Lamps, LEDs and Circuits

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Lamps, LEDs and Circuits

Lamps, LEDs and CircuitsDALIDALI uses a digital communications protocol but is almost aprogramming language for lighting control gear, allowingcomplete flexibility of control of lighting units. Grouping ofluminaires is via software as every luminaire is individuallyaddressable. As DALI allows bidirectional communication it ispossible to interrogate luminaires about their current operatingstate, fault conditions, etc., and to use a computer basedgraphical interface to control installations.8.15 Voltage dropWhen designing cabling for installation of luminaires it shouldbe remembered that there will be a voltage drop along thelength of the cable. This is due to the electrical resistance of thecable and means that the voltage measured at the end of thecable will be less than that measured at the start of the cable.The voltage drop for a given current carried is related to thecable materials and manufacturing process and is thereforeindividual to each cable type and manufacturer. Values arenormally quoted in terms of voltage drop per ampere per metre.Note that the wiring regulations give limits on permissiblevoltage drop.In the absence of manufacturers data the following formula forcalculating voltage drop may be used.2(Ix0.0175xL) U=Awhere U is the voltage drop across the length of the cablein voltsI is the current being carried by the cable in ampsL is the length of the cable in metresA is the cross-sectional area of a single conductorin mm 2Note this formula is for a twin copper conductor (phase andneutral) at 15˚C.This can have a major effect upon the lighting installation asa relatively small voltage drop can reduce the light output ofthe luminaire or for larger voltage drops can even preventthe luminaire from operating. An effect of this, especially forLamps, LEDs and Circuits | 225


Lamps, LEDs and Circuitsinstallations using high-pressure discharge lamps (e.g. forfloodlighting), is that luminaires closer to the supply transformermay produce more light than those furthest from the transformer.To help reduce the voltage drops in an installation the followingsteps may be considered;• The voltage drop of a cable is related to the cable gaugeor cross-sectional surface area. A cable with a largercross-sectional area will have less voltage drop than asmaller cable.• The voltage drop of a cable is related to the length of thecable. Longer cable runs will produce a larger voltagedrop. Therefore smaller cable runs should be used.• Increasing the number of transformers will allow thetransformer sizes to be reduced and also allow the lengthof the cable runs to be reduced.• The use of lower wattage lamps or fewer luminaires oneach cable run will reduce the loading on the cabling andtherefore the voltage drop (as voltage drop is related tocircuit current).8.16 FusingFuses are the simplest form of circuit protection. Whilst theyhave generally been replaced by electromechanical methodsof protection a benefit of fuses is that they can withstand muchhigher fault levels than other electromechanical methods ofprotection.However, circuit breakers are most commonly used forprotecting circuits on high voltage and low voltage circuits.For low voltage, low current applications typical of lightinginstallations miniature circuit breakers (MCBs) may be used toprotect the final circuit. Three different categories of MCB aredefined, giving different levels of performance depending uponapplication. These are;Type B used with resistive loads such as tungsten lightingType C used where a mixture of light inductive and resistiveloads are presentType D used where strong inductive loads such as motors orswitched mode power supplies are present226 | Lamps, LEDs and Circuits

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