Electric Lamps and Lighting Calculations, Definitions and Units | Electrical Earthing or Grounding

Impulses of radiant energy are emitted from various sources at various wavelengths, some of which affect the eye, causing the sensation known as light. Light can thus be expressed in terms of radiant energy wavelength between about 4000 and 7000 Angstrom. A given amount of such energy may cause different persons to experience light in different degrees.

The following are the principal terms and units involved in lighting applications and measurements:

Definition and units:
Luminous flux(F): This is the rate of passage of radiant energy measured in relation to the sensation of light received by the eye.

Lumen: this is the unit of luminous flux and is equal to the light flux emitted in an element solid angle by a uniform point source of light of one candle strength, i.e. the quantity of light falling on a surface of 1 sq.ft. of a sphere of radius/ ft. from a uniform point source of one candle. It is a measure of lamp output. Since the area of a sphere is 4πR2 where R is the radius, it follows that a consistent tip supply of one candlepower emits 4pi lumens.

Luminous Intensity(l): The luminous flux per unit solid angle emitted by appointing a source of light in a given direction.

Candle: The unit of power or intensity of a source of light. There are various standards such as the standard sperm candle, and Vernon Harcourt’s Tentative Lamp. International Candle(Bougie Decimale).

Men Spherical Candle, power(M.S.C.P): The intensity of illumination from a lamp varies in different directions. M.S.C.P. is the average value in all directions.
Since 1 c.p emits 4p lumens M.S.C.P. lumens/ 4π

Mean Hemispherical Candle-power, M.H.C.P: upper or lower. The average candle-power of a light source in all directions in a plane through the centre of the source and perpendicular to its axis.

Reduction Factor(r): The ratio of M.H.C.P/ M.S.C.P

Illumination (E): The luminous flux per unit area of the surface measured in foot candles or lumens.

Footcandle: A unit of illumination; the illumination produced on the surface of a sphere at 1ft, radius from a point source of light of one candle. 1-foot candle = 1 lumen per sq. ft. or 10.764 lux(metre candles).

Consumption of lamps: The electrical energy taken by a lamp (measured in watts).

Utilization Factor: An allowance which depends on the distribution of light from the fitting, the colour of the walls, and the proportions of the room.Depreciation Factor: The Proportion of the initial light which can be assumed after lamps have deteriorated and fittings become dirty. Maybe about 1.3 for a clean situation, and 1.63 for bad conditions.

Reflection Factor of a surface = Reflected light flux / Incident light flux
The illumination on a surface varies inversely to the square of the distance from the light source.

Illumination in foot-candles on the plane normal to an axis of light = Candle-power/ (distance)2

Radiation of different wavelengths produces eye sensations of different natures which are termed colour. Radiation of wavelength exceeding, 7000 Angstrom units is termed infrared and is invisible, though perceptible as heat rays. Radiations shorter than about 4000 Angstrom units in the ultra-violet region are also invisible, although they will affect a photographic plate and fluorescent powders.

Light output from electric lamps:

TypeWattageEfficiency (Lumens/watt)Initial Output Flux (Lumens)Average flux output through life of lamp (Lumens)
Mercury803830402240
Vapour1254252503750
2503690007250
400451800013600
Sodium4555.525002000
606539003120
8571.560804850
25010025000NA
400117.547000NA
Flouroscent803830402240
1254252503750
400381520012400

The approximate light output of electric discharge lamps

Electrical Earthing or Grounding

The term ground or grounding is familiar in countries like USA/Canada/Japan which is defined by IEEE Standard Dictionary as‘ Conducting connection to the earth. The term earthing or earth is spelt in UK, Europe and IEC Standards. Ground means Earth.
3 phase AC system neutral points are connected to the earth or a DC rectifier system to the earth is called neutral grounding or system grounding. Earthing connection of non-current conducting metallic accessories which are inbuilt with electrical installation devices or machines are called equipment body earthing. System ground refers to connecting neutral points to the earth. Equipment grounding refers to a connection or non-current carrying metallic parts to earth. Earth is the highest good conductor of electricity and can absorb unlimited charge with no rise in voltage. Hence the earthed point of the equipment always is at very low potential.

System earthing or grounding

Intentional connectivity of equipment or appliance neutral point to earth is called system earthing. If a neutral point is ground, the phase to earthing voltage is at earth fault condition so it does not increase to a high value. Earth fault protective mode becomes easy; hence it is a universal practice to have a neutral earthing at all voltage levels.

Equipment earthing or grounding

It is quite different from neutral grounding. Equipment earthing is connecting to earth from non-current carrying metallic spare parts. The non-current carrying metal parts include the motor body, switchgear structure, transformers, sheaths of heavy cable body of portable appliance body structure etc.

Equipment earthing ensures safety

The potential of an earthed device will not reach unsecured high values since they are connected to the earth. As the earth fault, current flow through the earthing and may readily does the operation of fuse or on earth fault relay, circuit breaker and faulty part are cut off automatically from the system. If equipment earthing is not provided or not maintained properly, the fault current will pass by contact through the human body to a ground source and cause highly dangerous shock.

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