What Is a Class 3 Electrical Equipment? Symbol, Examples

Class 3 electrical equipment – electrical equipment in which voltages are limited to extra-low voltage values as the provision for basic protection, and with no provision for fault protection [defined in the IEC 60050-195:2021].

The definition given gives a general description of class III electrical equipment from the point of view of providing protection against electric shock in operation. The live parts of class III electrical equipment are at extra-low voltage (ELV), by means of which they provide basic protection. Class III electrical equipment is not designed with any protective provisions designed to provide fault protection.

In the event of a single fault, touchable conductive parts of class III electrical equipment may be exposed to voltages that do not exceed the extra-low voltage. Therefore, protection in the event of a fault is actually provided by an extra-low voltage. That is, protection against electric shock in class III electrical equipment is based on the use of extra-low voltage.

Class III electrical equipment is protected against electric shock by the same protective provisions used for basic protection. Class III electrical equipment has voltages not exceeding extra-low voltages of 50 V on AC circuits and 120 V on DC circuits. Touchable conductive parts of this electrical equipment, if any, shall not be connected to protective conductors of the electrical installation of building so as to provide protective earthing.

Under normal conditions, when the insulation is intact, live parts must not be touched. Live parts cannot be short-circuited to touchable conductive parts of class III electrical equipment. Therefore, touchable conductive parts are not live. Under normal circumstances, class III electrical equipment may have touchable live parts which are not dangerous to humans or animals because they are of extra-low voltage.

Under single fault conditions, when the insulation of a live part has been faulted, resulting in an electrical connection between the live part and the accessible conductive part, no voltage exceeding the extra-low voltage may arise on the accessible conductive part of class III electrical equipment. Such voltages are not dangerous for humans or animals. Therefore, class III electrical equipment does not have protective provisions in place to protect against faults.

Class 3 Electrical Equipment Symbol & Marking

The equipment shall be marked with the graphical symbol of IEC 60417-5180:2003-02. This requirement does not apply where the means of connection to the supply is so shaped that it can only mate exclusively with a particularly designed SELV or PELV supply arrangement.

The requirements of Table 3 of Standard [2] also require class III equipment to be marked in the above way (roman numeral III in a diamond). The conditions for use of class III electrical equipment in a low-voltage installation, as specified in this table, allow it to be connected only to a SELV or PELV supply arrangement.

Class of equipmentEquipment marking or instructionsSymbolConditions for connection of the equipment to the installation
Class IIIMarking with the graphical symbol IEC 60417-5180:2003-02 (roman numeral III in a diamond)IEC-60417-5172-2003-02 (double square)Connect only to SELV or PELV systems
[Table 3 IEC 61140-2016] – Application of equipment in a low-voltage installation

Class 3 Electrical Equipment Examples

Class III electrical equipment is used in particularly hazardous conditions where there is an extremely high probability of electric shock. For example, portable electric lights used in confined spaces with conductive floors and walls, lights installed in fountains and swimming pools, and other electrical equipment designed to function under similar conditions must be class III electrical equipment.

Of the electrical equipment, which is often used by ordinary people at home and at work, we can include electrical equipment with an external power supply (scanners, laptops, monitors, smartphones, and so on). For the latter, safety is determined by the quality and degree of protection of the power supply.

Notebook as an example of class 3 electrical equipment
Figure 1. Notebook as an example of class 3 electrical equipment

The high electrical protection properties of class III electrical equipment are due in large part to its power supply. This electrical equipment is usually connected to the secondary windings of safety isolation transformers, which are sources of safety extra-low voltage. Safety isolation transformers reliably (with double or reinforced insulation) separate the conductive parts of class III electrical equipment from electrical circuits of the electrical installation of building, which have conductive parts connected to earth.

Class III Electrical Equipment Requirements

Requirements for class III equipment are set out in clause 7.5 of the standard [2].

Voltages

Equipment shall be designed for a maximum nominal voltage not exceeding 50 V a.c. or 120 V d.c. (ripple-free).

NOTE 1. Ripple-free is conventionally defined as an r.m.s. ripple voltage of not more than 10 % of the d.c. component. Maximum values for non-sinusoidal a.c. voltage are under consideration.
NOTE 2. According to Clause 414 of IEC 60364-4-41:2005, class III equipment is accepted only for connection to SELV and PELV systems.

Technical committees should determine the maximum permitted rated voltage of their products in accordance with IEC TS 61201 and the specified conditions of use of these products.

Standard [2] also states that internal circuits can operate at any nominal voltage that does not exceed the limits specified above.

In case of a single fault within the equipment, the steady-state touch voltage that may appear or be generated must not exceed the limits specified above.

Protective Bonding

Class III equipment shall not be provided with a means of connection for a protective conductor. The equipment may however be provided with means for connection to earth for functional (as distinct from protective) purposes where such a need is recognized in the relevant IEC standard. In any case, provision for the connection of live parts to earth shall not be made in the equipment.

The means for functional earthing shall have a distinctive from that used for a means for protective earthing and shall not be connected by a conductor identified as PE in accordance with IEC 60445.

References

  1. IEC 60050-195:2021
  2. IEC 61140:2016