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

Adrian Shiner ([email protected])
Wed, 14 Jul 1999 20:02:48 +0100


Simple...floating unused cables pick up all sorts of rubbish and then reradiate it.
Pick is by antenna action and capacitive coupling. I have seen 250 volts on a
floating cable in a bunch on an elevator. Also seen destroyed PCB drivers and remote
LED indicators when cables have been run 100's of feet through a building ceiling.
(Cure there is catching diodes from signal line to both supply rails at BOTH ends.
Simple cure is to ground unused cables, preferably at both ends (& mark them as
grounded to save some poor soul some grief in the future, if they are reused).

Generally comuter systems are very poorly designed for immunity, EMC wise. Perhaps,
playing at 5 volts and below, blinds designers to the total environment outside
their "computer box". A simple philosophy to follow is "To survive, be aware of your

Hope this helps


Georg Ramsch wrote:

> 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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