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RCD requirements of EV charging equipment
There are reported to be around 3000 EV charging points in the UK, with projected growth per year in the thousands due to the UK Government’s vision detailed in the DfT report ‘Making the Connection’.
This report recognises that for an effective take up of EVs, owners will need to access recharging at home, work and on the road. Without stating the obvious, the charging technology adopted at home will be different to that required at work and in public places, defined by the time available to charge, cost of charging and electrical infrastructure capacity. From a safety perspective, the association of untrained people with electrical equipment used outside in all weathers will no doubt send a shiver down the backs of those who have been unfortunate enough to witness the effects of electrocution.
The basic requirements for RCD protection can be found in the IET’s Code of Practice for Electric Vehicle Charging Equipment Installation, with references to BS7671 incorporating amendment 1. This code of practice was launched earlier this year, but does not cover in detail the issues associated with the selection and use of RCDs, leaving the installer to verify the selection of the type of RCD required. Consequently from a safety perspective and legal perspective, the ‘competent person’ (installer) should take careful note of the information detailed in clause 5.5.3, as the implications may not be clear until the point when the installation is subjected to final test and inspection or during follow-up inspections.
EVCE RCD specification issues
The Code of Practice for Electric Vehicle Charging Equipment states under section 9.1: “During and on completion of the installation, and before being put into service, the installation shall be inspected and tested to verify that the installation complies with BS 7671 and the charging equipment manufacturer’s instructions.”
What does this mean in practice? Well, the first point to make is clearly detailed in 5.5.3 even if the implications are not. ‘Type A RCDs are preferred, as they will provide some protection if the vehicle develops a DC fault to earth’. This clause goes on to explains that Type A are only suitable for use where the following applies: ‘Tripping is ensured for residual sinusoidal alternating currents, for residual pulsating direct currents, for residual pulsating direct currents superimposed by a smooth direct current of 0.006A, with or without phase-angle control, independent of the polarity.” It would not be safe to use Type A RCDs as the RCD tripping characteristics cannot be guaranteed; reference clause 133.1.3 page 20 of BS7671.
Some battery chargers used on plug-in electric vehicles (PEVs) can produce high levels of harmonic distortion, due to the technology employed and the non-linear loading of the supply during charging. Standard Type AC & A RCDs are only designed for use on supplies with standard 50Hz waveforms. High frequency AC flowing through RCDs designed for use with 50Hz supplies can cause overheating of the trip circuit components, leading to failure of the RCD due to incorrect application. The other issue associated with the rectifier circuits incorporated in battery charging technology is the transfer of DC earth leakage components into the AC supply under fault conditions. DC flowing through standard RCDs produces magnetic saturation in the RCD’s AC iron core. The RCD will not be able to detect the imbalance created by AC leakage currents; in this situation someone using the equipment, coming into contact with a current carrying conductor or conductive surface (live due to internal fault) and earth would potentially be at a high risk of electrocution.
RCDs are classed as a second line of defence and if selected, installed and maintained correctly, can reduce the risk of death or serious injury. It is very easy to overlook the basic selection process that is required to ensure that the correct device is installed, checked and signed off by the person responsible for this procedure, which is quite clearly defined in the Code of Practice. Making false declarations on test certificates is criminal and dangerous.
The Code of Practice defines a list of basic checks that must be performed, documented and agreed with the client prior to commencing any installation work; reference Annex C to E. Including the RCD selection criteria and Type of RCD proposed in the initial assessment should help avoid any arguments or issues at a later stage.
For example, section 8 details requirements for Commercial and Industrial locations; section 8.5 covers RCDs, in which it states Type A RCDs are preferred (8.5.3) – this is a minimum requirement but does not mean that it is suitable for the application.
Some EV charging points are supplied with basic Type AC RCDs; as previously mentioned, charging circuits interfere with the AC waveform affecting the operation of standard RCDs. Single phase EV charging points fitted with standard Type AC RCDs may not operate safely under certain fault conditions, exposing the employee to a high risk of electrocution in the event of an insulation fault.
Existing technology for in-cable controls does not check for levels of DC leakage current, hence the concerns with regard to the suitability of Type A devices for all possible leakage current conditions that may be present in Mode 2 installations. For sites where the AC charging facilities are designed for multiple vehicles (Mode3 – to deliver the power requirements), different car manufacturers will have varying requirements due to the individual charging characteristics of the vehicle concerned; the assessment must consider the worst case scenario.
The Government specification issued by the OLEV for e-car charging specifies Type A for Mode 3 single phase charge points. This must be on the assumption that there will be no harmonic distortion of the AC waveform and that any pulsating DC/smooth DC content, will not exceed the limits given in the existing product standards quoted in the code of practice - BSEN61008 and BSEN61009.
For mode 3 and 4 three phase EV charging, international car manufacturers already specify Type B RCCBs for the projects / installations that they are involved with, ensuring that they are covered legally with regard to their purchasing / installation specifications and more importantly, their customers’ safety.
When selecting Type B RCCBs, it is important to check the upper frequency limit detailed in the manufacturer’s technical data. Three phase EMC filters normally produce leakage currents in the kHz range, which is above the 1 kHz reference in the existing code of practice. But high-frequency electric shocks produce more heat than low-frequency shocks, leading to burns, coagulation, and necrosis of affected body parts.
Clear guidance with regard to RCD protection will no doubt come about as this relatively new market develops and the risks associated with using this type of electrical equipment outside are better understood. As previously mentioned, some international car manufactures are already specifying Type B RCCBs for use with 3 phase EVCE where the load characteristics cannot be determined or controlled (Mode 3). With Mode 4 there is no argument; the selection is straightforward – Type B RCCBs only. The amendment to the standard for low voltage electrical installations covering car charging installations, when it is published, should clarify the situation and lead to specific section under Part 7 of BS7671.
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