Hi Peter:
> Question 1: Can we rely on the two screws securing the appliance
> inlet as a means for bonding? Over the years, I have been told by
> various safety agencies that such construction is unacceptable.
The appliance coupler you describe MUST include an integral
EMC filter with metal cover, not just a plain, ordinary,
all-insulated appliance coupler. An ordinary appliance
coupler cannot be grounded by its own mounting screws as the
screws attach the insulated part of the coupler to the chassis.
If the appliance coupler is fitted with an integral EMC
filter, then the ground terminal of the appliance coupler
passes through the EMC filter to the filter metal cover to
a second earthing terminal. This extension of the ground
terminal must meet all the original requirements for the
appliance coupler earthing terminal. Usually, the EMC
filter earthing terminal is securely attached to the filter
cover such that the filter cover meets all of the
requirements of the coupler earthing terminal.
Given this construction, the coupler mounting screws can
provide the necessary bonding required for safety. In
fact, the bonding scheme has two screws, one of which
provides a redundant connection.
>From a certification house point of view, this construction
does not provide a separate and distinct scheme of bonding
the coupler earthing terminal to the chassis. And, it is
difficult to identify a mounting screw as the earth bonding
terminal with the symbol.
Furthermore, most safety standards require that earth
bonding connections be solely used for that function. Hence,
the mounting screws for the appliance coupler cannot also be
used for the earthing connection. This interpretation will
vary with your particular certification engineer as well as
with the particular certification house.
If you want the construction to be unequivocably judged
acceptable, use the separate wire to a separate stud.
Otherwise, you must be prepared to argue, and be prepared to
use a stud if you lose the argument.
This is not an issue of safety. It is an issue of clearly
and unequivocably complying with the standard.
> Question 2: Am I correct in assuming that the appliance earthing
> terminal for the above Listed/Certified products was considered as
> the main protective earthing terminal?
This is a good question without a clear answer.
The main protective earthing terminal would be the one marked
with the upside-down tree in a circle (IEC 417, 5017). Note
that this symbol is molded into the appliance coupler. It may
or may not appear on the EMC filter cover. Also note that the
stud is also marked with the same symbol. Which, then, is the
main earthing terminal?
According to IEC 60950, Sub-clause 1.7.7.1: "The wiring
terminal intended for connection of the protective earthing
conductor associated with the supply wiring shall be indicated
by the symbol (IEC 417, 5017)." The conductor from the supply
wiring is connected to the appliance coupler. Therefore, the
earthing terminal of the appliance coupler is the main earthing
terminal.
IEC 60950 seems to recognize the appliance coupler marking as
indicating the equipment main earthing terminal. See the
exemptions to Sub-clause 1.7.7.1.
However, many certification engineers and certification houses
believe that the point of attachment of the appliance coupler
earth terminal to the chassis is the main earthing terminal.
Again, if you want the construction to be unequivocably judged
acceptable, apply the marking next to the stud.
> Question 3: Why do most other ITE manufacturers use the "stud
> on the chassis method" for protective earthing terminal?
Already discussed. Tradition, and because such construction is
unequivocably acceptable.
Best regards,
Rich
-------------------------------------------------------------
Richard Nute Product Safety Engineer
Hewlett-Packard Company Product Regulations Group
AiO Division Tel : +1 619 655 3329
Effective 6/12/99: +1 858 655 3329
16399 West Bernardo Drive FAX : +1 619 655 4979
Effective 6/12/99: +1 858 655 4979
San Diego, California 92127 e-mail: [email protected]
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