Metering - Does your sub distribution measure up?


Sub and final distribution plays a crucial role in the measurement and control of  energy. It is time for distribution boards to evolve argues Steve Dyson, product manager for Hager

The whole nature of a commercial electrical installation has changed over the last few years. Just three or four years ago sub metering and the use of control devices or a building management system was a rarity. Not now.

In the face of growing concern about the environment the government has legislated. Whether or not you agree with this approach, it means that the electrical distribution system has evolved. It is now time for distribution boards to meet these changing needs.

The most important change came with the changes in Building Regulations in 2006. Part L2 of the Building Regulations for non-domestic buildings states that appropriate metering is included at the design stage so that building operators can monitor where the energy is consumed. It states that the meters and sub meters must account for 90% of the estimated consumption in a building.

Property owners and tenants must have accurate information about their energy usage. An overall figure does not meet this requirement; the information must show the different areas of consumption. They must be able to say for example how much of the fuel is used for lighting, power, air conditioning and so on. These regulations apply to all new premises with floor areas of 500m2 or greater.

An ideal solution is sub metering for each type of load. You should also note that the regulations state that separate metering is required for final distribution boards totalling 50kW or more. Most Type B boards exceed this figure when fully loaded.

Guidance on establishing a metering strategy can be found from CIBSE. The leaflet "TM39:  2006 Building energy metering" is an updated version of the General Information Leaflet "GIL65: Metering energy use in non-domestic buildings".  The latter can be downloaded free of charge from the internet.

So assuming that the easiest place to provide sub metering is at the distribution board, the installer generally needs to consider using a meter box and whether it offers enough space for fitting any CTs that are used with the meters - sometimes these are placed in separate cableways.

Such solutions add cost, need extra space and take time to install. The meter box should also complement the distribution board since sub distribution boards are often visible, so aesthetics are important. The whole solution can smack a bit of being an afterthought if you are not careful.

Most manufacturers will have a suite of products, which will help overcome such problems and it is possible to have custom built boards that incorporate metering. Produced as a special the latter may be an expensive option. I would argue that because sub metering is now such an integral part of most installations, a standard Type B distribution board should now allow metering to be fitted within it.

Using our new range of Type B distribution boards to illustrate the point, there is enough room next to the incomer to install metering. The metering pack is supplied wired with the appropriate CTs.

At this point it is worth reflecting on why sub metering is installed - it provides information so that businesses can reduce their energy usage. Regular main meter readings will provide some information about the overall energy consumption, but it reveals little about where energy problems might lie. Installing sub metering will help identify which end use or service is performing well or badly.

This will enable the operator to take targeted action and then measure the result of that action. Ultimately it should help businesses reduce their energy use and costs. Such thinking also helps to establish where to install sub metering. While we need to achieve the 90%?energy monitoring target, we want to achieve this cost effectively.

You may expect certain services to consume large amounts of energy, for instance lighting in an office can account for 40% of energy consumption. Certain areas or rooms in a building may also consume large amounts of energy - for instance a computer room in a school. Such judgements help to form the foundations of a metering strategy.

At this point it is worth referring to the Cibse leaflet TM39. It states metering end-use energy helps:
- establish the breakdown of energy use within a building i.e. where does it all go?
- provide a better perspective on building operation
- identify where energy use is greatest
- identify what the minor loads are
- promote a detailed assessment of demand patterns and benchmarking to identify end-uses that are untypically high
- allow patterns of energy use to be monitored
- reveal useful trends between, say, day/night, summer/winter, weekday/weekend
- provide one year moving averages cumulative sum plots comparing actual consumption with targets
- spot things going wrong before it is too late
- operators to understand and manage their buildings better, resulting in greater energy savings
- provide feedback to: building designers; building operators; manufacturers; government and supply side industry on performance achieved, helping them improve performance by setting better targets
- gain BREEAM credits
- designers complete the building log book
- demonstrate compliance with building regulations.

Of course installing the metering is only part of the story. To be effective the data must be both collected and then acted upon. Sadly it is only too common to see an energy management strategy fail because of this.

Now it is no longer good enough to simply just have metering. Since the end of last year it is a condition of selling or leasing commercial property that an energy performance certificate is provided - which means data is vital.

Buildings with poor energy ratings will suffer, as will those that cannot demonstrate good energy performance - whether this is because there is no sub metering or because the data has not been collected.

To aid collecting the data, meters should have a pulsed output offering remote measurement of kWh or for linking into an energy management system.

Having measured how and where the energy is being used, the next step is to take targeted action to reduce consumption. In part such action may be behavioural - encouraging people to turn off the lights for instance, but there are also many cost effective solutions that are more effective than the human memory!

As energy costs rise and the affects of energy performance certificates begin to take hold, action to reduce consumption will become more common. Again, it is at the sub distribution board where  you can take effective action.

We are finding the use of DIN rail mounted control devices such as time switches, lighting dimmers and twilight switches are becoming increasingly popular.

Tesco Homeplus stores for instance use a combination of a master keyswitch, digital time switches and photocells to switch luminaires via contactors. The lighting circuits are split so that 40% can be switched on for cleaning and or shelf stacking, with the remaining 60% switched on when the store is open to the public. The photocells will also turn the lighting on or off in response to natural daylight levels from the rooflights.

More sophisticated control systems also use the final circuits at the distribution board to control electrical consumption. Bus based systems for instance switch loads in response to commands from other devices. Again many of these bus devices can be fitted to DIN rails. The demand for more control devices means the distributions boards must have a range of extension boxes as part of their suite. Ideally they should be modular so that the installer can easily fit them on top, below or on the sides of boards.

In addition, the more circuits that a board has the finer the level of control available to the designer. It may be that separate lighting circuits are used for those luminaires nearest the windows for instance so that they can be switched or dimmed in response to natural daylight.

One thing is certain, the demand for metering and energy control is going to increase in the next decade. As one management guru said, "If you can't measure it, you can't manage it!" 
The challenge for electrical designers and installers is to help building operators both measure and manage energy consumption. The role of electrical sub distribution is at the heart of this process. It is time for manufacturers to introduce boards that make this job simpler to achieve.