Topband: Best Outlet sttrip

[Paul Christensen]

> I'm afraid I don't understand how a surge
> protector that clips an, e.g., 1KV spike on a 120 VAC line can end up
> doing more damage than no protection all. I understand that the
> clipped current pulse returns through the ground line and will cause a
> voltage spike on the ground, and I also understand that other
> interconnected equipment connected to different grounds may
> potentially see part of the spike, but on balance that seems to me to
> be a less dire situation than having no protection at all.

I think it comes down to the surge protection device (SPD) wiring 
configuration in relation to its placement in the electrical system. 
First, in most NA power systems, neutral and ground are connected at only 
two places:  (1) the secondary of the utility company's transformer; and (2) 
at the premises main panel board.   An SPD, if used, should first be placed 
at either the electric meter or panel board so that surge currents near the 
utility entrance may divert on a short path to ground/neutral.

On a branch circuit, the problem potentially becomes worse the further the 
distance of a common secondary SPD from the main panel (so-called "3 modes 
of protection" from devices using three MOVs).  Surge currents being induced 
from say...a shack entry point can divert a large current onto the grounding 
conductor, causing a large voltage differential to other grounding points on 
the same branch circuit.  On a branch circuit, surge current should never be 
diverted to a grounding conductor.  It may be safely diverted to the neutral 
conductor and even then, it's best managed if at least a portion of the 
surge potential can first be stored and then "bled" onto the neutral where 
that neutral is connected to ground at the main panel.  Common secondary 
protection SPDs meant for use on branch circuits don't meet this 
requirement, despite IEEE's recommendation that common "3 modes of 
protection" are safe for use on a branch circuit.  Secondary SPDs that do 
accomplish this goal are made by BrickWall, ZeroSurge, and SurgeX.

In a nutshell, my recommendation (for what little it's worth) is this:  (1) 
a secondary SPD on a branch circuit should only be used when a primary SPD 
is used at the utility company's meter or at the premises main panel; and 
(2) assuming condition 1 is met, then the secondary SPD should divert surge 
current only onto the neutral, and never the grounding conductor.

Finally, on the issue of balanced audio:  it costs manufacturers of consumer 
electronics and ham gear no more to balance all audio inputs with a 
"3-stage" instrumentation op-amp circuit.   It's far more important to 
balance each input this way than to balance audio outputs although balanced 
outputs are preferred for best system matching, especially on long cable 
runs or where distribution is complex (e.g., a broadcast or recording studio 
with cross-point switchers).  With a true instrumentation input, the input 
circuit does not care if the source is balanced or unbalanced.  It's simply 
a two terminal, floating device with extremely high common-mode rejection 
(CMRR) performance.  The best laid out instrumentation circuits carry that 
high rejection well into the HF range.

Paul, W9AC