RCD means Residual Current Device. They are so called because they work based on the residual current in a circuit. The RCD is an electrical safety device specifically designed to immediately switch the electrical current flow when current "leaking" to earth is detected at a level harmful to a person using electrical equipment. An RCD offers a high level of personal protection from electric shock.

RCDs also help to reduce the risk of fire by detecting electrical leakage to earth in electrical wiring and accessories. RCDs are designed to operate within 10 to 50 milliseconds and to disconnect the electricity supply when they sense harmful leakage, typically 30mA

In absence of an earth fault,
the vectorial sum of the currents (I1 + I2) is equal to zero; in case of an earth fault if the (I1 + I2) value exceeds the rated residual operating current I?n, the circuit at the secondary side of the toroid sends a command signal to a dedicated opening or trip coil causing the tripping of the circuit-breaker.

Types of Residual Current Devices (RCD)
There are three basic types of RCDs:
1. Switchboard mounted
2. Power point type and
3. Plug in (portable).
Switchboard mounted and power point types are referred to as non-portable RCDs. Portable RCDs are plugged into a fixed socket. A non-portable RCD installed at the switchboard is the best option in most wiring situations �residential & industrial - as it protects all the wiring and appliances plugged into the circuit.

Switchboard Mounted RCDs
These are non-portable units installed at the switchboard to provide protection of the complete installation, or protection of a selected circuit.

Power point (Fixed Socket Outlet units) RCDs
These are non-portable units consisting of RCD protection inbuilt into a fixed socket outlet to provide protection to equipment plugged into the outlet.

Portable Units RCDs
Various models are available from simple plug adaptors to units designed for specific equipment such as the portable unit.

The sensitivity and speed of disconnection of RCDs are such that any earth leakage will be detected and automatically switched off before it can cause injury or damage. The sensitivity of an RCD in terms of current is given by:

The choice of sensitivity of the RCD is a function of the resistance RE of the earth electrode for the installation. Typical values of RE and sensitivity, I in amps under a fault voltage of 50V and 25V respectively are given below:

I = 50V or 25V/RE

Need For Earthing

There are two methods of earthing, namely

Pipe earthing is done by permanently placing a pipe in wet ground. The pipe can be made of steel, galvanized iron or cast iron. Usually GI pipes having a length of 2.5 m and an internal diameter of 38 mm are used. The pipe should not be painted or coated with any-non-conducting material.

The plate electrode should have a minimum dimension of 600 x 600 x 3.15 mm for copper plate or 600 x 600 x 6.3 mm for GI plates. The plate electrode should be placed at least 1.5 m below the ground level. The earth conductor is to be securely connected to the plate by means of bolts and nuts. The bolts and nuts should be of the same material as that of the plate. The earth conductor should be carried in a GI pipe buried 60 cm below the ground level. The prate electrode should be surrounded by a layer of charcoal to .reduce the earth resistance.

The international standard IEC60364, part 4, and Reference 10 uses a set of diagrams to explain the five basic methods of earthing and providing the neutral of an electrical installation where it is required. The five methods are abbreviated TNC, TNS, TNCS, TT and IT.

The first letter denotes the source of power from a star-connected winding. T denotes that the star point of the source is solidly connected to earth, which is usually at a location very near to the winding.
I denote that the star point and the winding are isolated from earth. The star point is usually connected to an inductive impedance or resistance. Capacitive impedance is never used.

The second letter denotes the consumer. The consuming equipment needs
to be earthed. There are two basic methods that can be used to earth the body of electrical equipment. These methods are denoted by the letters T and N. The letter N is sub-divided into other letters, S and C, thus giving NS and NC and NCS.

T denotes that the consumer is solidly earthed independently of the source earthing method.

N denotes that a low impedance conductor is taken from the earth connection at the source and routed directly to the consumer for the specific purpose of earthing the consuming equipment.

S denotes that the neutral conductor routed from the source is separate from the protective earthing conductor, which is also routed from the source. This implies that five conductors need to be routed for a three-phase consumer.

C denotes that the neutral conductor and the protective earthing conductor are one and the same conductor. This means that four conductors need to be routed for a three-phase consumer.

 The different earthing types are illustrated with the diagrams that now follow:

(a) TNC Earthing System

(b) TNS Earthing System

(c) TNCS Earthing System

(d) TT Earthing System

(e) IT Earthing System

Wenner�s four-electrode method is popularly used to measure soil resistivity. In this method Four electrodes are driven into the earth along a straight line at equal intervals, S. The depth of the electrodes in the ground is always of the order of 10 to 15 cm. The earth megger is placed on a steady and approximately level base. The four electrodes are connected to the earth megger terminal as shown below:

A current, I , is passed through the two outer electrodes C1 & C2 and earth. A voltage difference V, is observed between the two inner electrodes PI & P2. The current I flowing into the earth produces an electric field proportional to its density and to the resistivity of the soil. The voltage V measured between the inner electrodes is, therefore, proportion to the field. Consequently, the resistivity will be proportional to the ratio of the voltage to the current, i.e., R.
The soil resistivity is then calculated as:

Where:
? =  Soil Resistivity of soil in Ohms-meter
S = distance between two successive electrodes in    meters
R = ratio of voltage to current or electrode resistance in Ohms
e = depth of burial of electrode in the ground

If the separation distance of the electrodes , S, is much greater than the depth of burial, e, of electrodes, then equation 1 above reduces to :

Modern equipment used for testing for soil resistivity use the Wenner four electrode system and the mathematical calculation above, is done automatically by these new set of equipment such that once setup properly, at the press of a button, the value of the soil resistivity is determined quickly.

One key determinant of a good earthing installation is the soil resistivity of the soil where the earth electrode is installed. If you are involved  in earthing buildings and electrical installations, here is a list of the typical soil resistivity values for different types of soil that you might encounter:

Characteristics of Earthing Rods
The rods used for earthing buildings may be:
(a) Copper or more commonly Copper clad steel. Copper clad steel are generally 1 or 2 meters long and provided with screwed ends and sockets in order to reach
considerable depths, if
necessary (for instance, the water-table level in areas of high soil resistivity).

(b) Galvanized steel pipe greater than or equal to 25mm in diameter or rod greater than or equal to 15mm in diameter with lengths greater than or equal to 2 meters long. Note however that where galvanized conducting materials are used for earth electrodes, sacrificial cathodic protection anodes may be necessary to avoid rapid corrosion of the electrodes where the soil is aggressive.

When earthing is done by rods, It is often necessary to use more than one rod as shown above, in which case the spacing between them should exceed the depth to which they are driven, by a factor of 2 to 3.

The total resistance (in homogeneous soil) is then equal to the resistance of one rod, divided by the number of rods in question. The approximate resistance R obtained is given by:

Where:
L = the length of the rod in meters
? = resistivity of the soil in ohm-meters.
n = the number of rods

Use of Vertical Plates
Rectangular plates, each side of which must be greater or equal to 50cm, are commonly used as earth electrodes, being buried in a vertical plane such that the center of the plate is at least 1 meter below the surface of the soil.
The plates may be:
(a) Copper of 2mm thickness or
(b) Galvanized steel of 3mm thickness
The approximate resistance, R, in ohms is given by:

Where:
L = the perimeter of the plate in meters
? = resistivity of the soil in ohm-meters.

Earthing or Grounding of electrical installation is a common practice. However, some common terms used in the practice could sometimes be tricky. Here, we have attempted to provide explanations for some of the more common terms used when earthing or grounding an installation. These terms are the ones used in the various national and international standards:

To understand some of these terms, the schematic above will be very helpful:

This is the conductor or group of conductors in intimate contact with, and providing an electrical connection with Earth.

This is the conductive mass of the Earth, whose electric potential at any point is

conventionally taken as zero.

These are earth electrodes located at such a distance from one another that the maximum current likely to flow through one of them does not significantly affect the potential of the others.

This is the contact resistance of an earth electrode with the Earth.

Earthing Conductor 

This is a protective conductor connecting the main earthing terminal of an installation to an earth electrode or to other means of earthing.

Exposed-conductive-part 

This is the conductive part of equipment which can be touched and which is not a live part, but which may become live under fault conditions. Usually all exposed conductive parts are connected to the earth electrode by protective conductors with the object of providing a low resistance path for fault currents flowing to earth.

Protective conductor 

This is the conductor used for some measures of protection against electric shock and intended for connecting together any of the following parts:

(a) Exposed-conductive-parts

(b) Extraneous-conductive-parts

(c) The main earthing terminal

(d) Earth electrode(s)

(e) The earthed point of the source or an artificial neutral

This is a conductive part liable to introduce a potential, generally earth potential, which is not part of the electrical installation. Examples of extraneous conductive parts include:

(a) Non-insulated floors or walls, metal framework of buildings

(b) Metal conduits and pipework (not part of the electrical installation) for water, gas, heating, compressed-air, etc. and metal materials associated with them.

Bonding conductor 

This is the protective conductor providing equipotential bonding.

This is the terminal or bar provided for the connection of protective conductors, including equipotential bonding conductors, and conductors for functional earthing, if any, to the means of earthing.

Equipotential bonding 

Equipotential bonding simply means terminating all the extraneous conductive parts to the earthing system of the installation in a process referred to as bonding. 

Bonding is carried out by protective conductors and the aim is to ensure that,

in the event of an incoming extraneous conductor (such as a gas pipe, water pipe etc.) being raised to some potential due to a fault external to the building, no difference of potential can occur between extraneous-conductive-parts within the installation.