GB2466966A

GB2466966A - Electrical housing including a pressure equalizing device

Info

Publication number: GB2466966A
Application number: GB0900648A
Authority: GB
Inventors: Geoffrey Lunn
Original assignee: VML TECHNOLOGIES BV
Current assignee: VML TECHNOLOGIES BV
Priority date: 2009-01-16
Filing date: 2009-01-16
Publication date: 2010-07-21
Also published as: GB0900648D0

Abstract

An electrical weatherproof housing 11 including an electrical component 14 and a volume adjustment means 30,42 which responds to changes in the internal pressure relative to the external pressure. Such changes in pressure are likely to occur in response to temperature differences between the interior and exterior of the housing. In one arrangement the volume adjustment 10 means 42 comprises a resilient deformable membrane provided in or on the housing. Alternatively the volume adjustment means 30 may include a moveable spring biased plunger 34 that changes position in response to a change in pressure (see figure 2). The housing is suitable for LED lamps.

Description

ELECTRICAL FITTING

The present invention relates to an electrical fitting and particularly to an electrical fitting which is suitable for use outdoors or in adverse conditions such as high humidity.

Electrical fittings of the type referred to in this patent generally comprise one or more electrical component located in a housing. The purpose of the housing is to provide an easy means of handling the fitting, a compact aesthetic appearance and a robust shell that prevents the electrical components being damaged or becoming dirty or damp. Some of those requirements are more critical than others, depending on the application of the electrical fittings. For instance, electrical fittings that are to be used in wet or humid conditions require a sealed housing that prevents the ingress of moisture. However, this is sometimes difficult to achieve for several reasons.

The cases of some electrical fittings provide a hermetic seal around the electrical components by permanently sealing the parts of the case during the assembly process and providing no opening in the case for accessing a battery compartment or for the passage of a power cable. However, some cases cannot be permanently sealed as the electrical components may need to be accessed and openings may need to be provided for the passage of a power cable or the mounting of an electrical connector or switch. In such instances, rubber seals or the like are provided around the openings or joints in the case, but over time these are likely to fail.

The temperature inside the case can rise or fall significantly dependant on external environmental conditions and operation of internal electrical components. This causes the pressure inside the case to rise all fall in accordance with the Combined Gas Law and more specifically Gay-Lussac's Law. These laws state that the pressure of a fixed volume of gas is proportional to its temperature and can be expressed as P11T1 = constant = P21T2, where T1 and P1 are the initial temperature and pressure and T2 and P2 are the final temperature and pressure. It therefore follows that Ti/T2 = AP. As the external pressure does not change significantly, any change in internal pressure is likely to create a pressure differential between the interior and exterior of the housing. If the pressure differential is significant, any seal provided on the housing is likely to be affected. For example a seal may be drawn inwardly if the internal pressure is below that of the ambient and drawn outwardly if the internal pressure is greater than the ambient pressure.

For example, a typical electrical fitting may be manufactured in a factory having an ambient temperature of 25°C and a pressure at sea level of I atmosphere and when switched on the electrical components are likely to raise the temperature inside the case by about 15°C. The electrical fitting may be used on an autumn day in Scotland where the temperature can fall to zero degrees at night and can rise to 25°C in direct sunlight. According to the equation above, once the electrical fitting is switched on, the temperature rises from 25°C (298K) to 40°C (313K) and the pressure increases by 6%. In the evening once the sun goes down and the electrical fitting is switched off the temperature in the case is likely to fall from 4000 (313K) to 0°C (273K) which creates a pressure drop in the case of 15%.

It is a principal object of this invention to provide an electrical fitting suitable for use outdoors or in humid conditions. A further object of the invention is to provide an electrical fitting having a sealed case, whereby the integrity of that sealed case is not compromised as the internal air temperature varies.

According to the invention, there is provided an electrical fitting comprising: -a weatherproof housing; -an electrical component within that housing; and -a volume adjustment means provided in or on the housing that is operable to vary the effective volume within the housing in response to changes in the internal pressure relative to the external pressure.

As described above, it is the pressure differential between the interior and exterior of the housing that is likely to damage the seals, so the volume adjustment means serves to equalise those pressures by adjusting the effective volume within the housing. In one arrangement, the volume adjustment means may be subject to the internal pressure and the external pressure and it may continue to adjust the volume until those pressures are equal. This arrangement is preferable because the volume adjustment means may not need calibrating.

In an alternative arrangement, the internal pressure of the housing and the pressure of a secondary source may both act against the volume adjustment means. For instance, the secondary source may comprise a thermally insulated canister containing air that will remain at a substantially constant temperature and therefore not be subject to the temperature related pressure changes to which the housing is subject. The volume adjustment means could operate simultaneously to vary the effective interior volume of the canister and the effective volume of the housing. This arrangement would require the volume adjustment means to be calibrated.

In one arrangement the volume adjustment means may be formed from a sheet of resiliently deformable material, such as a diaphragm, which deforms in response to a change in internal pressure to adjust the effective internal volume of the housing. Such an arrangement could be very simple and take the form of a balloon that may inflate or deflate depending on the change in internal pressure of the housing and the ambient.

The resiliently deformable material may be part of the housing so that it is at a junction between the internal and external volumes. For instance, an opening may be formed in the housing and the deformable material may be mounted to the housing to cover and seal the opening, such that as the internal pressure in the housing rises above the external pressure, the material deforms and bulges outwardly of the housing to increase the volume of the housing.

Conversely, when the internal pressure of the housing falls, the material will bulge inwardly of the housing. Elastic materials must be carefully selected according to their modulus of elasticity, as they must deform appropriately when subject to the pressure changes likely to occur in the housing. If the modulus of elasticity is too high, the material is unlikely to deform sufficiently when subject to the pressure changes in the housing and indeed the seals may be damaged.

Advantageously, the resiliently deformable material may be attached to support means mounted to the housing so as to permit removal of the deformable material should it become punctured, perished or otherwise worn.

For instance, the material may be adapted to overlie an opening in the housing and the support means may include an annular frame which may be fastened to the housing around the periphery of the opening. In so doing the material may be trapped between the housing and the frame to seal the opening.

In an alternative arrangement the volume adjustment means may include a movable member that changes position in response to a change in internal pressure to adjust the effective volume of the housing. The movable member may form part of the housing or it may be located inside the housing. In one arrangement the movable member may be adapted for pivoting movement, though in an alternative arrangement it may be adapted for sliding movement.

The particular type of movable member is likely to depend on the spatial relationship between the housing and the volume adjustment means and the pressure change inside the housing, which may depend on the electrical components to some extent.

For instance the movable member may comprise a plunger that may include a piston and preferably a spring arranged to bias the piston so as to maintain a positive pressure in the cylinder. It is advantageous to maintain a positive pressure inside the housing so that if the seals do for some reason fail, moisture from the exterior of the housing is unlikely to be drawn into the housing, instead air is likely to be drawn out.

In an alternative arrangement the volume adjustment means may be provided on a section of the housing such that the housing itself is modified to adjust its effective internal volume. For example, the housing may be substantially tubular and have one or more convoluted portion that concertinas as the internal pressure of the housing changes.

The shape of the housing and the materials from which it is made are likely to depend on the type of electrical fitting. For instance, a light fitting may be generally elongate and take the form of a tube having closed ends. So that the tube may be easily accessed to replace components and to facilitate easy manufacture and assembly, at least one of those ends may be provided with a removable end cap that sealing engages the housing. If access into the housing is unlikely to be required, the at least one cap could be bonded to the tube after assembly with the application of heat or a suitable adhesive.

Alternatively or additionally, the tube and cap could form an interference fit.

Preferably, the tube and cap form a close fit and a seal could be provided therebetween.

In a preferred arrangement the volume adjustment means may be provided on such an end cap, or indeed any cap designed to close the housing.

This may be more cost effective as the tube may be formed by an extrusion process without requiring subsequent machining, whereas the cap is likely to be manufactured by a moulding or possibly a machining process, both of which being suitable for forming the volume adjusting means. Furthermore, the volume adjustment means may take various shapes and sizes, but preferably it is at least partially hidden by the electrical component.

The electrical components may include at least one light source and the tube may be made from a transparent or translucent material to allow light emitted from the at least one light source to pass therethrough. Most preferably the at least one light source includes at least one LED, though other light sources such as incandescent or fluorescent sources could be used.

An aperture may be provided on the housing and cables connected to the electrical components may pass through the opening. The cables may be power cables or data cables and a seal may be provided between the periphery of the opening and the cables. Alternatively or additionally, connection means connected to the electrical components may be provided on the exterior of the housing and to which cables having co-operating connection mean may attach.

The housing may also include support means by which the electrical fitting may be secured to a wall, ceiling or other support.

In order that it may be better understood, but by way of example only, three embodiments of the invention will now be described in detail, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of an electrical fitting, which shows features that are common to the first and second embodiments of the invention; Figure 2 is a sectional view of the first embodiment taken along the line AA shown in Figure 1 and having a movable plunger to alter the effective internal volume of the electrical fitting; Figure 3 shows a sectional view corresponding to Figure 2, but with the plunger in a different position to reduce the effective volume; Figure 4 is a sectional view of a second embodiment taken along the same line AA of Figure 1, having a deformable material diaphragm shown at a position where it decreases the effective interior volume of the electrical fitting; Figure 5 is a sectional view of the second embodiment corresponding to Figure 4, but with the diaphragm shown in a position where it increases the interior volume; Figure 6 is a sectional view of a third embodiment of the invention having a different plunger; and Figure 7 is a sectional view corresponding to Figure 6, but with the plunger shown in a position to increase the internal volume of the electrical fitting.

Referring initially to Figures 1 to 3, there is shown a first embodiment of an electrical fitting in the form of a light unit 10 adapted for use outdoors or adverse conditions. The light unit comprises an elongate cylindrical tube 11, end caps 12, 13 mounted on opposed ends of the tube and a PCB 14 mounted inside the tube. The PCB 14 is generally elongate with LEDs 15 and associated circuitry disposed along its length and is secured in place within the tube by a suitable adhesive. The tube 11 is made from a transparent plastics material so that light emitted from the LEDs 15 is transmitted through the tube 11, though the material must also be sufficiently robust to protect the circuitry and resist scratching or cracking.

Each end cap 12, 13 comprises a circular plate 18 having opposed inner and outer faces 19, 20 and an annular flange 21 extending axially from its inner face 19. The plate 18 is diametrically larger than the flange 21 so as to define a lip 22 which bears against an end face of the tube 11 while the flange 21 locates inside the tube 11. The annular flange 21 has an inner circumferential face 25 which defines part of the interior volume of the light unit 10 and an outer circumferential face 26 in which there is formed a groove (not shown) and an 0 ring 28 is seated in said groove and arranged sealingly to engage the inner circumferential face 29 of the tube 11. Other seals may of course be used depending on the construction. The cap 13 includes an aperture 37 extending axially therethrough to accommodate a power cable 38 connected to the PCB 14, although the unit could be self-contained and battery powered.

Cap 12 is slightly different from cap 13 insofar as it includes a volume adjustment mechanism 30 for adjusting the effective internal volume of the light unit 10 in response to pressure differentials between the interior of the unit and the ambient environment. An inwardly turned lip 32 is provided at the free end of the flange 21, which, in combination with the inner circumferential face 25 of the flange 11 and the inner face 19 of the plate 18 defines a chamber 33. A piston 34 is arranged to slide along the chamber 33 such that its circumferential face engages the inner circumferential face 25 of the flange 21 with a gas tight seal. Holes 35 are provided through the plate, so that air may pass to and from the chamber 33 as the piston 34 slides therealong. A helical coil spring 36 acts between the piston 34 and the inner face 19 of the plate 18 to urge the piston in the direction of arrow B so as to maintain a positive pressure in the light unit.

In use, throughout the course of the day the air temperature in the light unit 10 can vary by as much as 40°C due to the changes in ambient temperature and also the heat generated by the LEDs and circuitry. This change in temperature causes a change in pressure inside the unit 10. When the temperature in the unit 10 increases so too does the air pressure and when the pressure is greater than the combination of the external pressure and the pressure exerted by the spring 36, the piston 34 slides in the direction of arrow C. When the temperature drops, the pressure inside the unit 10 also drops and becomes less than the combination of the external pressure and the pressure exerted by the spring 36, thereby causing the piston 34 to move in the direction of arrow B. Because the piston 34 responds to small changes in air pressure to effect an equivalent adjustment of the effective volume of the apparatus, the air pressure within the apparatus cannot change enough to damage the seals.

A second embodiment of the invention is shown in Figures 4 and 5.

Features of the invention that are common to the first and second embodiments have like reference numbers. In those figures, there is shown a lighting apparatus generally indicated 40 and comprising a tube 11 containing a PCB 14 and having end caps 13, 41 mounted at opposed ends. The end cap 13 is identical to the basic end cap 13 described in the first embodiment and attached to the tube in the same way. End cap 41 is different insofar as it includes a diaphragm of elastically deformable material 42 provided on the inner end of the flange 21. The material 42 must be of a non-porous type and form a hermetic seal around the flange 21 such that no air may pass therethrough. The location of the material 42 means that it is protected by the plate 18 from being damaged during transportation, installation and use of the lighting apparatus 40. A large opening could be formed in the plate 18 of the end cap 41 and the material 42 could be mounted on the plate to seal the opening, but it is more likely to be damaged this way.

In use, as the temperature and consequently the pressure inside the light unit decreases, the diaphragm 42 stretches inwardly thereby reducing the internal volume of the light unit and equalising the internal and external pressures. Conversely, as the temperature increases, the material expands outwardly as shown in Figure 5 to increase the effective volume and reduce the pressure. During assembly the material could be fitted in a neutral position, such that it is taught but not stretched in any direction.

A third embodiment of the invention is shown in Figures 6 and 7 which discloses a light unit generally indicated 60 having volume adjustment mechanism similar to that included in the first embodiment. Features of the third embodiment that are present in the first embodiment will be given like reference numerals to those of the first embodiment. Light unit 60 comprises a tube 11 containing a PCB 14 and having end caps 13, 61 mounted at opposed ends. The end cap 13 is the same as that described in the first embodiment, but the end cap 61 is slightly different. End cap 61 comprises a circular plate 18 having an annular flange 21 extending concentrically from the inner face 19 of the plate.

The volume adjustment mechanism comprises a cylindrical tube 65 extending eccentrically from the inner face 19 of the circular plate 18 and a cover 66 mounted to the free end of said tube 65. A piston 67 is slidably mounted in the interior of the cylindrical tube 65 and it has an outer peripheral face that engages an inner circumferential face 68 of the cylindrical tube 65 to form a sliding gas seal. An inner chamber 69 is defined between the cover 66 and the piston 67 and an outer chamber 70 is defined between the plate 18 and the piston 67. The inner chamber 69 communicates with the interior of the tube 11 through hole 71 formed through the cover 66. The outer chamber 70 communicates with the ambient atmosphere through an aperture 72 extending axially through the plate 18. In case the lighting unit 60 is to be mounted end to end with another similar lighting unit or against a wall where aperture 72 may be obstructed, a second aperture 73 is defined in the plate 18 extending radially between the bore 72 and the circumferential face of the cap 61. A spring 74 disposed in the outer chamber 70 acts between the plate 18 and the piston 67 to create a slightly positive air pressure inside the light unit 61.

In this third embodiment the PCB 14 is mounted on taller stilts 75 so as to form a space between the underside of the PCB and the tube 11, and in which space the cylindrical tube 65 locates.

Claims

CLAIMS1. An electrical fitting comprising: -a weatherproof housing; -an electrical component within that housing; and -a volume adjustment means provided in or on the housing that is operable to vary the effective volume within the housing in response to changes in the internal pressure relative to the external pressure.
2. An electrical fitting as claimed in claim 1, wherein the volume adjustment means is formed from a resiliently deformable material which deforms in response to a change in internal pressure to adjust the effective volume of the housing.
3. An electrical fitting as claimed in claim 2, wherein the resiliently deformable material is part of the housing.
4. An electrical fitting as claimed in claim 2 or claim 3, wherein the resiliently deformable material is attached to support means mounted to the housing.
5. An electrical fitting as claimed in claim 1, wherein the volume adjustment means includes a movable member that changes position in response to a change in internal pressure to adjust the effective volume of the housing.
6. An electrical fitting as claimed in claim 5, wherein the member is part of the housing.
7. An electrical fitting as claimed in claim 5 or claim 6, wherein the member comprises a plunger.
8. An electrical fitting as claimed in claim 7, wherein the plunger includes a piston and a spring arranged to bias the piston so as to maintain a positive pressure in the cylinder.
9. An electrical fitting as claimed in in any preceding claim, wherein the volume adjustment means is provided on a cap over an opening in the housing.
10. An electrical fitting as claimed in any preceding claim, wherein the housing is tubular with closed opposed ends.
11. An electrical fitting as claimed in claim 10, wherein at least one of those ends is provided with a removable end cap that sealing engages the housing.
12. An electrical fitting as claimed in any proceeding claim, wherein the volume adjustment means is concealed by or positioned beneath the electrical component.
13. An electrical fitting as claimed in any preceding claim, wherein the electrical components include at least one light source.
14. An electrical fitting as claimed in any preceding claim, wherein a connector electrically connected to the electrical components is provided on the housing and connectable to external components or a power supply.
15. An electrical fitting as claimed in claim 1 and substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.