MK Sentry RCD 100A 30MA 230V DP Type A 2 Module

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ii) RCD Type A: RCD tripping on alternating sinusoidal residual current and on residual pulsating direct current, suddenly applied or smoothly increasing. Assuming an AC Type RCD will suit every installation is incorrect and RCDs shall be selected according to the equipment which is likely to be used within the installation. Type AC RCDs shall only be selected where it can be assured that there is not and will not be any residual DC fault current on the circuit. RCDs work by monitoring the flow of current in a circuit's live conductors and, if the flow of current is imbalanced due to an earth leakage current of a set nominal value, the device will trip to cut off the supply to the circuit. Whilst RCDs have a nominal tripping current (IΔn), they may trip below the nominal value; for instance, a 30 mA RCD is required to trip at a current between 18 mA to 28 mA.

In some other countries in Europe (perhaps not coincidentally the ones commonly using 3-phase supply to homes), it's much easier to find type-A or even type-B. Requirement for a transient resistance feature < 3 kA and short 10 ms time delay. **Note: Type A transient resistant RCDs only meet EN 61008/9 for general UK 50Hz residual currents. 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? Type A RCDs are used for alternating sinusoidal residual current and for residual pulsating direct current up to 6 mA.

Type AC RCDs are in clause 531.3.3 of BS7671 2018, because they are still in EN 60364-5-53 2015. The UK have just adopted this line from the EN standard.

You might ask "Why are Type AC RCDs still referred to in the 18th Edition of BS7671, when most installations should be using Type A devices as a minimum requirement?" For existing installations, problems associated with the following may indicate that Type F is required in place of Type A:Circuit feeding a single-phase micro-wave oven with inverter-controlled output to the magnetron. A fault on the output side of the bridge rectifier will produce a pulsed fault current (If fig 1), generated at the supply frequency. For applications using single phase inverters for speed control (washing machines, GSHPs etc ), the design characteristics of a single-phase inverter will determine if it is safe to use Type A RCDs upstream. The method of installation requires 30mA RCD protection – e.g. Regulation 415.1 & 522.6.202. Dedicated circuit feeding an immersion heater, conventional oven or cooker, with resistance heating elements.