ELECTRIC POWER DISTRIBUTION UNIT FOR BUILDINGS

Abstract: An electrical circuit breaker unit (10) has an isolator switch (14), a residual current device (16) for sensing a current imbalance, and circuit breakers (18) for sensing current overload. A thermal sensor (22) is mounted within the casing (12) of the unit and triggers the RCD (16) to automatically disconnect the supply if the temperature in the unit rises significantly above ambient, such as might be caused by overheating due to a bad connection in the circuit breaker unit.

Description: ELECTRIC POWER DISTRIBUTION UNIT FOR BUILDINGS Background of the Invention This invention relates to an electric power distribution unit for buildings including a safety device. A large proportion of fires in domestic premises are caused by electrical faults. Of these, a large proportion originate in the region of the distribution device such as a fuse box, circuit breaker unit or the like. Fires will, for example, be caused by a poor connection in such a unit. It is therefore good and common practice to mount a smoke alarm in the vicinity of the fuse box or circuit breaker unit to detect any smoke which results from a fire at the unit. So long as the smoke detector is and remains in sound working condition, and there is someone in the building, this then alerts them to the fact that there is a problem. In addition, the fuse box or circuit breaker unit can be contained in a fire-resistant housing. This will give a degree of resistance, generally measured in a time period such as, for example, two hours. Much effort has in the past been directed to improving the operation of fuse boxes, circuit breakers and other overload devices, and the alarm provided thereby. These improvements are, however, largely directed to increasing the reliability of the units when an electrical overload takes place. For example, United States Patent US-A-4,706, 073 describes a circuit breaker which incorporates an alarm system. This alarm system includes sensors for sensing a change of light, sound, vibration, temperature, or ionisation level, which is produced by a thermo-electric or magnetic circuit breaker when sensing an overload condition. More particularly, the sensors detect noise or vibration produced by movement of the toggle switch of the circuit breaker. A separate sensor is provided for each circuit breaker in the unit, and in this way is able to detect which of the individual circuit breakers has tripped and provide an alarm indication accordingly. As described, the sensors are on the door of the circuit breaker unit. Amongst other things they can be sensitive to the temperature level or ionisation level which is present during an overload condition. However, such a system is designed only to enhance electrical current overload protection, and will not reliably detect heat generated, for example, by a bad connection in or to the circuit breaker. Also, it will only give an alarm signal to alert of the increasing danger, rather than eradicate and terminate the danger itself. United Kingdom Patent Application GB-A-2073974 describes an industrial circuit interrupter for automatically analysing the electrical conditions in a circuit being protected, which has a circuit breaker mechanism the contracts of which are separated under the control of a micro-computer. A bimetallic switch is mounted on the internal conductors of the circuit breaker and releases the circuit breaker when the conductors overheat. The bimetallic switch is also connected to the microcomputer to activate an alarm. United States Patent US-A-4208689 describes another circuit breaker which has a thermal tripping mechanism to trip the circuit breaker if the load conductors become overheated due to causes such as poor electrical connection to the load. For example the circuit breaker is tripped if the load bus bar temperature reaches 130°C. The circuitry within the circuit breaker is complex and thus expensive. A need exists for a safety device for an electrical power distribution unit which meets the specific requirement of sensing heat which might lead to the occurrence of a fire, and which is sufficiently small, simple and inexpensive for incorporation in units such as domestic circuit breakers. Summary of the Invention The present invention in its various aspects is defined in the appended claims, to which reference should now be made. Advantageous features are set forth in the appendant claims. The present inventor has recognised that the inherent safety of an electrical power distribution system for a building, particularly domestic accommodation, would be much improved by the inclusion of an ambient heat sensor in the region of the power distribution unit, such as to sense a temperature increase caused by heating within the unit such as might result from a poor electrical connection, the output of the sensor being operative automatically to disconnect electrical power from the unit or the relevant part thereof. The heat sensor is preferably arranged to trigger at a temperature significantly higher than the normal likely summer ambient temperature, but below that necessary, for example to melt a fuse. The sensor can sense the presence of heat before sufficient heat is generated to cause combustion, and thus actually prevent, as opposed to merely identify or alert, a fire. The unit includes a residual current device as the cut-out. Such devices are already incorporated in some commercially-available distribution boxes, and thus there is a minimum of additional equipment required. The heat sensor can take many forms, and can be coupled to the residual current device in different ways. The connection between the heat sensor and the cut-out may be one that does not require an electrical supply, and it may, in particular, be a mechanical connection. For example, the heat sensor may be constituted by a pivotally-mounted thermometer which changes position when a predetermined temperature rise causes the mercury to move and to change the centre of gravity sufficiently to cause the thermometer to pivo . A piece of spring wire can be connected between the thermometer and the residual current device to operate the latter in the case of such movement. Alternatively the connection can be electrical. If electrical power is required it can be supplied from the mains supply, which will of course be live if it is in danger of causing a fire. It is preferred not to use a battery, as this can run down, but this would be a less- desirable alternative possibility. The residual current device used in the unit is a known type of device which detects an imbalance in the current through the live and neutral conductors, indicating leakage to earth (ground) , and when such leakage exceeds a predetermined value, operates as a cut¬ out to disconnect the supply. The heat sensor can be connected to the residual current device, so that in the presence of excess . heat the residual current device is caused to trigger and disconnect the supply, even though the current through it may otherwise be balanced as between the live and neutral conductors, and even though there is no overload such as would trigger the normal overload protection. Brief Description of the Drawings The invention will now be described in more detail, by way of example, with reference to the drawings in which: Fiσure 1 is a front view of an electric power distribution unit for use in a domestic installation, with the front cover removed; Figure 2 is a side section through the unit of Figure 1 taken along the line II-II in Figure 1; Figures 3A and 3B show in schematic form two possible types of heat sensor and their connection to a cut-out; and Figure 4 is an electrical circuit diagram for the unit of Figure 1. Detailed Description of the Preferred Embodiment An embodiment of the invention will now be described in detail as an example of one way in which the invention can be implemented. However the invention can be put into practice in many alternative ways, as will be understood by the skilled reader of this specification. This embodiment will be described with reference to the drawings, and in Figure 1 is shown a circuit breaker unit 10 which has a generally rectangular casing 12, the front cover of which is removed and is thus not shown in Figure 1. The casing forms a substantially closed housing when mounted on the wall of a building. The unit contains an ON/OFF switch 14 to which the supply is connected, a residual current device (RCD) 16 which receives the live and neutral output of the switch 14, and a bank 18 of miniature circuit breakers connected to receive the live output of the RCD 16. Residual current devices are well- known and readily available. Individual circuits are then connected to the respective circuit breakers. In the event of an overload, the respective circuit breaker triggers, and a button 20 on the circuit breaker is automatically pushed outwardly at the same time as the circuit is broken. After the fault has been corrected the circuit breaker can be re-set by manual depression of the button. In the event of an earth (ground) fault, the RCD will detect a current imbalance and will trigger, cutting off the supply to all the circuits. After the earth fault has been corrected the RCD can be re-set. In accordance with this invention, the unit further includes within the housing 10, and preferably placed immediately above the bank of circuit breakers 18, a thermal sensor 22, illustrated purely diagrammatically in Figures 1 and 2 which is connected to the RCD 16. The thermal sensor 22 is located so that it is in the path of convection of any heat generated in the bank of circuit breakers. In the event of one of the circuit breakers overheating, the thermal sensor will trigger, and cause the RCD 16 to trigger and disconnect the supply. In this way, any danger of fire is averted. Figure 2 illustrates the location of the sensor above the circuit breakers 18. The manner in which the heat sensor 22 is connected to the RCD 16 is not shown in Figures 1 and 2, as it can take many forms. One possibility is shown in Figure 3A. A mercury thermometer 30 is mounted on a pivot 32, so that when cold, with the mercury fully within the bulb 34 of the thermometer, the thermometer tends to pivot clockwise as seen in Figure 3A, with its top end resting against a stop 36. When it heats up the mercury moves up the thermometer and its centre of gravity also moves to the left. When a dangerous temperature rise is sensed, the mercury will have moved for enough to cause the thermometer to pivot anti-clockwise. This movement is conveyed to the RCD 16 by a light wire 38 coupling the thermometer and the RCD. The movement mechanically moves the trigger element in the RCD. In an alternative arrangement, shown in Figure 3B, the connection is an electrical connection. In this case a bimetallic strip 40 is used as the heat sensor. This is connected at one end to the RCD 16 by an electrical conductor 42. The other end of the bimetallic strip closes against a contact 44 when the temperature rises to a preset value. This contact 44 is connected to the RCD by a conductor 46. The connection 40,44 is such as to draw a small current, e.g. through a resistor, from the live conductor to earth (ground) . Current flowing through contact 44 thus causes an imbalance in the live and neutral currents through the RCD, thereby causing it to trigger and disconnect the supply. Figure 4 shows the electrical connections, which are omitted from Figure 1 in the interests of clarity. Referring to Figure 4, the mains supply, illustrated at 24, is applied to the switch 14 and then to the RCD 16. The live output of the RCD is applied to a live bus 26 which distributes it to the circuit breaker bank 18. The neutral output of the RCD 16 is applied to a bus or distribution point 27. Individual circuits are then connected to the output of one of the circuit breakers, and to the neutral bus or distribution point 27. When excess heat is detected, the heat sensor 22 located adjacent to the circuit breaker bank 18 causes the RCD to trigger, in the same way that it would if the current in the live conductor and the current in the neutral conductor were not equal due to a circuit fault. Many variants of the system described are possible. The heat sensor could be located within the bank of circuit breakers 18. A separate heat sensor could be located in or adjacent to each of the individual miniature circuit breakers, each causing the RCD 16 to trigger. In any event, the sensor is set to trigger at a temperature above normal summer ambient temperatures, but below that at which combustion can occur. In this way any build up of heat preliminary to an actual fire occurring will be detected and trigger the unit before a dangerous situation arises. While the precise temperature at which the thermal sensor would need to operate will be a matter of empirical determination, it is believed that a temperature in the range 50°C to 80°C is likely to be appropriate. The principles of the embodiment described can be incorporated in other electrical power distribution units, such as a conventional fuse box, rather than a circuit breaker. The sort of heat that will cause a fuse to fail is usually less than that which would cause a significant danger of fire, and appropriate adjustment of the thermal sensor should ensure that it will not normally operate in the event of a fuse blowing. The principles disclosed can be used in a larger installation, such as with a three-phase supply.

Claims: CLAIMS 1. An electrical power distribution unit for use in a building, comprising: a casing (12) providing a substantially enclosed housing; a residual current device (16) mounted within the casing and operative as an automatically-operable electrical current cut-out device ,- a plurality of current overload sensors (18) within the casing and electrically connected in series with the residual current device for sensing current overload; and a thermal sensor (22) positioned within the casing to sense ambient heat in the casing generated in or adjacent to the overload sensors by overheating, the thermal sensor being connected to actuate the residual current device in the event that such heat is sensed. 2. An electrical power distribution unit according to claim 1, in which the thermal sensor (22) is mechanically coupled to the residual current device to actuate the latter. 3. An electrical power distribution unit according to claim 1, in which the thermal sensor (22) is electrically coupled to a control input connection of the residual current device to actuate the residual current device. 4. An electrical power distribution unit according to claim 1, in which the thermal sensor (22) is located at or near the top of the casing (12) , above the current overload sensors (18) . 5. An electrical power distribution unit according to claim 1, in which the thermal sensor (22) actuates the residual current device (16) when the ambient temperature reaches a predetermined temperature. 6. An electrical power distribution unit according to claim 1, in which the predetermined temperature lies in the range 50°C to 80°C. 7. An electrical power distribution unit for use in a building, comprising: casing means (12) providing a substantially enclosed housing; automatically-operable electrical current cut-out means (16) mounted within the casing means; a plurality of current overload sensor means (18) within the casing and electrically connected in series with the cut-out current means for sensing current overload; and thermal sensor means (22) positioned within the casing means for sensing heat generated in or adjacent to the overload sensor means which is caused by overheating, the thermal sensor means being coupled to actuate the cut-out in the event that such heat is sensed.

Inventors: GARRARD DAVID EARDLEY, GARRARD, DAVID, EARDLEY
Applicants: GARRARD DAVID EARDLEY, GARRARD, DAVID, EARDLEY
Application year: 1996
Main classification: H02H3
IPC codes: G08B17, H02H3