Is RCD Type B Better Than RCD Type A + RDC-DD at EV Charging?

Sep 11, 2021 | Applications, Technology

IEC61851-1: 2017 and IEC60364-7-222:2018 specify the protective measures against the residual current for Mode-3 EV AC charging systems as following

  • Option 1: RCD Type B complying with IEC62423 or IEC60947-2
  • Option 2: RCD Type A complying with IEC61008-1, IEC61009-1, or IEC60947-2 and RDC-DD complying with IEC62955

People might ask what the difference between the two protective measures is. The following table shows the protective difference between Option 1 and Option 2

Protective Measure Fault current detected Protective Difference
Option 1: RCD Type B (I∆n  = 30mA)
  • Sinusoidal AC residual current
  • Pulsating DC residual current
  • Composite residual currents with frequencies up to 1000 Hz
  • Smooth DC direct currents  per IEC 62423 / IEC60947-2
  • According to IEC 62423 / IEC60947-2, Type B RCDs shall operate in response to a steady increase of smooth direct residual current limits of 0.5 I∆n  to 2 I∆n (I∆n =30 mA for EV charging application)
  • Composite residual currents with frequencies up to 1000 Hz SHALL NOT exist in an EV AC charging system when OBC is designed with an appropriate isolation scheme.
Option 2: RCD Type A (I∆n  = 30mA) and RDC-DD (I∆dc = 6mA)
  • Sinusoidal AC residual current
  • Pulsating DC residual current
  • Smooth DC direct currents  per IEC 62955
  • According to IEC 62955, the Standard value of rated residual operating current  (IΔdc) is 6 mA, while the standard value of residual non-operating current (IΔndc) is 0,5 I∆dc (3mA).

The tripping range of Type B 30mA RCD for smooth direct residual current is between 15mA and 60mA, while the tripping range of RDC-DD for smooth direct residual current is between 3mA and 6mA.  The following diagram shows a type-B RCD of 30 mA installed at the final power distribution system. This type B RCD of 30mA does not trip in the occur of 6mA ~ 15 mA DC fault current – such as the DC fault current brought by OBC insulation fault. However, the DC fault current within 6mA ~ 15 mA can impair the function of the upstream type AC/A RCD.

In order to erase the risks, the following installation approaches for Type-B RCD installation are usually used.

  • Approach A: Replace the upstream  type-AC/A RCD with a type-B RCD
  • Approach B: Change the in-coming power supply of the type-B RCD from the out-going terminal of the upstream  type-AC/A RCD to the in-coming terminal of the upstream  type-AC/A RCD

For home charging applications, neither Approach A nor Approach B is a good solution from the perspective of product cost and installation convenience. Therefore, there is a clear trend of using the combination of  RCD Type A and RDC-DD for residual current protection for home EV charging.

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