,,,Those hum bars are for a large part caused by
> potential differences in the equipment connected at both
> ends. This potential difference is normally caused by
> voltage drop in power line neutrals.
>
> The same problem occurs with shielded audio cables. If the
> shield grounds to two pieces of gear that have connections
> to power line neutrals or different local grounds it
> introduces hum in the audio. I even have that problem with
> headphone leads in my shack.
For safety reasons, power line neutrals should only be connected to power
line loads. Any other connection is a ground fault and is dangerous. The
power line ground conductor normally has no current, and therefore no
voltage drop. Its purpose is to conduct fault current back to the
protection device, causing it to interrupt the power (blowing the fuse or
tripping the circuit breaker).
Having said that, I suspect that two different grounds might well have
different voltages. If they are connected by a wire, that wire will indeed
have current through it and a voltage drop across it. This voltage will most
likely be only a few microvolts or maybe millivolts. This is negligible for
power safety but is very detectable and probably annoying when it appears
unwanted at the input of a sensitive amplifier.
Just as a matter of interest, the National Electrical Code specifies maximum
voltage at a load of 5%. For a nominal 120 volt circuit, that would be 6
volts. Half of that is dropped across the hot wire, and half across the
neutral. Since line current doesn't flow in the ground wire, its drop is
zero volts. Thus, measuring at the load a fully loaded circuit would have 3
volts between the neutral and ground. That would be a huge voltage at an
amplifier input.
To read this voltage accurately a true rms voltmeter is required if the load
current is non sinusoidal. Load current is generally pulses in electronic
equipment, because the rectifiers do not conduct throughout the cycle.
73
Larry, N9DX
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