This invention relates generally to circuitry for protecting devices which receive a periodic input power signal, as generally described in commonly assigned application Ser. No. 07/580,957, now abandoned. More particularly, the present invention relates to diagnostic circuit protection devices which can be calibrated to protect a particular load or circuit.
There are circumstances under which fuses, circuit breakers, ground fault interrupters, or other currently used protection circuits do not provide adequate hazard and device protection. These presently used devices are not sensitive to all possible fault conditions. Sometimes the fault condition is such that it requires a power shut off time much shorter than these devices can provide, in order to prevent a hazard or equipment damage.
The present invention can be used in place of a fuse or a circuit breaker and provides protection under a broader range of conditions than do fuses, circuit breakers, ground fault interrupters, or other currently existing protection devices. It is capable of adapting to the fault conditions presented by the system in which it is installed. Unlike other protection devices, the present invention does not rely on built in fault limits. It is adaptable to the protection of common household appliances, air-conditioning units, light fixtures, medical instruments, and many other electrical units. Because it can be calibrated at the time of installation, it can be used with all types of electrical units as an add on device.
One inadequacy with currently available protection devices is their lack of flexibility in adapting to the peculiarities of the electrical system in which they are installed. Most protection devices are calibrated at the factory to protect at a single, preset fault limit. This is generally true of fuses, circuit breakers, and ground fault circuit interrupters. Similarly, electronic protection devices tend to be custom-designed to work with the power supply with which they are used. They are usually an integral part of the power,supply design, and are not adaptable for use as an `add on` device to an existing electrical system.
The typical electrical system comprises either a power source connected directly to the electrical unit (the load), or a power source coupled to the electrical unit through a matching device. The matching device comprises a device, such as a transformer, or a circuit comprised of other components. When used, its input is connected to the power source and its output is connected to the electrical unit.
Thus, one type of electrical system may comprise an alternating current power source coupled to an electrical unit through a transformer. The power source may be a standard United States alternating current source or may be a source conforming to a European or other standard. The transformer may be chosen from a variety of types, ratings, and constructions. A transformer is selected to match the power source to the load characteristics of the particular electrical unit to provide proper operating power. This is done by using a transformer with specific voltage and current ratings. The load characteristics of an electrical unit depend, among other things, on its type, physical configuration, and on the ambient conditions in its operating environment.
Proper installation, maintenance, and hazard protection of electrical systems is crucial to their safety and longevity. Currently, there are inadequacies in all three of these areas. The chief problems that an electrical system can suffer from are: (1) overloading; (2) underloading; (3) an imbalanced load (or electrical unit); (4) open circuits in the electrical unit; (5) short circuits in the electrical unit; (6) arcing; (7) power surges in the primary circuit; and (8) over-temperature conditions in either the matching device or the electrical unit. The present invention can either avoid or solve these problems for many systems. For example, the present invention can solve these problems in an electrical system comprising a gaseous light fixture, as discussed below.
Gaseous light fixtures are used in numerous commercial and residential applications. Neon sign and fluorescent light installations are common examples. The typical gaseous light fixture consists of an alternating current power source coupled to a luminous device through a transformer. The power source and load can vary as indicated above. For instance, the load characteristics of a luminous device depend on its type, physical configuration, and on the ambient conditions in its operating environment.
During fixture operation, the transformer steps up the source voltage connected to its primary coil and puts out a secondary coil voltage that is high enough to ionize the gas in the luminous device. The secondary voltage in a light fixture typically ranges from 110 volts to 15,000 volts, depending on the load characteristics of the luminous device. Many fixtures use a particular type of transformer called a luminous tube transformer.
A luminous tube transformer has a special shunt winding which will limit the output current to a maximum given level. The output current level depends on the instantaneous load conditions presented by the luminous device. Typically, luminous tube transformers for use with neon lights are designed to limit the output current to a maximum given level of 30 mA or 60 mA. In fluorescent light fixtures the maximum output current level of the ballast is typically around 800 mA. As described in commonly assigned patent application Ser. No. 07/580,957, the use of a luminous tube transformer in the fixture renders fuses, circuit breakers, and ground fault interrupters useless for detecting certain types of faults and preventing the hazards associated therewith.
The load characteristics for a given luminous device may vary considerably from those of other luminous devices of the same type. The variation may be due to any combination of differences in size, construction, or materials used. Changes in ambient temperature also change the load characteristics. Load characteristics also vary among the various types of luminous devices.
The transformer should be chosen to match the load at the time of installation. A properly loaded transformer has a longer service life, conserves electrical power, and maximizes the economic use of natural resources. Proper loading also promotes smoother operation over a range of varying conditions, such as large changes in temperature. A transformer is typically chosen so that it operates at 80% of its maximum rating when attached to a particular luminous device. The transformer has a shorter service life when overloaded or underloaded relative to this value. Transformer matching presents several problems.
To match the transformer, the sign maker estimates the necessary voltage and current ratings of the transformer based on the type, size, and configuration of the luminous device. Occasionally this choice is confirmed in the lab, but usually it is not confirmed. The transformer is then shipped along with the luminous device.
During installation of the fixture, an ammeter is sometimes temporarily inserted in the secondary coil circuit to verify proper matching of the transformer to the load, or to detect an improper load condition. The current draw should be approximately 80% of the transformer's maximum current rating. However, technicians often omit this cumbersome procedure and simply install the fixture. Thus, it is rare that proper matching is checked at any time prior to or during installation. Even when an ammeter is used, it only gives a current reading, but does not directly indicate to the technician any information about proper loading.
At the time of installation, the fixture can suffer from three distinct problems: (1) an overloaded transformer; (2) an underloaded transformer; (3) an imbalanced load.
During normal fixture operation, if a transformer draws substantially more current than 80% of its maximum current rating it is overloaded. This will result in a shorter transformer service life and increase the possibility of impaired system performance due to changing fixture parameters.
During normal fixture operation, if a transformer draws substantially less current than 80% of its maximum current rating it is underloaded. This will also result in a shorter transformer service life and increase the possibility of impaired system performance due to changing fixture parameters.
Some luminous devices are configured to present a balanced load at the transformer secondary. This is done by various methods, such as arranging a plurality of luminous tubes in series, in parallel, or in series-parallel combinations at the secondary output of the transformer. If the load is not properly balanced at the time of installation, transformer service life will be shorter. Imbalanced loads also require greater power consumption in most cases since greater phase angle differences between the voltage and current result from the imbalance. For this reason, electrical units are often designed to present a balanced load to the power source. Such techniques are quite common in three-phase power systems, to which the present invention can be adapted.
Aside from installation defects, during normal operation a fixture is susceptible to the following fault conditions: (1) open circuits; (2) short circuits; (3) arcing; (4) power surges in the primary circuit; (5) transformer over-temperature conditions; and (6) degradation over service life.
The risk of fire is particularly present when arcing or a short circuit occurs. Fixture construction is such that a short circuit or an arc may not result in a failure of components or a loss of power. This means that a dangerous condition may persist for a considerable time after the fault occurs.
For a short circuit, considerable heat can be generated in the wires as the transformer attempts to drive the short. Depending on the heat dissipation capabilities of the transformer, power to the shorted secondary may be maintained for a considerable amount of time. The heat generated may cause nearby materials to ignite. Even if a fire does not result, eventually the transformer will fail. The technician may not be aware of the short circuit fault when replacing the transformer.
Arcing likewise may continue indefinitely. High secondary voltages can sustain electrical arcs up to several inches in length. When this occurs, nearby flammable or combustible materials could easily ignite.
The three remaining operating faults do not carry as high a risk of danger. However, it is still a great advantage to turn off primary power to the transformer when they occur.
Delivering power into an open circuited fixture shortens transformer service life and may injure an unsuspecting person who believes the fixture to be off. Also, arcing may occur at any time an open circuit exists, depending on the air gap dimensions and ambient conditions. Changes in temperature, humidity, etc. can cause an open circuit to start arcing.
Power surges can potentially harm both the transformer and the luminous device. They also present some risk of hazard due to large voltage and current spikes. It is an advantage to shut down power to the fixture as quickly as possible when a surge occurs.
Beyond a certain temperature it is both dangerous and damaging to operate a gaseous light fixture. The primary power should be shut down whenever the temperature exceeds this level.
Currently, a device which can detect and protect against operating faults is disclosed in commonly assigned application Ser. No. 07/580,957 entitled Luminous Tube Protection Circuit. Although this device can detect all five faults listed above, it does so differently than the present invention and it does not offer diagnostic information. It cannot shut off the primary power as quickly as can the diagnostic circuit protection device of the present invention.
The Luminous Tube Protection Circuit of the related application time-averages the sensed secondary output voltage, does not sense transformer primary current, and does not use an analog-to-digital converter, so it lacks many of the advantages of the present invention.
One advantage of the present invention is that it can shut off power to the transformer primary much more quickly than the Luminous Tube Protection Circuit can. This is true for any fault condition.
Another advantage is that the present invention senses transformer primary current as well as transformer secondary voltages. This allows for accurate diagnosis and subsequent indication of the fault condition.
The present invention simply divides and converts the voltage (and current) values using an analog-to-digital converter, yielding a direct representation of the sensed waveform at a given point in time. This captures more information about fixture operation than does the Luminous Tube Protection Circuit, which time averages the transformer secondary voltages.
The digital sensing method helps provide a faster shut-off of the primary power when a fault occurs. It allows real time processing of the sensed information, to facilitate shut-off and indication. Digital sensing also facilitates permanent storage of fault condition waveform histories, which are then available during diagnosis and repair.
Finally, the present invention has the advantage of being able to calibrate itself to the peculiarities of the particular load at the time of installation, or at any other time the technician decides to recalibrate the diagnostic circuit protection device of the invention.
Although the foregoing discussion and the subsequent disclosure center around the use of the diagnostic circuit protection device to protect a gaseous light fixture, one skilled in the art can easily apprehend its application with other electrical units.