The primary screw has the ground symbol (upside down Christmas tree) in
a circle, all other locations you want to indicate as ground, called PROTECTIVE
BONDING SCREWS by the standard can use the symbol but without the circle, but
it is not required to mark them. At least that's the word from UL 60950 section
1.7.7.1. Heck the screws that bond the cover to the base and its connection to
the primary earthing connector are examples of this type of bonding. If you had
two separate chassis with signal and other non-primary power interconnects, you
might pass bonding through a braid strap or something as well. Reaching here,
because I can't think of a time I didn't cram everything into a single
enclosure.
Gary
-----Original Message-----
From: Chris Maxwell [mailto:[email protected]]
Sent: Tuesday, September 17, 2002 4:08 AM
To: David Sproul; Doug McKean
Cc: EMC-PSTC
Subject: RE: Earthing through screws.
David,
Your question isn't so "stupid" as you say.
It is correct that the "primary" grounding screw (the first screw connected to
the ground wire from the AC inlet) shouldn't be used for any purpose other than
grounding.
However, the standards do allow that screws and fasteners can be used to pass
this grounding on to other parts of the chassis. In that context (I guess you
could call them "secondary" ground screws), your question is valid. It is
worthwhile considering labeling these screws with a ground label; especially if
a user could remove this screw and still operate the product.
I guess what I'm trying to say is: your specific question may have an obvious
answer; but the fact that you were thinking about how to make the product safe
is just as important as quoting any standard chapter and verse.
It is never stupid to use logic and reason along with the standards.
Chris Maxwell | Design Engineer - Optical Division
email [email protected] | dir +1 315 266 5128 | fax +1 315 797 8024
NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA
web www.nettest.com | tel +1 315 797 4449 |
"A friend is a person who knows the song in your heart because they stopped
singing their own song long enough to hear yours :-)
> -----Original Message-----
> From: David Sproul [SMTP:[email protected]]
> Sent: Monday, September 16, 2002 5:46 PM
> To: Doug McKean
> Cc: EMC-PSTC
> Subject: RE: Earthing through screws.
>
>
> Hello Doug,
> I believe you are right. If I had taken the trouble to read EN60950 para
> 2.6.5.7, I would have saved my self from asking stupid questions.
>
> Thanks also to all those who responded to my orignial posting.
>
> regards,
> David Sproul.
>
>
>
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