[R-390] Isolation transformers [WAS: Corcom...]

[Charles Steinmetz]

Bill wrote:

>It is still a good idea to ground the frame of the R-390 to Earth so the
>antenna current can go somewhere, unless you are using a balanced
>antenna.

Not just for the antenna, but for electrical safety as well.  An 
isolation transformer should have a "pass-through" ground -- the 
third prong should be continuous from the radio to your house ground, 
but NO PART OF THE ISOLATION TRANSFORMER should be connected to it 
(except the neutral wire to the primary, but that is done at your 
service entrance, not inside the isolation transformer cabinet -- see below).

That is the difference between using an isolation transformer and not 
using one.  One side of a US 120v mains supply (the neutral) is 
connected to your house ground at or near the service entrance, so 
there is a current path from the grounded radio chassis to your house 
ground.  If all is well, no current flows in this path because the 
voltage at both ends is the same or very, very close to it.  But if 
something goes wrong, current CAN flow in this path.  Since there is 
no such path from the secondary side of a properly wired isolation 
transformer, no current can flow in it even if something does go wrong.

So -- what is the safety reason for the pass-through ground?  Because 
you might knock over a lamp, break the bulb, and have the hot wire 
touch the radio chassis.  Or, possibly more likely for a ham or SWL, 
a falling tree could take out your antenna and a power line, and 
connect them to each other.  Or your antenna could be hit by 
lightning.  If the radio chassis is grounded, you are protected (to 
the same extent fuses and GFIs protect you in normal 
circumstances).  If the radio chassis was not grounded, that current 
could flow through you and add "SK" to your call sign.

Best regards,

Charles