John,
This panel contains the IEC Inlet and fuse for the system. This is of course
at a Hazardous voltage level as the system is rated for an input of 100-240
VAC. The wiring is close enough to the panel that if something came loose it
could conceivably touch the panel. We feel it is better to err on the side
of safety, that is why we plan on grounding the panel.
Kurt Andrews
-----Original Message-----
From: Crabb, John [SMTP:[email protected]]
Sent: Friday, May 18, 2001 9:52 AM
To: 'Andrews, Kurt'; EMC-PSTC
Subject: RE: Protective Bonding-UL 60950 (replacement for UL
1950)
As a previous reply has suggested, but not in the same words,
surely one question that has to be considered is whether this hinged
panel is required to be earthed. The requirement in UL60950 2.6.1 a)
refers to "accessible conductive parts that might assume a HAZARDOUS
VOLTAGE in the event of a single fault".
I would suggest that if there are no single insulated hazardous
voltages
(typically wiring) touching this panel, or close to it, then why go
to
the trouble of earthing it. If a single fault caused a hazardous
voltage
to touch your metal enclosure, this should be taken care of by the
protection you have built in. You could even consider that in such a
case,
it would be better if the hinged panel was isolated from the rest of
the enclosure.
I will be interested to see what comments this brings out - I can
only make the point that I got both UL and CSA to agree that I don't
have to ground the doors of the safes in our Automated Teller
Systems,
based on the above arguments.
Regards,
John Crabb, Development Excellence (Product Safety) ,
NCR Financial Solutions Group Ltd., Kingsway West, Dundee,
Scotland. DD2
3XX
E-Mail :[email protected]
Tel: +44 (0)1382-592289 (direct ). Fax +44 (0)1382-622243.
VoicePlus
6-341-2289.
-----Original Message-----
From: Andrews, Kurt [mailto:[email protected]]
Sent: 16 May 2001 19:18
To: EMC-PSTC
Subject: Protective Bonding-UL 60950 (replacement for UL 1950)
Hi group,
We are designing a new piece of class I equipment to be evaluated
under UL
60950 (replaced UL 1950). The equipment has a metal enclosure. It
has a
metal access panel that is hinged at the bottom with a piano type
hinge. It
is secured at the top by two screws. This panel is hinged so that
the
customer's service personnel can have access to a PC board for
configuration
purposes. There are hazardous (AC mains) voltages behind this panel.
We are
concerned that this panel may not make a reliable earth connection
to the
rest of the metal enclosure, especially if the customer fails to
fully
tighten the two screws. We are planning on adding a wire from this
panel to
a side panel of the enclosure to reliably ground the panel to the
rest of
the enclosure. This wire will be on the inside of the unit.
According to UL
60950 this wire would be considered a Protective Bonding Conductor.
We are
planning on using a stranded 14 AWG wire with ring terminals on both
ends
for this Protective Bonding Conductor. It would be fastened to the
two
panels via threaded studs mounted in the panels. We plan on placing
the ring
terminals on the studs and securing them with toothed lock washers
and nuts.
I have a question about the requirements for this Protective Bonding
Conductor. According to UL 60950, clause 2.6.5.7, at least two
screws must
be used for each connection. Does this clause apply in a case such
as this?
Or is it meant to be used in cases where two metal panels are
fastened
together with screws to provide Protective Bonding between the
panels. I
don't really see how we can use two screws for each connection when
using a
wire for this purpose unless there is a crimp terminal that has two
rings on
it which I haven't seen. I have copied the clause from UL 60950
below for
your reference.
Any advice on this matter would be greatly appreciated.
Kurt Andrews
Compliance Engineer
Tracewell Systems, Inc.
567 Enterprise Drive
Westerville, Ohio 43081
voice: 614.846.6175
toll free: 800.848.4525
fax: 614.846.7791
http://www.tracewellsystems.com/ <http://www.tracewellsystems.com/>
2.6.5.7 Screws for protective bonding
NOTE -The following requirements are additional to those in 3.1.6.
Self-tapping (thread-cutting and thread-forming) and spaced thread
(sheet
metal) screws are
permitted to provide protective bonding but it shall not be
necessary to
disturb the connection
during servicing.
In any case, the thickness of the metal part at the point where a
screw is
threaded into it shall
be not less than twice the pitch of the screw thread. It is
permitted to use
local extrusion of a
metal part to increase the effective thickness.
At least two screws shall be used for each connection. However, it
is
permitted to use a single
self-tapping screw provided that the thickness of the metal part at
the
point where the screw is
threaded into it is a minimum of 0,9 mm for a screw of the
thread-forming
type and 1,6 mm for
a screw of the thread-cutting type.
Compliance is checked by inspection.
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