Low Pressure Mercury Vapour Electronic Discharge
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Published: Tue, 02 May 2017
A common discharge lighting luminary for use in an office environment would typically be a low-pressure mercury-vapour electronic discharge (fluorescent) lamp.
The lamp is constructed from a hollow glass tube with an aluminium cap at each end. Within the tube are two electrode coils, one at either end. The inner surface of the glass tube is coated in phosphor, and the tube, containing a small amount of mercury vapour, is filled at low pressure with an inert gas such as argon or krypton.
Construction of a typical fluorescent lamp
Block diagram showing fluorescent lamp and associated circuitry
An automatic starting switch (starter) is used to initialise the flow of electrons from a coated filament cathode, which then collide with mercury vapour atoms, exciting their electrons to a higher energy state. This higher energy state is unstable and returns to a lower, more stable level, and in so doing produces a very small amount of blue-green light and a large amount of ultraviolet radiation. When the UV radiation comes into contact with the phosphor coating it causes the phosphor’s electrons to reach a higher energy state, which when returning to a normal level give off visible light; the colour of which is dependant upon the chemical composition of the phosphor.
Fluorescent lamps are negative resistance devices, meaning that as the current increases, the electrical resistance decreases, allowing further increase in current flow. If the rise in current flow were to be uncontrolled the lamp would quickly self-destruct. To prevent this, a ballast device is used in order to regulate the current flow.
Fluorescent lamps are a well established standard for general lighting in industrial, commercial and domestic applications. They come in a range of standardised sizes, power ratings, white colours, and colour temperatures. Other benefits of using fluorescent lamps compared to, for example, incandescent lamps, is that they are more energy efficient and have a longer life, typically 10 – 20 times longer than an incandescent lamp. When switched on they illuminate almost immediately, there is no waiting period whilst they warm up to their operating temperature, which for a fluorescent lamp is room temperature. When switched off, they can be restarted immediately, unlike high pressure sodium lamps which must cool down.
One disadvantage of fluorescent lamps is that in some circumstances they may flicker at twice the supply frequency, causing a stroboscopic effect which, in a workshop type environment, may cause rotating machinery to appear stationary. One way to overcome this is by using lamps with a high-frequency electronic ballast.
A common discharge lighting luminary for use in an indoor sports environment would typically be a high-intensity discharge (HID) type lamp, such as metal halide or high pressure sodium.
High Pressure Sodium
Sodium vapour at high pressure and temperature is highly reactive with glass, which would rapidly fail as a result. So, for a high pressure sodium lamp, a ceramic arc tube body, known as translucent polycrystalline alumina (PCA) and manufactured from aluminium oxide, is used instead; and is itself enclosed by a protective outer glass bulb, which is either evacuated or contains an inert gas.
High pressure sodium lamp
(image from: http://www.hydroyard.com/)
The arc tube is evacuated of air and volatile contaminants, a dose of sodium-mercury amalgam and a filling gas of either argon or xenon are introduced. The type of gas filling is chosen for its ability to allow an arc to strike at low pressure. Tungsten electrodes, having a coating of electron-emissive material, connect to the electrical supply and the tube is hermetically sealed.
A high voltage pulse causes the gas within the arc tube to ionise, creating an arc between the two tungsten electrodes, and increases the temperature of the gas. Initially, the arc voltage, due to the low vapour pressures, is low. As the amalgam temperature increases so does the pressure, and thereby the voltage, taking a few minutes for the lamp to reach its optimum operating condition.
Once the lamp has been switched off, it needs to cool for a short while before being able to restart.
The ballast, as with the fluorescent tube, controls the current to ensure stable operation.
Diagram showing basic construction of a high pressure sodium lamp
Some of the benefits of using high pressure sodium lamps in an indoor sports environment are that they give good colour rendering which is important where teams need to clearly identify each other by their colours. HPS lamps also provide a whiter light which allows players to clearly identify the sports equipment, especially where that equipment is used in fast moving sports such as the ball in a game of squash.
Escape Route Lighting
It is essential that every workplace has a means of egress during an emergency, and this requires a system of emergency lighting be fitted to ensure that evacuation can be done in a safe manner should the normal lighting system fail. BS EN 50172:2004/ BS 5266-8:2004 details the requirements for emergency lighting systems.
Legislation requires that lighting be sited in ‘points of emphasis’ along the exit route, covering areas such as each exit door, intersections, changes of direction and floor level along the route, stairways, fire fighting equipment, alarm points, potential hazards, emergency escape signs, first aid points, equipment and machinery that requires shutting down in an emergency, outside and near to each final exit. The term ‘near’ is defined as within 2 metres, measured horizontally.
Examples of points of emphasis
At each exit door At each piece of fire fighting equipment
and alarm call point
At each intersection of corridors Near each first aid post
The escape route must attain a minimum level of illuminance, additionally, every compartment on the escape route must have at least two luminaires in order to provide some light should one fail.
BS 5266 Pt 7: 1999 (EN1838) details the Light Level Requirements, “a minimum of 1 lux anywhere on the centre line of the escape route for normal risks. A uniformity ratio of 40:1 maximum to minimum must not be exceeded. This illuminance must be provided for the full duration and life of the system. 50% of the illuminance must be available within 5 seconds and the full value within 60 seconds of supply failure.” (http://www.cooper-ls.com/dg_emersystem.html)
High Risk Task Area Lighting
Guidance for the requirements of provision of emergency lighting in high risk areas is given in BS 5266 Part 10: 2008.
The provision of emergency lighting in High Risk Task Areas must be sufficient enough to allow the safe shutting down of machinery and equipment in areas such as workshops, control and plant rooms, switchgear rooms, production lines, laboratories, or any other areas where potentially dangerous situations or processes are likely to occur, and which might affect the safety of the workforce or other occupants.
It is a requirement that for high risk areas the maintained illuminance should be not less than 10% of the normal maintained illuminance on the reference plane for the task, and not less than 15 lux, whichever is higher. It should have a maximum uniformity ratio of 10:1, and a response time of at least 0.5 seconds. It should also be free from stroboscopic effects.
Most emergency lighting systems fall into three types, Maintained, Non-maintained and Sustained.
A maintained system is one where the luminaire uses the same lamp for both standard and emergency use, can be switched on or off in the same manner as a normal light, but once the regular power supply is lost the lamp will illuminate, using its back-up battery pack. A maintained system has an M designation and a number indicating the emergency duration in hours, e.g. M2.
A non-maintained system is one that illuminates only when the mains supply fails. It is designated NM and, again, is followed by a number that indicates its duration of illumination.
A sustained system is a combination of the maintained and non-maintained systems, and contains two (or more) lamps within a luminaire. Each lamp is supplied independently, one by the mains supply and the other by the battery back-up for when the mains supply fails. This system is designated S and also has a number indicating the emergency duration.
Maintained systems should generally be used in locations such as pubs and bars and other premises where alcohol is served, along with public areas where the lighting levels can be reduced to below the levels required for escape route illumination.
Sustained systems can be considered for places that may require safe lighting at all times, e.g. hallways and stairwells in areas of accommodation such as hotels or halls of residence, where evacuation at night may be necessary, even if no power supply failure has occurred. As an energy and lamp-life saving measure it may only be necessary to have the lamps illuminate during night time, or other periods of low ambient light, activated by a motion sensor during normal operation, and automatically in emergency.
Non-maintained systems can generally be considered for all other situations.
A duration of emergency illumination lasting between 1 and 3 hours can be considered sufficient for most situations.
BS 5266 gives detailed guidance regarding which category may be most suitable for a given situation and location.
Back-up power supply
Providing a source of power in the event of a mains supply failure can be done by one of two main methods; either by battery or generator. There are two distinct types of battery systems, a self-contained system whereby each luminaire contains a battery, charger, and changeover device; and a central system where these items are located in one room and which supply all the luminaires in the system. Using the latter method, it is essential that the wiring be of a high standard as there may be a risk of loss of power due to fire damage, and also poor performance due to voltage drop over long cable runs. Therefore, the former may be preferable to overcome these drawbacks; installation is simpler and requires little maintenance other than routine testing. Where a back-up generator is used, it should be either run at all times or be able to start automatically and run up to provide the required output level in 5 seconds or fewer. Where neither of these conditions can be met, it should be supplemented by a battery back-up system, that is capable of running the emergency lighting system for one hour. In this situation the generator need not be required to start automatically, but be available to take over from the battery back-up as soon as possible.
Cooper Lighting and Safety states that illumination requirements for signage “must conform to the colours of ISO 3864, which defines that exit and first aid signs must be white with green as the contrast colour. The ratio of luminance of the white colour to the green colour must be between 5:1 and 15:1. The minimum luminance of any 10mm patch area on the sign must be greater than 2cd/mÂ² and the ratio of maximum to minimum luminance shall be less than 10:1 for either colour.” (http://www.cooper-ls.com/dg_emersystem.html)
Example of emergency exit sign conforming to ISO 3864
Number of luminaires required
The luminaires require an electrical input of 58 watts and suffer losses of 18%, which result in a power rating of 41 watts. Taking into account a correcting factor for this power rating of 1.04, multiplied by the Utilisation Factorâ€¦
â€¦the number of luminaires required is 50.28; rounding down to 50 to give a common sense workable number.
Taking a space/height ratio of 1.7, and the height of the luminaire above the working plane being 3.2 metres, the space between luminaires should be a maximum of 5.22 metres.
Arranging 50 luminaires to best fit a room of dimensions 20m x 15m gives an arrangement of 5 luminaires by 10 luminaires, with the length of each luminaire orientated across the width of the room.
The spacing between each luminaire across the width of the room would be 1.5 m with 0.7m between the luminaire and the wall at each end.
The space between the centre of each luminaire down the length of the room would be 2m with 1m between the centre of the luminaire and the wall at each end.
Both of these figures fall well within the maximum spacing determined by the space/height ratio.
Although to be mathematically accurate in determining the minimum number of luminaires required, one would usually round up; real world considerations need to be taken into account such as arranging the luminaires to fit the room’s dimensions, and the additional costs involved in adding substantially more luminaires required just to make a nice aesthetic pattern. In the question, no consideration has been given to the availability of natural light, nor the range of commercially available lamps and luminaires which could be more efficient and effective than the given figures suggest.
Diagram showing orientation and spacing of luminaires
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