Relay Requirements for Contemporary EV Charging Installations

Author : Claudiu Ciobotaru, Panasonic Industry Europe

18 April 2024

Figure 1: Technology forecast for vehicles sold worldwide [Source: Bloomberg]
Figure 1: Technology forecast for vehicles sold worldwide [Source: Bloomberg]

Switching functions in AC charging stations serve to protect the user and must satisfy certain normative requirements. These are defined in the IEC61851-1 and IEC62955 standards. This article discusses the use of relay components in next-generation charging infrastructure where low cost and physical compactness will both prove to be paramount.

The ambitious full-scale switchover from petrol-driven arrangements to fully electric vehicles (EVs) can only succeed if the accompanying infrastructure is significantly expanded. Corresponding promotional programs and new trends in the development of charging stations are intended to boost this market and permit everyone everywhere to charge their EVs. Bloomberg projections state that by 2040 half of all vehicles sold will be battery electric (with over 730 million of them on our roads by that time). Figure 1 gives more details, breaking down automobile technologies into internal combustion engine (ICE), plug-in hybrid EVs (PHEVs) and battery EVs (BEVs). To make such EV adoption possible, the charging infrastructure must also grow rapidly - and, above all, everyone must be able to charge vehicles at home overnight. 

In addition to high-performance DC chargers for ultra-fast charging, especially on motorways, AC charging stations (or AC wallboxes) are going to play a crucial role. Here, it will be possible for vehicles to be charged during shopping trips, while at work or after arriving back home. These stations (categorised as Mode 3 according to the standard) represent the vast majority of the charging outlets. 

As switching functions are essential to every charging station, suitable components need to be sourced. However, no conventional electromechanical system can be employed. PCB relays for use in charging stations must meet certain requirements defined in the prevailing standards.

Normative requirements
The IEC61851-1 ‘Electric Vehicle Conductive Charging Systems Part 1 - General Requirements’ standard serves as a fundamental basis for cabled charging. This defines all the core aspects of cabled charging devices up to 1000VAC or 1500VDC. Areas described here include insulation expectations, temperature limits, mechanical requirements, as well as the associated characteristics for the constituent switching elements. AC charging on a permanently installed charging station (in a home or an office building) is defined here as being Mode 3. 

If a relay is used as a switching element, reinforced insulation between coil contact, safe separation with at least 3mm contact opening and 50k load cycle operation are prescribed (in accordance with relay standard IEC61810-1). The HE-S relay from Panasonic has established itself in this market and has been proven there for years. This relay has 2 contacts with 35A/250VAC capacity each; at 22kW each contact must handle 32A/250VAC. Furthermore, an optional feedback contact is available that provides an elegant solution for detecting soldering on the main contacts (which is a facet that IEC61810-1 also stipulates).

Figure 2: An example of a HE-S relay
Figure 2: An example of a HE-S relay

Safety measures must also be implemented when installing an AC wallbox. Firstly, a load protection switch must be incorporated into the installation. Secondly, an FI switch (that triggers in the event of differential fault current) also needs to be included. Normatively, 2 options are described for switching off an FI switch if a fault occurs:

  •  An FI switch Type B arrangement.  
  •  An FI switch Type A arrangement with an additional mechanism for switching off in the event of a DC fault current that is greater than 6mA. 

As using a Type B FI unit is much more expensive than Type A, the 2nd option is primarily used. A contactor can be employed for shutoff, as likewise can a relay. The latter offers some advantages, however, in terms of:

  •  A more compact design for the charging station, as the switching element is moved from the DIN rail to the PCB.
  •  Energy saving because a relay consumes less energy at the coil.
  •  Monetary savings due to eliminating the need for manual installation of cabling on the contactor, with automated soldering of the relay on the PCB instead and a lower total-cost-of-ownership (TCO).

IEC61851-1 had been the reference for Mode 3 charging stations/wallboxes for many years, but a new standard has been in force since April 2018, entitled IEC62955 ‘Residual Direct Current Detecting Device (RDC-DD) to be Used for Mode 3 Charging of EVs.’ This standard also cites the same 2 options for implementing a shutoff of the FI switch. However, more stringent technical requirements are placed on the switching function. These are:

Figure 3: The HE-R relay from Panasonic
Figure 3: The HE-R relay from Panasonic

  •  Mechanical coupling of all switching contacts - In 3-phase systems, 3 phases and the neutral conductor are switched via a common actuator (thus requiring use of a 4-pole system). This contributes to increased safety in the event of a fault, as all phases are switched off at the same time.
  •  Increased requirements for the short-circuit capacity - Standards outline that no soldering of the contacts is permitted to be carried out after the short-circuit test.

Neither of these points can be met by the HE-S relay in a 3-phase charging system. This was the trigger for a new development at Panasonic Industry to continue creating an appropriate PCB relay solution. Once the new market requirements had been aligned with a number of customers, the successful result was the introduction of a relay capable of meeting all normative requirements - the HE-R. 

The HE-R relay 
This electromechanical component combines energy efficiency, cost-effectiveness and technical sophistication. The holding power for 4 contacts plus the feedback contact is just 490mW, thereby minimising its power consumption. Thanks to the compact design utilised (with 58mm x 35mm x 47mm dimensions), alongside the automated placement and soldering on the PCB, it presents a way of enabling both cost and space savings. It has a 40A/480VAC per contact rating, a short-circuit capacity up to 10kA (in accordance with IEC62955), a 32A nominal current, 3.6mm opening on the main contacts, etc. The newly designed linear drive of the contacts in combination with a very efficient coil/armature system have resulted in a switching element that facilitates implementation of efficient, streamlined and attractively priced wall boxes. Compliance of the short-circuit values according to IEC62955 documentation for a prospective short-circuit current up to 10kA at 32A nominal current (see Table 1) has been achieved using special patented techniques.

Use in charging stations

Combined with DC fault current detection, the HE-R relay can play a central role in protecting users from harm. In the event of a DC fault current, the coil of the relay is deactivated - meaning the contacts open and the circuit is safely disconnected. The contact distance of at least 3.6mm ensures the necessary insulation is upheld.

Table 1: Extract from IEC62955 documentation concerning short-circuit values to be met without soldering the contacts
Table 1: Extract from IEC62955 documentation concerning short-circuit values to be met without soldering the contacts

As IEC62955 is not a relay standard, acceptance can only be carried out together with the residual current device (RCD) unit. Nevertheless, the short-circuit tests on the relay were certified in accordance with this (up to a prospective short-circuit current of 10kA). This now makes it possible to integrate the FI function into the charging station, with the relay as a switch-off element.

What is more, the optional feedback contact provides the possibility of monitoring the main contacts in the event of soldering. This is reliably detected and the electronics in this case will place the system in a safe state. The standards require detection of soldering on the contacts, and the integrated mirror contact provides an elegant option for this (in accordance with IEC60947-4-1).

The modular concept behind the HE-R relay will allow other pin assignments with higher contact loads to be realised in the future, such as a 2-pole version up to 80A/277VAC. This means that various additional applications may also be covered and a whole range of new switching solutions introduced to the market. Looking further ahead, the technical capabilities to cope with intense short-circuit events can be incorporated too.

The long-term outlook 
Germany will increase investment in charging infrastructure to Euro 4.7 billion in 2024 via the Climate Fund. The pace of continued roll-out of such infrastructure in the UK is unfortunately less certain, with the current political administration slowing down on the urgency of its attempts to reach previously announced net zero targets. Whatever the situation that transpires, Panasonic Industry will be in a prime position to offer optimal solutions for charging infrastructure that significantly contribute to an efficient and secure future. 

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