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Circuit for Indicating and Removing an AC Line Cord Fault

IP.com Disclosure Number: IPCOM000106407D
Original Publication Date: 1993-Nov-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 2 page(s) / 93K

Publishing Venue

IBM

Related People

Clemmons, E: AUTHOR [+3]

Abstract

Disclosed is a circuit, for installation within a device, providing a visual indication of a phase to ground fault caused by the incorrect wiring of a line cord connected to the device. The circuit also removes the unsafe condition created by such a fault. This circuit senses the voltage difference between the ground pin of the appliance coupler to which the line cord is connected and either the neutral pin of the appliance coupler (in a 110 VAC device) or a convenient AC ground point (in a 220 VAC device). If this voltage difference exceeds a certain level, as determined within the circuit in relation to the breakdown voltage of a zener diode, a fault indicator is activated, and the connection within the device to the ground line of the line cord is opened by a relay.

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This is the abbreviated version, containing approximately 52% of the total text.

Circuit for Indicating and Removing an AC Line Cord Fault

      Disclosed is a circuit, for installation within a device,
providing a visual indication of a phase to ground fault caused by
the incorrect wiring of a line cord connected to the device.  The
circuit also removes the unsafe condition created by such a fault.
This circuit senses the voltage difference between the ground pin of
the appliance coupler to which the line cord is connected and either
the neutral pin of the appliance coupler (in a 110 VAC device) or a
convenient AC ground point (in a 220 VAC device).  If this voltage
difference exceeds a certain level, as determined within the circuit
in relation to the breakdown voltage of a zener diode, a fault
indicator is activated, and the connection within the device to the
ground line of the line cord is opened by a relay.

      This circuit 8 is shown in Fig. 1.  Terminal 10 of the circuit
is connected to the safety ground of the appliance coupler to which
the line cord is connected.  If the line voltage is 110 VAC, terminal
12 of the circuit is connected to the neutral pin of the appliance
coupler.

      In a device configured for a 220 VAC input, while terminal 10
is still connected to the input safety ground of the appliance
coupler, terminal 12 is connected to a convenient AC ground point in
the system.  As shown in Fig. 2, a convenient AC ground point 14 may
be found at the center tap of bulk capacitors 16, within a system
power supply, extending between terminals 17, to which 220 VAC is
applied.  Alternately, as shown in Fig. 3, a convenient AC ground
point 18 may be found at the center tap of an internal voltage
transformer 20, between terminals 21, to which 220 VAC is applied.

      Within the device, the safety ground wire 22 (typically a
green/yellow wire), extending between the safety ground of the
appliance coupler and the chassis of the device, is wired through
normally closed contacts 24 of a relay 26 within circuit 8.

      If the line cord is wired incorrectly in such a way that the
phase and ground wires are reversed, AC voltage is applied between
terminals 10 and 12 of circuit 8.  This voltage is rectified through
rectifiers 28, applying a full wave rectified voltage across
resistors 30, which divide the voltage.  Capacitor...