Patent Application
20110141635
The present invention relates to a ground fault circuit interrupter (GFCI) having protection against overheating.
A Ground Fault Circuit Interrupt (GFCI) monitors the balance of current passing through two wires that are carrying power, typically referred to as a line conductor and a neutral conductor. The current on the line conductor can be thought of as electrical charge going downstream, from the source to the load, while the current on the neutral conductor can be thought of as going upstream.
As long as the difference between current on the conductors stays within a given range, normally 5 ma, the GFCI does not interfere with normal operation. If the sensed current does not remain in balance, or within the given range, the GFCI interrupts current flow, thereby causing power to be shut off to the load. GFCIs are required by the National Electrical Code (NEC) in bathroom and kitchen outlets, i.e., where water is possible.
It has been proposed to use thermal protection for a receptacle in combination with a GFCI-protected power control panel. However, such proposals do not operate in a safe manner.
Accordingly, there is need for an improved way to provide thermal protection for a GFCI.
In accordance with an aspect of this invention, there is provided an electrical device having thermal overload protection. A current difference detector measures the difference in current between a line conductor and a neutral conductor. A control circuit prevents current flow on the line and neutral conductors when the measured current differences exceeds a current difference threshold. A temperature detection circuit detects a temperature of at least one of the line and neutral conductors and causes a current difference exceeding the current difference threshold when the detected temperature exceeds a temperature threshold.
The electrical device may be a circuit breaker, an electrical receptacle, an electrical plug, a power strip or a receptacle adaptor.
In accordance with another aspect of this invention, there is provided a method of providing thermal overload protection in an electrical device. The difference in current between a line conductor and a neutral conductor is measured. Current flow on the line and neutral conductors is prevented when the measured current differences exceeds a current difference threshold. A temperature of at least one of the line and neutral conductors is detected, and when the detected temperature exceeds a temperature threshold, a current difference exceeding the current difference threshold is caused.
U.S. Pat. No. 3,872,355 (Klein) relates to a fire protection circuit and device.
Power control panel 10 includes ground fault circuit interrupter (GFCI) 42. Line conductor 30, neutral conductor 32 and ground conductor 24 are each connected between power supply 23 and GFCI 42. GFCI 42 measures the current difference between line conductor 30 and neutral conductor 32, and when this exceeds a predetermined threshold, interrupts the current flow. Line conductor 46, neutral conductor 50 and ground conductor 51 are on the output (downstream) side of GFCI 42.
Duplex receptacle 62 includes two three-prong outlets. Each of the three prong receiving portions is connected to a respective one of line conductor 46, neutral conductor 50 and ground conductor 51. Thermal detector 74 is located in duplex receptacle 62, connected between a first conductor connected to neutral conductor 50 and a second conductor connected to ground conductor 51.
Thermal detector 74 may be a negative temperature coefficient (NTC) thermistor, as shown in
Thermal detector 74 is normally at a high impedance, so current essentially does not flow through detector 74. However, when the temperature exceeds a predetermined threshold, thermal detector 74 goes from a high impedance to a low impedance, so that current flows between the first and second conductors, simulating a ground fault on neutral conductor 50.
In an alternate configuration, plug 84 is inserted into an outlet of duplex receptacle 92, which is a conventional duplex receptacle. However, plug 84 has thermal detector 74 connected between its neutral blade and its ground pin.
Klein notes that alternatively, thermal detector 74 can be placed between the line (hot) conductor and the ground conductor, however, this is frowned upon by the various electrical codes.
UL and ANSI safety conventions prohibit circuits that function to deliberately create a fault to ground. Klein's technique of diverting current to ground intentionally creates a fault, which is unsafe and prohibited by safety conventions.
Two embodiments of an electrical device having thermal overload protection will now be discussed. The electrical device detects an abnormal temperature of the current carrying conductors, and causes a current imbalance so that a ground fault circuit interrupter (GFCI) triggers, thereby cutting off current flow and preventing thermal overload.
Resistor 273A is coupled between line conductor 130 and thermal detector 274A. Thermal detector 274A is coupled between resistor 273A and neutral conductor 150.
Resistor 276A is coupled between neutral conductor 132 and thermal detector 275A. Thermal detector 275A is coupled between resistor 276A and line conductor 146.
Resistors 273A, 276A have different resistances, to prevent current flow even if thermal detectors 274A, 275A switch at the same time. For instance, resistor 276A may have a value of 10 kOhms and resistor 273A may have a value of 20 kOhms, or vice-versa, or any other suitable values.
Thermal detectors 274A, 275A are identical units, and may be any suitable thermal detector, such as a bimetal, thermal reed switch or negative temperature coefficient (NTC) resistor. For manufacturability, a self-resetting bimetal is preferred. In some embodiments, a non-resetting bimetal is used so that an electrician must replace the triggered GFCI and investigate the problem. The bimetal in its normal open state does not have current passing therethrough and does not affect the operation of GFCI 242.
Use of two thermal detectors enables monitoring of four critical terminals: the line input, the line output, the neutral input and the neutral output. It will be understood that while conductors 130, 132, 146, 150 are present primarily to conduct current, they also conduct heat.
When the temperature sensed by bimetal 274A or 275A reaches a predetermined threshold, such as a temperature in the range 75° C.-200° C. depending on the bimetal selected, the bimetal snaps closed, diverting part of the current from one of conductors 146, 150 to one of conductors 130, 132, forcing an increased current difference between conductors 130, 132 that is sensed by control circuit 200 which in turn activates solenoid 202 and interrupts the current flow to the load and to all downstream elements such as receptacles (not shown).
The bimetal trigger threshold is chosen to be above normal operating temperature but below the temperature at which electrical materials are adversely affected such as plastic melting, i.e., in the range 75° C.-200° C.
Diverting current does not create a fault, so the configuration of
Resistor 373B is coupled between line conductor 130 and thermal detector 374B. Thermal detector 374B is coupled between resistor 373B and neutral conductor 150.
Resistor 376B is coupled between neutral conductor 132 and thermal detector 375B. Thermal detector 375B is coupled between resistor 376B and line conductor 146.
Resistors 373B, 376B have identical resistances, such as a value of 10 kOhms, or any other suitable values.
Thermal detectors 374B, 375B are identical units, and may be any suitable thermal detector. For instance, thermal detectors 374B, 375B may be bimetals in a normal closed position, so that the currents fed back from the output of transformers 220, 222 to the respective inputs of transformers 220, 222 are equal. When overheating occurs, one of the bimetals opens before the other due to differences in heat propagation time and/or components not being perfect identical, causing detection of a current imbalance and triggering the GFCI to stop current flow. Instead of a bimetal, a thermal fuse or NTC resistor or positive temperature coefficient (PTC) resistor may be used.
Receptacle 262 provides current from line conductor 246 and neutral conductor 250 to GFCI 242B. The output of GFCI 242B is provided to the two outlets of receptacle 262 and to downstream devices. GFCI 242B operates in similar manner as GFCI 242 of
The number of outlets is not crucial, that is, any number of outlets may be in the power strip.
In a variation, only one thermally protected GFCI is used for all of the outlets of the power strip.
Another embodiment of a thermally protected GFCI will now be discussed.
Thermal detector 174 is connected to neutral conductor 132 and neutral conductor 150. Line conductor 146 is configured to be in close physical proximity to thermal detector 174, but does not have electrical contact with thermal detector 174.
Heat flows to thermal detector 174 via three paths. A first path is through conductive contact with neutral conductors 132, 150. A second path is via radiation from line conductor 146. A third path is through the ambient temperature inside power control panel 110.
Thermal detector 174 may be any suitable thermal detector, such as a resettable or momentary contact bimetal, thermal reed switch or NTC resistor. The bimetal in its normal open state does not have current passing therethrough and does not affect the operation of GFCI 142A. When the temperature sensed by bimetal 174 reaches a predetermined threshold, such as a temperature in the range 75° C.-200° C., the bimetal snaps closed, diverting part of the current path around GFCI 142A, causing GFCI 142A to interrupt current to the load and downstream elements.
In a modification (not shown), another thermal detector is connected between line conductor 130 and line conductor 146, and is placed so that neutral conductor 150 generally surrounds the additional thermal detector. The additional thermal detector enables monitoring of the temperature at more points.
Duplex receptacle 162 provides current from line conductor 146 and neutral conductor 150 to GFCI 174B. The output of GFCI 174B is provided to the two receptacles of duplex receptacle 262 and to downstream devices. Thermal detector 174B is arranged in similar manner relative to GFCI 142B as thermal detector 174A of
Plug 184 has a face with hot and neutral blades and a ground prong. Inside the body of plug 184, GFCI 142C is coupled between the hot and neutral blades and cord 185, with thermal detector 174C arranged in similar manner as thermal detector 174A of
The number of receptacles is not crucial, that is, any number of receptacles may be in the power strip. The power strip may have one thermally protected GFCI per receptacle, or one thermally protected GFCI for the entire power strip.
U.S. Pat. No. 5,673,360 (Scripps) shows a portable humidifier with a bimetal thermal sensor. The thermal sensor is configured in a circuit with the GFCI between the load hot and load neutral. When the temperature at the overheat sensor rises above a predetermined level, the overheat sensor causes a transistor to close, by physically deforming to an open position, opening the electrical connection, which in turn causes another transistor to open, which directs the current through a resistor, which in turn causes a current imbalance in the GFCI. The GFCI senses the current imbalance in the line and neutral connections and trips.
U.S. Pat. No. 4,737,0769 (Masot) is directed to a temperature sensor mounted at an electrical outlet receptacle. When the temperature at the thermal sensor rises above a predetermined level, it sends a signal to a converter apparatus which is configured to send a signal through the circuit to a “short circuit or controlled overload device”. The resulting short circuit or overload is detected by a circuit breaker box, which then trips. In another embodiment, the signal is sent from the converter apparatus to the short circuit or controlled overload device via RF signal.
Although illustrative embodiments of the present invention, and various modifications thereof, have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to these precise embodiments and the described modifications, and that various changes and further modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.
