We don't have grounded chairs either.
In my American parochialism, I'd never heard of the 61340
specs, and none of our customers have ever mentioned
them either. Still, I'm going to order some copies
and have a look.
But I'm suspicious of such a "requirement".
1. Does the 61340 spec also require grounded floor?
If not, each chair would have to be grounded with a wire.
Nasty, cumbersome, and a heck of a tripping hazard.
2. Does the spec also require conductive clothing? I
don't see how else to make electrical contact between the
person and the grounded chair. And is it mandated that
such clothing cover one's seat when seated?
3. I don't buy the argument that the chair should
be grounded because it might be improperly used as a table.
By this logic, we should cover every horizontal surface
in the shop.
4. I don't think the reference to two grounded feet
means the operator's feet. I believe that conductive
heel straps or footwear are designed to meet grounding
requirements with the person's full weight on them. If
the individual is seated, there is insufficient contact
pressure with the floor to make a good ground path. When
seated, the operator should wear a wrist strap.
Bottom line, if the person is grounded through a proper
wrist strap, it shouldn't matter what they are sitting on.
We might get an argument from someone who is selling chairs.
Joe Kane
BAE SYSTEMS Platform Solutions
Johnson City, NY
-----Original Message-----
From: Gene Felder [mailto:[log in to unmask]]
Sent: Monday, March 21, 2005 6:14 PM
To: [log in to unmask]
Subject: Re: [TN] ESD Issue - non ESD Chairs on Manufacturing Floor
Per ANSI/ESD S20.20 Paragraph 6.2.2.1 Personnel Grounding Requirement
"All personnel shall be bonded or electrically connected to ground or
contrived ground when handling ESD sensitive items. When personnel are
seated at ESD protective workstations, they shall be connected to the
common point ground via a wrist strap system."
ESD protective Seating is an optional implementing process per Table 1of
ANSI/ESD S20.20; if used the minimum recommended technical requirement
range is < 1 x 10^9 ohms.
Included in the ESD Handbook TR20.20 section 5.3.5 is Static Protective
Seating "As mentioned in the floor materials section of this handbook,
the principle cause of static electricity in the workplace is cited as
the movement of people and materials. This routine movement,
particularly from a person seated in a chair or movement of the chair
itself across the floor, can generate significant voltages. This section
covers the use of seating in an ESD protected area for dissipation of
charge. Although not required in ANSI/ESD S20.20 as a primary means of
grounding personnel in the workplace, the use of chairs that meet
resistive requirements from the seat contact surface to the castors or
leg ends, may be an effective means of grounding personnel if a reliable
method can be found to bond personnel to the chair. In addition, if the
chair is used to connect personnel to ground than the maximum resistance
to ground from the person through the chair and flooring system should
be less than 35 megohms."
However, it is mandated per EN 61340-5-1. Per section 5.2.4 Seating
"The resistance from all areas of seating which may have human body
contact when in normal use, to either a floor contact point or a
groundable point, shall be as specified in table 1. [Rg =/< 1 x 10^10
ohms] When the floor is used as part of a grounding system, a minimum of
two wheels or two feet shall provide a path to EPA ground."
Its EN 61340-5-2 User Guide states:
5.2.4 Seating
"It is important that low charging and grounded seating is used, as
although seating should never be used as a primary means of grounding
there are occasions where, through human error, equipment breakdown or
bad design it can provide an important resistive path to ground.
Although not recommended, in some EPA seats may be used as impromptu
working surfaces or storage surfaces. If nonconforming seating is used
then ESIDS may be damaged in this way. Where grounded, if footwear is
used rather than wrist straps, when seated, many operators will spend
periods when footwear is not in contact with the ground. Charges and
potential can build up where there is no path to ground. In this case, a
path may be provided through the garments (see 5.2.5), provided they are
in contact with the seating. It should be emphasized that, where
operators are seated, a wrist strap approach should be used and seating
should form a secondary means of discharge. In the event that an ESID
coordinator should decide that there is no suitable alternative to using
seating as the primary grounding approach, resistance to ground should
comply with the same requirements as wrist strap systems, or for
flooring and footwear (i.e. 750 kQ to 35 MQ) (see 5.2.8). Seats in
compliance with IEC 61340-5-1 should carry suitable labels (see clause
4) to help with identification. It is permissible for those parts of
seats which are unlikely to come into contact with the body or with
ESIDS, for example the underside of the seat, to be made of
non-compliant materials."
Gene Felder
[log in to unmask]
-----Original Message-----
From: Dale Ritzen [SMTP:[log in to unmask]]
Sent: Monday, March 21, 2005 2:53 PM
To: [log in to unmask]
Subject: [TN] ESD Issue - non ESD Chairs on Manufacturing Floor
A sister company of ours has just undergone an ESD audit by one of their
customers. During the audit, they were cited for allowing non ESD chairs
(i.e. no drag chains and/or conductive wheels/legs) in the manufacturing
areas. I looked in IPC-A-610, in section 3.2 (ESD/EOS Safe Workstation)
and find no reference to non-ESD chair use. Simply put, if a solder
operator, assembler, etc. is seated in a non ESD chair, but wearing heel
straps (on a dissipative floor) and is jumpered to a grounded jack via a
wrist strap my take is that they are sufficiently grounded to perform
their work. I have never had any external ESD audit fail for this
reason.
Anyone else have any experience with this type of call? I don't have a
copy of ESD S2020 so I can't refer to it for help in this case. Thanks!
Dale Ritzen, CQA
Quality Manager
Austin Manufacturing Services
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