Electrical Safety With RCDs

Electrical Safety With RCDs
Electrical Safety With RCDs
Electric Vehicle Charging
Electrical Safety is achieved by using Electrical Protection Technology, but also by taking
additional precautionary measures to mitigate shock or fire risk. These involve Monitoring
and the use of products designed for outdoor or weather proof applications, etc.
An interesting example of the
application of Electrical Safety
is in the area of Electric Vehicle
Charging.
Figure 1 below demonstrates
a problem that needs to be
addressed.
IEC Requirements for RCD Protection in EV
Charging Installations
• According to IEC 60364-7-72 every connection point (plug/socket) must be fitted
with its own residual current operated protective device (RCD) with rated residual
current I∆n ≤ 30 mA
• Measures must also be taken to ensure protection in the case of smooth DC
residual currents that are higher than 6 mA
• This DC protection may be provided by a B Type RCD, or by means able to provide
protection in the event of a DC fault current ≥ 6mA DC flowing in the installation
AC Supply
Type AC or
Type A RCD
RCD Protects EV
and downstream
installation
Downstream
circuits
Electric Vehicle
Charging
E
Fig 1 - EV Charging
Figure 1 shows an EV being charged from an AC supply which is protected by an A or AC Type RCD. The supply cable connected
to the EV comprises of Live, Neutral and earth (ground) terminals. The EV could be charged in a garage or the car port of a house.
In the event of an insulation breakdown between the EV and ground, a DC current could flow from the EV to ground and back
through the AC supply, as demonstrated in figure 1a.
1
www.westernautomation.com
Electric Vehicle Charging
AC Supply
Type AC or
Type A RCD
Protection
removed from
downstream
installation when
RCD blinded
RCD Blinded by
DC fault current
IF≥6mA DC
E
Fig 1a - Problem: EV Insualation Fault & RCD Blinding
The DC current path is shown by the arrows, and it can be seen that the DC current passes through the RCD. AC and A Type RCDs
are intended to detect AC residual currents, but they can be blinded by a DC current of sufficient magnitude to the extent that the
RCD will now fail to see and respond to a subsequent AC fault current. IEC has determined that the risk of RCD blinding becomes
unacceptable once the DC current exceeds 6mA. If, during the period that the DC current is flowing, a person touches a live wire
somewhere else in the house on a circuit normally protected by the RCD, there is a strong risk that the RCD will fail to trip due to
the presence of the DC fault current and thus electric shock protection will have been compromised.
Western Automation has addressed this problem by developing a DC current monitor or detector that can be placed in series with
the EV supply as shown in figure 2.
AC Supply
Type F or
Type A RCD
6mA DC
Detector
RCD no longer blinded
Protection
maintained to
downstream
installation
DC fault
current
interrupted
E
Fig 2 - Install 6mA DC Detector with Relay or Contactor
2
www.westernautomation.com
Electrical Safety With RCDs
The DC detector will activate a contactor or circuit breaker to disconnect the EV in the event of a DC current ≥ 6mA flowing to
ground, and thus ensure that the RCD is not blinded.
Although this measure does not provide direct protection against the DC current fault, it does ensure that the RCD can continue to
perform its protective role and thus ensures the safety and integrity of the remainder of the AC installation.
ST = Self Test, EOL = End Of Life
Reliability & Product Failure
Reliability is the ability of the RCD to carry out its protective function when required. An RCD can be fitted in an installation for
many years, and even though it may be rarely if ever tested, people expect it to perform its protective function when required.
RCDs fail over time, but the occurrence of an electrical fault condition is not the optimum time to find out whether or not the RCD
is functioning correctly. RCDs are fitted with a test device and this should be operated by the user every few months. However, factors such as inaccessibility or inconvenience may discourage users from carrying out such tests. This problem has been overcome
with the next generation of RCDs, Self Testing RCDs. An example is shown below.
This SRCD will test itself every 2 – 3 seconds throughout its life, as indicated by the flashing ST LED. When the SRCD finally fails
(hopefully after many years of good service), the EOL LED will light to inform the user that the device is no longer capable of providing shock protection and must be replaced as soon as possible. The Self Test will not result in tripping of the SRCD, but the user will
have the option to operate the Test button to check that the RCD still trips correctly.
Weather Proof Housings For RCDs & GFCIs
Field studies on installed RCDs/GFCIs have been carried out in many countries such as German, Italy, Netherlands, Denmark and
USA. In all cases, defective devices were found in installations, with failure rates typically in the range 4 – 8%. In many cases, non
functioning RCDs were found to be located in areas with exposed or harsh environments, such as outdoors or in car ports or near
swimming pools or on farms, etc. Conventional RCDs & GFCIs are primarily intended for use indoors and are not intended for use in
exposed areas or harsh environments. The corrosive effect of chlorine from swimming pools is well known and yet people continue
to install standard RCDs in such locations. Less well known are the corrosive effects on farms caused by animal or vegetation gases.
The safety and protective functions of RCDs can easily be compromised when the RCD is used outside its “comfort zone”. This problem can be readily overcome by using RCDs in suitable weather proof housings for such applications. However, common problems
with standard RCD weather proof housing are;
i)
ii)
3
The RCD can only be accessed when the cover is open.
Users often fail to close the cover securely, thus undermining the weather proofing properties of the housing.
www.westernautomation.com
Electrical Safety With RCDs
Figures 3a, 3b & 4 below show an example of a WA GFCI incorporated into a weather proof housing that mitigates the above
problems.
Figures 3a & 3b: The housing comprises of a GFCI and two receptacle outlets integrated into the
weather proof box. Two distinctive features of this unit are;
i) The housing has an interlock switch that is open when the cover is open, and prevents power being applied to the internal socket
outlets until the cover is properly closed.
ii) The GFCI is accessible from the outside via a membrane cover. This provides user access to the GFCI to carry out manual testing
and reset operations, and to see the status of the GFCI at all times without any need to open the front cover.
An additional feature of this product is that the GFCI includes automatic Self Test, which it conducts every 3 seconds.
When the GFCI finally fails, the EOL (End Of Life) LED will be lit and power will be removed from the socket outlets.
Western Automation has developed a family of similar weather proof
housings for GFCIs and RCDs for use in N. America, UK and Europe.
The above are just two examples of where Western Automation has
taken a proactive approach to electrical safety.
Figure 4: View of the GFCI membrane.
4
www.westernautomation.com
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertising