Preventing Electrical Devices against Electrical Shock and Damage
Forward: In this article I explain the prevention of electrical devices against electrical shock to humans and electrical damage to the devices.
By: Chrysanthus Date Published: 1 Sep 2012
Introduction
Prerequisite
If you are not an electrician or electrical engineer, then read the following articles first in the order given:
- Electricity Basics and Explanation of Shock
- Alternating Current Basics and Explanation of Shock
- Explanation of Electrical Short Circuit
- Explanation of Lightning to Everybody
Electrical Damage
By electrical damage, I am referring to damage to the device caused by electrical short circuit in the device or some other error in the device. Short circuit inside the device can cause part or the whole device to burn. This rarely happen, but it can happen.
In this article the emphasis is on electrical devices like the TV Set with mains cable that is plugged to the wall socket. One of the terminals of a wall socket should be connected to the earth. The mains cable of the electrical appliance should have a conductor that is connected to the earth terminal of the wall socket.
Now, the casing of many electrical devices are made of metal, that can conduct electricity. This metal casing is deliberate. The earth conductor of the mains cable of the device has to be connected to the metal case.
Now with the earthing (of the case), if you touch the case of the device, even if the device is in operation, you cannot have a shock, because touching the case of the device is the same as touching the earth. For you to have a shock, there has to be a difference in voltage potential between what you touch and the earth. There is no difference in potential between the earthed casing and the floor (ground) on which your feet are.
If the house is note wired to standard, like some houses in developing countries, then touching the case while the device is operating can shock you. This would be because the case is not earthed through the mains conductor and the wiring of the house.
Electrical Damage and Fuse
If an error, say a short circuit occurs in the device, an unnecessarily high current may be pulled from the mains cable of the device. If this high current continues, the device can burn. Just after the mains cable in the device, is a fuse. Current must flow through a loop. So, from the mains cable, as current is entering the device, from one conductor, it meets a fuse. It passes through the fuse, then into the rest of the internals of the device. So if an error occurs in one of the components inside the device, and pulls more current than required, the fuse will blow (open circuited).
Some electrical appliances may also have circuit breakers, to serve a similar purpose like the fuse.
If the case of a device is not metal, then it should be an insulator, so that if a human touches the case, current from inside the device will not pass through the insulator and shock the person.
Be it metal or insulator, for esthetic (beauty) reasons, the casing of almost all devices are colored (painted). Despite that, the electrical prevention properties are still intact.
Damage to Device by Lightning
Assume that lightning strikes somewhere and crosses a telephone line and a TV cable line and an electrical power line. Also assume that these lines are coming to your house. Assume still that your house is near where the lightning strikes. Current surge from the lightning will propagate though the lines to your house (and neighboring houses).
Assume that you have a fixed phone connected to the phone line. Assume that the TV (coaxial) cable is connected to the antenna input (socket) of your TV set. Assume that the mains cable of your radio set is connected to a mains socket at the wall. Each of the devices may or may not be on (operating) as the lightning strikes.
This is what will happen: The current surge from the telephone line will be too strong for your telephone and will damage your telephone, entering the telephone from where the line is attached to the phone. The current surge from the TV line will be too strong for your TV set and will damage your TV set, entering the TV set by the antenna input. The current surge from the electrical power line will be too strong for your radio set and will damage your radio set, entering the radio set from where the mains cable is attached to the set.
In practice, lighting will hardly strike all the 3 lines outside your house. It can only do so if the lines are near one another. In practice, lightning often strikes single lines, not 2 or 3 together.
That is it for this article; I hope you appreciate it.