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Electric Vehicle Charge Point - RCD Safety

Electrical Engineering

27/04/2015

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 RCD’s 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

Type A RCD’s (Reg. 722.531.2.101) can be used with some types of EV charger, provided it can be determined that the smooth dc residual current does not exceed 6mA (check EV manufactures hand book). Some chargers produce transients at moment of switch-on, which could result in unwanted tripping of the RCD unless it is specifically designed to withstand the associated transient. Installing Type A’s under these conditions could result in problems for the installation and users.

Type A RCD operating principles

In Fig.1 the Hysteresis curve[1] 0 to B3 represents the +ve half of RDC toroid magnetic characteristic, the green area represents optimum operational area, 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 dc 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 dc content. This phenomenon is commonly referred to as “Blinding”.

 

RCD “Blinding”

 

The standards[2] relating to EV installations 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” of the RCD, 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 figure 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 dc leakage current at the EVCP and switching off the CP if the current exceeds 6mA d.c. there by protects the upstream RCD as well.

In figure 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 CP’s monitor the d.c. leakage content.

 

Specify the correct RCD

Local authorities and private companies offering public/staff 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 dc 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.

 

For complete safety requirements refer to the IET Wiring Regulations BS7671 2008 Amendment 3:2015 Section 722 and the IET Code of Practice for Electric Vehicle Charging Equipment Installation.   

 

The key points related to “SAFE” EV charge point RCD selection:

 

  • EVCP:  RCD selection – Check with EV Manufacture  / EV Model
  • Type A:  OK for 50Hz pulsed wave form with <6mA smooth dc content
  • Type KV:  Similar to A, but withstand <10mA dc and  <3kA transient
  • Type F :  Similar to KV, plus mixed frequency residual currents    
  • Type EV:  Similar to F , plus monitoring d.c. content and switch off if > 6mA
  • Type B:  For applications where the above parameters are exceeded

 

Chaz Andrews – Technical Manager, Doepke UK Ltd

sales@doepke.co.uk or www.doepke.co.uk

 

[1] Original drawing Source: www.zvei.org with additional text added
[2] BS7671 Amendment 3

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