Re: [OT] USB voltages and grounds

["Nick Austin" <nick.w.austin@xxxxxxxxx>]

Hi Paul,

This probably means that one piece of equipment has lost it's earth
connection.

The EU regs require electronic devices to carry interference
suppression, this is
normally an arrangement of high value resistors, and low value capacitors,
designed to remove interfence above mains frequency.  In those EU countries
that
provide mains earth connection this isn't a problem.  However because
most of the
EU an earth connection is not mandatory the interference suppression also
means that there is an electrical path between the equipment case and the
live
mains.  What you're meter is measuring is the off-load voltage which
is always going to be between 30% and 50% of the mains voltage.

What is important in this situation is the voltage present when a load
current is being drawn.  This can be calculated using ohms law if you
know the value of the resistance between equipment earth and live
mains.  Rather than measure AC voltage you need to measure AC current.
If you're using a multimeter you may need to connect the meter leads
to different sockets.

Say you now get a meter reading of 0.0003 amps, That means the
resistance path to live mains is 800,000 ohms.

So what happens when someone is holding the end of a USB cable and mains
earth at the same time.  A circuit is made from live mains through the
mains
suppression and through the person's body.  To determine the voltage of the
electric shock received we need to use ohms law again but this time the
circuit
path has changed.  We have two electrical resistance is series so first we
need
to know the value of the resistance of the human body.  This varies quite a
bit
but I'll use a mid-value of 30,000 ohms.  So the circuit path has a
total resistance
of 830,000 ohms and is connected to a source of 240 volts.  A current of
approx
0.00089 amps therfore flows.  The voltage across the human is
therefore 8.7 volts,
less than can be received from mishandling a battery and therefore a
very safe voltage.

Nick.

On 9/19/06, Paul Gale <groups@xxxxxxx> wrote:
> Following on from my previous post re USB cables sparking when
plugging in - I've just checked a couple of USB cables for different kit by
putting a voltmeter between the PC case and the USB cable shield. I get
around 90V and this is similar for several bits of kit.
>
> What would you expect to see?
>
> Paul.
>
>
> [Non-text portions of this message have been removed]
>
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