Our expert answers a question about the physical differences in the production of Type A and Type AC RCD units.
Question:
Hi, I've been wondering for a while but can't find any info on the subject. I know the differences between types of RCD but was curious as to what physical differences there were in the production between Type A and AC RCD units. What is it that makes an A RCD cope with the DC current? Is there something added to the unit or just a different type of component? Likewise with the other types of RCD. Thank you.
Answer:
Manufacturers may have different solutions to enable a Type A RCD to function correctly with < 6 mA smooth DC superimposed on a 50Hz AC sine wave residual current.
Low quality magnetic materials used in some cheap AC RCDs and the design of the trip relay are not suitable for use in Type A devices. This results in substatially different performance/reaction to AC residual currents with DC content. Basically, Type AC RCDs are not designed/tested to work with any smooth DC content in the AC sine wave.
The first requirement for Type A RCDs is that the material used in the primary magnetic circuit (B/H curve) is of sufficeint quality to ensure the DC content does not result in early saturation of the iron circuit.
The magnetic properties/design must function reliably for both + ve DC biased and - ve biased wave forms.
The characteristics and properties of the trip-relay built into the RCD must be sutiable for DC biased AC waveform.
Some Type A designs use additional electronics (PCB) in the secondery circuit.
– Chaz Andrews, Technical Manager, Doepke UK Ltd