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In alt.tv.tech.hdtv en wrote:

| I'm curous to know how surge suppression can work without a ground
| (earth) of any sort. Does the "black box" detect overvoltage and
| disconnect the power like an earth leakage safety switch?

Without a ground of any sort, not all types of surges can be protected against.
But some can.

If the surge is a differential one (some use the term transverse), then what
the surge suppressor can do is cancel it out by effectively short circuiting
it to itself. A differential surge involves two wires with the voltage on
each being of opposite polarity and equal level. The MOV component inside
the suppressor will normally not be conductive. But when the voltage is high
enough, it becomes a conductor. The arrival of a high voltage differential
surge will result in the MOV between those 2 wires to become conductive.

If the surge is a common mode one, AND if the surge has a slow rise time,
then a device that is interconnected to other wires or other devices can
be protected by allowing the surge to pass to all devices at the same level.
As long as the rise is not too fast, keeping all the incoming wires, and all
the interconnected devices, at the same level results in insignificant current
flows. That surge will either reflect back from the protected equipment to
the suppressor, and from there go back through all the connected wires (which
could be more than where the surge arrived from).

Most strikes have lower energy levels at high frequencies than what would
cause damage. The exact frequency level that needs to be considered depends
on the internals of the equipment. For example, where it has inductance to
one end of a sensitive component like a CMOS chip, and no inductance to some
other end, this could result in a very brief fast rise of voltage high enough
to damage the CMOS chip. In some cases an LC circuit can actually increase
the voltage level of high frequency components (at the resonant frequency).
For example if you have energy at some voltage at 200 MHz, an LC series
circuit resonant at 200 MHz will result in a higher voltage being present
at the connection between the L and the C. So even in cases where there is
not enough energy at high frequency in a surge to cause direct damage, it
can still happen on some devices (think of them as having a lower threshold
of damage to simplify this).

| This might be fine for a TV, but surely not for a computer.

If everything the computer is connected to is protected at a common point
in the same surge suppressor, you can have this kind of protection, even
on a computer. That does mean if you have a phone line connected to a
modem, you need to protect both the phone line itself and the power to the
modem, in common with the computer.

| I don't recall any computer I've owned that did not have a three wire
| connection to the mains. That and a MOV is OK for smallish surges, but
| I believe that for a large surge, the sort that will blow a telephone
| off the wall, one needs a large, short-path earth for the surge
| detector to dump the extra power down.

Such a surge is likely to have high levels of high frequency energy. The
effective protection against these rare events is a combination of somewhere
to divert the energy (like a ground path), and something (like an inductor)
to ensure the energy does get diverted.

One problem is that at the point of use, an alternate ground path is not
practical. The grounding wire of the power circuit coule be as much a source
of the surge as the neutral wire would be. The place to put the diversion
system is at the entrance to the building. Most surges that come in by other
paths besides the entrance to the building are induced surges that will not
have so much energy and even less at high frequencies.

Still, I have seen three incidents in which an induced surge damaged a device
that was not connected to anything at all (in two cases they were battery
powered devices, and in the third, it was disconnected before the storm but
suffered damage anyway).

| I've got a few plug in protectors here and there to sop up a small
| spike, but when a storm is within a few km, I pull the phone wire out
| of the ADSL router, and the plug out of the mains. If I'm working at
| the time, I might just keep a watch on the weather radar and count
| lightning fashes to thunder times. It's rare that I get interrupted. I
| have underground power and phone lines so that gives a little extra
| protection, I believe. I've been told that Australian phone lines are
| the most vulnerable, and the most urgent to protect or disconnect.
| I hope to be going wireless soon which obviates this problem.

Disconnecting provides even better (but still not 100%) protection. Yes, the
underground wiring helps. I don't know the issues with Australian phone lines.
I do the wireless thing myself and feel much more comfortable with it. Most
of the past damaging surges I've seen come in do that on phone and cable wires,
and much less often on power wires. That may be due to the more sensitive
aspect of equipment where it connects to these wires.

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