This invention relates to the general subject of overheating of electrical circuits, electrical connectors and electrical components.

It is well known that electrical fires are often caused at a point where there is poor electrical connectivity, which may be due to many factors, including incorrectly tightened screws, clamps or connection plugs, corrosion occurring at connection points and wires becoming loose due to mechanical fatigue over time or thermal trauma caused by successive expansion and contraction of metallic parts within an electrical distribution system. This is despite the presence of fuses which blow when there is excess current (but often overheat before doing so) and despite the presence of circuit breakers such as residual current detectors (RCD's) and residual current breakers with overload protection (RCBO's). Such electrical fires caused by overheated components are therefore a regular hazard even where all electrical connections have been properly made and all components meet the required technical specification.

The foregoing problems can be alleviated by the use of one-way thermal switches which exhibit a permanent closed circuit action when exposed to temperature exceeding a specified activation temperature, such as 80°C, thereby triggering an automatic power interruption to the main circuit being monitored. Such switches can be placed adjacent to known problem areas, such as next to the live terminal of a wall-mounted domestic plug socket, and in the event of overheating occurring the switch activates to thereby close the monitoring system and trigger an alarm which cuts off mains current and hence prevents further overheating at the source. However, it will be appreciated that where there are very many electrical connections around the circuit to be protected from thermal overheating a large number of thermal switches have to be used, and individual ones replaced upon the happening of an overheat event at the location where they are positioned. The installation of multiple thermal switches is therefore both time consuming and problematic where e.g. a large number of connections are present, each in close proximity to the other, such as in a domestic consumer unit, fuse box, distribution board etc. where multiple connections are made across a linear array of such connections, such as horizontally or vertically and where it is common to use busbars to which electrical connections are made by e.g. individual RCD's mounted in rows between oppositely disposed pairs of busbars.

According to a first aspect of the invention there is provided sensor apparatus for sensing overheating along a linear array of electrical connections, the apparatus including one or more linear carriages adapted to fit over the linear array of connections to be monitored, the or each carriage including a plurality of spaced-apart thermal switches sandwiched between a pair of metal busbars, the busbars being electrically connectable to a monitoring circuit which triggers an alarm when the circuit is closed by activation of one or more of the thermal switches.

With this arrangement multiple aligned connections can be monitored for overheating quickly and easily by, effectively, the use of a single component, being the elongate carriage containing multiple thermal switches at spaced intervals along its length.

The means by which the carriage can be fitted into place above a linear array of connections may conveniently take the form of clips comprising one or more projections, such as fins, extending below the carriage and being receivable between e.g. individual RCD's or micro-circuit breakers (MCB's) and the or each such clip may be moveably mounted or mountable to the base of the carriage to allow it to be fitted to a linear array of electrical connections of differing widths.

Conveniently, below each thermal sensor is an air vent which allows hot air resulting from an overheat event occurring immediately underneath the carriage below the thermal switch to trigger the switch and hence close the monitoring circuit to cause an alarm condition to be detected to thereby activate one or more circuit breakers forming part of the electrical system being monitored.

Preferably, the monitoring circuit for use with the apparatus is low voltage, such as 15 volts, with a maximum short term current of 300 mA such that the presence of the apparatus on e.g. a distribution board does not present an electric shock hazard, although it will be understood that higher or lower voltages may be used.

According to a second aspect of the invention there is provided heat sensing apparatus for detecting an overheat event at a releasable connection point forming part of an electric circuit, such as at a neutral terminal point on a distribution board, the apparatus comprising a hollow metal sensor plug containing an axially-wired thermal switch, the wires from each end being insulated and being connectable via a two-core cable to a monitoring circuit for monitoring closure of the thermal switch during an overheat event, and a terminal pin extending from the outside of the plug, the terminal pin being releasably connectable to an individual connector point of the circuit being monitored, such as a neutral terminal of a distribution board.

This arrangement allows for thermal switches to be installed anywhere around e.g. a distribution board where there is a connection point or terminal, where each thermal switch can therefore be activated in the event of overheating by heat transmission through the metal housing via the terminal pin.

According to a third aspect of the invention there is provided a combination sensor system in which sensor apparatus according to the first and second aspects of the invention are used together as part of the same monitoring circuit by which linear arrays of electrical connections can be monitored as well as individual electrical connections.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a front view of an electrical distribution board showing the apparatus of the invention in situ,

Figure 2 is a perspective view of a sensor carriage according to the first aspect of the invention,

Figure 3 is an enlarged and exposed partial view of the carriage of Figure 2,

Figure 4 is a view of a thermal switch and associated cable arranged according to the apparatus of the second aspect of the invention, and

Figure 5 shows a part sectional view of the thermal switch at Figure 4 mounted within a hollow sensor plug. Figure 1 is a front view of an exposed electrical distribution board or consumer unit shown generally at 1 comprising a linear array of neutral connectors 2 mounted above a linear array of MCB's and RCB's 3 at the end of which array is a mains switch 4. Copper busbars 5 provide common connections to various groups of the MCB's and RCBs 3 and exit cables 6 are then distributed around the building (not shown) within which the distribution board 1 is situated, all in a known manner. As will be apparent, a large number of electrical connections are present, each capable of being a source of overheating and although thermal switches may be utilised in an array around the distribution board to detect such overheating events it will be apparent that their installation would be time consuming and tedious, with the possibility of individual switches being improperly connected. To address this problem, the invention in its first aspect comprises sensor apparatus which includes, in this case, a pair of elongate carriages 7 (shown more clearly in Figures 2 and 3) which are clipped to respective ends of the MCB's and RCB's 3 at their respective electrical connection points and from one end each of which extends an insulated two-core cable 8. With this arrangement it will be noted that multiple MCB's and RCB's 3 and the mains switch 4 can be monitored by the presence of the carriages 7 at their respective electrical connection points, without the need to provide individual thermal switches 4 for each group of MCB's or RCB's 3 or at the mains switch 4.

Where an elongate carriage 7 is not required i.e. where there are only one or a few electrical connections to be monitored an alternative approach in accordance with the second aspect of the invention may be used in which individual thermal switches are mounted within hollow metal sensor plugs 9 (seen more clearly with reference to Figure 5), which include terminal pins 14 by which they may be connected to, in the case illustrated, three of the various neutral terminals 2, with each such sensor plug 9 also including a two-core exit cable 8 by which it can be connected to the rest of a monitoring circuit (not shown) for sensing when one or more thermal views has closed and hence current is flowing through the or each respective two-core cable 8. Again, this is a particularly convenient and simple way by which thermal switches may be positioned at convenient points around the distribution board 1 instead of or as well as the use of elongate sensor carriages 7.

Turning now to Figure 2 it will be seen that the elongate carriage 7 includes a vented top cover 10 which is removable to expose the inside of the carriage 7 as shown in Figure 3, being an enlarged view of one end of the sensor carriage 7. In the drawing a number of thermal switches 11 are shown positioned at spaced intervals therealong, with each end of each switch 11 being in electrical contact with a respective one of a pair of busbars 12 electrically connected at the end shown to an insulated two-core cable 8. Beneath each thermal switch 11 is a vent allowing heated air from an overheat event to flow past and, if of sufficiently high temperature to thereby trip the switch 11 to close the circuit between the busbars 12.

Depending from the carriage 7 is a removable clip in the form of a fin 13 which can be slotted between adjacent pairs of MCB's or RCB's 3 so as to releasably secure the carriage 7 to a respective linear array thereof in the manner as shown in Figure 1. Figures 4 and 5 respectively show how an axially wired thermal switch 11 can be mounted within a hollow metal sensor plug 9 (shown in section) whose terminal pin 14 can thereafter be inserted within and retained by an electrical connector in the manner as shown in Figure 1. The axial wires from the switch 1 1 are crimped to respective ends of the insulated twin-core cable 8 by the use of lugs 15 and the assembly then inserted within the hollow part of the sensor plug 9 in the manner as shown in Figure 5, thereafter to be sealed in place by a suitable bonding agent such as silicone sealant. The other end of the cable 8 can then be connected to a monitoring circuit (not shown) which remains open until activation of the thermal switch 11 by a heat event at the site where the terminal pin 14 is connected via conduction through the metal. As will be apparent, the plug 9 and twin-core cable 8 may be provided as a single unit in a "pigtail" arrangement which is particularly easy to connect and disconnect to both the electrical connector being monitored and the monitoring circuit. The invention in both these aspects therefore provides an elegantly simple solution to the problem of providing multiple thermal switches across or to multiple connection points including linear arrays of the type described.