Earthing and electrical appliance classes

Earthing or grounding is simply a connection that allows current to return to the earth.  It may be a physical connection to the ground, or can be a common return path or sink used in electronics or where no grounding is possible e.g. aircraft. 

Any electric current will eventually return to earth, and the protective earth system is a safety device that provides the path of least resistance for the current to take, instead of, for example, through a person.

Earthing and electrical appliance classesHistorically not well understood, it is now a requirement for all electrical installations.  Earthing can be via a special rod or electrode in the ground, or by connection to a copper mains water pipe which originates in the ground. 

The resistivity of the soil is a consideration, as the effectiveness of an earthing system can be affected by low soil humidity, low salinity, and low temperature.  Other benefits of earthing include dissipating static electricity or charge that can build up, which is critical for transmission and reception in radio and other communications. 

As voltage is a relative measure, it is useful to use ground as a reasonably constant reference for 0 Volts.  This is used in stabilising voltage, and the neutral in a power station is usually connected to the ground.  Grounding may help in the case of overvoltage, for example electrical power surges and lightning, but it should be noted that a building’s lightning conductor or lightning rod, if used, is completely separate from the mains electricity.

Appliance classes
Electrical equipment can be classified by the level of safety provided by earthing.  There are five categories in IEC 61140, although in normal or domestic use only Class I and Class II are relevant.  These classes allow users to quickly ascertain the degree of protection afforded by the design of the equipment.

Class O
Unearthed, not double insulated.  There is no earth connection, and only one layer of insulation between current-carrying components and the user.  If not banned outright, it is only permitted in dry areas.  An example is the European CEE7/2 plug, the design of which means it cannot be inserted into an earthed French or Schuko socket.
Class I
Class IThe equipment’s metal chassis is connected to earth via a green or green/yellow earth wire, and is powered by a 3 pin plug (2 pin plus earth).  Any unsafe current is returned to earth by the earth wire and not through the user, reducing the risk of shock. 

If the current to earth is significant, it will blow any fuses or trip any circuit breakers present in the circuit.  Equipment may also be double insulated; the terms ‘earthed’ and ‘double insulated’ are not mutually exclusive.  The symbol is the electrical earth sign inside a circle.

Class OI
Instead of the earth connection through the mains plug, there is a dedicated earth directly between the equipment and the ground.  If specific earth wire such as 6491X or H07V-R is not used, the wire must have a green and yellow sheath to identify it.  This dedicated earth takes the shortest and straightest path to limit ground loops, and to prevent interference and noise that may be introduced from the mains; this is used in sensitive environments such as recording studios etc.
Class II
Class IIUnearthed equipment which is double insulated, i.e. has two or more layers of electrical insulation between current-carrying components and the user. 

European equipment must be marked “Class II”, “double Insulated” or the concentric squares symbol.

Class III
Class IIIThe device is powered by extra low voltage, generally 50V AC or less, which poses a low risk of electrocution if in contact with the user.  A similar but unrelated use is the 110V power supply on building sites which is actually 55V split phase, so 110V phase-to-phase but only 55V phase-to-ground.  In the event of a cable being cut, the smaller voltage difference to ground lessens the risk of shock.

In the context of mains leads, earthed equipment requires a 3 core cable with three pole plug and connector, while unearthed equipment needs a 2 core cable with two pole plug and connector. 

If the equipment is earthed it must have an earthed cable and earthed plug.  There are some small variations, for example the 2 pin Japanese with flying earth wire which is connected separately, or the three pin UK plug which needs an earth pin to open the socket’s safety shutters even on ungrounded equipment.  For safety, the design of the plug and socket is usually such that the earth makes first contact when plugged in, and breaks contact last when unplugged. 

A common method is the have the earth pin on the plug longer than line and neutral pins; alternatively the earth contact within the socket may be advanced or before the other contacts.

The addition of an earth pin usually polarises a plug by ensuring it can only be plugged in one way round.  A notable exception is the Italian plug, which being symmetrical can be inserted in either direction. 

The modification of the earth pin can be used to create non-standard plugs for ‘clean’ circuits for use in electronic or data applications.  The UK plug can have a T shaped earth, round earth pin, or all three pins rotated 90 degrees.  The Australian plug replaces the flat earth pin with a round one, and the South African modifies the round earth pin so that it has a flat side.  The Danish Data plug modifies the live and neutral pins instead.

3 core, and follow black-white-green (line-neutral-earth) for North America and JapanAll Morvan Trading’s power cables are 2 core or 3 core, and follow black-white-green (line-neutral-earth) for North America and Japan, and brown-blue-green/yellow (line-neutral-earth) for the rest of the world. 

3 core, brown-blue-green/yellow (line-neutral-earth) for the rest of the worldThree phase power used in the higher rated IEC 60309 connectors has more cores, but earth will still be green or green/yellow. 

Control cable may have many black cores individually numbered, but the earth wire will still be green or green and yellow. 

Electrical installations pre-2006 in the UK under BS 7671 may have the old core colouring system of black red and green, but earth is still easily identifiable. 

This harmonisation of core colours originates in CENELEC Harmonisation Document HD 384.5.514.3: Identification of conductors and HD 308 S2:2001 Identification of cores in cables and flexible cords.  As such, the earth, ground or protective conductor should be easily recognised anywhere in the world.


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Morvan Trading Ltd has over 40 years experience in the powercord business.

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