>The units I'm referring to are 120V/15A surge protectors with RF
>filtering. Give me a hint.
The mechanism is this. You have two pieces of gear plugged into two different
branch circuits, and there is a low voltage connection between them. For
example, one is a computer and the other is your ham transceiver. (or one is
a computer and another is a printer). There's a shunt mode suppressor on the
computer, but not the ham gear (or a different one on the ham gear), and
you've got an audio cable or control cable running from the computer to the
ham gear.
Now, here comes lightning, with lots of volts for several hundred
microseconds, and the MOV's conduct it onto the green wire(s). This energy is
not simple DC, it's a huge impulse, and IEEE studies show that the spectrum
of the energy is concentrated between the hundreds of kHz and ten MHz or so.
There can be hundreds or even thousands of amps for those microseconds, and
the wires have inductance (and some resistance, but the inductance usually
dominates), so the voltages on the green wire at those outlets can rise to
some very high voltage. But the inductances are not EQUAL, nor will the
lightning-induced voltages be equal, so the voltage from outlet A to outlet B
can be several thousand volts. The current may not be enough to melt the
wire (only because it doesn't last long enough), but the voltage can be high
enough to fry a bunch of semiconductor junctions in your equipment. And if
the wire melts, chances are the voltage spike will fry the gear first.
On the other hand, let's say you use the BIG MOV's at the service entrance,
and all the grounds in your building are carefully bonded together. Now, when
a surge comes in, it goes straight to the building ground, and the whole
ground bus goes up in voltage in proportion to that current (and the
inductance of the ground path). Now, there's relatively little voltage
difference between the grounds (chassis) of equipment on branch circuits,
because all of them are going up together. You can still have voltage
induced on the interconnect wiring, but it's a lot less.
73,
Jim Brown K9YC
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