Re: [SI-LIST] : EMC on large distributed systems

owsley@us.ibm.com
Wed, 14 Jul 1999 10:33:26 -0400

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My vote (guess) is for number (2). And some large AC motor switching on and off
and possibly an improperly wired building. Use a 'scope an monitor the mains
(all lines) at several points around the site. We found significant ground
shifts during some motor operations and improper wiring caused it.

Bill Owsley, EMC Engineer
EMC Design - Do It First... Do It Last... But It must be Done...

Georg Ramsch <georg.ramsch@po.uni-stuttgart.de> on 07/14/99 05:10:30 AM

Please respond to si-list@silab.eng.sun.com

To: "'si-list@silab.eng.sun.com'" <si-list@silab.eng.sun.com>
cc: (bcc: Bill Owsley/Raleigh/IBM)
Subject: [SI-LIST] : EMC on large distributed systems

Hello, All !

A friend of mine has a serious problem regarding EMC/SI within a large computing
center.

Spurious data connect problem pop up and dissapear intermittent without the
chance to do some analysis.
The computer systems are conected via hubs/routers/switches, type of connects
are: HIPPI,Ethernet,SCSI,ATM, Fibrechannel.

In addition unused, unplugged older cables remain in the raised floor after the
disassembly of the
computersystems, which belonged to them (were not removed, because if you try
to pull one certain cable out of a bunch then you may damage active cables or
disrupt connectors).
Estimated amount is about some 10th of kilometers representing some 100s of kgs.

The discussion up to now is as follows (in brief): any comments are welcome!

1) What may be the influence of unplugged cables (mostly shielded) running under
raised floors,
which might pick up noise energy and conduct it somewhere else?

2)Regarding switches: through data cables (like HIPPI, SCSI) the cabinets and
GND levels of the attached devices
are electrically coupled. So if you have a GND shift / GND bounce in
one of the devices, this bounce may propagate like a wave through the whole
system of attached
devices and perhaps trigger HW faults (normally switching treshold is about
1.5 V and 0.8 V are the budget for GND shift of the switching transistor and
additional 0.7 V is the noise budget !). True or false ?

3)EMC measures on single devices: influence of devices attached within the
system, where closure/cabinet and GND
of the electronic circuits are connected (some product designer do so to
improve EMC/EMI behaviour)?

All GND levels of all devices would then be connected via one or more
"entrance point" to the overall
chassis structure (assumed, that the cables connect the GNDs of the attached
devices).
Given that case the structure of attached devices would perform a very large
antenna, would it susceptible to short wave radio communication f.e.?
Influence of radio transmitters in the nearer area ??

3) A try (if possible): shorter cables are always better, as they reduce latency
and inductive
path between devices. Often cables are used which are premanufactured to
standardized lengths, but those
length are not needed.

4)Air conditioning effects: if the humidity goes down below 30% rel., then the
airflow in big cabinets
may cause local charge fields, which may discharge thunderstorm-like
(story from an air conditiong engineer responsible for the ac systems at a
computing center).
How long does it take to build up those charge fields, given a swing within
the rel. humidity ?

>>> At the moment we do not regard this as a reason, but pros and cons are
needed.<<<

(A similiar effect is reported from the cleaning of bins of oil tankers; here
the water hoses have a GND
connect at the tip to avoid the buildup of charges on the walls. The discharge
could provoke explosions
of the air/gas mixture inside the bin. Ref.: Jearl Walker / The Flying circus
of physics.

Thanks in advance

CU :-)

Georg

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