[Boatanchors] BA's and 3-wire cords

[Brian Clarke]

There is one major concern. And that's the formation of ground loops.
It's more of a problem with audio frequency testing. If the object under 
test [DUT] and the test equipment all have mains leads with Earth 
connections, and the audio paths are single-ended, then quite 
sizeable hum Voltages can build on the shield between the DUT and 
the test equipment, giving rise to erroneous results. Same can 
happen between any mains-powered audio equipments.

An industry solution is to have the most dangerous piece of 
equipment with a direct Earth and put, say, a 10 Ohm resistor in 
series with the mains Earth lead of each piece of peripheral gear. 
The 10 Ohm will be much greater than the resistance of any 
shielded lead and so the hum potential will be attenuated 
considerably.

The better solution, at audio frequencies, is to use twisted-pair, 
shielded, balanced feed lines between pieces of equipment. Then 
you can use normal 3-wire mains cords without 10 Ohm attenuators.

73 de Brian, VK2GCE.

  Dick asked:

  Is there any situation when it would not be advisable to use
  the 3-wire cord on test equipment or ham gear?
  73, Dick KF4NS