EP1308670A1

EP1308670A1 - Explosion-protected luminaire

Info

Publication number: EP1308670A1
Application number: EP01660198A
Authority: EP
Inventors: Tuomas O. Luomala
Original assignee: Centaurea Oy
Current assignee: Centaurea Oy
Priority date: 2001-10-30
Filing date: 2001-10-30
Publication date: 2003-05-07
Anticipated expiration: 2021-10-30
Also published as: EP1308670B1; DE60141027D1; ATE454590T1

Abstract

The invention relates to an explosion-protected luminaire (1) comprising an illumination source (2), a plastic casing (3), and means (15) for providing electric power to illuminate the illumination source (2). The plastic casing (3) comprises at least a transparent middle block (20), and non-transparent first (18) and second end blocks (19). The electric power is supplied to the illumination source (2) through at least one barrier (12) to limit the maximum power supplied to the illumination source (2). The illumination source (2) comprises light emitting diodes (10).

Description

The present invention relates to an explosion-protected luminaire comprising an illumination source, a plastic casing, and means for providing electric power to illuminate the illumination source, the plastic casing comprising at least a transparent middle block, and non-transparent first and second end blocks.
Hazardous locations mean such areas in which a risk of explosions exist. The risk of explosions may be due to existence of explosive materials in such areas. These hazardous locations are, for example, petrochemical plants, coal mines, etc. In addition to that, such hazardous locations may exist in normally safe areas. For example, fuel containers may be stored in service buildings. It is extremely important to eliminate any risks of sparking in such areas. The sparking must not be induced for electrostatics, inductivity, electric devices (e.g. luminaires), breakage of an electric device or any other reason. For the reasons mentioned above many requirements are set for electric devices which may be used in hazardous locations. These kind of devices must be protected against sparking, the electric power supplied to the devices must be limited, warming-up of surfaces of the devices must not exceed certain limits, etc.
The determination that areas can be classified as hazardous locations is based on the following:

the possible presence of an explosive atmosphere such as flammable gases, vapors, or liquids, combustible dusts or ignitable fibres and flyings;
the likelihood that the explosive atmosphere is present when equipment is operating; and
the ignition-related properties of the explosive atmosphere that is present.

Many Classifications of Hazardous Degrees exist around the world. For example, an Ex Classification is used in Europe.
In Europe three Zones are defined where explosive mixtures of gas or powder and air can exist. The most dangerous zone is Zone 0 in which the mixture is continuously or almost continuously present. Zone 1 is defined as a location where the mixture is likely to occur in normal operations. In Zone 2 locations the mixture is unlikely or will only exist for a short time.
The devices which are intended to be used in hazardous locations must be accepted by a Notified Body before the devices are allowed to be used in such locations.
Electric devices to be used in locations of Zone 0 and/or Zone 1 are very demanding and many points of view have to be taken into account when designing such devices. The manufacturing of prior art Explosion-protected luminaires has been very difficult and expensive since e.g. the electric power to be supplied to the luminaire has to be limited and since the temperature loss of the luminaire has to be kept within certain limits. These limitations cause difficulties to design a luminaire which can produce enough illumination. One furher aspect in designing explosion-protected luminaires is the impact resistance of the luminaire because strong impacts may exert to the luminaire in usage. The luminaire may e.g. drop. The explosion-protected luminaire must not induce any risk of explosion, such as sparking and/or overheating even when it is broken.
The patent US 5,381,321 describes an explosion-proof luminaire comprising a transparent tube in which the source of illumination is placed. The source of illumination is a fluorescent lamp. The control electronics for controlling the fluorescent lamp is situated inside the casing of the luminaire. Therefore the control electronics must not generate sparks or over-heat even if the luminaire breaks down. Furthermore, the power supply which is normally situated outside the hazardous location must limit the electric power supplied to the luminaire especially when the luminaire is used in hazardous locations (such as Class 1 according to the U.S. National Electrical Code). In the design of the luminaire any possible fault situations have to be taken into account. Such fault situations are, for example, a breakage of the fluorescent lamp caused by e.g. dropping of the luminaire or the luminaire is otherwise affected by an impact. Another disadvantage with this kind of luminaire is that the fluorescent lamp generates current spikes into the voltage supply wirings. Therefore, if the insulation of the wirings is flawed, the spikes can even induce an electric breakdown in the wiring. This electric breakdown can generate an explosion. The explosion-proof luminaire according to the patent US 5,381,321 does not fulfil the requirements of Zone 0 of the Euronorm Standards.
Another explosion-protected luminaire comprising a transparent tube is known in which pressurized air is blown into the casing of the luminaire. The pressurized air rotates a generator which generates electric power to the luminaire. The pressurized air also cools down the fluorescent tube. The luminaire is so designed that if the supply of pressurized air stops, the fluorescent lamp cools down before the temperature of the casing of the luminaire rises too high. This kind of luminaire has the disadvantage that it is large and quite heavy, about 50 to 70 kg therefore it is not very practical to be used in maintenance and service work.
One other aspect when designing explosion-protected luminaires for maintenance and service work is the size and weight of the luminaire. The luminaire should be as small and light as possible. Fluorescent lamps are not very small wherein it is not easy to reduce the size of the luminaire.
The weight of the luminaire can be reduced by using plastic casings, e.g. antistatic plastic casings. Ordinary, impact-proof plastic can cause electrostatic sparking which is not desirable when such luminaires are used in hazardous locations. However, if ordinary plastic material is used instead of antistatic plastic material, the risk of electrostatic sparking has to be taken into account in designing the luminaires.
It is an aim of the present invention to eliminate the above-mentioned drawbacks and to present a considerable improvement to the prior art explosion-protected luminaires. The present invention is based on the idea that the illumination source is made of light emitting diodes and the electric power supplied to the illumination source is limited by barriers. More precisely, the explosion-protected luminaire according to the present invention is primarily characterized in that the electric power is supplied to the illumination source by barriers to limit the maximum supplied power, and that the illumination source comprises light emitting diodes.
Considerable advantages are achieved by the present invention when compared with prior art explosion-protected luminaires. The illumination source of the explosion-protected luminaire of the present invention comprises light emitting diodes (LED) which are not very sensible to impacts and they don't induce electric spikes (voltage and/or current) if they are broken. The LEDs also have the advantage that they don't generate too much heat so there is no need to arrange any cooling apparatus inside the casing where the LEDs are situated. The use of LEDs also makes it possible to use barriers for limiting the electric power supplied to the illumination source. This has not been possible before with normal illumination sources, e.g. fluorescent lamps, because the barriers limit the current level too low for generating enough illumination by illumination sources used in prior art explosion-protected luminaires.
In an advantageous embodiment of the present invention a grounded metal sheet is provided to reduce the touchable area of such parts of the plastic casing which are not antistatic. Therefore, the electrostatic sparking can be eliminated.
In the following the invention will be described in more detail with reference to the attached drawings, in which

Fig. 1a: describes an explosion-protected luminaire according to an advantageous embodiment of the present invention,
Fig. 1b: describes as a simplified cross-section of a cable which can be used with the explosion-protected luminaire according to an advantageous embodiment of the present invention,
Fig. 2: describes as a simplified circuit diagram the electrical structure of an explosion-protected luminaire according to an advantageous embodiment of the present invention,

In Fig. 1a there is described an explosion-protected luminaire 1 according to an advantageous embodiment of the present invention. It comprises an illumination source 2 which is protected by a casing 3. The power is supplied to the illumination source 2 by a cable 4 comprising the necessary amount of wires for the power supplement. It is also possible to use more than one power supply cable 4, if necessary and if required by safety regulations.
The cable 4 comprises a set of pairs 5 of wires 6 (twisted pairs) as shown in the simplified cross-section in Fig. 1b. Each pair 5 is covered by a shielding 7, preferably a metallic membrane or braiding. The structure of the cable 4 reduces a risk of electrical breakdown between wire pairs of the cable. All the pairs of the cable are further surrounded by an outer shielding 8, which can also be e.g. a metallic membrane or braiding. The cable is preferably a so called Robo® cable, which is the trademark of Robotech. The cable can also comprise connectors 9 to connect/disconnect the cable. The connector has contact element (not shown) for each wire 6 of the cable and also for each shielding 7, 8 of the cable. It is obvious that the number of connectors 9 is not restricted to one but the explosion-protected luminaire 1 can comprise more than one connector 9 and more than one cable 4. The connector(s) 9 is/are also explosion-protected, e.g. intrinsic safe connectors.
The illumination source 2 comprises LEDs 10. The amount of the LEDs can vary in different embodiments. The amount of the LEDs affect the illumination produced by the illumination source 2. On the other hand, the number of LEDs can be limited by safety regulations. For example, the maximum electric power supplied to the explosion-protected luminaire must not exceed certain limits. This means that the power supply means of the explosion-protected luminaire comprises means 12 to limit the electric power supplied to the illumination source 2, as will be described later in this description. The LEDs of the illumination source 2 are preferably so called bright LEDs which has the advantage that the illumination produced by them is much stronger than illumination produced by normal LEDs. The colour of the illumination is preferably white, but it is obvious that other colours can be used within the scope of the present invention. As an example of such LEDs is L5-W54N-BS produced by Sloan Ag company. This type of LED can produce a maximum illumination of 8000 mCd when supplied voltage is 3,6 V and supplied current is about 20 mA.
The LEDs 10 of the illumination source 2 are advantageously divided into groups 11, wherein power is supplied individually to each group 11. The LEDs 10 in a group 11 are connected in series such that the voltage drop across each LED 10 is within allowed limits. In this advantageous embodiment the series connection further comprises a series resistor 14. The number of LEDs within each group depends e.g. on the supply voltage, on the allowed supply voltage of the LEDs (voltage drop), and on the resistance of the resistor 14. Normally the supply voltage is about 3,6 V for bright LED and the current consumption of the bright LED normally is about 20 mA. Part of the groups of the LEDs can be connected in parallel wherein it is not necessary to arrange power limiter 12 for each group 11 of LEDs. The power transferred via the pair of wires to a paralleled connected groups 11 of LEDs defines the maximum number of groups 11 of LEDs which can be connected in parallel. In the advantageous embodiment of the explosion-protected luminaire the cable comprises six pairs of wires. The total number of LEDs is 60 wherein each pair of wires supply power to 10 LEDs. Assuming that the supply voltage from the power source 15 is 12 V and each group 11 of LEDs comprises equal number of LEDs there are two serially connected LEDs in each group 11 and five groups 11 are connected in parallel to be supplied by one pair of wires. The voltage drop across the two LEDs is approximately 7.2 V wherein the voltage drop across the series resistor 14 has to be approximately 4.8 V (=12 V - 7.2 V). It should be noted that above described example is a non-restrictive example of the present invention.
Advantageously, the LEDs 10 are arranged to a base plate 13 as a matrix. Therein all the LEDs illuminate to substantially same direction but it is obvious that other arrangements can be used within the scope of the present invention. The base plate 13 is e.g. a normal printed circuit board (PCB) and the LEDs 10 can be fixed to it by soldering.
The power is supplied to each group 11 of the LEDs through power limiters 12 which are also called as barriers. Also terms safety barrier and intrinsic safety barrier are known for such devices. In the following, the term barrier is used for such power limiting devices. The barrier has means to limit the output power of the barrier within safe limits to limit the energy delivered to the instruments located in the hazardous area. The barrier keeps the voltage and current from exceeding prescribed limits even under conditions where faults occur. Therefore, in each pair of wires of the cable the power (voltage x current) is within acceptable value for hazardous locations. The total power supplied to the illumination means 2 can then be as much as the maximum power of one pair of wires times the number of wire pairs. If the same amount of power would be supplied by only one pair of wires that could mean an unacceptable high value for hazardous locations.
Although the barriers 12 limit the output power to a certain safe limit, one must still take into account the total power supplied by all the barriers so that the total power entering the cable is not too high. Therefore the illumination of the illumination source can not be increased without limits by increasing the number of LEDs and the number of wire pairs.
The power source 15 comprises a transformer 16 for transforming the input voltage to a suitable level. For example, the input voltage is 230 VAC (normal mains voltage in Europe) or 110 VAC (normal mains voltage in U.S.A.) which is transformed to 12 VAC. The regulator 17 converts the AC voltage into DC voltage. The DC voltage is then supplied to the power limiters 12. The power source 15 can have means for supplying power to more than one illumination source 2. For example, the power source 15 can produce power to three illumination sources 2 therein at least the power limiters 12 are tripled so that there are three groups of power limiters 12 each of them supplying power to one illumination source 2.
The power source 15 is arranged into a flame-proof casing (e.g. gases of class 2c) to prevent any risks that a possible malfunctioning of the power source 15 induces an explosion outside of the casing.
In the following, the structure of the casing 3 of the illumination source 2 according to an advantageous embodiment of the present invention is described in more detail. The casing 3 comprises two end blocks 18, 19, and a transparent middle block 20. The illumination source 2 is inside the tubular middle block 20 to enable the illumination of the illumination source 2 to spread outside from the casing 3. The middle block 20 is preferably of plastic which is covered by antistatic coating known as such. The antistatic coating is necessary to eliminate the electrostatic sparking of the middle block 20.
The end blocks 18, 19 are made of antistatic or non-antistatic plastic. Usage of non-antistatic plastic material enhances the risk of electrostatic sparking. Therefore, in an advantageous embodiment of the present invention the end blocks 18, 19 comprise so called impediment plates 21 to reduce large planar areas in the end blocks 18, 19. Further, the first end block 18, which is also used as feeding through conduit for the cable 4, comprises a metallic sheet 22. The metallic sheet 22 is grounded to the outer shielding 8 to prevent the electrostatic sparking of the metallic sheet 22. The meaning of the metallic sheet 22 is to further reduce the touchable area of the plastic first end block 18.
It should be noted that the present invention is not restricted to such embodiments in which the casing 3 is made of non-antistatic plastic material. Further, the casing 3 or part of it can also be made of other materials than plastic, e.g. of glass.
The explosion-protected luminaire 1 of the present invention can be used in hazardous locations of any class and still enough illumination is provided for normal maintenance and service work of e.g. aeroplanes, petrochemistry plants, coal mines, etc. The explosion-protected luminaire 1 can also be used in locations which are not classified as hazardous locations.
It is obvious that the present invention is not limited solely to the above-presented embodiments, but it can be modified within the scope of the appended claims.

Claims

An explosion-protected luminaire (1) comprising an illumination source (2), a casing (3), and means (15) for providing electric power to illuminate the illumination source (2), the casing (3) comprising at least a transparent middle block (20), and non-transparent first (18) and second end blocks (19), characterized in that the electric power is supplied to the illumination source (2) through at least one barrier (12) to limit the maximum power supplied to the illumination source (2), and that the illumination source (2) comprises light emitting diodes (10).
The explosion-protected luminaire (1) according to claim 1, characterised in that at least part of the casing (3) is made of plastic material.
The explosion-protected luminaire (1) according to claim 2, characterised in that the plastic material is non-antistatic.
The explosion-protected luminaire (1) according to claim 3, characterised in that at least the first end block (18) of the plastic casing (3) is at least partly covered by at least one metallic sheet (22) to minimize the touchable area of the non-antistatic plastic part of the casing (3).
The explosion-protected luminaire (1) according to any of claims 1 to 4, characterised in that the light emitting diodes (10) of the illumination source (2) are arranged into groups (11), that light emitting diodes (10) of each group (11) are connected in serial.
The explosion-protected luminaire (1) according to claim 5, characterised in that at least two groups (11) are connected in parallel to form one or more sets of parallel connected groups (11), and that power is supplied to each set of parallel connected groups (11) by one barrier (12).
The explosion-protected luminaire (1) according to claim 6, characterised in that power is supplied to said illumination source (2) by a cable (4) comprising at least one pair (5) of wires (6) for each set of parallel connected groups (11).
The explosion-protected luminaire (1) according to claim 7, characterised in that each pair (5) of wires is covered by a shielding (7).
The explosion-protected luminaire (1) according to claim 8, characterised in that said shielding (7) is a metallic membrane.
The explosion-protected luminaire (1) according to claim 8 or 9, characterised in that the pairs (5) of wires are covered by an outer shielding (8).
The explosion-protected luminaire (1) according to any of claims 7 to 10, characterised in that the cable (4) comprises an explosion-protected connector (9).
The explosion-protected luminaire (1) according to any of claims 1 to 11, characterised in that it comprises at least two illumination sources (2), and a power supply cable (4) for each illumination source (2).
The explosion-protected luminaire (1) according to any of claims 1 to 12, characterised in that said end blocks (18, 19) of the casing (3) comprise impediment plates (21) to reduce large planar areas in the end blocks (18, 19).
The explosion-protected luminaire (1) according to any of claims 1 to 13, characterised in that it is arranged to be used in hazardous locations of zone 0 according to Ex Classification.
The explosion-protected luminaire (1) according to any of claims 1 to 14, characterised in that the means (15) for providing electric power are arranged into a flame-proof casing.