Factors responsible for choosing proper cable
Mainly two factors are important for selecting the proper cables and both of them are equally important.
- Ability to carry the current load: The cable should be
able to carry the required amount of current without being
overheated. It should be able to do it during its whole life time
and in extreme conditions of temperature.
- Sound earthing: The proper and stable earthing is very
necessary for the running of all electrical systems. It is essential
for two reasons.
- To check the voltage to a safe level for securing the user from any unwanted damage.
- To permit the fault current to trip the fuse in a short time.
In any wiring system, over-current is one of the two main safety hazards that should be in control. It is very risky and dangerous because it lead to a risk of fire. Most of the fire accidents are caused due to electrical faults. A fuse or MCB of the same current rating as the cable should be selected to work properly.
Over current is an increment in the amount of current above certain level. Cables become heated when current flows through them and it is a normal process. The level of heat is permissible only up to a specific point and any thing above it would be dangerous.
Types of over-current
Over current usually occur due to two reasons.
- Overload: Overload usually occurs due to running of too
many electrical appliances at the same time or due to the use of a
heavy duty appliance. When the current flowing in circuit is 50% to
100% higher than normal then overload occurs. It don't create any
instant damage but as the duration of overload increases the chances
of damage also increases. If the overload isn't controlled
immediately then the wires will overheat and melt down. It may cause
fire.
- Short-circuit: A short circuit is produced when the wires become loose and touch each other like live wire touches neutral or earth. This connection will lead to low resistance and it can cause the current to increase hundred or thousand times.
Over load and short circuit protection both is provided by same device. It may be a fuse or an MCB. As the current exceeds a certain limit, the fuse blows or the MCB trips and in both cases the flow of current is disrupted.
Fuses:
Fuse is a small device consisting of a piece of wire of proper length and thickness. When current in the circuit exceeds the permissible limit the wire in the fuse gets heated and melt thus breaking the flow of current.
Types of Fuses
- Cartridge Fuse: These fuses have wire encased
in a packed cylinder having a contact at each end.
- Semi-enclosed (Re-wirable fuses): These fuses can be re-wired with fuse wires. Though they are still used at a large scale but it has one major disadvantage. During rewiring it is very common to replace the fuse wire with the improper one which may lead to future dis functioning.
MCB (Miniature circuit breaker):
It is one of the most used electrical protection devices. It is used as an ordinary switch and over-current circuit breaker as well. It has a convenient lever in front of it for manual operation.
How to select the appropriate cable size
In domestic wiring there is no need of calculating the sizes of cable. Usually for power circuits cables 2.5mm cables are used and for lighting purposes 1.5mm cables are needed. But if long cables or high electrical appliances are to be installed then selection of proper cable size is necessary.
