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Type B RCD vs an RDC-DD?

What is the difference with an in-built RDC-DD?

By Martin Scottorn, Final Distribution Product Manager, Schneider Electric NZ

If you are an electrician keeping up with the latest changes to the NZ Wiring Rules AS/NZS 3000:2018 or working on installing an electric vehicle charger in a home, you might have heard about the use of Type B RCDs or Type B RCCBs. The new Wiring Rules will mandate their use in specific applications (see my previous video on the topic below) once cited.

What electricians and home owners need to be aware of is that there is an exception to the rule when installing a Mode 3 Electric Vehicle charger that incorporates a Residual Direct Current Detecting Device (RDC-DD) conforming to IEC 62955. The dedicated final sub-circuit feeding these types of EV chargers only need be protected by a Type A RCD.

However, there is more to it than first meets the eye. An RDC-DD will disconnect power when the 6mA DC earth leakage limit is exceeded. As most people charge their electric vehicles overnight, it is unlikely they will become aware of the tripping of the device until they wake up in the morning and find their vehicle isn’t charged, probably causing a lot of hassle and inconvenience.

In the same circumstances a Type B RCD would only disconnect the power if the earth leakage current was dangerous. For example, in the presence of 10mA DC earth leakage current a Type B RCD will not trip as this amount of DC earth leakage current is not dangerous and the customer’s car will continue to be able to charge uninterrupted unlike a charger being protected by an RDC-DD in conjunction with a type A RCD.

6mA of DC earth leakage is not actually dangerous, which is why a Type B RCD won’t trip. However, DC earth leakage of 6mA can blind a Type A RCD inhibiting its correct operation. Hence an RDC-DD will disconnect the power when the 6mA DC earth leakage is exceeded because this is the maximum safe permissible DC earth leakage a Type A RCD can still function correctly at.

As Type B RCDs do not become ‘blind’ they do not need to disconnect the power with such little DC earth leakage and continue to supply power until a dangerous earth leakage current exists.

So yes, Type B RCDs are a little bit more expensive, but they also provide a more reliable solution for the home owner with less likelihood of tripping, and therefore less chance they will wake up in the morning without a fully charged electric vehicle and wondering why.

In summary; though a 6mA detection device (RCD-DD) inside the charger can provide protection, a type B RCD ensures better continuity of service and protection because it will detect DC current and its tripping value is much higher than 6mA DC. Unlike an RCD-DD, it will also detect earth leakage current at frequencies higher than 50/60Hz.

Webinar: Discover the new RCCB – Type B

Find out the key differences of B type EV and B-SI type RCCB’s, where to use them and why.

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