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Due to the constantly increasing values of illumination for the various visual tasks, the number of lamps and lighting fittings and the consumption of electric energy are growing too, so that the maintenance of illumination installations becomes more and more important. The discharge lamps, which are preferably used in lighting engineering, have added to the maintenance problem because of their lamp shape, light yield and service life. The deterioration of the illumination or luminance of a lighting installation is caused by a number of factors, for example:
- Ageing of the lamps (reduction of light flux)- Premature failure of lamps
- Getting dusty of lamps and fittings
- Contamination of the limiting surfaces of the room with indoor lighting installations
- Changing of the optically effective parts of the lighting fittings.
Further influence is exercised by deviations of voltage as well as adverse ambient temperatures for fluorescent lamp fittings. Most of all, the dust depositing on lamps and lighting fittings contributes to the decrease in illumination or luminance.
The reduction of the illumination or luminance occurring in the course of operation is expressed in calculations by the depreciation factor (v).
The depreciation factor (v) is the product of the partial depreciation factors (v1, v2 and v3):
v = v1 x v2 x v3
For the majority of indoor lighting installations the ‘normal’ degree of getting dusty is calculated. The degree of ‘much’ getting dusty is applied mainly to installations in surroundings with much dust generation such as cement plants or in connection with coal.
Maintenance work at a lighting installation must be done at the latest if and when the maintenance illumination has reached the minimum admissible value. Here the following values apply:
Indoor, sports fields and outdoor lighting
E ‘ ‘ m = 0.8 En
E ‘ ‘ a = 0.8 En
Street lighting
E ‘ ‘ m = 0.7 En
L ‘ ‘ m = 0.7 Ln.
From this follows, that the maintenance of indoor lighting installations has to be done at the latest, if and when the nominal illumination on the illuminated surface has lowered to the value of 0.8 En, with street lighting installations to the value of 0.7 En. The right time for maintenance is found out by regular measurings of the illumination at fixed control measuring points, at least every half year.
The regulations on maintenance on the basis of permissible minimum operating values in the form of the maintenance illumination obliges the user to provide for the measuring of the illumination at regular intervals. In this connection, it is recommendable to work out a maintenance schedule which should include the following details:
- Position of the control measuring points
- Time of the control measurings
- Lamp replacement
- Cleaning of lamps and lighting fittings
- Renovation of the rooms
- Replacement of optically effective parts of lighting fittings.
The main maintenance times at a lighting installation are the replacement of lamps and the cleaning and - according to the respective kind of lighting installation - the replacement of optically effective parts of fittings. The replacement of lamps can be done in the form of replacing single lamps or groups of lamps.
Replacement of single lamps
With single lamp replacement, each lamp is replaced immediately after failure. On the one hand, this method is very troublesome and hardly justifiable with larger installations. On the other hand, there are always lamps of very different age in one installation as a consequence of this method, which may result in disturbing effects with indoor lighting installations due to different light colours and luminance values of the lamps.
Single lamps should be replaced only in rooms with a low number of lamps or if the failure of a lamp of a great luminous flux would cause a considerable drop of illumination.
Replacement of groups of lamps
In many countries, the replacement of groups of lamps has proved good. Group replacement means, that after a definite period of time all lamps of one lighting installation are replaced by new ones, irrespective of the number of lamps that would still be serviceable. This method of lamp replacement is followed mainly with fluorescent lamps and high-pressure mercury vapour lamps.
The replacement of all lamps of a lighting installation by fresh ones complies with economical findings in the field of lighting engineering. The lamps which are exchanged despite that they are still usable have in any case only a little portion of their original light flux left due to age and dust, so that their further use would be uneconomical during the last part of their total service life. Among others, group replacement has the following advantages as to economy and lighting engineering:
- Better effective power of the lamps - higher lighting level with the same energy costs.
- More accurate planning of the costs for the fresh lamps.
- Better appearance of the lighting installation.
- Better working atmosphere.
- Uniform light colour and luminance of the lamps.
- Simultaneous carrying out of cleaning of the lighting fittings.
- Less maintenance work.
From the economical point of view, the best time for the group replacement of lamps is at approximately 80 % of the average service life of the lamps, because later the light flux drop and increasing number of failures would have an unfavourable bearing on the illumination level.
With lamps of little switching frequency or in permanent operation the time of lamp replacement can be postponed to 100 % of their service life.
In addition to the group replacement schedule the lighting installation should be checked after approximately 800 to 1000 hours of operation in order to replace the lamps that have prematurely failed individually.
On the occasion of group replacement of lamps the lighting fittings should be cleaned.
Complex replacement
Often, the complex replacement of high-pressure mercury vapour lamps and the lighting fittings reflectors and/or protective glasses as optically effective parts of the fitting represent the main part of regular maintenance in street lighting.
The time of this is determined by the reflector behaviour and the service life of the lamp. So, complex replacement is done preferably at intervals of three years. For example: high-pressure mercury vapour lamps of 80 to 400 W and a service life of 12000 h are replaced after three years of operation.
In addition to this, a reflector replacement after 1.5 years is considered as optimal. This method of lamp and reflector replacement is very effective in improving the level of illumination and must be approved especially from the economical point of view.
The decisive factor in the decrease of illumination is the dust that deposits on lamps and lighting fittings. According to the local conditions and the kind of lighting fittings used the illumination can decrease up to 60 % within one year due to the deposition of dust. Taking the corrosion of fitting components into consideration, the values with outdoor lighting components installations may be still worse.
It is understandable that freely radiating lamps or luminous rows or lighting fittings of the light distribution ‘uniform’ count among the most contaminated ones. Reflector fittings or enclosed lighting fittings suffer only little from dust deposition. Regular cleaning is of technical and economical importance.
Indoor lighting installations of a normal degree of getting dusty should be cleaned twice a year, for installations with very little dust deposition one cleaning per year is sufficient.
With outdoor lighting installations in most cases one cleaning per year is carried out in the course of which - in case of much reflector corrosion - the reflectors of the lighting fittings must perhaps be replaced.
Effective maintenance requires the appropriate auxiliary means.
Maintenance equipment and devices
For maintaining indoor lighting installations, a number of devices and equipment, e.g. bridge crane are used. Devices and equipment for maintenance work should meet, the following requirements:
- little floor area
- large working platform
- light-weight construction
- adjustable height
- mobility.
Shall the maintenance work be done from a bridge crane, special safety measures have to be observed. The bridge crane is allowed to be used for maintenance work at lighting installations only, if there is a secured gangway along the crane track providing an escape-route in danger.
All this shows that in the projecting phase already the future maintenance is determined. The kind of lighting fittings as well as their arrangement must guarantee the user unhindered maintenance.
For outdoor lighting installations, especially for street lighting, a number of vehicles is available equipped with ladders and/or with a universal mounting tower.
On principle, maintenance work is allowed to be done only at a dead installation.
The work should be done by two people.
It is recommendable to take the lamps out of the fitting for purposes of maintenance. The lamps are cleaned best by two sponges - one for washing, one for rinsing.
The lamp sockets can be cleaned by a brush. With aluminium parts strongly alcaline agents should be avoided. For glass and enamel only non-grinding cleaning agents must be used.
For plastic parts, normal household cleaning agents will do; yet they will not produce an antistatic effect. Washable plastic surfaces dry in the air; in no case they must be rubged dry, because by this they would charge up statically. After cleaning it is recommended to apply an antistatic coating.
Questions for repetition and knowledge tests
1. What are the reasons for the increasing importance of maintenance of lighting installations?
2. By what criteria is the maintenance work scheduled?
3. What are the most important devices used in maintenance?
4. What is understood by ‘complex replacement’?
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