Effect of current through human body

Effect of electric current through the vital parts of human body depend on the duration, magnitude and frequency of this current. Most dangerous consequence could be ventricular fibrillation, a condition of inordinate action of main chambers of the heart, resulting in immediate arrest of blood circulation.

Currents at 50 Hz about 0.1 A can be lethal. But human can sustain larger currents at 25 Hz or DC or at frequencies in the range of 3,000 - 10,000 Hz.

Current depends on voltage applied and body resistance. Resistance is mainly offered by skin. Skin resistance increases with thickness and diminishes with moisture / perspiration. Except for skin; blood vessels, intravascular spaces etc. offer conduction system.

What is Earthing?

Earthing is a connection done through a metal link between the body of any electrical appliance, or neutral point, as the case may be, to the deeper ground soil. The metal link is normally of MS flat, CI flat, GI wire which should be penetrated to the ground earth grid. The purpose of earthing is to ensure a low resistance path between earth and metal body. Any short circuit or fault current that occur in the system will be dissipated to earth. The object of an earthing system is to provide as nearly as possible a surface under and around a station which shall be at a uniform potential and as nearly zero or absolute earth potential as possible. The purpose of this is to ensure that in general all parts of apparatus, other than live parts, shall be at earth potential, as well as to ensure that operators and attendants shall be at earth potential at all times. Also by providing such an earth surface of uniform potential under and surrounding the station, as nearly as possible, there can exist no difference of potential in a short distance big enough to shock or injure an attendant when short-circuits or other abnormal occurrence take place. Earthing associated with current-carrying conductor(neutral) is normally essential to the security of the system and is generally known as system Earthing, while earthing of non-current carrying metal work and conductor is essential to the safety of human life, of animals and of property and is generally known as equipment earthing.

Lightning Protection for Low Voltage Devices

Low Voltage Devices are more prone to damages due to Lightning Surges and High Voltage Surges. There are many ways to protect these devices from Lightning Surges. The first step is to protect the supply system from High Voltage surges and lighting. This include providing proper earthing(grounding) and providing Lighting arresters at various key locations. The second method is to provide surge arrestors at different point where there is a risk for lighting surge. The third method is to provide SPD, Surge Protection Devices. Surge protection devices open up when there is a heavy surge and protect the device.

What is the difference between earthing and grounding?

What is the difference between earthing and grounding?

Earthing is used in European electrical installation standards (IEC), Grounding is used in US electrical installation standards (NEC, UL). But both are describing the same idea. By definition both are same but the usage is different. An earthing system or grounding system (US) is defined as an arrangement of connections and devices necessary to earth equipment or a system separately or jointly for providing protection against voltage surges and shock accidents.

Control Panel Earthing

Most of the Control panel-manufactures used to connect all the panel doors to the Earth bar provided inside the panels. Some clients demand for earthling rear panel covers. Is there any logic behind this demand? It is an added safety to earth the rear covers of panels also. Generally we do not do this as rear covers are screwed permanently and as such while we access the rear covers, the panel would be made dead and hence no danger. The standards and rules say that each touchable metallic parts should be earthed, so the rear cover, even screwed, can be touchable it should be earthed.