[RFI] Wiring practices to minimize emissions

James Gordon Beattie, Jr. w2ttt at att.net
Tue Jul 12 14:13:32 EDT 2016

Jim, et al,

Breaking DC paths with optical interfaces reduces inductive and radiated emissions.

Further, twisted pairs are good, shielded twisted pairs properly bonded and grounded are better.

Vy 73,

Gordon Beattie, W2TTT


Sent from AT&T Mail on Android

From:"Jim Brown" <jim at audiosystemsgroup.com>
Date:Tue, Jul 12, 2016 at 12:53
Subject:Re: [RFI] Wiring practices to minimize emissions

On Tue,7/12/2016 8:55 AM, nm8rmedic via RFI wrote:
> I would appreciate some input from the group on talking points.  This could also be the chance to influence wiring design for many of this manufacturer's lift station packages.

Key issues are:

1) Include in your specification for equipment that all shield 
connections are bonded to the shielding enclosure at the point of 
entry/exit. This addresses what we in the pro audio world call "the Pin 
One Problem." It is a MAJOR cause of RFI, both emissions and 
susceptibility. Likewise, require that all "green wire" (equipment 
ground) conductors be bonded to the chassis/frame/shielding enclosure at 
the point of entry, NOT via internal wiring, NOT to a circuit board. 
Also, specify that this connection be carefully inspected to verify that 
is it is not insulated from the frame/chassis/enclosure by paint!

2) Require the use of twisted pair for ALL circuits. When the circuit is 
"ground" referenced, require that the ground-referenced connection be to 
the shielding enclosure at the point of entry/exit.

3) Pay VERY careful attention to every element of variable speed motor 
systems, including their control wiring. Specify twisted pair, a pair 
per phase of the wiring between the controller and the motor, specify 
that controllers and the motors they control be as close together as 
physically possible, and that the chassis of the controller and the 
frame of the motor be bonded together, with the bonding conductors 
following the path of the power conductors.

4) Specify that all wiring be installed in steel conduit that is 
continuous over its entire length and bonded to the equipment on both ends.

The common element here is that most emissions at HF and below are 
common mode and are radiated on wiring connected to the noisy equipment. 
Even DC and low frequency AC circuits should be assumed to have strong 
RF noise components as a byproduct of how the power is generated, 
controlled, and used. Those variable speed motor controllers are an 
extreme case -- the power they provide to the motors are rectangular 
waveforms, with their width adjusted to control motor speed. Repetition 
rates are typically in the range of 10-20 kHz.

Henry Ott's classic EMC text is a great reference, and is widely 
considered the "bible" by most EMC experts. 
http://www.hottconsultants.com/book.html It's published by Wiley, and is 
available from all major book sellers. You want the latest (2009) edition.

73, Jim K9YC

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