Metal Halide Lamps
Have you ever wonder how this street lighting works? Or does your boss told you to check on the perimeter lighting, parking lighting or even indoor lighting. Or you just wonder how this things work. Well here is our chance to see the metal halide lamps up close. To check the possible problems you might encounter in dealing with high intensity gas discharge lamps.
In this topic we will study about Metal Halide Lamps. It is a kind of ( HID ) high-intensity discharge lamp which produces high output for a compact size lamps. It is also efficient compare to other light source such as fluorescent lamps and incandescent lamps. It is used for general lighting purposes. Can also be used for indoor lighting application because for their wide spectrum and bluer or redder color due to the different metal halides that they used inside it’s compact arc tubes.
Basically it is composed of the following parts. In this example, we will consider that they used metal halide lights as street light.
1]. T he bulb – the one that emits luminous light. Like any other gas 0r vapor discharge lamps, metal halide lamps produce light by passing an electric arc through a mixture of gasses. The arc tube contains a high pressure mixture of argon, mercury and some metal halides that changes the color of the light. The heat generated by the arc vaporized the mercury and the metal halides which produces light in the process as the temperature and pressure increases.
2]. Ballast – it regulates the current and voltage of the arc to work properly.
3]. Starter – it generates high voltage ( 1.5 KV ) pulse to start the arc.
4]. Capacitor- it helps in the ignition of the arc stream . It holds the charge to be released to start the arc that will start the lamp. Often it is just the capacitor that needed a replacement to start the lamp when it encounters a problem.
5]. Photoresistor ( optional ) – This sensor activates or deactivates our circuit or the light bulbs. It is mainly dependent to the light intensity sensed by the sensor. For example when it is already dark in the street the the sensor will activate the light and when the sun comes up it will turn off the light automatically.
Let us consider the circuit diagram.
This is the wiring diagram of our circuit. The ballast is a type of autotransformer which can be connected to different voltage level depending on what available voltage supply in the area. As you can see in the picture, you can connect in to 277 volts/ 240 volts/ 208 volts/ 120 volts. In connecting the circuit you will not be lost because there are markings in the wires, numbers or even colors to guide us. Take a look at this picture below.
This wiring marks can also be seen in the wiring diagram so you will know where it is connected. Most equipments or machines have these diagrams if not visible it should be in the manuals, where it is always best to read the manual or the instruction before starting any operation or repairs. But sometimes these manuals, diagrams and markings may not be always be available for us, this is where your skills and knowledge come in to play, but we will not discuss it here in this topic.
What to check or inspect if the lamp is not working:
1] Check first if there is power going to our circuit. Check the voltage across or between common and 240 volts ( let us assumed that the available source is 240 volts ). If there is no voltage then the problem is in the supply. May be the circuit breaker or fuse trip off. If that is the case we may have some kind of short circuit. It is always safe to full down the whole fixture and troubleshoot it in your maintenance room because these fixtures are usually mounted high above the floors and it will be difficult and dangerous if you are doing it up above. So bring it down and make sure that the power has been cut off before you do it. Though some people would probably do it in it’s location where ever it is, well as far as I’m concern these people are highly trained and skilled but if you are beginners then you have to think of your safety first.
2. If there is power between the common and 240 volts, then our problem is in our circuit. You don’t need to climb up to the fixture to check on the power or voltage, just check in the fuse box or panel where the power first came from. ( referring to figure ” circuit diagram” ). Cut off the power and check the capacitor. Short circuit first the terminals of the capacitor to remove any charge that it carry. Using a tester you can check the capacitor. If you are using analogue tester measure the resistance across the two terminals. A good capacitor will try to approach” 0″ resistance and gradually approach infinity. Interchange the two test probes you will get the same result. A bad capacitor usually accompanied by leaking fluids in the sides or deformed casing of the capacitor. If you see any of these sign replace it and check if it will work now.
3]. Check on the lamp. You can tell just by looking at its appearance if it has some problems like burned arc tube or some broken lines. It is best to have a new or a working spare parts available that you can just interchange it immediately and know if that part is working or not. That way you can save time and some effort. But it is not always the case and so you will need to know some method to use to check on the parts.
4]. Check on the ballast. One way of checking the ballast is to have a megaohm meter. Take a look at the picture.
Check the insulation resistance of all the lines or wires in the ballast to our ground. It should read just like in the picture above. If it falls down like this on the picture below then we have a problem in the ballast.Replace the ballast and try energizing the circuit.
You can also use megaohm meter tester to any electrical equipments or machines that have windings or coils to check the insulation resistance of the winding specially on high voltage application where the value of insulation resistance is significant to determine if you are in a safe condition to operate.
5]. Check on the Photo Resistor Switch. Referring to figure ” Street light ” . It is located on top of the fixture a round and dark color device on the top. To check this just replace it with a working one or a new one to see if the problem is in the photo resistor switch.