The selection and use of RCDs as prescribed in BS 7671 amendment 1:2020 is on the face of it pretty simple.
Unless Electrical Separation is used, the each charging point will either need to be protected by a Type B RCD or a combination of both a Type A (or F) RCD and an RDC-DD which provides DC residual current protection, usually rated at 6 mA.
In EVE - the Electric Vehicle Equipment Installation Course, we explore the implications of this in detail, not only at the survey and design stage, but also the practical implications encountered during installation.
Explained in as few a words as possible, Electric Vehicles can produce DC currents that feed back into the electrical installation. DC currents exceeding 6 mA will blind Type A, Type F and also the standard type AC RCDs (used in split-load boards for years). This exposes users of the installation to an increased risk of a fatal shock as the devices intended to provide additional protection may not operate!
The 6 mA RDC-DD function is then to stop any connected vehicle from charging if the 6 mA DC threshold is exceeded - the problem is that currently some cars will generate DC currents well in excess of this - so if one of these is plugged into a charging point with 6 mA DC protection then the driver is going to be disappointed when the vehicle doesn't charge.
Type B RCDs are designed to have an increased immunity to a wider range and type of DC currents, meaning that they will not operate when a car with higher than 6 mA DC is plugged in. There are a couple of challenges when using Type B RCDs though, one is that you will need a slightly bigger wallet to accommodate them and the second is that because they operate at higher DC current levels they can't be used downstream of other types of RCD (with or without the 6 mA DC protection)
This then means that the EV installation designer may have some head scratching to do, especially when designing an EV charging point system that will be attached to an existing installation. EVE explores this topic in detail.
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