On Wed, 23 Mar 2005 08:03:09 -0800, Dan Richardson wrote:
>At 07:40 AM 3/23/2005, Jim wrote:
>>The ground should probably NOT have a toroid on it.
>
>Jim,
>
>I have been wondering about that. In the March issue of QST page 48
"Short
>Takes" they review the ICE model 475-3 AC line filter. While no schematic
>is show they do, however, show a picture of the unit. Looking at it, it
>appears to have a toroidal choke in series with the ground lead.
>
>Comments?
The word "probably" in my reply is based on the potential for a conflict
between safety grounding and RFI suppression. You MUST bond all grounds
within an installation for safety, but these conductors can radiate RF
current that is flowing on them. The obvious answer is to choke that
current. BUT -- the reasons for the bond are 1) to provide a path for a
fault current to blow a breaker or fuse; and 2) to equalize the very high
potentials that can exist with a lightning strike. The choke will likely
cause no problem with #1, but it CAN cause a problem with #2, because much
of the energy in a lightning event is in the middle of the AM broadcast
band, and, of course, continues above that range.
Remember, it is the impedance of the bond at the frequency of the
lightning strike that determines its effectiveness. We know this
intuitively re: the lightning bonds for towers, masts, etc. -- lightning
will jump across any bumps and grinds that we put in its way in the form
of coils, loops, zig-zags, etc. Thus, to avoid compromising lightning
protection, any choke added on such a bonding conductor must be designed
to have minimal impedance in the part of the spectrum where lightning
energy is concentrated. What might happen if there's a choke there and it
has too much Z? First, it will overheat and self-destruct, but second,
the lightning may not be discharged quickly enough to avoid damage
elsewhere.
Jim Brown K9YC
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