WO2014029772A1

WO2014029772A1 - Method of and system for isolating led luminaires from electrical surges

Info

Publication number: WO2014029772A1
Application number: PCT/EP2013/067314
Authority: WO
Inventors: Johan SCHLERITZKO; Yves Borlez
Original assignee: Schreder
Priority date: 2012-08-20
Filing date: 2013-08-20
Publication date: 2014-02-27
Also published as: ZA201501245B; CN204634080U; AU2013305063A1

Abstract

Described herein is a system for isolating an LED luminaire (610) from an electrical surge, the LED luminaire comprising a housing (614) to accommodate an LED driver (616) to drive a PCB (618) fitted with LEDs (620), the PCB and LEDs being connected to a heat sink (622), the system comprising a reinforced isolating member (624, 628) being provided within the electrical path from a mains power supply (626), for providing electricity to the LED driver (616), to ground, so as to isolate the luminaire from ground as opposed to simply grounding the luminaire to earth.

Description

METHOD OF AND SYSTEM FOR ISOLATING LED LUMINAIRES FROM

ELECTRICAL SURGES

Field of the invention

This invention relates to a method of and system for isolating, and therefore protecting, light-emitting diode (LED) luminaires from electrical surges, and in particular, surges generated by either lightning strikes or faults on high voltage lines Background of the invention

LED based luminaires are gaining widespread acceptance, primarily due to their inherently high efficiency and low power consumption.

However, these luminaires use delicate electronic components that are more sensitive to high voltage surges than more traditional lamp technologies.

Luminaires are conventionally protected from high voltage surges by lightning protection circuits or devices implemented in different ways.

During an abnormal condition generated either by a lightning strike on the ground or a fault on a high power line, current can flow through the ground and generate high potential differences between the groundings located several hundred meters apart from each other. Furthermore, the high density of current flowing through a lightning strike is capable of generating, by inductive coupling, a relatively high common mode voltage on the supply cables of the luminaires. Different earthing points may be used which can have very different potentials during the lightning strike or a fault on the power line.

Surges can also be generated in a differential mode. In particular, they can be caused as a result of an imbalance of the connecting wires during inductive strikes or by overvoltage propagating over the supply line during transients. Usually, this mode can be protected by appropriate surge arrestors (for example, gas discharge tube, meta! oxide varistors, semiconductor transient suppressors, etc.) arranged between the two wires of the power supply. Such components allow efficient protection if they are connected in such a way to provide a clear path for the lightning strike, which would then prevent any destruction of the equipment.

Thus, as described above, several mechanisms are capable of producing high voltage strikes that either raise the local earth potential with respect to the power lines or the power lines with respect to the local earth. In both these cases, such a difference of potential will put certain electrical constraints on the Iuminaire.

In many cases, electronic ballasts cannot perform the role of a perfect isolator between their primary and secondary sides. This is primarily due to the parasitic capacitance between the windings or even the electromagnetic compatibility (EMC) capacitance that is usually used between the primary and secondary side of the transformer within the ballast. Additionally, the LED circuitry is not perfectly isolated with respect to the frame, as electrical safety constraints require it to be only of about 1500V. This frame is usually connected to the ground via either a protective earth wire connection or the pole itself, if it is made of a conductive material.

As a result, when a high voltage difference of potential is present between the mains wires and the ground to which the Iuminaire body is connected, a disruption can occur inside the LED driver and LEDs themselves and the housing frame that can generate destructive currents for either the driver or the LEDs.

The ground return is a key element for typical surges. In order to protect the Iuminaire, it is required to find ways to increase the level of the disruption voltage between the main lines and the frame of the iuminaire or to provide a shorter path for the strike to exit between the lines and the earth. One such way is by providing a short path from the main lines to ground where a reliable Protective Earth (PE) connection is available in the luminaire, that is, Class I type luminaires according to IEC 60598-1 . In such cases, the PE connection can be used to provide a short return to ground for the strike. In this case, metal oxide varistor (MOV) devices can be used between each of the lines and the PE connection.

In cases where no PE is available, a functional earth is sometimes provided. This can be either a connection to ground with no safety related function, or a direct connection to earth via the metallic frame of the luminaire and the metallic pole to which the luminaire is fitted. In this case, due to electrical safety requirements, no MOVs are allowed to be directly connected between the lines and this functional earth. Some solutions use an additional gas discharge protector to achieve the proper isolation required by Class II type luminaires, but there is always a trade off between providing a sufficient isolation for safety and allowing the strike to jump from the lines to the ground.

In CN-A-2015891 56, an LED street light is described in which a lightning arrester is provided on a heat dissipation body external to an LED lamp group. A power input wire is connected to the lamp group via the lightning arrester which is electrically connected with a lightning grounding wire. When a lightning strike occurs, high voltage currents generated by the strike are directed to the lightning arrester and then to ground via the lightning grounding wire.

In CN-A-2010127625, a lightning protective LED street lamp is described having at least two drivers, each driver being connected to at least one LED and being provided with a protection circuit for absorbing surge pulses. At least one secondary driver is provided having a protection circuit so that the street lamp can still operate even though one of the drivers may be damaged or LEDs are partially turned off due a lightning strike, in this case, the overall cost of the street lamp is increased due to duplication of drivers which are required to ensure that the street lamp can still operate under certain lightning strike events.

Moreover, in both the documents described above, the protection circuits disclosed are directed to protection against direct lighting strikes and do not address the problem associated with high voltage surges generated by indirect lightning strikes and the associated ground potential rise which occurs as a result, or as a result of a fault condition on a high power line. Ground potential rise occurs when large amounts of electricity enter the earth. When currents of large magnitude enter the earth from a grounding system, not only will the grounding system rise in electrical potential, but so will the surrounding soil.

This has the disadvantage that grounded luminaires also become susceptible to this rise in electrical potential causing damage to electrical and electronic components within the luminaire, and in particular, luminaires which cannot utilize PE protection such as Class II luminaires according to IEC 60598-1.

Summary of the invention

The aim of the present invention is thus to provide an effective solution for isolating, and therefore protecting, LED luminaires from electrical surges, taking the above considerations into account.

According to the invention, there is provided a system for isolating an LED luminaire from an electrical surge, the LED luminaire housing comprising a housing to accommodate an LED driver to drive a

PCB fitted with LEDs, the PCB and LEDs being connected to a heat sink, the system comprising a reinforced isolating member being provided within the electrical path from a mains power supply, for providing electricity to the LED driver, to ground, so as to isolate the luminaire as opposed to simply grounding the luminaire to earth.

In an embodiment, the isolating layer is capable of sustaining the voltages that are usually encountered during surges. Typically, these voltages can reach a value up to 10kV, which is higher than the usual isolation requirements needed for electrical safety.

In an embodiment, the reinforced isolating member comprises an isolating layer sandwiched between the PCB and the heat sink.

In an alternate embodiment, the reinforced isolating member comprises an insulating spacer between the luminaire housing and a pole to which the housing is to be mounted.

In yet a further alternate embodiment, the reinforced isolating member comprises an isolated pole to which the housing is to be mounted.

In an embodiment, the luminaire is typically a class II luminaire according to IEC 60598-1.

Brief description of the drawings

For a better understanding of the present invention, reference will now be made, by way of example, to the accompanying drawings in which:-

Figure 1 shows one way in which a surge may be generated and the problem of different ground potentials in the context of an electrical strike;

Figure 2 shows a further way in which a surge may be generated, namely a surge generated in a differential mode;

Figure 3 shows one possible way in which current can flow as a result of a surge, namely through the heat sink of the luminaire and then through the frame; Figure 4 shows a basic circuit diagram for protecting a luminaire by providing a short path to ground, in cases where a reliable PE is available;

Figure 5 shows one way of protecting the luminaire, according to a first aspect of the invention;

Figure 6 shows another way of protecting the luminaire, according to a second aspect of the invention; and

Figure 7 shows yet a further way of protecting the luminaire, according to a third aspect of the invention.

Description of the invention

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.

The term "Class I luminaire" as used herein refers to a luminaire in which protection against electric shock does not rely solely on basic insulation but which also includes an additional safety precaution connecting accessible conductive parts to a protective (earthing) conductor in the fixed wire of the installation. Due to such a connection, accessible conductive parts cannot become live in the event of failure of the basic insulation.

The term "Class II luminaire" as used herein refers to a luminaire in which protection against electric shock does again does not rely solely on basic insulation but which also includes additional safety precautions, such as, double insulation or reinforced insulation. These luminaires do not have provision fro protective earthing or reliance upon installation conditions. It will readily be appreciated that, although embodiments of the present invention will be described with respect to Class II luminaires, these embodiments are not limited thereto.

Figure 1 illustrates schematic layout of a high-to-!ow voltage transformer housing 100 connected to a plurality of luminaires 1 10, 120, 130, 140 and to a high voltage electricity pylon 150, the transformer,, luminaires and high voltage electricity pylon being grounded as shown at 105, 115, 125, 135, 145 and 155 respectively. Here, each luminaire is mounted on a post as shown. Although not shown, it will be appreciated that the luminaires are spaced a few tens of meters away from one another and each luminaire may be spaced several hundreds (or even thousands of meters) away from the transformer housing and high voltage electricity pylon. Similarly, the electricity pylon 150 and the transformer housing 100 can also be spaced several hundreds or thousands of meters, away from one another. As discussed above, during an abnormal condition generated by such a lightning strike on the ground (or a fault on a high power line), current can flow through the ground and generate high potential differences between the groundings with the high density of current flowing through a lightning strike generating, by inductive coupling, a relatively high common mode voltage on the supply cables (not shown) of the luminaires 110, 120, 130, 140. The different earths 105, 1 15, 125, 135, 145, 155 shown in Figure 1 , namely earth 1 15, 125, 135, 145 for the light poles on which the luminaires 110, 120, 130, 140 are mounted, earth 105 for the transformer housing 100 and earth 155 for the high voltage pylon 150, can have very different potentials during the strike or a fault on the power line.

Turning now to Figure 2, a schematic cross-sectional view through a Class I type luminaire 200 is shown. The luminaire 200 comprises a housing 2 0 in which an LED driver 220, a printed circuit board (PCB) 230 containing a plurality of LED elements, shown generally as 240, a heat spreader or dissipation device 250 and a window 260 are mounted or positioned. The PCB 230 is connected to the LED driver 220, which, in turn, is connected to a power or mains supply 270. As shown, the PCB 230 is mounted on the heat spreader or dissipation device 250 above the window 260 so that light generated by the LED elements 240 can be transmitted outside the housing 210 to illuminate an area or region of interest (not shown). A protection earth (PE) connection 280 is also provided on the housing 210 which provides a short path to ground under lightning strike conditions. Surges can be generated in a differential mode, the surges being caused as a result of an imbalance of the connecting wires during inductive strikes or by overvoltage propagating over the supply Sine during transients. In this case, a surge is generated due to an imbalance between LED 240' and the LED driver 220, the flow of current being indicated by arrow Ά'.

Usually, as is also shown in Figure 2, appropriate surge arrestors {for example, gas discharge tube, metal oxide varistors, semiconductor transient suppressors, etc.) can be connected between the two wires of the power supply as indicated at 290. Such components allow efficient protection if they are connected in such a way to provide a clear path for the strike, preventing any destruction of the equipment within the housing 210 of the luminaire 200.

Figure 3 is similar to Figure 2 but illustrates the flow of current due to a lightning strike 300. Features which have previously described with reference to Figure 2 are numbered the same. In this case, current flow due to the lightning strike 300 flows as indicated by arrow 'B' with current flow from each LED through the heat spreader or dissipation device 250 being indicated by arrow 'C. In this case, the LED circuitry is not perfectly isolated with respect to the frame or housing 210, as electrical safety constraints require it to be only of about 1500V. This frame or housing 210 is usually connected to the ground via either a PE connection 280 or the pole itseif, if it is made of a conductive material. As shown in Figure 3, when a high voltage difference of potential is present between the mains wires and the ground, a disruption can occur inside the LED driver and LEDs themselves as well as in the housing frame that can generate destructive currents for either the LED driver or the LEDs.

Figure 4 illustrates the use of a PE connection at the mains supply. As shown, MOVs are connected across the mains supply and also across a connection between the neutral wire and the PE connection. The PE connection provides a short return to ground for a lightning strike.

Referring first to Figure 5, according to one embodiment, an LED luminaire 612 is shown for which isolation of electrical components is provided with respect to ground. The invention comprises a system 610 for isolating an LED luminaire 612 from an electrical surge is shown. The LED luminaire 612 comprises a housing 614 to accommodate an LED driver 616 to drive a PCB 618 fitted with LEDs 620. The PCB 618 and LEDs 620 are connected to a heat sink 622 to facilitate heat transfer and dissipation.

The system 610 comprises a reinforced isolating member 624 being provided within the electrical path from a mains power supply 626, for providing electricity to the LED driver 616, to ground. The present invention thus aims to isolate the luminaire 612 as opposed to simply grounding the luminaire 612 to earth.

In an embodiment, the reinforced isolating member 624 comprises an isolating layer 628 sandwiched between the PCB 618 and the heat sink 622. An envisaged possible shortcoming of this arrangement is poor thermal conductivity within the luminaire housing 614. For this reason, the materia! for the isolating layer 628 may comprise a polyimide material, such as, a KAPTON foil (a polyimide film having good dielectric properties), which not only provides good isolation properties but is also relatively thin, so as to also facilitate relatively good heat dissipation. KAPTON is a trademark of the E. I. du Pont de Nemours and Company (commonly referred to as DuPont) of Wilmington, Delaware, USA.

In an alternate embodiment, as shown in Figure 6, the reinforced isolating member 724 for the luminaire 700 comprises an insulating spacer 730 between the luminaire housing 714 and a pole 732 (typically, metallic) to which the housing 714 is to be mounted. The mains supply 734 for the luminaire passes through the pole 732 to the LED driver 716. A lightning strike 740 to ground does not have any affect on the LED driver 716 and the LEDs 740 within the luminaire housing 714 due to the presence of the insulating spacer 730.

The insulating spacer 730 could be made of any mechanically resistant plastic or even wood to provide a decorative effect. A minimum length, typically at least 5cm, is required to ensure proper clearance between the post 732 and the luminaire housing 714.

Finally, with reference to Figure 7, in yet a further alternate embodiment, the reinforced isolating member 824 comprises an isolated pole 834 to which the housing 814 of the luminaire 800 is to be mounted. The isolated pole 834 could be made of fiber glass. As before, the LED driver 816 and heat spreader or dissipation device 822 is mounted within the housing 814, the LED driver 816 being connected to a mains supply 830 and to a PCB containing LEDs 840.

Advantageously, in the arrangements shown in Figures 6 and 7, during the common mode surge as described above with reference to Figure 3, the whole !uminaire potential will rise. However, since no current will flow within the luminaire, the luminaire will not be damaged.

Although the present invention has been described with respect to specific embodiments, it will readily be appreciated that these embodiments are non-limiting and other embodiments are also possible which provide electrical isolation of the electronic components within the luminaire from ground.

Claims

CLAIMS:

1. A system for isolating an LED luminaire from an electrical surge, the LED luminaire comprising a housing to accommodate an LED driver operable for driving a PCB fitted with LEDs, the PCB and LEDs being connected to a heat sink, the system comprising a reinforced isolating member provided within the electrical path from a mains power supply, for providing electricity to the LED driver, to ground, so as to isolate the luminaire from ground.

2. A system for according to claim 1 , wherein the reinforced isolating member comprises an isolating layer sandwiched between the PCB and the heat sink.

3. A system according to claim 2, wherein the isolating layer comprises a layer of polyimide material.

4. A system according to claim 1 , wherein the reinforced isolating member comprises a mechanically resistant material located between the luminaire housing and a pole on which the luminaire housing is mounted.

5. A system according to claim 4, wherein the mechanically resistant material comprises a plastic material.

6. A system according to claim 4, wherein the mechanically resistant material comprises wood.

7. A system according to any one of claims 4 to 6, wherein the mechanically resistant material comprises a length of at least 5cm.

8. A system according to claim 1 , wherein the reinforced isolating member comprises an isolated poie on which the luminaire housing is mounted.

9. A system according to claim 8, wherein the isolated pole comprises fiber glass.

10. A luminaire including a system according to any one of the preceding claims.

1 1. A luminaire according to claim 10, comprising a Class II luminaire.