North American Network Operators Group

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## nanog

• From: David Lesher
• Date: Sun Mar 13 18:12:56 2005

```Speaking on Deep Background, the Press Secretary whispered:
>
>
>
> Perhaps someone who knows EE can enlighten me?

I'll try.

The #1) is BS. #2) is true.

If ANYTHING is, it's square waves. [Huh?]

Virtually all surge protectors have MOV - Metal Oxide Varisters -
within. These are devices with s sharp knee on their Voltage vs
Resistance curve. In other words, they do not conduct for a 120V
level, but do at 200, shunting that overvoltage to ground.

Line power is a sine wave. The cheapest inverters make square
waves {because it's how an inverter works; one transistor comes on,
making a positive-going waveform; the second one takes its turn,
making a negative-going one. Back and forth.} But the transitions
are quite abrupt. Better UPS's use a stepped squarewave, maybe
3 levels, instead of one. The be\$t make sine waves...

[Side note: the usual switcher supply on a PC/Sun/etc does not
give a damn re: sine vs square wave; as the first thing it does
is make DC out of the AC and filter it..]

Now, via magic called Fourier, sharp transitions mean lots of short,
high voltage pieces-parts. [trust me, or read up..you can create
any square wave from an infinite series of sine waves..]

So the ""risk"" I hear talk about is this. The MOV's in a strip
conduct slightly on those squarewavish USP output waveforms. This
results in 2 things, gradual loss of MOV efficacy, as they
degrade slowly with use; and the worry that iffen the MOV's
absorb too much squarewave crap, they'll explode. See, MOV's
turn transients into heat. Line transients are assumed to be err
transient & tolerable. Continuous stuff is another issue...that
can overheat the MOV and cause it to worship the Smoke God.

But then, so do big power hits. If you ever shake a strip and hear
it rattle, the MOV's were #1 on the Smoke God's list. And in the
WORST cases, the heat melts the plastic strip/sets it on fire.

So let's ban surge strips on UPS's...But wait... lots of machines
have MOV's inside their power supplies.. Shhh!!!

But note that the OP does not have a MOV issue; he has an inspector
offer no surge protection. He likely will need to first pin
the inspector down on what rules he's allegely broken, however.

--
A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433