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‘EARTHING’, a must for every one to understand its importance for Electrical Safety

By on September 21, 2014

The word ‘earthing’ is used in UK whereas ‘grounding’ in US but meaning the same. Earthing has a significance in electrical engineering to the fact that earth has very high capacitance of .0007 F, a very high value assuming it be infinity, and not to cause any potential change (V=Q/C) when gaining or loosing even enormous charge during lightning, fault current etc. It is, therefore, convenient to accept earth body possessing equipotential, and thus, accepting as reference potential. Every electrical appliance body is connected to earth and the reason for word ‘earthing’ (or grounding) to find relevance. Earthing  provides path for the fault current to flow through the earth and helping the protection relay to sense the high current and trip the circuit. Screenshot - 20-09-2014 , 18_16_49

Power distribution transformer output supply  is a 4-terminal output of which three phases (R,Y, and B) and one neutral as shown in the diagram. The neutral is connected to earth at the sub-station and all metallic non-current carrying parts of the installation. One of the phase and neutral is given to your residence along with an earth connection generated near your residence by digging a pit and inserting a pipe. Therefore, there are three wires in your house called phase, neutral and earth. Phase and neutral are connected to power the equipment whereas earth terminal is connected to the body of the equipment through earth terminal of the plug and socket.

Protection Class of Electrical Appliance

There are four classes namely 0, I, II and III classes of equipment for the need of protection and given as follows:

0 Class: Products that are Class 0 features a single level of insulation and body of the appliance not connected to earth. This means that 1 fault could cause an electric shock to its user or at the very least cause a spark that can easily escalate to a fire. Such products are no more used and one should be carefully that he is not buying such appliances.

I Class: Products  in this class have an earth circuit built-in and an earth wire in the plug. All metal parts of the products are earthed via the protective conductor. A Fault in the appliance should trip and over current device like a fuse or circuit breaker protecting the end user. Kitchen appliances are good examples of I class such as kettles, irons, washing machines, microwave oven, fridges, etc.

II Class: These products does not have  earth circuit and no earth wire in the plug. These products are commonly known as ‘double insulated’ with floating potential such that one fault will cause development of higher voltages. Such products are Electronic appliances such as TV, Laptop, mobile charger, TV setup box, etc.

III Class: These products operate at SELV (Separated Extra Low Voltage) and the device cannot produce enough voltage to risk the user getting an electric shock.

Design features of three pin Plug and Socket for electrical safety

  1. The earth terminal is thick and longer so that earth terminal is connected first, ability to handle fault current and easy identification. Suppose you are not using three terminal plug-socket and instead using only two terminal plug. Have you imagined what will happen, if there is fault inside the equipment where phase is shorted with the body? The user if touching the body will get fetal electrical shock. Now, if it is three pin plug, the body is now connected to electrical earth and heavy current will flow from phase towards to ground resulting electrical protection to trip the circuit and you will come to know that there is some electrical fault inside the equipment.
  2. An insulated sleeve is provided at the shank of the phase and neutral pin to save from accidental touch with the terminal when there is a gap between the socket and plug.
  3. There is a shutter on the three pin terminal which opens when earth terminal enters first, touches the earth connection and opens the phase and neutral through a lever mechanism for phase and neutral wire to contact. This follows the basic principle of
  4. In some plugs, a fuse is also in-built.
  5. These are class I products.

Three pin twin socket for 15 & 5 Ampers

AC power plug

Three pin AC plug



Have you ever thought why plug for mobile charger, laptop, TV, Setup box is having only two pin and how safety is taken into consideration

  1. All electronic devices are low voltage devices fed by power supply where 230V AC is converted to desired low voltage level.
  2. Double insulation is provided to avoid any fault in AC circuit
  3. DC system is at floating voltage system and one fault in the equipment will not cause any problem as it will not get return path.
  4. Some manufacturer uses three pin plug and where the third terminal is a dummy to provide stability to the plug and also to drop the lever to open the phase and neutral points.
  5. These are Class II or III products.

Earth Leakage

The fuse or other protective MCB will not  work always if body  get connected with the live terminal through high resistance. The appliance will still keep on working with current leakage. Such leakage raises the level of ground potential on your  switch board making it unsafe. It is important to identify such defects. Two types of protection system developed namely leak voltage detection  by ‘Earth Leakage Circuit Breaker’ and current unbalance detection by ‘Residual Current Detection’

What is ELCB?

A voltage-operated ELCB detects a rise in potential between the house earth connecting equipment frames, metallic conduits, water pipes etc. and a distant isolated earth reference electrode. It operate on detecting potential of around 50 volts to open a main breaker at the main distribution board to isolate the supply to the premises. ELCB was introduced at the beginning of 20th century but associated with spurious tripping. Indian Railways also introduced directive for providing ELCB at offices and residential complex but there were not much takers due to technical problem of correct adjustment. During the period there were cases of electrocution while spreading wet cloth on metallic string for drying during monsoon season when electric current leaked through walls with no protection circuit to work. 

What is RCD?

Current always travel in a close circuit of  insulated wire. In case, the insulation weakens, there is a tendency of the current leaks to the ground, and the entire current will not return through neutral wire. It means, the current through phase and neutral will be different in such fault condition. A current through the wire produces electro-magnetic field around it and if the circuit is designed in which the electro-magnetic field produced by the current of phase and neutral opposes each other, than it shall be possible to detect if there is any difference between the two. This is the principle followed in RCD and found wide application for electrical safety. RCD is for detection of leaking current and RCCB is having RCD as well as circuit breaker to break the circuit. This is how the RCCB looks like. The leakage current shown is 100 mA and if the difference of current that is going in and coming out of the system exceeds 100 mA the position changes automatically to off position.


The best safety option every house hold shall have.

There are many makes of RCCB available in the market. You may heard of the advertisement on TV ‘laga laga….’ and talks about residual current detection. RCCB and MCB together makes your house  safe from electrical shock.

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There Are 3 Comments

  1. Jankimchandra Jayswal says:

    Hello to all R.R.B, Sr.S.E / J.E. aspirants and thanks to admin of the website / blog creating a valuable effort towards R.R.B. Sr.S.E / J.E. preparation guidance information includes details of nature of job of different categories,few study materials,examinations’ recent trends etc. in detail.I request other friends such as Vikram Singh and others to upload previous year questions papers from 2004 on wards or otherwise sent it personally to my email:[email protected] myself is a Graduate in Mechanical Engineering preparing for R.R.B. Sr.S.E as well as J.E. examinations 2015.

    Thanks & Regards


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  3. Rajesh Sharma says:

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