Sender: <marv@lancaironline.net>
To: lml@lancaironline.net
Date: Fri, 21 Apr 2006 09:25:28 -0400
Message-ID: <redirect-1075501@logan.com>
Received-SPF: none
 receiver=logan.com; client-ip=148.78.247.53; envelope-from=hwasti@starband.net
From: "Hamid A. Wasti" <hwasti@starband.net>
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.0.2) Gecko/20021120 Netscape/7.01
MIME-Version: 1.0
Subject: Re: [LML] Re: Avionics=Black Art
References: <list-1074011@logan.com>
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit

> Posted for "Tom Gourley" <tom.gourley@verizon.net>:
>
> There are probably as many techniques and opinions on how to wire
> in the shield as there are readers on the LML but the one I use is to 
> fold the
> shield back over the the outer jacket of the wire bundle, solder a 
> wire to it,
> and then put heatshrink over it.

Why bother doing that?  If you are going to terminate a shield in a wire 
that goes into a pin, you are wasting your effort.  The wire and the pin 
is going to have a high enough impedance at high frequencies to be 
rendered totally useless.  There is only one way to terminate a shield: 
 To the back-shell of a properly grounded connector.  Unfortunately you 
are SOL if the idiot designing the avionics does not know about proper 
grounding.

As for single or double termination; a single terminated shield does not 
provide any protection whatsoever against the current induced by a 
nearby lightning event, as tested by the cable bundle tests in DO-150E, 
Section 22.  With a single terminated shield, all the induced current 
has to travel through the wires into the pins and the devices connected 
to it are unlikely to survive.  A single terminated shield does provide 
protection against ground loops created by poor wring or poorly designed 
avionics.  The latter category, unfortunately, is a lot more common that 
most people realize.

Sorry that I have had to sit this discussion out due to tremendous 
pressure at work.  On the whole, the information provided has been 
correct and well explained, but when people start talking about doing 
thing like terminating shields to pins and leaving off flyback diodes 
from relays/solenoids, something has to be said.

Regards,

Hamid