I will not really trust the gripping action of star washers unless such
thing are custom made and fitted for their intended design. In general
star washers tend to loosen their grip over time especially if the chassis
has to experience some form of vibrations.
This is an element of dogma...
Personally I would would expect to find from a cabinet of electronic
equipment running from a normal 50 Hz mains supply, a good Earth connection
made to the equipment metalwork using a corrosion-resistant technique and
situated close to where the mains lead enters the cabinet.
All equipment metalwork should be electrically bonded in a manner which
does not rely on 'hopeful' electrical conduction through anti-corrosive
treatment like anodised aluminium and paint. Careful attention to the
assembly process have to be done to weed out things like ball-bearings
races, nylon runners and coasters, or other insulating materials.
Conduction through painted panels should not be dependent on the gripping
action of star washers. The design should be such that no currents flows
in any part of the metal work. The objective is to ensure that any part of
the metalwork can be relied upon as an effective electrostatic screen and
not the reverse, a radiator....
regards
:-)
Tim Foo
|---------+--------------------------------->
| | Kazimier_Gawrzyjal@Del|
| | l.com |
| | Sent by: |
| | owner-emc-pstc@majordo|
| | mo.ieee.org |
| | |
| | |
| | 03/22/02 08:56 AM |
| | Please respond to |
| | Kazimier_Gawrzyjal |
| | |
|---------+--------------------------------->
>-------------------------------------------------------------------------------------------------------------------|
|
|
| To: cdwe...@stargate.net, emc-p...@ieee.org
|
| cc: (bcc: Wan Juang Foo/ece/staff/npnet)
|
| Subject: RE: chassis bonding - star washers enough?
|
>-------------------------------------------------------------------------------------------------------------------|
Chris,
Admittedly I'm not expert on materials or on grounding principles/issues
but the relevance of a rock solid bond to ground is usually beyond dispute.
In a previous life, I've found star washers had a varied level of
difficulty in penetrating painted materials ...it all depended on the type
of "paint" being used. For example, the powder coat type of "paint" found
on numerous enclosures and frames is extremely durable and you might find
that star washers will barely make a dent much less provide a good bond
connection. Due to the power levels involved we would make sure that bond
connections were masked off as opposed to relying on a star washer for
adequate bonding. At times we found they were a point of failure when
conducting the Earthing test to the CSA C22.2 No. 0.4 standard as invoked
by the binat....could get them glow a nice shade of red.
I don't have them handy but I do recall reading some studies indicating
ground connections due to the use of star washers or the like were a likely
source of ignition under extreme transient events....IEEE I think.
My personal opinion and not that of my employer.
Regards,
Kaz Gawrzyjal
Dell
-----Original Message-----
From: Chris Wells [mailto:cdwe...@stargate.net]
Sent: Thursday, March 21, 2002 6:40 PM
To: 'emc-pstc'
Subject: chassis bonding - star washers enough?
Looking for some feedback on bonding various parts of a painted metal
chassis together for safety and EMC performance issues.
I know we have a lot of experts out there on this topic and can remember a
number of threads on the topic.
Please help me pull it together:
When screwing the cover on the base will star washers cut through
paint, even epoxy paint and provide a reliable connection?
Are there special versions of electrical star washers that work
better than others?
What about oxidation of the mating surface? I understand star washers
can make a gas tight seal.
Should there be a special treatment to the painted area where the
screw/washer come together?
Do PEM nuts act as a good mating mechanism for the screw? There are
many types are some better than others? What about the self
clenching types?
One can get screws with captive washers. Are there any issues with
these? Preferred Vendors and types?
There are various chemical materials to keep screws from backing out
and are applied to the threads of a screw such as those from Loctite.
Do these interfere or help? Are some formulas better than others?
Under the paint if the base metal is steel are there any treatments
or plating that help? For Steel, for Aluminum?
If the chassis is plated steel instead like zinc chromate yellow.
What changes?
Proper plating on screw and star washer? (assume painted steel)
Any advice on the topic welcome!
Thanks in advance!
Chris Wells
Senior Design Engineer
Cutler-Hammer
christopherdwe...@eaton.com
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