IEC 60755, the residual current-operated protective devices specified in this standard are primarily intended to provide protection against electric shock hazards, which can be dangerous and potentially fatal.
IEC 60755 defines the operational characteristics for these devices, and the details of how they should be implemented to achieve the desired level of protection are provided in various sections of IEC 364(1).
There are two basic requirements for protection against electric shock hazards: protection against indirect contact and protection against direct contact.
Protection against indirect contact involves systems that prevent hazardous voltages from occurring on accessible metal parts, which must be earthed in all cases. Thus, these metal parts will only become live under fault conditions (e.g., earth fault conditions).
In such conditions, the hazard does not arise from direct contact with a live conductor by the user but from the user touching earthed metal parts that are in contact with a live conductor.
The rules state that a residual current device with sufficient sensitivity (IEC 60755, Section 5.14, Table IV) provides a high level of protection even in the event of direct contact with a live conductor.
However, it is explicitly stated that the use of residual current devices for this purpose is not considered a separate protection method but rather a complement to direct earthing.
In other words, the primary function of residual current devices is to provide protection against indirect contact, but with devices that have sufficient sensitivity (e.g., operating residual currents not exceeding 30 mA), they provide additional protection as a benefit when other protection methods fail, such as in the event of direct contact with a live conductor.
However, it is clearly stated that the use of residual current devices for this purpose is not seen as an alternative protection method, but as a complement to direct earthing.
The operational characteristics specified in IEC 60755 are based on the rules developed in accordance with the information covered in IEC 479.
These devices also provide excellent protection against the possibility of fire caused by earth leakage currents that can persist for a long time without triggering overcurrent protection devices.
IEC 60755 covers residual current devices (hereinafter referred to as RCDs) that are primarily intended to provide protection against electric shock hazards, with nominal voltages not exceeding 440 Volts in alternating current and nominal currents not exceeding 200 Amperes.
IEC 60755:
Residual current devices that compare the residual current (IEC 60755, Section 2.3.4) to a reference value (IEC 60755, Section 2.3.5) and open the protected circuit when the residual current exceeds the reference value (IEC 60755, Section 2.3.6),
Devices that perform one or both of the above functions, but a group of devices acting together to perform all three functions,
are applicable.
Whether or not overcurrent protection is included as an integral part, complementary or specific rules are required for residual current devices intended to perform one or two of the above functions.
In addition to the above, if other functions are also performed, IEC 60755 applies along with the standards for these additional functions.