RCDs and Protective Devices
[ Last Updated 27 March 2006 ]
Q. Why Has Increased Importance Been Placed on the 0.4 Seconds Protective Device Operating Time Requirement?
A. To ensure that, in the event of an active fault to earth fault, the duration of potentially hazardous touch voltages is maintained within safe limits. See the reply under EFLI.
Q. Is a 27/25 amp MCB Suitable to Protect 10 Amp Socket Outlets in Final Subcircuits?
A. Yes, provided that maximum voltage drop and the cable maximum current rating (when de-rated for thermal insulation) is not exceeded, a 27/25 amp MCB is permitted. The requirement for the protective device rating to not exceed twice the socket outlet rating has been deleted from the Electricity Regulations 1997.
Q. Will it be Permissible to Use Plug-In MCBs to Replace Rewireable Fuses?
A. It is permissible to use plug-in MCBs in lieu of rewireable fuses, but the live connections in the base should be screened.
Q. Is It a Requirement to Replace a Faulty MCB with a RCBO?
A. No, it is acceptable to replace a faulty MCB with another MCB (or with a RCBO) of the same current rating able to break the maximum short circuit current at the point of connection.
Q. Is It Permitted To Replace a Faulty MCB with a RCBO?
A. Yes, it is acceptable to replace a faulty MCB with a RCBO able to break the maximum short circuit current at the point of connection.
Q. For Socket Outlet Over Current Protection, is a 16A MCB Suitable for a 15A Socket Outlet (15A MCB Not Made)?
A. Yes, see the answer to question 2.
Q. There are RCD Types AC, A and B. What are They Used For?
A. Type AC RCDs operate with a.c. residual currents only flowing. Type A operate with a.c. and pulsating d.c. residual currents flowing. Type B operate with a.c. and constant d.c. residual currents flowing. Due to the high cost of Type B, these RCDs are rarely found in New Zealand. In New Zealand, RCDs used for protection against electric shock are required to operate when tested with a.c. residual currents and pulsating d.c. residual currents. So now, only type A or B may be installed in New Zealand.
Q. Will RCDs Operate With Two or More in Series?
A. Two or more in series are permitted, and if each RCD is of identical residual current rating either or both RCDs may trip in the event of fault. If it is necessary to use this configuration, consider discrimination.
Q. Is an RCD Acceptable in Lieu of Mechanical Protection? If So, What Are the Conditions?
A. No. Notwithstanding AS/NZS 3000 clause 3.9.4.6, regulation 70 (1) does not allow this.
Q. What is the Reason for Mandating RCDs in Domestic Residences?
A. To provide an increased level of personal protection against electric shock.
Q. Why is RCD Use in Commercial Electrical Installations Not Mandated?
A. As the obligation to provide an electrically safe work site, is an employer's responsibility.
Q. Is Retrospective Installation of RCDs Required in Commercial Installations?
A. No. The Electricity Regulations do not require the retrospective installation of RCDs unless it is a specialised area (e.g. a damp area).
Q. Is Retrospective Installation of RCDs Required in Domestic Installations?
A. No. The Electricity Regulations do not require retrospective installation of RCDs unless there is a change in use of the area (e.g. change to a damp area) or a new final sub-circuit is being carried out. For a new final sub-circuit, the RCD may be either on the switchboard or on the first socket outlet. (See Amendment A to AS/NZS 3000.)
Q. Will an RCD Trip in the Event of a Supply Failure?
A. Most non-portable (switchboard mounted or socket outlet units that contain an RCD) RCDs do not trip in the event of a supply failure. However, portable RCDs are required to trip to "OFF" in the event of supply failure.
Q. Is there a Requirement for Socket Outlet Labelling, if RCD Protection is Provided?
A. No. Labelling is not required by AS/NZS 3000 but it is recommended if the labels are supplied.
Q. What is the Optimum Number of RCDs in a Domestic Installation?
A. No recommendation is made in AS/NZS 3000. The number is dependent on the size and layout of the installation and consideration must be given to the effect on the occupants and services in the event of a RCD tripping due to a fault. In general, it is recommended that a minimum of two RCDs be provided for an average domestic installation with approximately half the lighting and socket outlets on each RCD.
Q. Where are the Requirements for RCD Testing Listed?
A. For new installations, refer to AS/NZS 3000 Section 6 and for existing installations, refer to NZS 3019 Section 4. For portable RCDs, refer to AS/NZS 3760:2001.
Q. Do Appliances and Refrigeration Equipment Require RCDs?
A. Yes, unless they are connected to a permanent connection unit.
Q. For Fluorescent Fittings and Refrigeration Equipment, What are the Options if the Leakage Current is High and the RCD Trips?
A. For the RCD to trip, the leakage current must have reached hazardous levels. Test and repair the fitting or equipment to reduce leakage currents or replace it.
Q. Will there Be an RCD Tripping Problem due to High Inrush Currents at Turn-on with LV Transformers, Light Dimmers, or Welders?
A. Modern RCDs manufactured to approved Standards have good levels of immunity to tripping caused by short term transients at turn-on of highly inductive loads. Consideration may need to be given to the number of final sub-circuits protected by an RCD.
Q. What is the History of RCD Tripping Problems in Kitchens in Australia?
A. There have been no reports of RCD nuisance tripping problems in Australia. However, problems have arisen, both in Australia and New Zealand, where the RCD line and load terminations are transposed. With some types, this will destroy the RCD.
Q. Has Consideration been Given to Insurance Claims Resulting from RCD Tripping on Refrigerators?
A. This is outside the scope of the Electricity Regulations, however, the Australian experience is that the tripping of RCDs protecting refrigerators is the same as for other circuits i.e. very rare.
Q. Is it Permissible to Fit an RCD Bypass to Prevent Loss of Supply to Refrigerators, etc. in the Event of the RCD Tripping during Periods when the Property is Uninhabited?
A. No. A RCD bypass must NOT be fitted in any circumstances.
Q. With an RCD in the Switchboard, What are the Earth Fault Loop Impedance (EFLI) Testing Requirements?
A. An RCD provides a higher level of fault protection, and therefore, if final sub circuit volt drop limits are also complied with, EFLI measurements may be omitted.
Q. Why are RCDs Required on Lighting Circuits?
A. To provide protection if metallic roofing becomes live.
Q. Are Two or More RCDs Mandated on Lighting?
A. No.
Q. Do Security Systems have to Be on an RCD?
A. Yes, unless the are permanently connected.
Q. Is RCD Protection Required for Emergency Lighting Systems?
A. If mains voltage wiring is being used for the emergency lighting system type area, wiring in a domestic installation, RCD protection is required. In other electrical installations, RCD protection is optional.
Q. Why are there Different Requirements for RCD Protection of Domestic and Residential-type Areas Lighting Final Subcircuits?
A. In residential-type areas, there is less risk of roofing being enlivened.
Q. In Domestic Installations, which Final Subcircuit Does Not Need RCDs?
A. Any subcircuit supplying only permanently connected equipment or the socket outlet provided for a free standing cooking appliance.
Q. Are there any Changes to the Requirements for RCDs in Industrial Damp Locations Subject to OSH Coverage?
A. There are no changes from existing requirements. Some additional guidance is given in AS/NZS 3000 clause 2.5.3.3.
Q. Is RCD Protection Mandated for Domestic Garages?
A. For new final subcircuits originating from a switchboard, an RCD is required. If an existing final subcircuit, which does not have RCD protection, is extended, RCD protection is not required. However in this type of situation it is recommended that the socket outlets have RCD protection.
Q. What are the RCD Requirements for Permanently Connected Loads?
A. Permanently connected loads do not require RCD protection unless the load is mandated to be RCD protected.
Q. In a Domestic Installation with Three Phase Socket Outlets, is RCD Protection Required?
A. Yes.
Q. When Surge Protection Devices (MOVs) are Fitted to an Installation should they be Upstream or Downstream of the RCD?
A. The IEC 60364 recommendation is to connect any surge protection devices upstream of RCDs. IEC 60364-5-53, 534.2.5, P 33
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