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From: Pat Carpenter
Subject: Re: ESD immunity - large chassis / enclosure redesign question.
X-Newsreader: Forte Agent 1.92/32.572
Date: Mon, 16 Sep 2002 22:58:57 +0100
NNTP-Posting-Date: Mon, 16 Sep 2002 22:59:00 BST
Organization: ntlworld News Service
If you have a serviceable unit then make sure that ESD procedures are
1 If possible have an anti-static floor, mat or other material on the
floor in front of the unit (don't for get that wood is a good neutral
material). Define it in the installation manual, also define the
operating environment (I was caught out by a unit installed at the
docks but they forgot to mention the large radar transmitter on the
roof next door!)
2 Drill ESD wrist strap points (banana sockets) into the frame so that
the service guy has some place to earth to. You could leave straps in
place so that they are there already for use, but makes it hard to
write the good QA procedure for strap integrity checking.
3 Enclose all the sensitive electronics in a metal box, use a hinged
lid with copper/brylium seals. Use twist handles (increases pressure
on seals) or at least spring twist screws (i.e not detachable so they
won't get lost and don't forget that the more you use the less that
will be re-done up).
4 An old favorite is if you have wheelable units, passing by, they
often pick up very large static charges and if either bumped into the
equipment or sometimes just pass very close-by they can transfer the
5 Maybe go the whole way and specify the torque settings on all
fixings (these will be different depending on what metal is fix to
what i.e steel/copper, alumimum/steel, etc)
6 Above all specify the manufacturing and servicing procedures
exactly. If possible get hold of a video of ESD and its effects and
show it to all your production and materials purchasing staff. It's
very easy to allow a change to a part by the purchasing department
without realizing its significance. If the equipment is large,
expensive and has a service pack (containing special tools etc.) why
not include a copy of the video with the pack?
7 Define all items and processes (i.e. connectors, cables, wiring
layouts) which affect ESD performance and create a symbol to be placed
on all diagrams, BOMs and purchase lists which defines that nothing
can be changed to that part/process without sign-off from a named
person or EMC designer. Also a good idea to have a symbol for safety
components and processes, again requiring sign-off from the person
responsible for safety design. It's too easy to buy a different part
or just alter a layout without knowing why it was done that way!
8 As for the ESD fixing of the withdrawable unit, braids with copper
tags (crimped and soldered) are better but be very careful of damage
points, i.e. being un-done and then pinched or run over by the unit if
it has wheels. Make sure all fixing points are marked and one of the
servicing instructions is to visually and physically check the braids.
Of course have a separate periodical test/inspection procedure.
9 The IEE (the British organization nor the IEEE) has an excellent
manual on maintenance of equipment as opposed to a site
infrastructure. Has good advice on periodic testing and risk analysis,
for safety purposes, but can be adapted or added to for ESD.
Hope some of the above is of some help, if you think I could be of
further use please just email me any time.
On 15 Sep 2002 22:19:35 +0900, Tom Sato wrote:
>> We are reworking a large product to make it more serviceable. We are
>> remodeling the chassis (approximately 2m x 1m) so that a section (1m x 1m)
>> will be able to be withdrawn for servicing etc. The structure is a steel
>> frame with powder coated metal covers. During compliance testing we had
>> quite a challenge to pass the ESD tests.
>> I am trying to determine the best way to couple the removable section to the
>> main chassis to optimise ESD immunity yet be easily and simply disconnected
>> for the withdrawal. An important constraint is that the unit that is
>> 'wheeled out' cannot be tightly (mechanically) coupled to the main chassis.
>Although it would depends to some factors including structure of the
>product (especially the structure around the removable section),
>if finger strips can be used naturally, I suppose there is no reason
>to reject the solution.
>Bonding strap may not good for ESD when it can be long, and I think
>it is not ideal for your situation anyway because it would be necessary
>to remove the strap when removing the removable section.
>Regardless of the bonding method, you may need to think of the route
>of the possible ESD events, too, so that the ESD current would not
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