Hi Bill:
> I have a request for interpretation of requirement. Which symbol is correct
> for use on a chassis for protective earthing - the upside down tree or the
> circle upside down tree or the pitchfork?? (Note:- the ground on the back
> of the box is generally marked "GND" and is screw and locking washer
> configuration). What is the correct marking to satisfy a NRTL for a
> marking?
You have asked about four different symbols:
1) Circle.
2) Protective earthing (upside-down tree)
3) Chassis common ("pitchfork")
4) "GND"
The circle is a symbol for imperative. This means
that something MUST be done or is permitted to be
done. (The circle with a diagonal line is a
negative imperative -- some action which must not
be done or is not permitted to be done.)
(Unfortunately, there is ambiguity in the circle
symbol between "must" and "permitted.")
The protective earthing symbol (upside-down tree)
means that this terminal is connected to earth.
The circle combined with the protective earthing
symbol means that this connection point (terminal)
MUST be connected to earth. Therefore, this
terminal MUST be used for the wire which connects
the equipment to the earth.
The chassis common ("pitchfork") means that this
terminal is connected to the chassis or common
point (or functional earth) of the circuit. The
chassis may also be connected to earth.
The correct marking which will satisfy an NRTL
depends on the standard being applied to the
product. The symbols you mentioned are IEC
symbols and are not necessarily required by the
standard which will be applied by the NRTL. Some
USA standards allow the use of GND or the color
GREEN for terminal identification instead of the
IEC symbols.
Best regards,
Rich
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