Electrical Test and Electric Motors

In all cases, the service life of lamps is reduced by frequent ignition, except for induction compact fluorescent lamps and LEDs.

Fields of application, advantages and disadvantages:

The principle of light emitting diodes is the emission of light by a semi-conductor as an
electrical current passes through it. LEDs are commonly found in numerous applications, but the recent development of white or blue diodes with a high light output opens new perspectives, especially for signaling (traffic lights, exit signs or emergency lighting).

The average current in a LED is 20 mA, the voltage drop being between 1.7 and 4.6 V
depending on the color. These characteristics are therefore suitable for an extra low voltage power supply, especially using batteries. A converter is required for a line power supply.

The advantage of LEDs is their low energy consumption. As a result, they operate at a very low temperature, giving them a very long service life. Conversely, a simple diode has a weak light intensity. A high-power lighting installation therefore requires connection of a large number of units in series.

These diodes are used particularly where there is little power available.

Related News:

Royal Philips Electronics has announced the introduction of LivingColors, an exciting and innovative new form of home lighting using LEDs which allows you to alter the color of the light in your home, so that you can adapt the lighting to your circumstances and create the atmosphere of your choice. Read about it here.

Fluorescent tubes
These were first introduced in 1938. In these tubes, an electrical discharge causes electrons to collide with ions of mercury vapor, resulting in ultraviolet radiation due to energization of the mercury atoms. The fluorescent material, which covers the inside of the tubes, then transforms this radiation into visible light.

Fluorescent tubes dissipate less heat and have a longer service life than incandescent lamps, but they do need an ignition device called a “starter” and a device to limit the current in the arc after ignition. This last device called “ballast” is usually a choke placed in series with the arc. The constraints affecting this ballast are detailed in the rest of the document.

Compact fluorescent lamps
These are based on the same principle as a fluorescent tube. The starter and ballast
functions are provided by an electronic circuit (integrated in the lamp) which enables the use of smaller tubes folded back on themselves.

Compact fluorescent lamps were developed to replace incandescent lamps: they offer significant energy savings (15 W against 75 W for the same level of brightness) and an increased service life (8,000 hrs on average and up to 20,000 hrs for some).

Standard compact fluorescent lamps take a little longer to ignite and their service life is reduced according to the number of times they are switched on. So, if the ignition frequency is multiplied by 3, the service life is reduced by a ratio of 2.

Incandescent lamps are historically the oldest (patented by Thomas Edison in 1879) and the most commonly found in common use.

They are based on the principle of a filament rendered incandescent in a vacuum or neutral atmosphere which prevents combustion.

A distinction is made between:

• Standard bulbs

These contain a tungsten filament and are filled with an inert gas (nitrogen and argon or krypton).

• Halogen bulbs

These also contain a tungsten filament, but are filled with a halogen compound (iodine, bromine or fluorine) and an inert gas (krypton or xenon). This halogen compound is responsible for the phenomenon of filament regeneration, which increases the service life of the lamps and avoids them blackening. It also enables a higher filament temperature and therefore greater luminosity in smaller-size bulbs.

Halogen bulbs are widely used in automobile headlamps.

The main disadvantage of incandescent lamps is their significant heat dissipation, resulting in poor light output, But, they have the advantage of a good Color Rendering Index (CRI) due to the fact that their emission spectrum is fairly similar to the eye’s reception spectrum.

Their service life is approximately 1,000 hours for standard bulbs, 2,000 to 4,000 for halogen bulbs.

The simplest way to explain the difference between a neutral and a ground is as follows:

A neutral carries the “unbalanced” current in a three phase system or is the current return path in a single phase system.

The ground is a “safety” system that, under normal conditions, does not carry any current. If a hot wire touches an enclosure wall, the current from the short will follow the ground wire back to the transformer instead of arcing through the poor guy standing next to the cabinet. (At least that is the theory.)

You are correct in that the neutral is connected to (or “bonded”) to the ground at the point of origin but that is really only to create a “zero” or common point at the beginning of the system. After that point, the ground and neutral have two completely separate functions.

Source: http://www.electricalknowledge.com/forum/topic.asp?TOPIC_ID=6015

A megger checks winding insulation by sending a high dc voltage through the windings. in this way, the megger will read a resistance in megohms. if the reading is low, it means there is a leakage of this dc voltage to ground through the insulation, indicating deterioration in the insulation.

A megger will pick up grounds that a regular ohmmeter can not. When you go on those calls and find the breaker tripped, can’t find a reason, restart and system takes right off, it’s time to dust off the megger.

Source:  http://hvac-talk.com/vbb/showthread.php?t=159252

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