Browse Prior Art Database

Line Fault Sensing Technique

IP.com Disclosure Number: IPCOM000101814D
Original Publication Date: 1990-Sep-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 86K

Publishing Venue

IBM

Related People

Keidl, SD: AUTHOR

Abstract

Disclosed is an apparatus for detecting lethally miswired line power provided to a load. The disclosed circuit detects all possible ways of lethal miswiring. Detection is immediate upon plugging into the miswired circuit and does not depend on the existence of a fault current.

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Line Fault Sensing Technique

       Disclosed is an apparatus for detecting lethally miswired
line power provided to a load.  The disclosed circuit detects all
possible ways of lethal miswiring.  Detection is immediate upon
plugging into the miswired circuit and does not depend on the
existence of a fault current.

      The circuit works on the principle of current flow through a
capacitance formed between a wire "antenna" and surrounding ground.
The antenna is one plate of the capacitor, air is the dielectric, and
surrounding ground is the other plate.  The capacitor created in this
way is small but sufficient to cause a detectable current flow when
the antenna is connected to a live wire.  Fig. 1 shows the basic idea
for a single-phase 110-volt line.  If the neutral or green wire
ground of a circuit becomes live through a miswiring of the service,
a capacitive current flow occurs through the current sense resistor
R1.

      Refer to Fig. 2 for circuit operation.  The sense of line
potential on the two service wires that should normally be at ground
potential is made through R2 and R3.  If either neutral or green wire
ground become live, current flows through R1.  This current will
result in a voltage drop across R1 greater than Vtrip, and the
comparator made up of Q1 and Q2 will switch NAND gate 1.  Vtrip is
set sufficiently high to compensate for noise and background current
flow.  Gate 1 is used as a current amplifier and filter for NAND gate
2. Filter capacitors C1, C2 and C3 reduce circuit line noi...