Earthing and 5 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. in 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.
Human detection of electricity starts at low voltages: under 2 V AC for a startle effect, and 20 V AC for a muscular reaction (both in dry conditions). Electric shock is defined as the physiological effects upon a human or animal body from an electric current passed through it. Common symptoms of electric shock include muscular contractions, immobilisation, burns, difficulty breathing (up to respiratory arrest), and heart function disturbance (including cardiac arrest).
Historically 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 of the local earth. 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 zero Volts. This is an ideal, and local earth in a real world application may not be 0V. 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. Appliance class symbols provided where the exist:
Appliance Class O
Unearthed; not double insulated. There is no earthing connection, and there is only one layer of insulation between current-carrying components and the user. This is sometimes also known as basic protection or simple separation. The insulating material may be air, in which case an outer enclosure or barrier is normal.
Class O has been removed from the majority of national standards. If not banned outright, it is only permitted in dry areas. It is sometimes allowed in non-conductive environments, where high-impedance barriers or obstacles of at least 50 kΩ (e.g. a wall or floor) exist between the circuit and the user.
In the setting of a powercord, it is only to be used where voltage to earth is less than 150V. An example is the old European CEE7/2 plug, the design of which means it cannot be inserted into an earthed French or Schuko socket.
Appliance Class I
The equipment’s metal chassis is connected to earth via a green or green/yellow earthing wire, and is powered by a 3 pin plug (2 pin plus earth). Any unsafe current is returned to earth by the earthing 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, making it a protective earth, and may also be marked ‘PE’, or have green / yellow colouring (plain green in USA).
Appliance Class OI
Instead of the earth connection through the mains plug, there is a dedicated clean earth directly between the equipment and the ground. If specific earth wire such as 6491X or H07V-R is not used, the noiseless earth wire must have a green and yellow sheath to identify it. This dedicated earth provides what is known as functional earthing by taking 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.
Appliance Class II
Unearthed equipment which is double insulated, i.e. has two or more layers of electrical insulation between current-carrying components and the user. This is defined as basic insulation (which insulates current carrying parts from physical contact) plus supplementary insulation (which protects agains faults).
An alternative means of compliance is reinforced insulation which is a single layer but equivalent in protection to double insulation. Another is basic insulation plus electrically protective screening or shielding. Otherwise, a residual current device (RCD) with IΔn ≤ 30 mA. All of these may be referred to as (electrically) protective separation.
The two different protective measures must not affect each other. This means the simultaneous failure of both is such low probability as can be disregarded in general purpose settings.
European equipment must be marked “Class II”, “double Insulated” or bear the symbol of the two concentric squares.
Appliance Class III
The 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.
Grounding in mainsleads
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.
All Morvan Trading’s power cables are 2-core or 3 core and follow harmonised core colours brown – blue – green/yellow (line – neutral – earth) for international use. The exceptions are:
- black – white – green (line – neutral – earth) for North America, Japan and Taiwan;
- red – black – green/yellow (line – neutral – earth) for India.
Three-phase power used in the higher-rated IEC 60309 connectors has more cores, but the 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 colour coding is still used in India today.
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.
Grounding: is it safe without an earth?
This very much depends on the application. Almost all equipment is double-insulated nowadays, and circuit breakers much more sensitive, so electrocution is less of a risk than previously. We always recommend using and eathed plug and earthed opower cable where suitable.
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