Safety Measures for Electric Vehicle Charging

Electric Vehicle Charger | Robotics and Automation News

According to International Energy Agency, the global stock of electric vehicle (EV) and plug-in hybrid electric vehicle (PHEV) reached 7.2 million units in 2019, with an annual average increase of 60% over the period 2014-2019. Bloomberg New Energy Finance estimates that the EV and PHEV could represent up to 50% of the total worldwide sales of vehicles in 2040. 

Increasing use of Electric Vehicle will require an intense growth of charging infrastructure. The vehicle charging needs connection to an electricity supply, the question of electrical safety when charging is central. 

The so-called EVSE (electric vehicle supply equipment) are intended to be installed in various environments : in the vicinity of private buildings (e.g. large charging stations for office building), in public area (e.g. car parking with charging stations) or in residential facilities; the vehicle and the charging station may be located outdoor, in a wet environment and in contact with children, or a person not aware of the risk of electricity.

Related Article: How to Select Circuit Protection Devices?

Safety measures are required by IEC 60364 (Part 7-722) 

International series of standard for Low Voltage Electrical Installations (IEC 60364 series) contains a new part dedicated to supply of electric vehicle. First edition was published in 2015 to define a set of safety measures for EV charging. The second edition was published in 2018 to address more specifically the various possible cases of EV charging. 

The charging of electric vehicle requires specific measures to cover the various applications and environments, such as 

  • outdoor charging in presence of water, snow, salt, ice, dust; 
  • charging in public areas with the presence of children, persons not aware of risks of electricity, risks of mechanical shocks on the EV supply equipment; 
  • charging in residential premises (private homes or multi-house dwellings), with no scheduled maintenance; 
  • charging of EV fleets for industrial or public actors, where the availability of the vehicle is essential. 

IEC 60364 part 7-722 requires electrical safety protective measures to address the above-mentioned applications. 

  • Protection against short-circuits and overloads 
  • Protection against electric shocks and risks of electrocution 
  • Protection against overvoltages 

Moreover, the Electric Vehicle Supply Equipment (wallbox or parking stations) shall be in accordance to the relevant part of IEC 61851 series.

Protection against short-circuits with circuit-breakers 

Like for any final circuit supplying a load, IEC 60364 part 7-722 requires to provide, so-called overcurrent protection. In practice, this means protection against short-circuits or overload in the final circuit, which is really meaningful having in mind that some charger can be rated up to 22 kW or 50 kW, meaning permanent load current of 32 A or 63 A. 

Protection shall be provided by circuit-breakers complying with their relevant standard, namely IEC 60898-1 or IEC 60947-2, installed in the distribution switchboard. Compliance to these standards provides safe behavior during the entire life of the installation. This includes the case of high short-circuit (e.g. 6 kA, 10 kA or 20 kA), overload in the circuit, temperature rise behaviour when nominal current is passing, ageing, behaviour of terminals, insulation, electrical or mechanical endurance.

Protection against electric shocks with 30mA RCD 

When considering that EV charger can be located in public areas, outdoor, with presence of water, presence of children; when also considering that an electric vehicle is a large conductive area, which could come in contact with the human body, the question of protection against electric shocks shall be considered carefully. 

  • Firstly, to cover an insulation fault, the most common safety measure is to connect all accessible metal parts to the earth (Protective Earth) and to disconnect the supply in case of fault. This is function is performed either with a circuit-breaker in TN earthing system or with a medium sensivity RCD for TT earthing systems. 
  • Secondly, the effect of currents on human beings is covered in IEC 60479 series, and the threshold of ventricular fibrillation is defined (see Figure 1, curve c1). For this reason, one of the most important requirement of IEC 60364 part 7-722 is to require that each connected point be protected by a 30 mA RCD.

Protection against overvoltages

The power surge generated by a lightning strike near an electricity network propagates into the network without undergoing any significant attenuation. As a result, the overvoltages likely to appear in a LV installation may exceed the acceptable levels for withstand voltage recommended by standards IEC 60664-1 and IEC 60364. The electric vehicle being designed with an overvoltage category II, according to IEC 17409, therefore, it should be protected against overvoltages that could exceed 2,5 kV.

Example of Electric Vehicle charging installation 

The EVSE (Electric Vehicle Supply Equipment) shall be integrated in the electrical installation in order to comply to the safety measures of IEC 60364-7-722, and thus provide protection against short-circuits, electric shocks, and overvoltages. Each EVSE shall be supplied by a switchboard with a dedicated circuit with circuit breaker complying to IEC 60898-1, a 30 mA type B RCD complying to IEC 62423 and, where the connected point is accessible to public, by a surge protective device complying to IEC 61643 series.

Recommended single line diagram for charging electric vehicle | Schneider Electric

Alternatively, IEC 60364-7-722 also considers the case where protection against electric shock is done by a 30 mA Type A RCD, complying to IEC 61008 or IEC 61009 series, in conjunction with a function of detection of 6 mA DC residual current, complying to IEC 62955, in the EVSE, as given in Figure 3. This alternative solution is usually applied for more simple application.

This 6 mA DC detection function is known as Residual Direct Current Detecting Device (RDC-DD). It should be mentioned that
  • The function monitors any DC residual current in the circuit; 
  • RDC-DD function is a detection function, it does not provide protection; 
  • RDC-DD shall comply to IEC 62955 
  • RDC-DD shall be used in conjunction with a 30 mA Type A RCD, installed in the LV switchboard. 

In case the DC residual current exceeds 6 mA the RDC-DD function included in the EVSE will immediately switch off the contactor of the EVSE thus stopping charging the electric vehicle, in order to be compatible with the use of a Type A RCD. However this value of DC residual current is not dangerous.

Alternative single line diagram for charging electric vehicle | Schneider Electric

  • Title: Safety Measures for Electric Vehicle Charging
  • Author: Jean-François REY
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1 comment:

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