If you want to install one EV Charger, what type of RCD will you choose? Type A or Type B? Why choose so?

What is an RCD?

A household switchboard


An RCD or Residual Current Device is a clever piece of equipment that has become one of the most ubiquitous electrical safety devices in the world. Put simply, it’s designed to protection against electrical
injury or death by rapidly switching a live circuit off in the event that a person or animal touches it.


How does an it work?


In a balanced electrical circuit, electric current flows from the supply, through the load (for example an EV charger) and back to the supply. The current flowing in both the live and neutral conductors is equal, and the RCD remains closed.

A diagram of an EV RCD under normal operation
A circuit under normal conditions, with the electrical current balanced in both the supply and return path.

In the event that say, a person uses a faulty charging station or cable and touches a live conductor, electric current will begin to flow through their body to the ground (earth). In this event, the current flowing in the live and neutral conductors is no longer balanced, as some of the current has taken an alternative path to earth. The RCD quickly opens, thus interrupting the circuit and protecting the person.

A diagram of an EV RCD tripping
RCD protection responds to an event where a person touches a live conductor. It detects that the supply and return currents are no longer balanced, and opens the circuit.


Relevance to EV charging stations


In many countries, including New Zealand and Australia, it is a requirement to install an RCD upstream from the EV charger (at the main supply or distribution board), even if the EV charger has one built in. Part of the reason for this is that it will protect a person in the case that the supply is broken between the distribution board and the EV charger (for example a person cuts the supply cable with a power tool).


Type B RCDs

For some time, it was not unusual to see common, reasonably priced ‘Type A’ RCDs installed on the EV charger supply. More recently however, a significantly more expensive type referred to as ‘Type B’ has been encouraged by various guidelines and standards. Take for example this guide from Worksafe NZ.

The reason for these recommendations is that in theory, the charging circuitry in the power electronics of a modern EV has the potential to introduce harmonics or ‘smooth DC residual currents’ while charging. This DC residual current could potentially ‘blind’ a Type A RCD, rendering it incapable of responding to a situation in which there is a genuine electric shock risk.

A Type B on the other hand provides a more comprehensive detection capability for various residual fault currents, including smooth DC currents.

The Residual Direct Current Detection Device

The installation of a Type B RCD in series with the EV charger is one option to remove the risk of a Type A RCD becoming inoperable, but it can add at least $300 – $400 to an EV charger install. An alternative option is to choose a charging station with a built in RDC-DD (Residual Direct Current Detection Device), alongside a more cost-effective Type A RCD. The RDC-DD will detect any DC residual current of over 6mA and safely stop the charging session, before allowing the Type A RCD to operate in a potentially compromised manner.

In the latest IEC standard for Mode 3 electric vehicle charging stations (IEC 61851-1:2017) section 8.5 refers to the EV supply equipment requiring either:

  • RCD Type B or;
  • A Type A RCD and appropriate equipment that ensures the disconnection of the supply in case of DC fault current above 6mA

The standard used as an example of the appropriate DC detection equipment is IEC 62955.

Not only does IEC 61851-1:2017 permit the use of an RDC-DD and Type A instead of a Type B, but New Zealand’s own Worksafe has released an addendum to the Worksafe Electric Vehicle Charging Safety Guidelines permitting the use of RDC-DD:

“They (Guidelines) do not prevent the use of an RDC-DD in compliance with IEC 62955 in combination with a type A RCD as an alternative to the use of a type B, provided the RDC-DD and the type A are installed in accordance with the relevant IEC Standards.”

So how do I know if I need to install a type B RCD?

When installing an EV charger, you should ensure that its supplier has provided an appropriate SDoC (Supplier Declaration of Conformance). If installing a Type A RCD, you should ensure that the charging equipment that you are installing includes an RDC-DD compliant with IEC 62955, as not all charging stations do. All Evnex charging stations include a IEC 62955 compliant RDC-DD, and this is stated explicitly in our SDoC.

“All Evnex charging stations include a IEC 62955 compliant RDC-DD.”