[c-nsp] Multiple power supply failures. Advise needed

[John van Oppen]

I have never seen a piece of network gear that is AC which does not have
the electrical ground bonded to the chassis, I was under the impression
that bonding is required for safety.    The only time I have ever seen
this is a floating positive side on -48v gear, but even that is not
terribly common.

We have several POPs that are on the tops of hills where we are exposed
to lighting and I can say that not having all of one's grounds bonded is
a really bad idea.   Contrary to popular belief, grounds are not really
required to be at the same potential to the ground but rather to be the
common potential across the entire site, preventing current from flowing
across the data circuits between pieces of gear by providing the lowest
resistance path for any leakage or differentials between the gear.
This common electrical plane is earthed due to safety and static
dissipation reasons since it would be bad from a safety prospective to
have the racks at a different potential than the ground the people are
in the building are standing on (let alone a pain for copper data
circuits).

--John

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Justin Shore
Sent: Tuesday, September 01, 2009 3:03 PM
To: Michael Ulitskiy
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] Multiple power supply failures. Advise needed

Unless you scrapped the paint off of every joint between the chassis 
through the mounting brackets to the rack then you aren't guaranteed a 
good connection.  That's why most telco screw kits come with the star 
washer to help scrap the paint of the rack and why most telco equipment 
frames and mounting kits are a non-painted alloy.  Data equipment isn't 
generally made to the same standards.  So for example if you rack up a 
3750 you're using non-painted mounting brackets on a painted 2-post. 
The chassis is also painted so you most likely aren't making a 
connection between the chassis and the bracket and thus not the 2-post.

The ground in the power plane should never be connected within the 
chassis to the chassis itself.  The power plane should never share 
anything common with the chassis.  The chassis should always be grounded

separately.  Now beyond the panel at the site ground they'll likely meet

up again but within the powered equipment they should never touch.  Ie, 
the ground conductor in the L5-20R that your colo provider dropped in 
your cage should not internally connect to the chassis of the device. 
The electronics within the device should be insulated from the chassis 
and the chassis should have an external ground connection that you 
connect either to the frame or to a ground bar on the frame.  Depending 
on the equipment (thinking telco for a minute) the equipment is 
sometimes insulated from the frame and connects to a ground bar that is 
also insulated from the frame as well.  There are a lot of telco 
standards out there that are meant for specific applications.  Bottom 
line, always ground the chassis with the supplied hardware either to a 
grounded frame or to a ground bar within the frame that goes back to a 
site ground bar.  Not all manufacturers adhere to those standards
though...

Justin


Michael Ulitskiy wrote:
> Sure, but what the proper grounding is? Does it mean that I have to
run a 
> dedicated grounding wire to every piece of equipment?
> The racks are properly grounded (according to provider) and every
server is screwed to them. 
> The power is provided via NEMA L5-20P twisted lock connecter with
proper grounding 
> (according to provider). There I currently have tripp lites followed
by managed APC PDUs.
> All equipment is plugged in into APC grounded outlet. Does it not
qualify for "proper grounding"?
> 
> I also personally went there with a voltmeter and check for voltage
between metal parts 
> per Seth Mattinen suggestion and I found 0 voltage. This may sound
silly, but I'm taking any chances.
> What else I can do?

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