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Re: OT (most likely) strange modem disconnection when using USB drives



Tim wrote:
> On Tue, 2007-10-02 at 12:47 -0500, Mikkel L. Ellertson wrote:
>> If you can, check out the "polarity" of the outlets.
>> I know this is usually used to refer to DC circuits, but you can run
>> into problems if the phases on an AC circuit are reversed on one or
>> more outlets.
> 
> One of the first things I checked, was to grab the multi-meter and check
> voltages between each of the three pins.  'twas as I expected them to
> be.
> 
> Australia:  We only have grounded outlets for general purpose outlets,
> and special purpose outlets are a bit of a rarity.  Although we do have
> two-pin plugs, for some equipment.  The GPO looks like:
> 
> Mains supply  /   \  Mains current return
> 
>                 |    Protective earth (equipment chassis)
> 
> If they'd had any sense, they would have put the earth up at the top,
> I've been giving a huge zap in a bank when a metal plate fell into the
> top of an outlet.  They don't like hearing loud bangs and screams in
> banks.
> 
Yes, they would. We have the same problem in the US, though that is
slowly changing. Then again, we still have houses that were wired
with non-grounded outlets.

> There's 240 Volts on the mains supply pin, the return and earth connect
> together at the distribution box.  Plugs can only be inserted one way,
> thanks to the angle of the blades.
> 
One thing that can be interesting is the voltage difference between
the ground connection on two different outlets. There should not be
any, but in practice there is always some. It when the difference is
more then a few millivolts that you run into problem. You can also
run into problem if the difference between the earth and return
wires or between 2 return connections is too great.

> We also have earth leakage protection breakers.  If there's a difference
> in current between the supply and return line, such as some of it is,
> instead, going through the earth, something else, or a person, the power
> is cut off.  Well, so says the theory...  /me briefly brushes finger
> past a mains wire, can definitely feel the voltage, and not a mild
> tingle, circuit breaker didn't break the circuit.
> 
We call them Ground Fault Circuit Interrupters. (GFCI) But the trip
point depends on the circuit. For the normal 15/20 amp circuit, that
should be more then enough to trip it. For a main breaker, it takes
a bit more to trip them. (I have done it.) I can easily trip the
branch circuit GFCI breakers with a magnetic coil voltage tester.

> Before someone says, "DON'T DO THAT!"  I have some twenty-odd years of
> working in electronics, and the mains voltage is smoooooth compared to
> getting a 24,000 Volt shock from the inside of equipment you're working
> on.  You'd swear that you could count the cycles.  Getting a bite is
> just an occupational hazard, you get used to ripping your hand away at
> the slightest hint of pain.
> 
Now, when it comes to touching how wires, remember the old
electrician's trick - touch with the back of your finger. The shock
will make your mussels contract, and break the connection. This is
especially important with DC circuits, as you may not be able to let
go if you do it the other way.

> It's a bit like trying to tell a motor car repairman that he cannot do
> anything under the bonnet while the motor is running.  It's just not
> practical.  ;-)  But I will say "DON'T DO THAT!" to anyone who's not
> familar with working with electricity.
> 
40+ years of experience. I have learned a few tricks over the years.

Mikkel
-- 

  Do not meddle in the affairs of dragons,
for thou art crunchy and taste good with Ketchup!

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