Ground a metal roof? [RE-wrenches]

[Jason Fisher]

Windy,

It's a good question but I don't believe there will be a good answer. I have
asked this of many electrical trades people and have always gotten the same
response:

-Is the roof part of the "structural" steel of the building?
-Is the roof "likely" to become energized?

If the answer to both of these is "no" then the answer is always "it is not
required by Code." Since your question is concerned with "protection of
electrical systems in and around the building" this may or may not be an
issue. Many, and I lean in this direction, believe that isolation of
equipment is a good preventive strategy from power surges. In this case, if
the metal roof is not connected in any way with other grounded members (i.e.
building steel) then its isolation may reduce the likelihood that it will
attract any surge and channel it to the power equipment. I know nothing will
ever be truly isolated but if the connections made have very high impedance
than there should be less chance the surge would follow this path. More of a
concern, as I see it, is a relatively low impedance connection that has the
likelihood of breaking down under the surge. In this case, a ground wire
that burns up during a surge can ignite fires and/or cause arcs to jump,
possibly hitting other equipment or persons.

My undereducated guess (would this be a hypocrithesis?) is to leave the roof
ungrounded unless there is equipment connected to it or other conductive
pathways to ground (i.e. metal framework or antennas). Then, I would make
sure all the electronic equipment is well grounded and connected to a common
ground yet ensuring that no ground path exists where the sensitive equipment
is in the middle. For example, run the equipment ground from the array
directly to the grounding electrode system then run the ground from the
inverter to the same electrode system, while avoiding running a grounding
wire between the two.

On the subject of PV laminates bonded to the metal roof however, or with PV
panels and racks attached to the metal roof, particularly when using single
conductor interconnects (MC tails), I would say the roof should definitely
be grounded. Then the question becomes HOW? Sorry I can't answer that, my
time is up!

Best,

Jason Fisher
Owner

Aurora Energy LLC
P.O.Box 3315
Annapolis, MD 21403

voice / fax: 410-268-3684
email: jason at aurora-energy.com

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