THE BENEFITS OF MAXIMUM DEMAND SURVEYS

There are many benefits to performing a detailed maximum demand survey on a premises, the most obvious of which being the accurate measurement of actual energy use over time rather than using an estimation. But there is also a lot of interesting analytical information that can be obtained which could be useful in identifying energy use, and potential areas for reduction and savings.

The energy balance within modern properties is constantly changing and this has been further accelerated by the current ‘cost of living’ crisis, with the trend towards more home working also having had a notable affect, both on the home and on the now vacated business premises.

Traditionally a ‘normal’ dwelling, for example, would be considered to have two Ring Final Circuits, two Lighting Circuits, one Immersion Heater, one Cooker and one Shower. However, this is not particularly representative of many modern dwellings, and doesn’t consider the benefits of heating appliance efficiency improvements, the availability of low energy electrical equipment, and the effects of PV, EV, heat pumps, etc.

So, how should you go about accurately determining the maximum demand for a property?

The complexity of the diversity calculations required make this a daunting task, further exasperated by the fact that the figures in the IET Onsite guide are way out of date and have large margins for error built into them.

A recent case study performed at a large town house converted into 5 flats in Richmond, London showed exactly how far out calculated maximum demand can be compared to what’s really being consumed in a domestic situation.

Extrapolating this out to larger commercial and industrial installations would give rise to even more opportunity for errors.

The conversion into flats had occurred over 50 years ago, with the existing 100A single-phase service having been split by the DNO into a 6-way singlephase supply. Five for the flats and one landlord’s supply for communal areas.

This installation had worked very well since then with no known overloading issues, however, the old service head and tails were looking their age, and conversations with the DNO were underway about updating the supply. It was at this point that it was suggested the existing supply was potentially inadequate for the premises, and that the property owner would need a new, larger, DNO supply to be connected, at his expense!

Faced with the huge cost implications of having a new supply connected to the property, the owner set about obtaining accurate and reliable maximum demand data.

They reached out to a local electrician who came and fitted a Chauvin Arnoux PEL103, power and energy logger, along with a 1-metre-long flexible current sensor which was capable of looping around all 6 tails at once, and therefore capturing the total current in them all.

The PEL103 was set to log current for a period of 12 days to capture the full range of loading on the property and locked safely away in the utility cupboard.

By performing this activity in the colder weather, the results would include any heating loads that might not be present in the summer months.

Analysis of the logging data was a simple process using the PEL PC software, which displayed a log of the current sampled every second both as a table of values, and as a current vs. time graph.

The graph of the total log (Figure 1) showed the maximum current drawn from the supply during the logging period was 48.83A which spanned a period of approximately 2 minutes. The average current drawn from the supply during the logging period was 8.85A.

The data showed that the maximum current drawn from the supply, during the 12-day logging period was less than half the 100A main fuse/cut-out rating, and far less than the textbook diversity calculation would suggest.

The electrician created a report for the property owner detailing his findings and confirming that the installation was indeed operating well within its supply capacity, and a larger supply wouldn’t be required.

A significant cost saving for them and victory for maximum demand measurement over diversity calculations!

In certain situations, it may be useful to be able to identify the types of loads that are consuming energy, particularly if the maximum demand of a premises is close to or exceeds the main fuse/cut-out rating, or if clues are needed in identifying items that are using energy out of hours.

Utilising power and energy loggers with fast sampling rates, such as the PEL103 or PEL104, which can store results every second or 200 milli-seconds respectively, it is possible to achieve exactly that by studying the graphical data produced.

In the previous Figure 1, that shows a graph of the total current demand over the entire 12-day log, it is possible to see the peeks which occur at the same time every morning as the occupants first get up, and again in the evening when they return home from work and start to use cooking and heating appliances. This can be seen in more detail in Figure 2 which is zoomed into a 24-hour period.

Starting at midnight on the left and advancing in time one hour per square we can see the life of the tenants laid out for us. The kettles and toasters of the early risers appear as peaks of roughly 10A lasting from a few minutes. We can also see a block of current between 6am and 7am, probably a heater pretimed to give morning warmth or hot water. Further peaks are seen around lunchtime and again in the evening when the property reaches its Maximum Demand of around 48A, although only for a few minutes.

The points of maximum demand were when the various cooking activities in the five flats overlapped with each other in the evening, and where the base loads were naturally higher due to increased activity within the flats from TVs, PC’s, lighting etc.

We can deduce this by comparing the current log with a log of 3rd harmonics over the same time. (Figure 3). The high percentage of harmonics prior to the red cursor line being due to non-linear loads such as audio/video/IT equipment being the dominant consumers of energy, but as soon as the much bigger, linear loads (kettles and cookers) are switched on, which produce no harmonics, the percentages drop, only returning to their higher levels once dinner was cooked! The benefits of using maximum demand measurement over diversity calculations are clear and many. In this example it saved an enormous amount of unnecessary work and expense by proving the suitability of the supply to the actual load(s), and while the nature of the loads was somewhat irrelevant as the maximum demand was so low compared to the 100A main fuse/cut-out rating, such information can be extremely useful in different circumstances. www.chauvin-arnoux.co.uk