Cable Selection Considering Over Current and Electric Shock

Electrical cables are like the veins of all electrical systems. As veins carry blood in our body, they carry electricity to run a number of appliances. The selection of electrical cables should be done according to the current, voltage and power requirements.

While selecting a electrical cable, you must keep two things in mind.

• The cable you choose should be able to carry the current load without overheating. The cable should withstand the extreme conditions of temperature during its entire working life.
• The cable should provide sufficient sound earthing, to allow the fault current to trip the MCB or fuse in a short time, and to limit the voltage to which people are exposed to a safe level.

The two major safety hazards that needs to be controlled in a wiring system is Over-Current and Electric Shock. Protection against over-current also provides protection against electric shock. Over-current is the increase in the amount of current above the level for which it is designed. Over current are very dangerous as they lead to a risk of fire.

When current flows in electrical cables it is normal that they become warm, and heat is generated. The level of heat generated by electrical cables is safe only when it is kept within reasonable boundaries. The standard PVC-insulated cables are designed to run at temperatures up to 70 degrees Celsius. Anything beyond that can be a risk.

Overload occurs when the current flowing is too high for the system. They don't impose any immediate damage to the cables or surroundings. The danger is supposed to enhance if the duration of the overload remains for a longer period. In that situation, due to overheat the covering will melt leaving the bare conductors exposed. The heat can lead to cause fire. The main reasons for overloads in domestic setting are : plugging a heavy duty appliance into a supply insufficient for that, and using too many consumer appliances at the same time.

A short-circuit occurs when live and neutral or live and earth that bypasses an appliance is connected. This connection will have a very low resistance and the current flowing can be hundreds or thousands of times higher for the system. This is called short-circuit current or the fault current. The short-circuit can happen if the wires in a mains electrical plug become loose and touch one another. Handling short-circuits is not only important to protect cables, it is a part of the protection against electric shock too.

In domestic installations, short-circuit protection and overload protection both are provided by the same device, either an MCB or a fuse. An RCD can be used for additional shock protection. When the amount of current flowing exceeds a specified limit for a certain time, the fuse will break in the fuse or the MCB will trip. In both the cases, the circuit will be opened and there will be no further flow of current.