Hi Chris:
> What is clear from IEC60065 is that if a product is capable of being
> switched on or off or both by a timer or a data link, a front panel
> mechanically operated switch is not required. What is not clear is the
> definition of off.
Hmm. For me, the question is: What is the safety function
of "off"?
If there is a safety requirement for an "off" function, then
we can easily define "off" in terms of the safety function
"off" provides.
If the product meets all of the safety requirements when it
is "on" (and unattended), then I would suggest there is no
operator safety function that requires an "off" function.
IEC 60950 implies two "off" functions.
One is a "soft" off, where the "off" controls all circuits
except the circuit that controls the "off" function. An
example of this is the computer or monitor whose "off" and
"on" states are controlled by a key control or software/firmware
control.
The other is the "hard" off, where the control physically
switches all power (except, optionally, the power to the EMC
filter when it is on the supply side of the switch).
IEC 60950 mandates a "hard" off function. This may be either
a switch, or the appliance coupler, or the plug, or a control
that is part of the installation (for permanently-connected
equipment). The purpose of the "hard" off is to protect the
serviceman when he is working on the mains circuits. (The
"soft" off does not satisfy this requirement.)
In IEC 60950, the "off" function does not provide protection
for the user/operator.
However, the "hard" off function must be readily accessible
to the user/operator in case of an unanticipated hazardous
condition that can only be made safe by disconnection of the
supply.
So, as far as I know, there are two safety functions of the
"hard" off:
1. Protection of the serviceman while servicing mains
circuits.
2. Protection of the operator in the event of an
unanticipated hazardous condition that can only be
made safe by disconnection of the supply.
In a safety standard from a long time ago, there was a
requirement that safety shall not rely on conduction (or
lack of conduction) in a gas, vacuum, or semiconductor. I
believe that to be a good rule. This rule precludes the
use of a "soft" off for safety purposes.
Best regards,
Rich
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