This article orignally appeared in the July issue of the IET's Wiring Matters. Read the full version here.
The pressure is on to achieve net zero. The roll out of electric vehicles, solar photovoltaics (PV) and heat pumps is set to be a big part of reaching that target, so it comes as no surprise that new challenges have been coming thick and fast for the designers and installers of electrical installations to accommodate new technologies into homes.
Renewable technologies require specific protection in order to be safe for use in line with BS 7671:2018+A2:2022. These protective devices are sometimes difficult to retrofit into existing consumer units due to the age of the existing unit or space constraints, so installers sometimes need to either replace with a larger consumer unit or install a separate consumer unit to supply electric vehicle charging equipment. If there is space inside the property adjacent to the existing consumer unit, then this is fairly straightforward and simple. However, if there is no space, then installers need to find other solutions. Some properties have external meter boxes which might be tempting to use as the location of one of these consumer units. However, the Energy Networks Association issued a statement regarding using these enclosures for this purpose:
While the meter cabinet is the customer’s, it is a space designed for the use of electricity industry apparatus only and no allowance is made for additional equipment. For safety reasons, we would not recommend that any internal wiring, including a consumer unit is installed within the cabinet.
One option which has been growing in popularity is to use an external consumer unit fitted externally adjacent to the meter cabinet.
This article looks at some of the main design considerations for such an install in a domestic environment.
What should a designer and installer consider?
External consumer units in domestic premises will need to conform to BS EN 61439 series and in particular, BS EN 61439-3 Low-voltage switchgear and controlgear assemblies. Distribution boards intended to be operated by ordinary persons (DBO).
All electrical installations need to consider the general requirements of BS 7671:2018+A2:2022 and mitigate any potential hazards at the design stage. For an external electric vehicle consumer unit, there are some specific considerations:
- Weather
- Temperature variances
- Exposure to the sun
- Corrosive or pollutive substances which would include coastal air with a high salt content
- Condensation
- Impact protection
- Protective multiple earthing (PME) supplies
- Tampering
- Rated current of circuits within the consumer unit.
- Weather, temperature variances, sun exposure, and corrosive or pollutive substances
Weather, temperature variances, sun exposure, and corrosive or pollutive substances can all have a detrimental effect on electrical equipment. Electrical equipment is designed to operate within certain parameters, so any venture outside of these parameters means their reliability and/or safety cannot be assured.
Traditionally, an external consumer unit was cited in locations away from the weather such as under soffits of roofs of summer houses and out buildings, or housed in additional enclosures. This provides additional protection against the weather and prevents the user having to operate any protective devices in direct rain or snow which could be hazardous.
The weather in the UK can vary greatly and it is important for an installer to understand what that means. Driving wind and rain can penetrate even the best of seals so, using the correct entry method is essential. If attempting a rear entry from inside the property into an enclosure, ingress of moisture will almost be inevitable at some point in the future. Driving wind and rain will get past tiny gaps between any silicon used and brickwork, and then track inside either the enclosure or into the house along the cables.
The temperature in the UK also varies and although these assemblies and devices are tested to the extremes (-25° C/+40° C with an average daily max of 35° C), it is important to remember that as temperatures change, so do the electrical resistances of conductors and thermally operated protective devices. In the cold they will operate slower and as temperature increases they will operate faster.
The enclosure selected will need to have suitable protection from the effects of the sun.
According to PD IEC TR 61439-0:2022 Low-voltage switchgear and controlgear assemblies – Guidance to specifying assemblies:
Solar radiation can have two impacts on assemblies and enclosures exposed to it.
Heating effect caused by the complete spectrum of sunlight, usually referred to as solar irradiance, and
UV radiation which has a shorter wavelength than that of visible light and can degrade synthetic material such as plastics.
These effects could make some plastics become brittle. Some plastics are formulated to be more UV stable than others.
PD IEC TR 61439-0:2022 8.5.2 mentions that the heating effect caused by the complete spectrum of sunlight is not addressed in the BS EN 61439 Low-voltage switchgear and controlgear assemblies series, except for assemblies for PV applications. Outdoor assemblies are required by the BS EN 61439 series of standards to be UV stable in temperate climates and mentions that if exposed to intense sunlight, the specifier should consult with the manufacturers on ways to increase the UV resistance.
A metallic enclosure is more susceptible to corrosion from the rain and coastal air due to higher salt content. In order to demonstrate resistance to corrosion, design verification tests are carried out in accordance with the BS EN 61439 series on representative samples or parts of the assembly to recognized standards.
The tests are accelerated life tests, which use chemicals and processes to stimulate in a short time exposure of the assembly to normal atmosphere for its intended life. All assemblies are required to be resistant to a basic level of corrosion. Two levels are included for ferrous metallic parts which are severity A and severity B. Severity A is indoor equipment and the internal parts of outdoor equipment and severity B is external parts of outdoor equipment located in normal environments. PD IEC TR 61439-0:2022 mentions:
For outdoor equipment where exceptionally long service without maintenance is required, or where particularly onerous conditions prevail (for example exposure to sea spray), additional protection and/or measures can be necessary.
Other considerations
As time moves on, electricians will be conducting Electrical Installation Condition Reports (EICRs) and coming across enclosures like this and it is important to know which ones have been verified by the manufacturer and which ones someone have just been put together themselves from an empty enclosure and devices. This is important as the rated current of assembly verified by the manufacturer may be less than the device marked ratings. Different devices may have different heating effects. It is important that a manufacturer has verified the assembly to BS EN 61439-3.