The invention relates to a lighting armature with built-in protection circuit for use in environments where explosion hazard may exist. Armatures for use in such spaces have to meet special requirements.

Especially, a too high surface temperature or the occurrence of a spark may not lead to an explosion if there is an explosive gas mixture present in the environment. For most armatures, the assumption is that the armature cannot be made so gas-tight as to prevent penetration of an explosive gas mixture. For this reason, in practice, other measures are taken to prevent explosions. Widely used for general workspace lighting, for example, on drilling rigs or in chemical plants, are armatures with cold start fluorescent lamps, for example, 36 Watt T8 types with a diameter of 26 millimeters and a length of 120 centimeters. These lamps are placed in relatively heavy and large armatures, while an operating device for igniting the lamps and stabilizing the lamp current is used which is protected in a special manner from explosions that might occur as a result of internal faults or heat upon malfunctions in the operating device. Often, for example, a sand filling is used for the operating device in combination with a mechanically very strong case. In this type of armatures an emergency light function may be integrated. In that case, an accumulator battery is mounted, a charger circuit for the accumulator battery is included, and upon failure of the mains voltage at least one of the lamps, generally with reduced power, is provided with voltage, via the operating device, supplied by the accumulator battery.

The disadvantage of the existing armatures is that they are large and heavy, that frequent switching of the light source results in the lamps at start-up wearing relatively fast due to the so-called cold lamp start to which modern standard low pressure fluorescent lamps are very poorly resistant. As a consequence, in many situations, replacement of the lamps is necessary relatively often, which renders the costs of maintaining the light installation high. It appears that, in practice, in armatures for the above- mentioned application, where in addition to air and water vapor all kinds of aggressive gases or mists are present in the outside air, in certain situations these aggressive gases, mists or water, especially in tropical environments, penetrate into the armature. This may be noxious to the components, lead to unsafe situations due to undesirable creepage currents or to corrosion of the internal electrical connections. This may lead to failure and it may also jeopardize the safety of the armature.

Nowadays, light sources are available that do allow such frequent switching and yet have a similar output, designated in lighting technique as efficacy, to fluorescent lamps, such as Light Emitting Diodes, further referred to as LEDs, and induction lamps. At the same time, however, these light sources generally do not meet the requirements imposed on electrical equipment in environments with explosion hazard, for instance, regarding air and creepage paths between the electrical connections, and also the operating equipment necessary for feeding the light sources has to be protected in the manner mentioned in the foregoing. The life of these light sources, however, is so long that replacement of the light source during the life of the armature is not necessary.

In the case where LEDs are used as light source, an additional facet plays a role, viz., that LEDs, unlike current light sources, do not have a gas-tight glass and metal envelope, but the semiconductor chip is often covered only by a layer of silicone material. As a result, due to the ambient influences mentioned, the LED too may degrade prematurely. This holds also for the electronic components of the supply circuitry of the light sources.

The object of the invention is to provide an armature for lighting of spaces with explosion hazard, having, in a particular embodiment, the possibility of emergency light functionality, having a low weight, which can be manufactured at relatively low cost, while the internal components are very well protected from the ambient conditions outside the armature, especially from aggressive gases, aggressive mists or very high humidity, and where there is no need to replace the light source during the life of the armature.

This object is achieved by filling the space within the armature, which is enclosed by the housing thereof and the light window and in which the light source(s) or supply circuit(s) are arranged, with at least a non- explosive gas, for instance, pure nitrogen. Further, the housing and the light window are manufactured from materials that poorly transmit or do not transmit gas and moisture (water vapor), so that gas and moisture can only poorly penetrate or cannot penetrate from outside of the armature into the space. Non-gas transmissive materials are metals and glass. Theoretically, the armature may be wholly built up from glass, while the supply of electrical energy takes place via melted-in metal pins, and the heat dissipation of the internal parts proceeds also via the glass.

More practical is the design of metal and glass whereby a poorly gas-transmissive material such as butyl rubber is used as sealing between metal and glass. Other possibilities are the use of plastic in combination with foils having poor gas-transmissive properties. Such materials are under development as replacement of glass, for instance, for use as packaging for foodstuffs, as beer bottle, and for packaging medicines or keeping blood plasma.

For practical construction of an armature, a requirement to be fulfilled is that during the life of the armature the penetrating moisture and oxygen can be absorbed by moisture- and oxygen-absorbing materials, while costs for, and volume of, these absorbing materials are to be reasonable. For a life expectation of, for instance, 20 years and an armature volume of 2 liters, a reasonable oxygen absorption is 2 liters, and the oxygen absorbing T NL2011/050852

material weighs approximately 20 grams. Such moisture- and oxygen- absorbing materials can be included in the space enclosed by the housing and the light window.

For the transmission of both oxygen and nitrogen and water vapor, it is a useful assumption, under the operating conditions, that the

transmission is proportional to the partial pressure difference across the material, proportional to the surface bounding outer and inner atmosphere, where sorption and desorption of the gases takes place, and inversely proportional to the thickness of the layer through which the gas or the water vapor is to diffuse.

This means that the oxygen transmission per day may not be more than 0.27 cm ³ of oxygen. It can be calculated from material specifics what materials satisfy such a requirement.

For butyl rubber a typical permeation is 9 cubic centimeters of oxygen per day per square meter per Bar of pressure difference given a thickness of 1 centimeter.

Over a period of 20 years, this means for a sealing ring having a perimeter of 30 centimeters and a thickness of 1 millimeter and an oxygen partial pressure difference of 0.2 Bar, that per day 0.0018 cubic centimeter diffuses and in a period of 20 years an oxygen volume of 13 cubic

centimeters will diffuse in.

The same applies if an armature is designed for use in tropical environments, and internally the air humidity is kept close to 0% relative humidity.

A molecular sieve material such as Zeolite with 3 angstrom pores can take up up to 20% of its own weight of water vapor. Such a moisture- absorbing material can be included in the space enclosed by the housing and the light window. Using 40 grams of this type of Zeolite, 8 grams of water can then be adsorbed, and the maximum allowable moisture influx is 0.001 grams per day. NL2011/050852

The specified moisture permeability of a commercially available butyl rubber Adcotherm 32 is 0.2 grams per square meter per day for a thickness of 2 millimeters according to American standard ASTM F1249. Assuming a sealing ring width of 10 millimeters and a height of 1

millimeter and a perimeter of 30 centimeters, the gas absorption and desorption surface is approximately 1 square centimeter, and given a diffusion length of 10 mm, a transmission of 4 micrograms of water per day is found, or 0.3 grams over a period of 20 years. At higher temperatures and air humidity, water transmission will increase strongly, for instance, by more than a factor of 3, but even so, there is still an amply sufficient margin present then.

In principle, the glass can be replaced by plastic with a sufficiently low moisture and oxygen transmission, but since the surface for sorption and desorption will then be much greater, in the order of 100 to 1,000 square centimeters and a practical thickness will be no more than 2 to 4 millimeters, the requirements to be imposed on this material are much higher than those to be imposed on a sealing ring.

According to one embodiment, the armature includes a monitoring circuit, which reliably detects oxygen being present, and upon an

established deviation interrupts the power supply to the electronic circuit of the operating device and the connected light source. In a further preferred embodiment, a fault tolerant circuit and protection method is used with the aid of a microcontroller and a pressure sensor, made with the aid of Micro Electro-Mechanical System technology, further to be referred to as MEMS. Further, preferably, a temperature sensor is used and a relative humidity sensor. The microcontroller determines on the basis of the signals generated by the pressure sensor, the temperature sensor and, if present, the relative humidity sensor, whether the housing is still intact and there can be no oxygen present, so that cut-off of the equipment is not necessary. If it is derived from the signals that oxygen may be present, the electrical L2011/050852

6 equipment is switched off, so that explosion hazard is precluded. The elaboration will in most cases be completed by an oxygen absorbing material, for instance, based on very fine iron powder in combination with a catalyst, such as marketed, for instance, under the name of Ageless by Mitsubishi Gas Chemical Company, which binds oxygen diffusing in. Such an oxygen absorbing material may be included in the space enclosed by the housing and the light window. Also, moisture absorbing materials may be used, so that, especially in tropical environments, internal condensation, for instance at night if the temperature falls and the light source is not switched on, is prevented. The invention will now be described in more detail, partly in relation to the drawings. In the drawings:

Figure 1 shows a cross section of a possible embodiment of the armature according to the invention, with LEDs as light source.

Figure 2 shows a cross section of a possible voltage lead-through, used in the armature according to the invention.

Figure 3 shows an electrical block diagram of a possible

embodiment of the protection circuit and light source supply circuit of the armature according to the invention.

Figure 4 shows the electrical block diagram of a variant of a possible embodiment of the protection circuit and light source supply circuit of the armature according to the invention.

Figure 5 shows a detail of an embodiment of an extension of the circuits from Figure 3 and Figure 4 to enable an emergency lighting function.

Figure 6 shows a characteristic of the pressure change as a function of the temperature and the relative air humidity in the armature.

Figure 7 shows a possible embodiment of the relative humidity sensor circuit.

Figure 8 shows a cross section of a third embodiment of the protection circuit for the armature according to the invention. Figure 9 shows an electrical diagram of the third embodiment of the protection circuit for the armature according to the invention.

Figure 10 shows the temperature curve of components in the third embodiment of the protection circuit according to the invention.

The armature, in the preferred embodiment, consists of a housing

1, manufactured from a material that transmits little or no gas and moisture, for example, manufactured from aluminum, and a light window 2, which consists of a material that transmits no or very little gas and moisture, for example, glass. Arranged between these two parts is a sealing ring 3, for example, of butyl rubber. As is customary with insulating double glazing for use in residences and utility buildings, an extra adhesive layer may be provided on the outer side of the butyl rubber or other primary sealing because in practice the penetration of moisture and gas is thereby impeded still more. Situated in the space enclosed by the housing and the light window is a printed circuit board 4, with the electronic circuit thereon having arranged thereon a pressure sensor 5, a temperature sensor, which in the exemplary embodiment is integrated in a microcontroller 6, and preferably a relative humidity sensor 7. Also possible are embodiments where the temperature sensor is integrated in the relative humidity sensor. Also arranged in the preferred embodiment are an oxygen absorption material 8 and a moisture absorbing material 9, for instance on a support of gauze or perforated plate 10. Further provided, preferably on the same printed circuit board, are the circuit to provide the light sources with power, and the circuit to interrupt the input voltage in case of penetration of oxygen.

If for the light source LEDs 11 are used, these are mounted on a support 12, which conduct the heat generated by the LEDs to the metal housing of the armature. This results in a small temperature difference between the LEDs and the housing. This support 12 may consist of ceramic material, but also of FR4 printed circuit board with special provisions to increase thermal conductivity, such as through-metalized holes. The housing preferably has cooling fins 13, so that the junction temperature of the LEDs remains low enough to ensure a very long life, for instance, more than 100,000 hours for a light decrease of less than 30% of the original value. Further, in the preferred embodiment, internally a mirror 14 is arranged, so that a large part of the generated light is reflected, and the glass window may be etched or otherwise provided with a light diffusion layer, for instance, by using corrugated glass or frosted glass. Mirror and glass may then be so shaped that a uniform spread of the light is obtained, while no blinding occurs owing to the glaring light spots of the LEDs not being visible, while yet the light is spread over a large surface, as is the case with the fluorescent lamp-based, customary armatures for use in explosion hazardous environments. Also other internal constructions may be used, such as lenses, in particular if the armature is designed more like a spotlight and normally there is no direct view of the light sources from the surfaces to be lighted. Also provided in the housing are lead-through openings to supply the mains voltage to the internal circuit and, in the case of an armature with emergency light, to realize the electrical connection with an externally arranged accumulator battery. The electrical connections for the mains voltage and/or the accumulator battery may be designed as indicated in Fig. 2. Fig. 2 shows one such connection. Practically, however, the lighting armature will typically be provided with two of such

connections. Massive metal pins 15 are shown, included in a rubber lead- through piece 16, which is arranged in cylindrical openings 17 of the housing 1. On the inner side of the armature, an additional insulation piece 18 may be arranged to satisfy the requirements regarding air and creepage paths. Other forms of massive lead-through may be used, such as massive rectangular tongues. For the lead-through piece 16, instead of rubber, also a molding mass may be used, such as epoxy or polyurethane with better electrical insulation properties than rubber. While the gas transmissivity of T/NL2011/050852

9 such a molding mass is greater than that of rubber, the extra gas diffusion that may occur is acceptable in view of the great length and small surface area of the lead- through piece.

During production the armature is provided with a pure nitrogen filling. The simplest manner of achieving this is to provide an oxygen absorbing material 8 with sufficient capacity to wholly absorb the oxygen present in the air, and with sufficient capacity left to absorb the calculated oxygen diffusion from outside during the life of the armature.

Another method is closing the armature in a protected nitrogen environment with the desired pressure, preferably between 65 and 80 kiloPascal at a temperature of 25 °C. A third possibility is to provide one or more fill openings to which can be connected a pump to pump out the air and a nitrogen source to admit the nitrogen. These fill openings are then closed afterwards and, if necessary, additionally sealed.

The filling pressure of the armature is chosen such that under all operating conditions an excess pressure is present on the outside of the armature, and that further for the average operating conditions the partial pressure of the nitrogen inside and outside the armature is virtually equal, so that diffusion of nitrogen through the sealings is limited to a minimum.

In particular, it holds that the non-explosive gas is nitrogen, while the partial pressure of the nitrogen in the lighting armature is at least substantially equal to the partial pressure of nitrogen at the earth's surface under atmospheric conditions. The diffusion of nitrogen through rubber and plastics, such as epoxy, is considerably smaller than that of oxygen under equal conditions. This fact, together with the partial pressure of the nitrogen inside and outside the armature being preferably virtually equal, makes the diffusion of nitrogen negligible.

A first variant of the electrical diagram of the protection circuit of an armature according to the invention is given in Fig. 3. The mains voltage is presented on terminals 20 and 21. A protection or monitoring circuit 25, T NL2011/050852

10 for instance, supplied from the mains voltage via a capacitor 23 and a resistance 24, drives the relay 22. This relay is switched on only if it is concluded from the signals of a temperature sensor 25" built into the protection circuit, a pressure sensor 26, and a relative humidity sensor 27, that the armature is sufficiently gas-tight and no oxygen can be present in the armature. The manner in which this is done is described later. The relative humidity sensor 27 may be implemented as a capacitor with a capacity that depends on the air humidity.

The circuit arranged in the armature may further be provided with circuitry familiar to the skilled person, to satisfy mains loading

requirements regarding harmonic currents and generated interference voltage as well as elementary electrical safety requirements, and further with circuitry to stabilize the current through the LEDs 34 (indicated as LEDs 11 in Fig. 1). Thus, the exemplary circuit shown in Fig. 3 consists of a fuse 33, an interference filter 38, a boost converter 36 which prevents harmonic currents in the mains, a smoothing capacitor 40 and a direct voltage-to-direct current converter 37 which converts the direct voltage on the smoothing capacitor 40 to a stabilized direct current through the LEDs 34. Further, a connection 31 with a high-frequency receiver or transmitter- receiver 32 may be arranged, which, for instance, can pass on information about desired dimming of the lamps. Further, the monitoring circuit 25 may be combined with a control circuit 25', which may be implemented partly in software, which sets the operating parameters of the partial circuits, for instance by measuring the rectified voltage via connection 28, and

controlling the boost converter 36 via setting signal 29 and the direct voltage-to-direct current converter 37 via signal 30, if desired additionally based on the information coming in via the high-frequency

transmitter/receiver 32. The elements 20, 21, 22, 28, 29, 33, 35, 36, 37, 38 and 40 together form the supply circuit of the LEDs 34. NL2011/050852

11

The monitoring circuit 25 including, if present, the control circuit 25' and the supply thereof, continue to be connected with the mains voltage if a defect in the sealing of the armature is established. That is why these parts must be protected in a supplementary manner to prevent explosive hazard, for instance, they must meet the requirements applying to

equipment for use in environments where explosion hazard may exist, for instance, the regulations for cast-in circuitry, for enhanced safety and for intrinsic safety.

There is a variant embodiment possible, however, that utilizes a bistable remanence relay. Such a relay can be brought in the conductive state by a pulse voltage in the production of the monitoring circuit, and thereafter remains energized as a result of remanent magnetism, without external power needing to be furnished for this purpose. To open the relay contacts, only a short pulse in opposite direction needs to be presented, and no separate power supply of the monitoring circuit is needed, but power may be drawn from one of the converters, as described below in the second exemplary embodiment.

A second exemplary embodiment is shown in Fig. 4. In Figs. 3-5, 7-9, corresponding parts are provided with a similar reference numeral. The difference with the earlier-described circuit is that the protection circuit 25 now does not operate a relay, but a short-circuit transistor 39, which, in case of an established leak in the armature, induces a large current in the input circuit as a result of which fuse 33 will interrupt. The short-circuit current is here limited by the resistance of the interference filter 38. Dimensioning should be such that, for instance, the bonds to the chip of transistor 39 will not interrupt before the fuse interrupts. Further, an extra buffer capacitor 48 is included, which has sufficient charge to energize the short-circuit transistor sufficiently long to cause the fuse to melt. Further, in this embodiment, the protection and control circuit 25 is preferably supplied, in normal operation, from a voltage coming from the direct voltage-to-direct T NL2011/050852

12 voltage converter 37. It is possible, however, in case of malfunction of one of the converters 36 or 37, to supply the protection circuit 25 via a series resistance from the rectified mains voltage. In that case, the current consumption of the protection circuit is strongly reduced, for instance, by switching, in case of a microcontroller, to a very low clock frequency, and to interrupt the power supply to the sensors. Because presently no sensing of the necessary parameters can take place anymore, in this case, as a precaution, the short-circuit transistor 39 will be energized. An energy buffer in the form of a capacitor 48 ensures that also in fault conditions cut- off can still take place. If necessary, the circuit may be provided with a hardware watchdog circuit, which in the case of an inoperative processor cuts off the transistor independently and supplied from this capacitor.

After interruption of the fuse 33, the whole circuit is connected to the mains voltage only with one pole. Therefore, no energy supply to the circuit can take place anymore, and that is why there is no chance of sparking or heat development in the parts that could lead to an explosion hazardous situation. In the case of a grounded metal wall, as is generally the case for this type of armature, however, the parts in the armature do need to have the required air and creepage paths with respect to the grounded parts, for instance, as described in the standards for explosion prevention protective method "enhanced safety".

If desired, as a short-circuit transistor, one of the transistors that are present in the above-mentioned direct voltage-to-direct voltage

converters can be used. Also, the circuit can be made redundant in this manner, by using, if the short-circuit transistor were to fail, as a second one, one of the transistors from the direct voltage-to-direct voltage converters.

A third exemplary embodiment of the protection circuit is shown in Fig. 9. The protection circuit 25 in this embodiment activates a heating element 57, for instance, a wire-wound resistance with Nichrome wire wound on a ceramic cylinder -shaped support. Due to the heat-up of the resistance, the thermal fuses 55 and 56 are interrupted, as a result of which the supply voltage for the armature presented on the terminals 20 and 21 is subjected to bipolar cut-off . The advantage of bipolar cut-off is that the remaining circuit is wholly potential-free after a leak has been established. This is especially necessary if the distances between voltage-carrying parts and earth, on which, if there is a protective atmosphere present, no particular requirements are imposed, as a result of a defect no longer meet the requirements regarding explosion safety in a situation with a

nonprotective atmosphere in which an explosive gas mixture may be present, or where voltage -carrying parts, as a result of a defect, may be touched.

In particular, when grounded metal parts occur in the construction, and the potential of the input voltage on both poles can have a value deviating from the earth potential, it is important to cut off both poles of the input voltage. If multiple voltage sources are present, such as the mains voltage and an accumulator voltage for the purpose of emergency lighting, it may be necessary to cut off more than two poles.

Although hereinbelow bipolar cut-off is described with two thermal fuses, the same principle can also be used for single-pole cut-off, or for triple-pole or multipolar cut-off, for instance, to additionally cut off an accumulator connection for emergency power supply.

The mechanical implementation of this manner of protection is shown in Fig. 8. In a part of the armature housing 1, there is included, combined with the input of the supply voltage from terminals 21 and 22, a cup-shaped space, which is filled with molding mass 62. Within the molding mass chamber there is a chamber 65, which is free of molding mass and may be filled, for instance, with nitrogen. The chamber 65 is surrounded by a cap 59 placed on a printed circuit board 60. The thermal fuses 55 and 56 are pressed on opposite sides against wire-wound resistance 57 by a silicone ring 64 and a clamping ring 63, for instance, a so-called ear clamp. The output of electrical connections proceeds via conductors 68, and consists of the connections for the supply voltages after the thermal fuses 55 and 56 and the separate connections of resistance 57. If the protection circuit establishes a leak in the sealing, the resistance is provided with voltage, for instance, via a switching transistor 58. In the diagram shown in Fig. 9, the voltage is connected after rectifier 35, via transistor 58. For a reliable operation throughout a large supply voltage range, use can be made of pulsating switching-on of transistor 58, such that the power supplied to the resistance is independent of the supply voltage. To that end, the duty cycle of switching on should be inversely proportional to the square of the effective value of the supply voltage. If, for instance, the resistance takes up a power of 10 Watt at the minimum supply voltage of 90 Volts, then at 180 Volts the duty cycle has to be 25%, and at 270 Volts 11%.

A reliable cut-off of both thermal fuses requires the resistance 57 to be heated up relatively fast. The temperature curve is outlined in Fig. 10. The power of the resistance is, for example, the quadruple of nominal. As a result of the heat capacity of the ceramic support, the temperature increases virtually linearly in time. Due to the thermal resistance and heat capacity of the thermal fuses, the temperature in the thermal switching material in the fuse lags with respect to the temperature of the wire-wound resistance. Thus, with a fuse that interrupts at 125 °C, the temperature T57 of the resistance may already have run up to 200 °C. At time tl fuse 56 interrupts, so that the energy supply to the resistance stops also. Due to the high temperature the resistance now has, however, the second fuse 55 will nonetheless interrupt also. A typical response time for a wire-wound resistance dimensioned with a diameter of 5 mm and a length of 10 mm against which two thermal fuses have been clamped in a plastic housing dimensioned 5mm by 4mm by 2 mm, is 20 seconds at the above-mentioned temperatures and a starting temperature of 25 °C. T/NL2011/050852

15

At lower starting temperatures the response time is longer and the maximum temperature of the wire-wound resistance is higher at equal supplied power to the wire-wound resistance. At high starting

temperatures, response time is shorter. Thus, at - 40 °C in the above- mentioned example, response time will be 30 seconds and at + 80 °C

10 seconds.

It is possible to implement the protection circuit in a different manner, for instance, by using a MOSFET transistor as heating element instead of a wire-wound resistance. In that case, though, the current through the MOSFET transistor must be controlled, and the maximum temperature must remain lower, to prevent uncontrolled short-circuiting of this transistor whereby fuse 33 will interrupt and the intended bipolar cut-off is not achieved.

It is possible to provide the armature with an emergency light function. One possible way of realizing this is indicated in Fig. 5. Arranged extra with respect to the variant according to Fig. 3 (of which in Fig. 5 only the parts 36, 37 and 40 are shown) or extra with respect to the variant according to Fig. 4 (of which in Fig. 5 only the parts 36, 37, 39 and 40 are shown) or extra with respect to the variant according to Fig. 9 (of which in Fig. 5 only the parts 36, 37 and 40 are shown) are a charging circuit 43, an accumulator battery 44, a direct voltage-to-direct voltage converter 45 and a diode 47. The short-circuit transistor 38 in Fig. 5 is thus a part that belongs only to the variant according to Fig. 4, but in Fig. 5 is included at a different place than in Fig. 4, as will be further elucidated hereinafter. Other embodiments are possible. The accumulator battery is preferably arranged outside the nitrogen protected environment of the armature, because small amounts of hydrogen that may escape through the rubber sealing rings of, for instance, Nickel Cadmium accumulator cells, would unbalance the gas equilibrium in the armature by pressure increase, as a result of which the protection circuit would respond. Other possibilities of realizing the emergency light function are possible, as is known to the skilled person, for instance, with only one bidirectional direct voltage-to-direct voltage converter, which takes care of both the charging of the accumulator and the emergency light provision, or the possibility of controlling all or a part of the LEDs 34 directly from direct voltage -to-direct voltage converter 45. In this exemplary embodiment, a second fuse 49 is included in the accumulator line. The short-circuit transistor 39 in this example, in deviation from the variant according to Fig. 4, is included parallel to the input of direct voltage- to-direct voltage converter 37, so that with direct current coupled converter 36, when making transistor 39 conductive, both fuse 33 interrupts the energy supply from the external mains voltage and fuse 49 interrupts the energy supply from the accumulator. Diodes 47 and 35 in this exemplary embodiment separate the direct voltage-to-direct voltage converters 45 and 36.

According to an alternative embodiment the monitoring circuit 25 can interrupt the energy supply from the accumulator to the supply circuit for the light sources by opening one or more relay contacts.

The armature according to the invention can also utilize a different light source, for instance, an induction lamp with associated supply circuit. Protection from explosive hazard can then take place in the manner described above.

The characteristic on which the protection circuit is based is shown in Fig. 6. This is the general gas law, whereby the following holds, for constant volume of the enclosed gas: p/T = constant with p being the total pressure and T the absolute temperature.

The software in the protection circuit verifies continuously whether this ratio remains constant. Nitrogen diffusion is small in proportion to oxygen diffusion. At equal partial pressure difference, equal temperature and equal air humidity, the diffusion rate of nitrogen is approximately a factor of 5 lower than that of oxygen. T/NL2011/050852

17

An important datum is that the diffusion through the rubber sealing ring, and also through comparable materials, is very strongly temperature-dependent. At high temperatures of, for instance, 45 °C, the diffusion rate is many times greater than at 25 °C and at low temperatures, for instance, - 20°C, many times lower.

Specifically at high temperature, the internal partial nitrogen pressure will be slightly greater than in the outside air, which will lead to a small decrease of the total pressure.

Oxygen diffusing-in is completely bound by the oxygen absorbing material. This is an irreversible chemical process. If a leak has come about in the sealing, so that the gas diffusion becomes many times greater than has been assumed in dimensioning, a deviation of the p/T curve will occur. At a temperature so high that the partial pressure of the internal nitrogen becomes greater than that of the outside air, generally greater than about 80 kPa, the pressure will initially decrease until the oxygen absorbing material is saturated, after which, in fact, a pressure increase occurs. The pressure increase, however, due to the greater diffusion rate of oxygen and the much greater partial pressure difference, proceeds much faster, in the order of a factor of 20 faster. This means that the slow pressure decrease which occurs at continuously high temperatures can be corrected by the software, because this does not lead to a hazardous situation, whereas upon incipient fast pressure increase, cut-off will ensue.

At low temperature the diffusion of gases is so small that only when there is a fairly large leak will any pressure increase occur, initially by nitrogen pressure increase and, not until the oxygen absorbing material is saturated, by a much faster increase of the oxygen pressure. The

protection circuit will keep track of the pressure-temperature curve in a non-volatile EEPROM memory and thus be able to intervene in case the curve deviates. A reference curve can be generated directly after fabrication by having the armature traverse the whole ambient temperature range for T NL2011/050852

18 which the armature is designed. Doing so, automatically, temperature dependencies of the sensors, non-linearities and offset deviations of the sensors and deviations from the general gas law are compensated.

Especially at higher temperature, as in tropical environments, the relative humidity plays an important part. In contrast to oxygen absorption, the available moisture absorbing materials are based on a reversible physical principle. This means that at a particular relative humidity, there is a particular weight increase of the absorbing material.

In tropical environments the possibility exists of condensation upon temperature fluctuations, in view of which a moisture absorbing material will be chosen that can keep the maximum humidity low enough. In these environments, depending on the chosen type of moisture absorbing material, in some cases the relative humidity will increase during the life of the armature. In other environments, it can fluctuate and in very cold dry environments even decrease during the life of the armature. With the available materials for sealing between metal housing and glass light window it is not possible to preclude moisture transport entirely. The influence of moisture on the internal pressure, however, is precisely known, so that this can be corrected for with a calibrated relative humidity sensor. This is outlined in the curves of Fig. 6. Accuracy must be such that the reliable detection of oxygen diffusing-in is not hindered.

There are also moisture absorbing materials consisting of molecular sieves, such as synthetic zeolite with pores of 3 or 4 angstroms. In that case, as long as the material is not saturated, an air humidity close to 0% is maintained. If the internal parts of the armature tolerate such a low air humidity, a correction of the curves for air humidity may be dispensed with, and if desired the relative humidity sensor may be omitted. However, the relative humidity sensor may also be used to indicate that the moisture absorbing zeolite material is saturated. This may also be an indication of a 11 050852

19 too large leak in the sealing, so that upon increasing air humidity the armature can still be cut off as a precaution in a manner already described.

Especially, constructions are conceivable where, by outgassing of particular internal components, deviations from the reference pressure- temperature curve occur. However, also in the use of the zeolite material mentioned, a part of the nitrogen may be absorbed, which also results in a deviation from the reference curve. In this case, cut-off can be done exclusively on the basis of the signal of the relative humidity sensor. This can be done exclusively if in all conceivable leak situations or end of life situations, the moisture absorbing material becomes saturated sooner than the oxygen absorbing material. Accordingly, in relative terms, a sufficient excess of oxygen absorbing material must be present in the armature.

Because the temperature dependency of moisture and oxygen transport through pores of material but also the oxygen and gas transport through small cracks or differently shaped openings in the sealing exhibit similar shapes it is well possible to choose the minimum required excess of oxygen absorbing material such that always the moisture absorbing material becomes saturated. In principle, a pressure sensor can then be omitted.

However, as an extra monitoring, and, for instance, also for reliably detecting deviations in a very dry, cold environment, it may be desirable to maintain the pressure sensor.

A systematic deviation of the curve, whereby, viewed in the long term, a fairly abrupt pressure rise occurs, indicating that the oxygen absorbing material is saturated, can be reliably detected. To satisfy the current requirements of explosion safety, the oxygen concentration must remain smaller than a certain limit value. This means that the sensors on average must have a sufficient accuracy. By applying signal averaging over a long time and using sensors that do not exhibit any systematic drift, the desired accuracy can be realized. T/NL2011/050852

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A relative humidity sensor having good utility is a capacitive sensor which has, for instance, a dielectric of polyamide or other polymer. Such sensors have a well-defined temperature coefficient and will have a high reliability and reproducibility in the protected nitrogen environment. A possible circuit for coupling this type of sensor to the protection circuit is shown in Fig. 7. An inverting amplifier 50, together with a series resistance 54, a coil 52, a capacitor 53, and the air humidity sensor which in this example is designed as a capacitor 27 with capacity depending on the air humidity, forms an oscillator circuit. The frequency on output 51 is now a direct measure of the air humidity. Characteristic value for the capacity 27 is between 100 and 300 picoFarad, depending on manufacturer and type of sensor, while the capacity varies 15 to 20 percent between 0 and 100 percent relative humidity. The result of this, given proper dimensioning of the circuit of Fig. 8, is a frequency variation in the order of 6 to 8 percent, which is very reliable and can be measured with high accuracy.

The pressure sensor, as described in the foregoing, can be made with MEMS technology. Here too, the protected nitrogen environment is a guarantee of long-lasting accurate operation of this part. The operation is based on the elasticity of the silicon wall of a closed chamber which bends as a result of the varying pressure. By piezo-resistive resistances included in a bridge circuit, such bending is converted to an electrical signal. The sensitivity may be in the order of a good 1 millivolt per kiloPascal pressure at 5 Volts supply voltage. This signal can be amplified by differential amplifiers built into particular types of microcontrollers, and then be digitized by an analog-to-digital converter.

Many types of temperature sensors can be used. Advantageously, a sensor included in the microcontroller is to be used.

The sensors must be calibrated before use, if they do not have the required accuracy beforehand. In principle, the pressure-temperature curve can be measured afterwards in the armature when mounted, as described earlier. If cutting-off is done exclusively or mainly on the basis of the signal of the relative humidity sensor, the necessity for precision calibration is less.