Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
2002-07-10
2004-03-16
Toatley, Jr., Gregory J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
Reexamination Certificate
active
06707652
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is directed to electrical switching apparatus and, more particularly, to electrical switching apparatus such as, for example, receptacles including terminals, such as screw terminals, for electrical conductors, such as copper wiring.
2. Background Information
Electrical switching apparatus include, for example, circuit switching devices and circuit interrupters such as circuit breakers, contactors, motor starters, motor controllers and other load controllers.
Circuit breakers are generally old and well known in the art. An example of a circuit breaker is disclosed in U.S. Pat. No. 5,341,191. Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. Molded case circuit breakers, for example, include at least one pair of separable contacts which are operated either manually by way of a handle disposed on the outside of the case or automatically by way of an internal trip unit in response to an overcurrent condition.
Ground fault circuit interrupters (GFCIs) include ground fault circuit breakers (GFCBs), ground fault switches and other ground fault contactors, motor starters, motor controllers and other load controllers.
Arc fault circuit interrupters (AFCIs) include arc fault circuit breakers (AFCBs), arc fault switches and other arc fault contactors, motor starters, motor controllers and other load controllers.
Ground fault and/or arc fault switches include ground fault and/or arc fault receptacles (GFRs/AFRs), and cord-mounted or plug-mounted ground fault and/or arc fault protection devices (e.g., ground fault and/or arc fault protection circuitry at the alternating current (AC) plug end of the AC power cord of an appliance, such as a hair dryer).
A typical GFCI includes an operational amplifier, which amplifies a sensed ground fault signal and applies the amplified signal to a window comparator. The window comparator compares the amplified signal to positive and negative reference values. If either reference value is exceeded in magnitude, a trip signal is generated.
A GFCI may employ, for example, the well known dormant oscillator technique for sensing a load side grounded-neutral condition, without requiring a connected load. Two magnetic elements are employed. The first magnetic element is a differential current transformer, which produces an output proportional to the difference in the current flowing to the load through the line conductor and the current returning from the load through the neutral conductor. The difference is the ground current. The second magnetic element is a voltage transformer, the primary of which is energized by the output of a ground fault sense amplifier, which is part of the GFCI electronics. The transformer has two single turn secondaries formed by passing line and neutral conductors through its core. The polarities of the primary and secondary windings of the transformer are such that the ground fault sense amplifier output induces a voltage on the secondary of transformer, such as the neutral conductor, which voltage increases the ground current caused by a load neutral-to-ground fault. This increased ground current increases the sense amplifier output, thereby resulting in a positive feedback condition increase in the ground current. If the load neutral-to-ground impedance is less than about 2 ohms, this positive feedback may become unstable, which results in a monotonic increase in the induced ground fault current in the neutral conductor until the ground fault trip level is exceeded and the receptacle trips. Both conductors are passed through the core to cover the case where the input leads are reversed.
A glowing contact is a high resistance connection, which can form at the interface of a copper wire and a screw terminal, for example, of a receptacle. The resulting temperature rise at this connection point can melt the wire's insulation and damage the receptacle. It is desirable to be able to detect this condition and interrupt the current before the glowing contact fault progresses to a hazardous condition.
The hazard associated with aluminum wiring has been known and understood for thirty years. The connection of an aluminum wire conductor to the terminal of a wiring device is unstable, since the aluminum, over time, tends to flow, thus, making the aluminum wire-to-terminal a high resistance connection. The resulting I
2
R heating causes local heating that can melt the wire's insulation and the receptacle. It was believed that simply returning to copper wire would resolve this problem. Unfortunately, this is not true. Furthermore, most people, outside of the standards and wiring device industry, are unaware of the glowing contact problem. Also, the lack of wide spread public knowledge of the glowing contact problem may follow from the fact that there has been no known solution to this problem.
It is very easy to create a high resistance or glowing contact at a receptacle terminal using copper wire. See, for example, Sletbak, J., et al., “
Glowing Contact Areas in Loose Copper Wire Connections
,” IEEE, 1991, pp. 244-48.
The hazards associated with glowing contacts, including contacts made with all combinations of copper, brass and iron are known. See Yasuaki Hagimoto, “
Japanese Reports on Electrical Fire Causes
,” http://members.ozemail.com.au/~tcforen/japan/index.html, 1996, 12 pp.
In a similar manner that aluminum oxide creates the aluminum wire problem, the culprit associated with a glowing contact is copper oxide. There are two recognized mechanisms for creating a high resistance copper oxide contact: arcing; and fretting. The arcing mechanism involves, for example, a loose receptacle screw terminal and slight movement of the wire while it is carrying a current. Every time the electrical connection is broken, a single electrical arc discharge can occur.
FIG. 1
shows the voltage across the terminal-to-wire connection in the upper trace (about 170 V peak) and the current through that connection in the lower trace (about 15 A peak) for different intervals of an electrical connection being broken while carrying current. This pair of voltage and current traces is broken into three intervals I,II,III. The first interval I shows normal operation in which there is negligible voltage across the terminal-to-wire connection, which has a relatively low resistance, with an alternating current flowing through that connection. During the second interval II, there is a significant increase in the resistance of the terminal-to-wire connection, due to a single arcing half cycle. Hence, there is a corresponding significant increase in the voltage across the terminal-to-wire connection, along with a corresponding reduction in the magnitude of the alternating current flowing through that connection. Finally, during the third interval III, the terminal-to-wire connection becomes an open circuit and the voltage across the terminal-to-wire connection is the line voltage. As a result of the open circuit, there is essentially no current flowing through that connection.
While there is essentially very little power dissipated in the terminal-to-wire connection during the first and third intervals I,III, relatively significant arcing and power dissipation occurs in the second interval II. To the extent that the second interval II may become relatively periodic or persistent, then oxidation can occur at the copper wire-screw interface where the half cycle arcing has occurred with each breaking of the wire-screw connection. This copper oxide layer at the wire-screw interface can also occur due to the mechanism of fretting or a rubbing action with no arcing.
By Paschen's laws, it is not possible to create a sustained copper-to-copper through air arc discharge in a 120 V
RMS
circuit with a resistive load. An arc is formed when the contact breaks, although it extinguishes at the first zero current crossing, since the voltage is too small for a “re
Eaton Corporation
Moran Martin J.
Nguyen Danny
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