Greenhouse lighting fixture with safety device

The invention relates to a greenhouse lighting fixture according to the preamble of claim 1.

A greenhouse lighting fixture according to the preamble of claim 1 is known from WO2006/046858. This fixture is particularly suitable for lamps such as the Philips Green Power TD 1 ,000 W EL 400 V. This is a high-pressure sodium lamp which is specifically adapted to use in greenhouses. Lamps of this type have an elongate glass body having a first end and a second end opposing the first end. The ends are both provided with a "pinch" in the glass, that is to say, a substantially flat part which is produced by pinching during production the still warm glass of the glass body. An electrode is located close to each end, in the glass body. Each electrode is connected to an electrical conductor element which extends to outside the glass body. In the case of the aforementioned type of lamp, the electrical conductor element comprises a metal pin which is made for example of molybdenum or a molybdenum alloy and to which - outside the glass body - a Litz wire is fastened. In the case of some variants of this type of lamp, the conductor element also comprises a conductor plate which is generally arranged in the pinch of the glass body. During the mounting of a lamp in the fixture, the Litz wire is brought into contact with an electrical contact element of the fixture. By connecting the contact elements of the fixture to a power supply, the lamp can subsequently be switched on. Because the conductor elements of the lamp protrude from the glass body at both ends, and the lamp thus has to be electrically connected at both ends, a lamp of this type is often referred to as a double-ended lamp. Lamps having a connector of this type are also used in stadium lighting.

A drawback of the known lamps having an electrical connector of this type is that they are relatively sensitive, compared to lamps having for example a screw fitting, to manufacturing faults in the electrical conductor element and to the occurrance of an inadequate electrical contact during the placing of the lamp in the fitting. Examples of manufacturing faults which have been noted include a defect in a connection between the metal pin and the Litz wire and a defect in the connection between the conductor plate in the pinch and the connecting part of the metal pin of the conductor element. An example of an assembly fault which has been noted is the fact that the Litz wire does not rest fully against the contact element of the fitting. The consequence of faults of this type can be the generation of an undesired electrical arc somewhere in the region between the contact element of the fitting and the electrode on the side in question of the lamp. The location of the arc depends on the location of the defect or the assembly fault.

The production or the presence of an arc of this type is generally not detected by the electronics which control the lamp. This is due to the fact that the voltages and amperages occurring in the arc do not significantly differ from that which is conventional during normal operation in greenhouse lighting. As a result, an undesired arc of the type described hereinbefore has been found in practice to last often for several minutes. This can lead to damage to the lamp, the fixture, the electronics which control the lamp or to objects in the direct environment of the fixture.

The object of the invention is to provide an improved greenhouse lighting fixture.

The invention achieves this object by a greenhouse lighting fixture according to claim 1. The safety device which is provided ensures that if an undesired arc is generated or present in the region between the contact element of one of the fittings and the electrode of a lamp arranged in the fixture on the side of the contact element in question, this arc or the generation thereof is disturbed so that the risk of the occurrence of an undesirable situation, and thus the risk of consequential damage, is at least limited. The safety device can be activated as a result of the generation of the arc (for example by reacting to a spark) and/or as a result of the presence of an arc.

Reliable functioning of the safety device is important. In this case, not only is it necessary for the safety device indeed to be activated if an arc occurs, but it is also very desirable for the safety device not to issue a "false alarm", i.e. not to be activated if there is no arc or if the arc is being generated. In order to achieve this, the safety device is configured to be activated by the arc itself, and not by for example a derived electrical parameter, such as the amperage of the electrical current flowing through the fixture.

In an advantageous embodiment, the safety device is activated as a result of the rise in temperature which is generated by the arc. The temperature of the arc plasma is very high, as a result of which the temperature in the fitting and the direct environment thereof increases to values which are much higher than in normal operation. The use of a safety device which is activated as a result of the rise in temperature which the arc brings about therefore also provides a reliable safety device which does not become active unnecessarily.

In an advantageous embodiment, the safety device is arranged in one of the fittings, but more preferably the safety device is present in both fittings. Placement in the fitting ensures that the safety device is present in the direct environment of any arc. This ensures that the safety device is activated rapidly after the generation of arc should that occur.

The safety device according to the invention can also be used in fixtures for lamps

comprising comparable conductor elements which are used for other applications, for example for fixtures for stadium lamps.

In a first advantageous embodiment, the safety device is based on a chemico-physical concept. In this embodiment, the greenhouse lighting fixture according to the invention comprises a safety device having a gas-generating element, which gas-generating element is configured to be activated by an arc. The gas-generating element is configured to after activation emit a gas, which gas disturbs the arc. Preferably, a rise in temperature in the greenhouse lighting fixture as a consequence of the presence or possibly the generation of an arc leads to the emitting of gas by the gas-generating element. The gas-generating element disturbs the arc, for example by smothering said arc.

In practice, it has been found that nitrogen gas is effective for disturbing an arc, for example as a result of the smothering of the arc. Materials are known which, on heating, emit sufficient nitrogen to smother an arc. Materials of this type have a high nitrogen content. The nitrogen is for example released if the heating causes decomposition of the material. In addition, the process of decomposition and any other occurring processes, such as evaporation or sublimation, extract energy from the arc, and this provides a supplementary arc-disturbing effect.

A first example of a material which emits, under the influence of the heat which is generated by an arc, sufficient nitrogen to be able to smother an arc is dicyandiamide (DCD). Dicyandiamide is also known by the names 2-cyanoguanidine, cyanoguanidine, dicyanodiamide, N-cyanoguanidine, 1 -cyanoguanidine, guanidine-1-carbonitrile, dicyandiamine, didine, DCD and dicy. The IUPAC International Chemical Identifier is 1/C2H4N4/c3-1-6-2(4)5/h(H4,4,5,6); the molecular formula is C2H4N4.

A second example of a material which is suitable for this objective is melamine. Melamine is also known by the names 1 ,3,5-triazine-2,4,6-triamine, cyanurotriamide, cyanurotriamine and cyanuramide. The molecular formula is C3H6N6.

The gas-generating element can wholly or partly be made from the gas-generating material. It is also possible for the gas-generating material to be accommodated in a matrix, for example epoxy resin or nylon.

It has been found to be advantageous to shape or else to arrange the gas-generating element in such a way that it at least substantially encloses - if a lamp is attached in the fixture - the electrical conductor element of the lamp. If an arc is then produced close to the electrical conductor element, then this enclosure prevents the generated gas from being

blown away freely. As a result of the fact that the gas is kept in direct proximity to the electrical conductor element, it can disturb the arc more effectively. Preferably, the environment of the electrical conductor element is shaped in such a way that alternative paths which the arc might select, for example to be able to escape the smothering effect of the gas, are physically blocked. The shaping and the placement of the gas-generating element can contribute to this.

In a possible embodiment which has been found to be effective, the gas-generating element comprises a body made of epoxy resin containing therein 30 - 60 per cent by weight of dicyandiamide, preferably 50 to 55 per cent by weight of dicyandiamide. It is also possible to use dicyandiamide in a matrix of nylon. The combination of melamine and nylon is attractive from the point of view of production technology, but a combination of melamine and epoxy resin is also possible.

It is possible for a fitting or a part thereof to be provided with a layer of gas-generating material from which, on heating, gas is released for disturbing an arc which has been produced. A layer of this type can be made for example of epoxy comprising dicyandiamide or melamine.

The invention also provides an embodiment wherein the safety device operates on the basis of a thermal/mechanical effect. In this embodiment, the safety device comprises a pre- tensioned spring element having a pre-tensioned position and a relaxed position. The spring element is configured to disturb, during a transition from the pre-tensioned position to the relaxed position, an arc in the region between a contact element of a fitting and the electrode on the side of the fitting in question of a lamp arranged in the greenhouse lighting fixture. The safety device also comprises a locking element for the pre-tensioned spring element. The locking element is, at normal operating temperatures of the greenhouse lighting fixture, sufficiently rigid and/or strong to hold the pre-tensioned spring element in the pre-tensioned position but is configured to allow, under the influence of an increase in the temperature as a consequence of an arc in the greenhouse lighting fixture, the spring element to move to its relaxed position.

It is for example possible to make the locking element of plastics material which softens or even melts as a result of the rise in temperature in the fitting as a consequence of an arc. As an alternative to the plastics material or in addition thereto, a low-melting metal can for example be used. It is also possible to use a for example liquid-filled capsule. The material in the capsule expands under the influence of the rise in temperature as a consequence of the arc, as a result of which the capsule breaks at a given moment. The breaking of the capsule can then allow the spring element to relax.

The movement of the spring element from the pre-tensioned position to the relaxed position can be used in various ways for disturbing the arc. In the first place, the spring element can in its relaxed position cause a short circuit. The current which then starts to flow through the fixture is then so high that said current is detected by the lamp electronics which switch off the lamp. It is also possible for the short-circuit current to damage the lamp electronics and/or the ballast in such a way as to cause failure thereof, as a result of which the power supply is subsequently interrupted. This leads to damage to the lamp electronics and/or the ballast, but limits further consequential damage as a consequence of an arc. The spring element can cause short-circuiting as a result of the fact that it brings the electrical conductor element of the lamp into contact with an earthed part of the fixture (or brings it so close thereto that the short-circuit current starts to flow between the electrical conductor element and the earthed part) or by even making a connection between the electrical conductor element and an earthed part of the fixture. It can also make a connection between the electrical contact element of the fitting and earth.

In the second place, the spring element can, as a result of its movement from the pre- tensioned position to the relaxed position, cause rapid displacement of the electrical conductor element of the lamp with respect to the contact member of the fitting. If this movement is sufficiently rapid, then the arc plasma cannot follow the displacement and the arc is broken. This effect occurs most reliably if the displacement causes the distance between the electrical conductor member and the contact element of the fitting to increase.

In a further variant, the safety device is further provided with a shield. In the case of this variant, the spring element brings the shield into the path of the arc during the transition from the pre-tensioned position to the relaxed position. The shield, which is made preferably of an electrically insulating material, disturbs the arc in this way. Suitable materials for the shield or components thereof include for example PTFE, ceramic (for example beryllium oxide or aluminium nitride), poly(4,4'-oxydiphenylene pyromellitimide) (sold under the name Kapton) or biaxially oriented polyethylene terephthalate (sold under the name Mylar). These materials can also be arranged in a matrix of for example epoxy resin, nylon or another material. In a variant, a gas-generating material can be arranged - additionally or otherwise - in or close to the shield. .

The invention also provides an embodiment wherein the safety device operates on the basis of an explosion. In this embodiment, the safety device comprises a cartridge having an explosive charge. The cartridge is configured to go off under the influence of a rise in temperature caused by an arc in such a way that the explosion disturbs the arc.

In an advantageous embodiment, the cartridge is attached to or in direct proximity to the fitting. As a result, the cartridge is located in direct proximity to the location where an arc can possibly be produced. As a result of such placement of the cartridge, the cartridge will then also react rapidly to a rise in temperature caused by an arc. If the cartridge is activated soon after the production of the arc, the arc will be short-lived. This significantly reduces the risks which an arc entails.

In an advantageous embodiment, the cartridge comprises a mixture of potassium chlorate and antimony sulphide, also referred to as a "primer" or "gun powder". Cartridges of this type are for example used in alarm guns. They are readily available and in principle present, as a result of their limited charge, few risks.

In a further, alternative embodiment, the safety device comprises temperature sensors. These temperature sensors are configured to detect an increase in temperature as a consequence of an arc. The temperature sensors are connected to a controller which switches off a lamp attached in the greenhouse lighting fixture if an increase in temperature as a consequence of an arc is detected.

In a further, alternative embodiment, the safety device comprises a switching element made of bimetal or of memory metal. The shape of a switching element of this type is dependent on its temperature. The switching element has a first and a second shape. The first shape is assumed at normal operating temperatures of the fixture, the second shape in the case of an increased temperature. This increased temperature at which the switching element assumes its second shape is preferably selected so as to be so high that the increased temperature is attained only as a consequence of an arc in the fixture.

The switching element is shaped in such a way that at normal operating temperatures no influencing of the normal path of the electrical current through the fixture and the lamp occurs. However, if the temperature in the fixture rises as a consequence of the generation of an arc, then the switching element assumes a different shape which causes the electrical conductor element of the lamp or an electrical contact element of the fitting to be brought into contact with an earthed part of the fixture, so that a short circuit is produced. The higher amperage of the current which is the consequence thereof is detected by the lamp electronics, as a result of which the lamp is switched off. It may also be the case that the short-circuit current destroys the lamp electronics and/or the ballast. This is in itself less desirable, but can be acceptable because this also results in elimination of the arc, therewith limiting any consequential damage.

A variant of this embodiment provides a shield which is brought into the path of the arc by an

actuator made of bimetal or memory metal and thus disturbs the arc. The rise in temperature accompanying an arc causes in this variant the actuator to change its shape and as a result to displace the shield.

In a further, alternative embodiment, the safety device comprises a melting element made of a low-melting conductive material, for example solder or a comparable alloy, which, in the event of an increase in the temperature as a consequence of the generation of an arc, melts and generates a short circuit, as a result of which the arc is disturbed. As an alternative, use may be made of a capsule comprising an electrically conductive liquid. As a result of the rise in temperature as a consequence of the arc, the liquid in the capsule expands, causing the capsule to break at a given moment. The conductive liquid is then released and can generate a short circuit. The conductive liquid and/or the molten material can generate a short circuit by generating an electrical contact between the electrical conductor element of the lamp or an electrical contact element of the fitting and earth, for example in the form of an earthed component of the fixture.

The invention will be described hereinafter in greater detail with reference to the drawings showing in a non-limiting manner a number of embodiments.

In the drawings:

Fig. 1 shows a greenhouse lighting fixture;

Fig. 2 shows an assimilation lamp in the form of a high-pressure gas discharge lamp, of the type which is suitable to be received in a greenhouse lighting fixture according to the invention;

Fig. 3 is a cross section of a mounting of one of the ends of a lamp according to Fig. 2 in a fixture;

Fig. 4 is a perspective view of a first end of a lamp which rests on the known bottom fitting part of Fig. 3; Fig. 5 shows a bottom fitting part 41 which is provided with a first embodiment of a safety device according to the invention;

Fig. 6 is a cross section of the assembly of the bottom fitting part and the lamp taken along the line A - A from Fig. 5;

Fig. 7 shows a variant on the embodiment of Fig. 5 and Fig. 6; Fig. 8 shows a first variant of a greenhouse lighting fixture with a safety device based on a thermal/mechanical principle;

Fig. 9 shows a second variant of a greenhouse lighting fixture with a safety device based on a thermal/mechanical principle;

Fig. 10 shows a third variant of a greenhouse lighting fixture with a safety device based on a

thermal/mechanical principle;

Fig. 11 shows a fourth variant of a greenhouse lighting fixture with a safety device based on a thermal/mechanical principle;

Fig. 13 shows a following embodiment of a fitting for a greenhouse lighting fixture according to the invention;

Fig. 14 shows a fourth embodiment of a fitting for a greenhouse lighting fixture according to the invention; and

Fig. 15 shows a further embodiment of a fitting for a greenhouse lighting fixture according to the invention.

Fig. 1 shows a greenhouse lighting fixture 1 of a known type. Fixtures 1 of this type generally comprise a housing 2 for receiving lamp electronics or a ballast. The fixture 1 further comprises a connector 3 for coupling the fixture to an electric power supply and a suspension 4 for attaching the fixture 1 to a carrying construction 5. Furthermore, a reflector 6 is usually attached to greenhouse lighting fixtures of this type for the reflecting, in the direction of the crop to be illuminated, of light which is produced by a lamp attached in the fixture 1.

Fig. 2 shows an assimilation lamp 10 in the form of a high-pressure gas discharge lamp of the type which is suitable to be received in a greenhouse lighting fixture according to the invention. A lamp of this type is typically configured to be operated at a voltage of approx.

400 V. The capacity is typically 1 ,000 W. This lamp 10 comprises an elongate glass body 11 having a first end 12 and an opposing second end 13. At both ends of the lamp 10, a "pinch"

16, 17 is formed in the glass. This is a substantially flat part - in this example it has an H- shaped cross section - which is produced by pinching during production the still warm glass of the glass body. A lamp having the shape as shown in Fig. 2 is also referred to as a double-ended lamp.

A first electrode 14 and second electrode 15 are located in the glass body 11 of the lamp. When the lamp is switched on, a light-generating arc is present between the first electrode 14 and the second electrode 15.

In order to be able to connect the electrodes 14, 15 to an electric power supply, a first electrical conductor element 18 and a second electrical conductor element 19 are respectively provided. Each electrical conductor element comprises in this example a first pin 20, 21, a conductor plate 22, 23, a second pin 24, 25 and a Litz wire 26, 27. In some cases, a sleeve 28, 29 is present to strengthen the connection between the second pin 24 or 25 and Litz wire 26 or 27. The conductor plates 22, 23 serve to be able to accommodate differences in thermal expansion between the glass and the metal parts of the lamp.

In the case of lamps as shown in Fig. 2, manufacturing faults can occur in the connections between the various components of the electrical conductor elements. Faults of this type can cause the generation of arcs.

Variants of double-ended lamps as shown in Fig. 2 are also used for other applications, for example in stadium lighting. The safety device according to the invention can be used also in other applications of double-ended lamps.

Fig. 3 is a cross section of a known mounting of one of the ends of a lamp 10 according to Fig. 2 in a fixture. Both ends of the lamp are in this way mounted in the fixture.

The lamp fitting 40 has a bottom fitting part 41 and a top fitting part 42. The pinch 16 of the lamp 10 rests on the bottom fitting part 41. A spring 46, which is connected to the top fitting part 42, presses the pinch 16 against the bottom fitting part 41.

The Litz wire 26, which protrudes from the end in question of the lamp 10, rests against the bottom fitting part 41. The top fitting part 42 is provided with a contact element 43 which is connected to an electric power supply. The contact element 43 provides the electrical connection between the lamp and the power supply. A spring 44 ensures that the contact element 43 is pressed against the Litz wire.

Fig. 4 is a perspective view of a first end 12 of a lamp 10 which rests on the known bottom fitting part 41 of Fig. 3. This view also clearly shows the H-shaped cross section of the pinch 16. The bottom fitting part 41 is provided with two springs 48 which secure the lamp.10. A support 47 is also provided for supporting the Litz wire 26 when said Litz wire is pressed against the contact element 43 of the top fitting part 42 (not shown in Fig. 4). The situation in the second end 13 of the lamp is similar to the situation shown in Fig. 4, albeit mirror- reflected of course.

Fig. 5 shows a bottom fitting part 41 which is provided with a first embodiment of a safety device according to the invention. In this embodiment, the safety device comprises a gas- generating element 70. In this example, the gas-generating element is embodied in the form of a body 71 which is made of a matrix, for example epoxy resin or nylon, comprising for example melamine or dicyandiamide, for example in 30 - 60 per cent by weight. A slot 72, through which the electrical conductor element 18 of the lamp can be fed, is present in the body 71.

Dicyandiamide and melamine have the property of producing a relatively large amount of

nitrogen gas during decomposition. If an arc is produced in proximity to the body 71 , the temperature in the vicinity of the arc will rise markedly. When the decomposition temperature of melamine or dicyandiamide is reached, the melamine or dicyandiamide starts to decompose and nitrogen gas is released. This nitrogen gas smothers the arc and/or dispels the arc plasma.

Fig. 6 is a cross section of the assembly of the bottom fitting part and the lamp taken along the line A -A from Fig. 5.

Fig. 6 clearly shows that the gas-generating element 70 encloses most of the electrical conductor element. This is advantageous because the nitrogen gas which is produced by the gas-generating element 70 is then kept for the most part in direct proximity to the arc. As a result, the arc is smothered more effectively and more rapidly.

Fig. 7 shows a variant of the embodiment of Fig. 5 and Fig. 6. In the case of this variant, an additional gas-generating body 71* is provided in addition to the gas-generating body 71 shown in Fig. 5 and Fig. 9. This additional gas-generating body 71* is attached to the top fitting part 42. Even better enclosure of the electrical conductor element 18 is attained in this way.

In further variants (not shown), a plurality of gas-generating elements can also be arranged in the fitting or elsewhere in the fixture.

The variants of the invention with one or more gas-generating elements have the additional advantage that physical paths which an arc might follow are blocked as a result of the placement of the one or more gas-generating elements. In this way too, the gas-generating elements disturb arcs which may have been produced.

In a further variant (not shown), the fitting can at least partly be provided with, instead of or in addition to a gas-generating element as shown in Fig. 5 - 7, a layer of dicyandiamide or melamine in a matrix, for example epoxy resin or nylon. A layer of this type then performs the same function as for example the gas-generating body 71 from Fig. 5 and Fig. 6.

Various variants of a second embodiment of a greenhouse lighting fixture according to the invention are shown in Fig. 8 - 11. These variants are all based on a thermal/mechanical principle.

Fig. 8 shows a first variant of a greenhouse lighting fixture with a safety device based on a thermal/mechanical principle.

In the variant of Fig. 8, the safety device comprises a pre-tensioned spring element 80 and a locking element 81. The spring element 80 has a pre-tensioned position and a relaxed position.

Fig. 8A is a cross section of a fitting having an end 12 of a lamp 10 arranged therein. Pinch 16 is pressed against the bottom fitting part 41 by a spring 46 which rests against the top fitting part 42. Litz wire 26 rests against contact element 43 to provide the electrical connector of the lamp 10. Spring 44 presses the contact element 43 against the Litz wire 26. In the variant of Fig. 8, the top fitting part 72 is provided with an earthed bolt 45.

In the example of Fig. 8, the electrical conductor element 18 of the lamp 10 has a weak spot 101. This can for example be the consequence of a weak connection between the Litz wire 26 and the second pin 24 as a consequence of a manufacturing defect. In this example, an arc 100 is produced as a consequence of this weak spot, for example as a result of the fact that the connection becomes so warm as a result of its relatively high electrical resistance that material melts away and an air gap comes into being between the second pin 24 and the Litz wire 26. In this air gap sparking-over can occur.

The locking element 81 during normal operating conditions holds the spring element 80 in its pre-tensioned position (Fig. 8A shows the spring element 80 in the pre-tensioned position). In this case, the locking element 81 is for example a plastics material band which is arranged around the spring element 80.

The arc 100 emits a considerable amount of heat, causing the temperature in the fitting to soar. Just a short time after the start of the arc, the temperatures in the fitting are significantly higher than the temperature during normal operation. This increased temperature causes the locking element 81 to melt and/or soften, as shown in Fig. 8B.

As a result of the melting and/or softening of the locking element 81 , said locking element loses so much rigidity and/or strength (for example as a result of the decreasing of the modulus of elasticity or as a result of the reduction in size of the effective cross section as a consequence of the melting-away of material) that said locking element is no longer able to hold the spring element 80 in the pre-tensioned position (as shown in Fig. 8A). As soon as this is the case, the spring element will move from the pre-tensioned to the relaxed position.

Fig. 8C shows the situation with the spring element 80 in the relaxed position which it can assume now that it is no longer held by the locking element 81 in the pre-tensioned position. The spring element 80 presses the part of the electrical conductor element 18 that is still

secured to the lamp 10 upward, against the earthed bolt 45. This produces a short circuit. It can occur that the short-circuit current starts to flow as soon as the electrical conductor element 18 enters the region of the earthed bolt 45. It is not strictly necessary, for the purposes of operation, for the electrical conductor element 18 to come to lie precisely against the earthed bolt 45.

The short-circuit current thus created destroys the ballast or the lamp electronics, as a result of which the electric circuit is interrupted. As a result, the arc disappears. It may also be the case that the high current strength of the short-circuit current is detected by the lamp electronics, as a result of which said electronics switch off the lamp, preferably before any damage occurs.

If the spring element 80 passes sufficiently quickly from the pre-tensioned position to the relaxed position, the arc can also already be disturbed as a result of the rapid displacement of the part of the electrical conductor element 18 that is still secured to the lamp 10 with respect to the other part of the electrical conductor element 18 (the part that is still clamped between the bottom fitting part 41 and the contact element 43).

Fig. 9 shows a second variant of a greenhouse lighting fixture with a safety device based on a thermal/mechanical principle.

The variant of Fig. 9 is for the most part similar to that of Fig. 8. In this case too, there is a pre-tensioned spring element 80 which is held by a locking element 81 in a pre-tensioned position (as shown in Fig. 9A).

After the production of arc 100, the locking element 81 melts and/or softens (see Fig. 9B). As a result, it will ultimately have to allow the spring element 80 to pass to its relaxed position, as is shown in Fig. 9C.

In the variant of Fig. 9, the spring element 80 raises the lamp end 12, counter to the action of spring 46. As a result of this raising of the lamp end, the part of the electrical conductor element 18 that is still secured to the lamp 10 is pressed against the earthed bolt 45, so that a short circuit is produced. In a variant (not shown), the lamp 10 is secured by springs 48 as shown in Fig. 4 instead of by a spring 46 as shown in Fig. 9. Springs 48 of this type can be shaped in such a way as to allow the lamp end 12 to be raised more easily.

It can occur that the short-circuit current starts to flow as soon as the electrical conductor element 18 enters the region of the earthed bolt 45. It is not strictly necessary, for the purposes of operation, for the electrical conductor element 18 to come to lie precisely against

the earthed bolt 45.

As described in relation to Fig. 8, the short circuit leads to the arc being disturbed by means of an (eventual) interruption of the current.

In the case of the variant of Fig. 9 too, the arc can also already be disturbed as a result of the rapid displacement of the part of the electrical conductor element 18 that is still secured to the lamp 10 with respect to the other part of the electrical conductor element 18 (the part that is still clamped between the bottom fitting part 41 and the contact element 43).

Fig. 10 shows a third variant of a greenhouse lighting fixture with a safety device based on a thermal/mechanical principle.

In the variant of Fig. 10 too, the safety device is provided with a pre-tensioned spring element 90. Again, this spring element 90 also has a pre-tensioned position and a relaxed position. Locking element 91 holds the spring element 90 in the pre-tensioned position, as is shown in Fig. 10A. In the example of Fig. 10, the spring element 90 is connected to earth. In the example of Fig. 10, the spring element 90 has a first leg 92 and a second leg 93. In the pre- tensioned position of the spring element 90 the legs 92, 93 lie more or less parallel to each other, whereas in the relaxed position of the spring element 90 they form a clear angle (which is not equal to 0°) to each other.

If an arc 100 occurs for example as a consequence of a weak spot 101 in the electrical conductor element 18 (see Fig. 10A), the temperature in the fitting rises to values lying well above the normal operating temperature.

As a result of the increased temperatures, the locking element 91 softens and/or melts. Fig. 10B shows the situation in which a part of the locking element 91 has melted away. The locking element 91 has increasing difficulty in holding the spring element 90 in its pre- tensioned position.

Fig. 10C shows the situation in which the locking element 91 has no longer being able to hold the spring element 90 in the pre-tensioned position, for example as a result of the fact that it has melted or as a result of the fact that, under the influence of the increased temperature, the material of the locking element 91 has softened in such a way that the locking element 91 can no longer produce the force to hold the spring element 90 in its pre- tensioned position. In the situation of Fig. 10C, the spring element 90 has assumed its relaxed position. In this case, the second leg 93 of the spring element 90 comes to rest against the part of the electrical conductor element 18 that is still connected to the lamp. As a

result of the fact that the spring element 90 is connected to earth, this causes a short circuit.

The short-circuit current thus created destroys the ballast or the lamp electronics, as a result of which the electric circuit is interrupted. As a result, the arc also disappears. It may also be the case that the high current strength of the short-circuit current is detected by the lamp electronics, as a result of which said electronics switch off the lamp, preferably before any damage occurs.

In a variant (not shown), the spring element 90 makes contact, in its relaxed state, with the electrical contact element 43 of the fitting instead of with the electrical conductor element 18 of the lamp. In this variant, the spring element is earthed.

In still another variant (not shown), the spring element 90 does not make contact, in its pre- tensioned position, with earth, with the electrical conductor element 18 and/or with the electrical contact element 43. In this variant, the spring element 90 makes contact, in its relaxed position, with earth and with the electrical conductor element 18 of the lamp or with earth and the electrical contact element 43 of the fitting.

In a further variant (not shown), the safety device comprises a catch or pin which is fastened to a spring element. The catch or pin is pressed by the spring element against a wall made of material having a relatively low melting or softening temperature when the spring element is in the pre-tensioned position. The rise in temperature caused by an arc causes the wall to lose its rigidity and/or strength, as a result of which the spring presses the catch or pin through the wall (or the wall is pressed wholly or partly away). The catch or pin subsequently causes short-circuiting or that the arc is disturbed in a different manner, for example as a result of the blocking of the path which the arc follows.

Fig. 11 shows a fourth variant of a greenhouse lighting fixture with a safety device based on a thermal/mechanical principle.

In the variant of Fig. 11, the safety device comprises a switching element 110 made of bimetal or of memory metal, which switching element 110 can assume a first shape and a second shape. The switching element 110 assumes its first shape at the normal operating temperatures prevailing in the fitting of the greenhouse lighting fixture.

Fig. 11A shows the switching element 110 in its first shape. In this first shape, it preferably does not rest against the electrical conductor element 18 of the lamp.

If an arc 100 is produced for example as a consequence of a weak spot 101 in the electrical

conductor element 18 (see Fig. 11A), the temperature in the fitting rises to values lying well above the normal operating temperature. At values which lie well above the normal operating temperature, and which are preferably so high that they can be caused only by an arc, the switching element 110 passes from its first shape to its second shape. This is shown in Fig. 11B.

Fig. 11C shows the switching element 110 in its second shape. Now, the switching element 110 presses the part of the electrical conductor element 18 that is still connected to the lamp against the earthed bolt 45 of the top fitting part 42. As a result of the fact that the bolt 45 is connected to earth, this causes a short circuit. It can occur that the short-circuit current starts to flow as soon as the electrical conductor element 18 enters the region of the earthed bolt 45. It is not strictly necessary, for the purposes of operation, for the electrical conductor element 18 to come to lie precisely against the earthed bolt 45.

The short-circuit current thus created destroys the ballast or the lamp electronics, as a result of which the electric circuit is interrupted. As a result, the arc also disappears. It may also be the case that the high current strength of the short-circuit current is detected by the lamp electronics, as a result of which said electronics switch off the lamp, preferably before any damage occurs.

In a variant (not shown), the switching element 110 makes contact, in its second shape, with the electrical contact element 43 of the fitting rather than with the electrical conductor element 18 of the lamp. In this variant, the switching element 110 is earthed.

In still another variant (not shown), the switching element 110 does not make contact, in its first shape, with earth, with the electrical conductor element 18 and/or with the electrical contact element 43. In this variant, the spring element 90 makes contact, in its second shape, with earth and with the electrical conductor element 18 of the lamp or with earth and the electrical contact element 43 of the fitting.

Fig. 12 shows a variant of the embodiment of Fig. 11. In the variant of Fig. 12, blocking element 115 is present instead of switching element 110. This blocking element comprises an actuator 116 made of bimetal or memory metal and a shield 117. Under normal operating conditions, the actuator 116 of the blocking element 115 has the first shape, as is shown in Fig. 12A.

As a result of a rise in temperature in the fitting as a consequence of an arc 100, the actuator 116 passes over to its second shape, as shown in Fig. 12B. As a result, the shield 117 is brought into the path of the arc 100 (see Fig. 12B). The shield is made of electrically

insulating material, as a result of which the arc is interrupted and/or blocked. This is shown in Fig. 12C.

Once the fitting has cooled down again, the actuator 16 returns again to its first shape, as is shown in Fig. 12A.

It will be clear to a person skilled in the art that the shield 107 can also be used in combination with a pre-tensioned spring element such as has been described in relation to the embodiments of Fig. 8, 9 and 10. In that case, the spring element brings the shield 117 into the path of the arc when the spring element passes from its pre-tensioned position to its relaxed position.

In a further variant (not shown), the actuator or the spring element is not permanently connected to the shield. When the actuator has its first shape or the spring element assumes its pre-tensioned position, the shield is detached from the spring element or the actuator. When the actuator passes over to its second shape or the spring element to its relaxed position, the actuator or the spring element displaces the shield - for example by pressing against it - in such a way that the shield enters the path of the arc and interrupts and/or blocks the arc.

In the case of a further variant (not shown), the safety device comprises a catch or pin which is fastened to an actuator made of bimetal or memory metal. The catch or pin is pressed by the actuator against a wall made of metal having a relatively low melting or softening temperature or is located after a wall of this type when the actuator has its first shape. The rise in temperature caused by an arc ensures that the wall loses its rigidity and/or strength, and the actuator assumes its second shape, as a result of which the spring presses the catch or pin through the wall (or the wall is pressed wholly or partly away). The catch or pin subsequently causes short-circuiting or that the arc is disturbed in a different manner, for example as a result of the attachment of a shield in the path of the arc.

Fig. 13 shows a following embodiment of a fitting for a greenhouse lighting fixture according to the invention. In this embodiment, the safety device makes use of an operating principle on the basis of an explosion.

In the embodiment of Fig. 13, a cartridge 120 is attached in the top fitting part 42. This cartridge is of the type which goes off if an activation temperature is exceeded. It will be clear to a person skilled in the art that the cartridge can also be accommodated elsewhere in the fitting, for example in the bottom fitting part 41. The cartridge contains an explosive charge, for example a primer or gun powder (e.g. a mixture of potassium chlorate and antimony

sulphide).

If an arc 100 is produced for example as a consequence of a weak spot 101 in the electrical conductor element 18 (see Fig. 13A) ₁ the temperature in the fitting rises to values lying well above the normal operating temperature. At values which lie well above the normal operating temperature, and which are preferably so high that they can be caused only by an arc, the cartridge 120 is activated, so that an explosion ensues.

Fig. 13B shows the situation in which the temperature in the fitting has risen under the influence of the arc to the activation temperature of the cartridge 120. The charge of the cartridge generates an explosion, producing a pressure wave 121. This pressure wave blows the arc 100 apart.

Preferably, the pressure wave also causes that the distance between the points between which the arc ran is increased, as is shown in Fig. 13C. This hinders the start of a new arc. In the example of Fig. 13C, the part of the electrical conductor element 18 that is still connected to the lamp is bent, so that the distance to the part of the electrical conductor element 18 that is no longer connected to the lamp has increased.

Fig. 14 shows a further embodiment of a fitting for a greenhouse lighting fixture according to the invention.

In this embodiment, the safety device comprises a melting element 130 made of conductive material having a relatively low melting point, for example solder or Woods metal. This melting element 130 is attached to the top fitting part 42.

The bottom fitting part 41 comprises a recess 132 through which the electrical conductor element 18 of the lamp passes. An earth plate 131 is attached in this recess. The earth plate 131 is connected to earth.

If an arc 100 is produced for example as a consequence of a weak spot 101 in the electrical conductor element 18 (see Fig. 14A), the temperature in the fitting rises to values lying well above the normal operating temperature. At values lying well above the normal operating temperature, the melting element 130 starts to melt. In this case, drops 133 of the molten material fall into the recess 132 (see Fig. 14B).

The drops 133 fill the recess 132 with molten material 134 (see Fig. 14C). As a result of the fact that the electrical conductor element 18 passes through the recess 132, the electrical conductor element 18 will at a given moment - once enough molten material 134 has fallen

into the recess - produce electrical contact between the electrical conductor element 18 and the earth plate 131. As a result of the fact that the earth plate 131 is connected to earth, this produces a short circuit.

The short-circuit current thus created destroys the ballast or the lamp electronics, as a result of which the electric circuit is interrupted. As a result, the arc also disappears. It may also be the case that the high amperage of the short-circuit current is detected by the lamp electronics, as a result of which said electronics switch off the lamp, preferably before any damage occurs.

In a variant (not shown), molten material causes short-circuiting by making an electrical connection between the electrical contact element 43 of the fitting and earth.

Fig. 15 shows a further embodiment of a fitting for a greenhouse lighting fixture according to the invention.

In the embodiment of Fig. 15, temperature sensors 140 are attached in the fitting. The temperature in and/or near the fitting is monitored by means of these temperature sensors.

If an arc 100 is produced for example as a consequence of a weak spot 101 in the electrical conductor element 18 (see Fig. 13A), the temperature in the fitting rises to values lying well above the normal operating temperature. If the temperature exceeds a preset threshold value (which lies well above the normal operating temperature and is preferably so high that said increase in temperature can be caused only by an arc), then the controller of the greenhouse lighting system or the lamp electronics switch(es) the lamp off by switching off the electric power supply. As a result, the arc also disappears.

In the variant of Fig. 15, four temperature sensors 140 are attached in the fitting. It will be clear to a person skilled in the art that a different number is also possible.

A number of possible embodiments of the greenhouse lighting fixture with a safety device according to the invention have been described hereinbefore. It will be clear to a person skilled in the art that the various embodiments and individual aspects thereof can also be combined with one another. It will also be clear to a person skilled in the art that the described embodiments of the safety device can also be used in fixtures for other applications in which comparable lamps are used, either double-ended or not, such as for example in fixtures for stadium lighting.