5.6Limitations of residual current devices#
RCDs provide additional protection against electrocution that is already provided by insulation, barriers, obstacles, placing out of reach, electrical separation, automatic disconnection of supply, good earthing, fuses, circuit breakers and good circuit design including low earthing system impedance.
RCDs have been designed to monitor the vector difference in current between the phase and neutral.
Consideration should be given as to whether the installation of the RCD will provide the particular additional protection that may be required in an application.
As with all electrical devices, RCDs have their limitations. Some are:
- 1.Electrocute is defined as ‘to kill by electricity’. The RCD does not and cannot protect against electric shock but given the right circumstances, it can protect against electrocution.
- 2.RCDs are not intended to replace conventional overcurrent protection as the primary means of protection against the effect of electric shock.
- 3.RCDs cannot replace the long-established MEN protection system where reliance is placed on an effective and properly installed earthing system.
- 4.RCDs can provide additional protection in areas where excessive leakage current in the event of failure of other protection devices could cause significant electrical risk.
- 5.RCDs do not replace other forms of primary protection – they are in addition to these other measures. It is taken for granted that good electrical design has already been carried out and RCDs are added as appropriate to help lower residual risk.
- 6.RCDs do not provide protection against phase-to-phase or phase-to-neutral faults, including arcing faults and short circuits (except where the short circuit is to earth).
- 7.RCDs do not provide protection against overload.
- 8.RCDs do not provide protection against voltages imported into the electrical installation earthing system through the supply system neutral conductor.
- 9.RCDs do not protect against reverse polarity connections.
- 10.RCDs may require upstream fault limiting protection.
- 11.RCDs may not operate in very cold conditions or corrosive environments.
- 12.RCDs cannot differentiate between current flow through an intended load and current flow through a person.
- 13.The leakage current normally found in an electrical installation can cause nuisance tripping of RCDs. Examples:
- a.current leakage as a result of the capacitance between live conductors and earth in electrical cabling installations (this can be minimised in wet underground installations by using appropriate cable such as cross linked polyethylene (XLPE) / HDPE)
- b.current leakage as an integral part of the operation of some electrical equipment such as fluorescent and HID luminaires and smoothing filters
- c.current leakage associated with high frequency currents flowing to earth through parasitic capacitances due to pulsed DC-type component in equipment such as switch mode power supplies of electronic equipment, and
- d.current leakage resulting from common mode over voltages due to lightning strikes or distribution system switching causing transient currents to flow to earth via the installation capacitance.
- 14.The standard value of residual non-operating current for a 30 mA RCD is 15 mA. Manufacturers generally calibrate RCDs at 22.5 mA ± 3 mA. Nuisance tripping may therefore be more of a concern where there is excess leakage current.
- 15.RCDs are not recognised as a sole means of basic protection against contact with live parts but may be used in addition to insulation, barriers and obstacles.
