Hey Cameron -
Damaging currents can be either a.c. power currents due to different ground
potentials, a.c. fault currents, or large lightning surges (due to external
strikes as well as intrabuilding). These phenomena are the reason shielded
cables can not be consistently connected at both ends between distant
equipment, and leads to the use of hybrid grounding schemes. Even with
isolation, there are no guarantees (can't protect against direct strikes).
This is not to say that you can't use a shielded cable connected at both
ends, but you have to understand the grounding environments into which the
equipment is being deployed. Your circuits which are referenced to earth
will see these same effects.
Regards,
Mark Gill
EMC/Safety/NEBS Design
C-MAC Engineering Design Services
-----Original Message-----
From: Cameron O'phee [mailto:O'p...@ali.com.au]
Sent: Wednesday, December 06, 2000 11:37 PM
To: 'EMC - PSTC Forum'
Subject: Ground potentials and communications.
Hi All,
I believe there would be safety considerations when using non-isolated RS
485 for communications between machines that may be separated by large
distances in a building and consequently be powered from different
circuits/phases. The communication circuits ground is connected to the
chassis ground of the machine. I am concerned that ground potentials
between machines on different circuits could be enough to be a safety
concern however the safety standard (AS3260) our equipment is tested to does
not seem to cover this scenario. Any thoughts or information?
Regards,
Cameron O'Phee.
EMC & Safety Precompliance.
Aristocrat Technologies Australia.
Telephone : +61 2 9697 4420
Facsimile : +61 2 9663 1412
Mobile : 0418 464 016
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