When it comes to EV charging RCD selection, is ‘Type A’ good enough?

When it comes to EV charging RCD selection, is ‘Type A’ good enough? Chaz Andrews, Technical Manager at Doepke UK, gives his take.

EV charging equipment, in the event of certain faults, relies on automatic supply disconnection to protect the user and other members of the public. To reduce the chances of receiving a fatal shock, RCDs must operate before the residual current reaches a dangerous level and within the defined disconnection time. Understanding the basic operating principles of RCDs helps with regard to the specification of the correct Type of RCD, based on the characteristics of the EV charger connected to the charge point.

EV chargers

The regulations place the responsibility on the specifier/designer to verify the d.c. residual current content when selecting RCDs, and EVCPs may contain as a minimum requirement Type A RCDs – Reg. 722.531.2.101, provided it can be determined that the smooth d.c. residual current doesn’t exceed 6mA. Some chargers will not work with Type A RCDs.

The various stages of an EV charger produces common mode leakage currents, mainly as a result of the capacitors used in the PFC section and residual currents with modified wave forms incorporating d.c. components which could exceed 6mA.

Values of leakage current at 50Hz can be relatively low, in the order of 2.5-3.5 mA per charger.

The switching frequency of the converter stage and the associated harmonics can generate leakage currents 20 times that of the 50Hz value at higher frequencies. Transients at the moment of switch-on could result in unwanted tripping of the RCD unless it’s specifically designed to withstand the associated transient. Installing Type A’s under these conditions could therefore result in problems for the installation and users.

Type A RCD operating principles

In Fig 1 the Hysteresis curve1 0 to B3 represents the +ve half of RDC toroid magnetic characteristic, the green area represents optimum operational area, and yellow represents the area of increasing magnetic saturation. A 50Hz residual current IRac, (I) equal to the tripping value sensitivity, produces a magnetic field 0 to B1 for the +ve ½ cycle. The resultant change in this field as IRac passes through zero for –ve ½ cycle induces a proportional voltage (II) in the trip circuit winding and would result in the RCD tripping.

A residual current with +ve biased d.c. component IRdc > 6mA, passing through the toroid will shift the operating point of the magnetic material on the H axis to the right. Now if a residual current IRac with the same value as (I) flows the combined current IRdc+ IRac (III) produces a magnetic field B2 to B3. Although the amplitude of (III) is similar to (I), the resultant voltage (IV) induced in the trip circuit winding is lower and not sufficient to trip the RCD due to the effect of the smooth d.c. content. This phenomenon is commonly referred to as “Blinding”.

RCD “Blinding”

Electrical engineers selecting RCDs for EV charge point applications shouldn’t think purely in terms of smooth d.c. residual current. The standards relating to EV installations actually refer to “d.c. component”, i.e. any current with a permanent +ve or –ve bias that does not pass through zero on every ½ cycle.

The level and characteristics of these currents in normal operation and under certain fault conditions can result in reduced sensitivity or “Blinding” due to magnetic saturation of the RCD sensing circuit, and therefore reduced protection for the public.

The design characteristics of EV chargers vary and consequently EV manufactures specify the type of RCD that can be used safely when charging from mode 1, 2 or 3 installations.

EV infrastructure – RCDs

In installations containing RCDs in series, the residual currents flow through both the EVCP RCD and the upstream RCD, as can been seen in the following examples.

In Fig 2 the feeder pillar distributes power to 1 EVCP, the leakage currents and residual currents flow through both upstream and downstream RCDS, monitoring the level of d.c. leakage current at the EVCP and switching off the CP if the current exceeds 6mA d.c. thereby protecting the upstream RCD as well.

In Fig 3 the upstream RCD mounted in the feeder pillar will be subjected to combined leakage currents of the connected EVCPs and must be selected accordingly. In this case a Type B is required upstream; as the combined d.c. leakage current may exceed the safe limit required for the specification of Type A. In this example the individual CPs monitor the d.c. leakage content.

Specifying the correct RCD

The level of d.c. residual current, common mode leakage currents and transient currents are determined by the EV battery charging circuit design characteristics and therefore dependent on the EV make/model connected to the EVCP.

Local authorities and private companies offering public charging facilities have no control over the EVs that will be connected to their EV infrastructure, and therefore need to cater for the worse case. Options to monitor d.c. residual currents within the EVCP improve the safety performance of installations containing Type A RCDs, where different manufactures vehicles are connected to the EVCP.

Some chargers may require specific RCD features such as: transient resistance to prevent nuisance tripping, operation with mixed frequency leakage and residual currents.