Electrical code Compliance - Grounding / Bonding

Author

Grounding / Bonding

maxelrod

2006-12-06, 1:37 pm

In a data center containing equipment racks with servers, etc, it has
always been my position to ground the rack, and bond the
cases of all equipment to the rack using a suitable bonding wire.

This complies with NEC 740.15

"All exposed noncurrent-carrying metallic parts of an information
technology system has to be grounded as noted in Article 250"

and NEC 250.4 (2)

"Any noncurrent-carrying conductive material enclosing electrical
conductors or equipment, or part of equipment must be
connected to earth"

Further, everything I have read indicates this is best practice.

However; there are those that state that the third prong on the
electrical plug provides this and is sufficiant.

I would be interested in the opinion of this community.

Thank you

Sean

2007-01-10, 3:25 am

maxelrod wrote:
> In a data center containing equipment racks with servers, etc, it has
> always been my position to ground the rack, and bond the
> cases of all equipment to the rack using a suitable bonding wire.
>
> This complies with NEC 740.15
>
> "All exposed noncurrent-carrying metallic parts of an information
> technology system has to be grounded as noted in Article 250"
>
> and NEC 250.4 (2)
>
> "Any noncurrent-carrying conductive material enclosing electrical
> conductors or equipment, or part of equipment must be
> connected to earth"
>
> Further, everything I have read indicates this is best practice.
>
> However; there are those that state that the third prong on the
> electrical plug provides this and is sufficiant.
>
> I would be interested in the opinion of this community.
>
> Thank you
>
There is no guarantee that the third prong of the power plug will be at
ground potential or even connected to ground unless you have inspected
the installation yourself.

Further, there usually exists some resistance in the ground wire from
the plug to the point where it is actually grounded, which means that
there is the opportunity for the 60 Hz mains to cross couple into your
ground wire and pin and thence into your equipment. Not enough to
damage the equipment, but enough to mess things up considerably. This
is exacerbagted if the ground wire from the third pin is less than
perfectly grounded.

Those who are telling you that the third pin is a good ground need their
fortunes told.

Sean

Paul E. Bennett

2007-01-10, 5:25 pm

Sean wrote:
[color=darkred]
> maxelrod wrote:

This would be the case the world over too.
[color=darkred]

You started by talking about equipment racks in a data centre. I would
expect that this is based on the 19" racking systems and as such the inlet
to such equipment would be a permanently wired supply. In this situation
all equipment in the rack that is bolted to the framework should be
electrically bonded to the rack with the appropriate strapping.

If you are using three prong plugs to equipment modules within racks then
you need to consider whether the equipment is earthed to the rack by
strapping or should rely on the third prong. This is like using portable
appliances as the system modules. In these cases you need to satisfy
yourself that the earthing to the power sockets in the cabinet are
adequately strapped to the earthing system in the rack and that the module
has passed a portable appliance test which checks out earth continuity and
quality as part of the testing whether or not it is practical to strap the
module case down to the rack (this would be the consideration for network
analysers, scopes and other test equipment that may be installed on shelves
in the racks for test purposes only and not the data equipment itself).

--
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