[c-nsp] Grounding issues between devices

[Bill Wichers]

> Here's an interesting problem. At one of our sites we have a 2950 switch
> sitting directly on a 2620 router. These devices have rack ears attached
> but they are not rack mounted. A tech from a different group just
> informed me that he was at that site earlier and needed to move the
> network equipment, and he got quite a shock when he grabbed the rack
> ears of the switch in one hand and the rack ears of the router in the
> other. He described it as being significantly more uncomfortable than a
> shock from a 9v battery.

I would suspect static discharge first, but that should be very short
duration. Anything sustained indicates a problem. If you have DC powered
equipment there should be a ground connection to each device that is
obvious to check (a green wire), if it's AC powered equipment it's
possibly you have a faulty power cord or someone has broken off the "third
prong".

Remember that copper Ethernet (and obviously any fiber interface) is
electrically isolated between pieces of gear, so wherever the problem is
it has to be a non-isolated metallic interface (usually power, IMHO)

> The weird thing is that the chassis are touching, so I wouldn't think
> he would have been shocked even if there was some sort of grounding
> problem or some other electrical issue. Since the devices are touching,
> he wouldn't have been the preferred path. I'm not quite sure he got
> shocked, but he said it is repeatable.

The paint is a good enough insulator to prevent an electrical connection
between adjacent devices. You need a solid electrical connection, and the
rack ears are *not* supposed to be used for that. With either DC or AC
equipment there should be a solid ground path from each device back to
some central point (DC distribution panel, AC plug strip, etc.), and that
central connection should eliminate any voltage differentials between
individual pieces of equipment.

You probably have some kind of grounding problem on the power side of your
equipment, which you should certaintly check and fix. I'd look at any
plug-type power connectors being loose first if you think the feed to the
POP itself is OK.

     -Bill



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Waveform Technology
Systems Engineer