Codes vs. common sense (Was module grounding) [RE-wrenches]

[William Miller]

At 11:01 AM 11/4/02 -0500, you wrote:
>
>I agree that a metal to metal ground should be enough, if worried about it
>put a star washer between the frames and the mount and that should keep as
>good ground as possible and if you remove one module, it causes no problem
>the rest are still grounded.
>
>Why is it that we seem to be held to a higher standard than the rest of the
>industry??
>


Bob and other colleagues:

As a professional installer, one makes many decisions on how to put a
system together.  These decisions must be based on personal experience,
common sense and the codes that apply.  The real key is to determine which
of these three to believe when they are in conflict.

While it conforms to common sense that two metallic items bolted together
must have continuity, apparently the UL and other labs have tested this and
found it to be unreliable or inconsistent when those items are module
frames and module racks.  I am the one that raised this issue most
recently, citing code that apparently supported the idea of metal to metal
bonding.  However, my resources can not compare with the lab's testing
resources-- special instrumentation, staff and locations to set up test
scenarios, etc.-- so I have to acquiesce to those with those resources.

It occurs to me that the code reference may have been intended for ferrous
metal parts in an indoor environment.  In any event, apparently this
special circumstance of anodized module frames being bolted to aluminum
racks is a common enough installation practice that it merited special
testing.  And I personally am glad it got extra scrutiny-- I want my
installations to be absolutely bullet proof, backed up by specific testing.
 I don't believe we are being held to higher standards, rather, we have
specific installation requirements and are scrutinized with those in mind.
For comparison, look at the code requirements for medical facilities,
swimming pools, theaters, and many other special situation.  They have
their own special requirements as well.

I see a danger if installers flaunt the code.  It sets a bad precedence and
encourages sub-standard installations.  In a sense, RE installers are NOT
pioneers.  We follow a long history of those that have studied electricity,
its application and its misuse.  I never forget these primary rules:
Electricity kills and it acts in mysterious ways.  I never read about an
electrical house fire without feeling the chill of responsibility.  Any one
of us could cause a fire with one careless installation.  It is complete
hubris to believe otherwise.



Having said the above, I still don't understand why DC grounds have to be
sized to the largest current carrying conductor?!?!  this makes no sense at
all.  Bill Brooks, can you explain this?  Or better yet, leave me in
blissful ignorance....



Respectfully


William Miller

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