Type AC RCDs and the effects of DC leakage current

Type AC RCDs have been the default standard for decades in the UK, however due to the increased number of electronic components, accessories and equipment in use which effectively manipulate wave and current forms, there have been issues identified with DC leakage. In the event of a fault within electronic equipment that contains DC, such as a faulty EV inverter, a direct DC leakage current from a Solar PV panel or indeed the variable speed drive contained within a washing machine, the current can “leak” to earth and into the system via the neutral (as in most systems the earth and neutral are combined in some way shape or form).

The DC leakage current is not only undetectable by an AC type RCD, but it also detrimentally impacts the disconnection time for the RCD under fault. The larger the amount of DC current the greater the impact on the RCD. DC current can “saturate” the ferrous core of an AC RCD, which subsequently interferes with the AC sine wave.

In a normal operation of a type AC RCD, the magnetic fields which are produced when current is passed through the line and neutral coils within the RCD, equalise, ensuring no current is generated in the tripping coil, thus the device remains energised. In the event of an AC fault condition as the current entering the device (via the line conductor) does not equal the amount of current returning on the neutral conductor. I.e. a greater amount of current (and thus stronger magnetic field) in the line conductor than the neutral, the tripping coil is energised. If the imbalance is of sufficient magnitude, i.e. greater than the rated residual operating current (I∆n) for the RCD, the device opens the circuit.

In fact, the DC current impacts the ferrous core itself, which can become shocked, stunned, or saturated. When the current is travelling in the positive direction all the iron (ferrous) molecules are positively charged, and when the current is travelling in the negative direction all the molecules flip direction and become negatively charged. This pattern continues constantly in line with the AC sine wave of the supply. However the introduction of DC current permanently impacts the characteristics of the positive coil, which means that more current is needed, or a stronger magnetic field is produced than for the negative coil. So much so that some of the iron molecules remain positively charged even when the current is returned via the neutral coil. The means that the ferrous core remains slightly magnetised. As the amount of DC current is increased, it increases the degree of residual magnetism (or remanence) within the ferrous core and once all the molecules remain positively-charged it is termed “saturated.”

If a fault was to occur at this stage, there would be insufficient magnetism between the positive and negative coils to produce an imbalance and thus generate sufficient current in the tripping coil to cause disconnection of the RCD. Or more accurately, a much larger amount of fault current would be needed to cause the device to operate. This could be seven or eight times larger than the I∆n for the RCD, and much larger than the minimum current required to cause ventricular fibrillation (heart attacks) and subsequently death.

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