Fault Protection. Any tripping current dependent on the resistance of the earth path. (Regulations 411.4.204; 411.5.1; 531.2). Residual Current Devices (RCDs) are safety switching devices. They are designed to provide safety for three types of protection; these are: The RCD type should not be confused with the different types of circuit-breakers which are manufactured according to their time/current characteristics. Circuit-breakers for overcurrent protection of cables are available in types B, C and D in accordance with BSEN 60898-1. It's quite easy to see how an RCBO with a B Type time-current curve could be mistaken for a Type B RCD. The DC link values of capacitance and voltage, do not result in smooth dc values < 10 mA, being imposed on the AC pulsed wave form. In the 18 th Edition, the term for discrimination has been changed to selectivity. It is important to achieve selectivity when installing multiple RCDs in series. A common misconception is that an RCD with a higher mA sensitivity rating will provide selectivity, but this is not the case due to the instantaneous operation of the device. The only way to achieve selectivity with RCDs is to install a time-delayed device upstream.

If the inspector is concerned that residual DC current may affect the operation of Type AC RCDs, the client must be informed. The client should be informed of the potential dangers which may arise and an assessment of the amount of residual DC fault current should be made to determine if the RCD is suitable for continued use. Depending on the amount of residual DC fault current, an RCD which is blinded by residual DC fault current is likely not to operate which could be as dangerous as not having an RCD installed in the first place. In-service reliability of RCDsA Type S RCD is a sinusoidal residual current device incorporating a time delay. It can be installed upstream from a Type AC RCD to provide selectivity. A time-delayed RCD cannot be used for additional protection because it will not operate within the required time of 40 mS.

Single-phase inverter designs, applied in washing machines and handheld appliances, containing variable speed features, may produce mixed frequency leakage currents. Appliances containing inverters and motors, can be used with Type A RCDs, if:Type A RCDs are used for alternating sinusoidal residual current and for residual pulsating direct current up to 6 mA.

Where electric vehicle charging equipment is likely to create a residual DC fault current, manufacturers may provide the correct type of RCD within the charging equipment. However, problems could arise if the charging equipment is installed downstream of an existing Type AC RCD, which could be blinded by the residual DC fault current. In some types of DC charging equipment, the AC input side is galvanically isolated from the DC output which, hence, provides electrical separation. This means that faults on the DC output side, connected to the vehicle, would not be detected by an RCD on the input side of the circuit. What are the hazards and problems?NOTE 4: For guidance on the correct use of RCDs for household and similar use, see PD IEC/TR 62350. UK electricians can still make informed technical decisions. This is explained in Chapter 12, 13 and Chapter 30. The Electrician is responsible, not the Technical committee, to make the decision on which device is appropriate for the installation. The revised RCD guidance in 531.3.3 is precise. The appropriate Type of RCD must be selected, based on the characteristics of the residual current and the equipment that is likely to be connected to the RCD protected circuit. and the RCD should be selected based on the characteristics of the connected loads. The initial verification required under 641.1 and 301.1 (ii), covers the requirements for the verification that electrical equipment (any appliance supplied or connected, as part of the existing fixtures and fittings) is compatible with other electrical equipment such as RCDs installed to provide additional protection e.g. for a domestic installation, document the appliance RCD requirements with the justification for the consumer unit specification. NOTE 3: For RCD Type B, tripping is achieved for residual pulsating direct currents superimposed on a smooth direct current up to 0.4 times the rated residual current (IΔn) or 10 mA, whichever is the highest value. This is explained in Chapter 12, 13 and Chapter 30. The Electrician is responsible, not the Technical committee, to make the decision on which device is appropriate for the installation.