There are so many factors to consider when you’re choosing
meter… where do you start?
I’ll start by saying “sorry” but this week we’re going ‘techie’ … we are often asked what things need to be considered when choosing an electrical energy meter. So here goes …
Is the meter information going to be used to bill a tenant or customer?
The first question to ask should always be…How will you use the energy information produced? If you are using the meter for billing, the LAW requires that the meter is “MID” (Metering Instrument Directive) approved – this ensures that the meter is accurate enough to be used to bill a tenant or customer. There is no way around this one … people are being fined regularly for using non-compliant meters.
Oh and by the way, don’t get caught out with your existing meters. If you are unsure about whether your current meters are sufficiently compliant or need to be upgraded ask a metering specialist for more advice (we’d be very happy to help if we can!)
Do you want to read the meter manually or automatically?
If you are happy to only read the meter manually, all you need to ensure is that the meter has a clear, easy to read display. However, if you have lots of meters spread over a large geographic area, manual reading quickly becomes a chore. Thankfully, there are now several, cost effective ways to automate the collection of energy data but to do so your meter will need an ‘interface’ that will allow you to read it remotely. The interface you need depends on the next question …
Are you interested in ‘energy consumption’ or ‘energy quality’?
If all you are interested in is energy consumption information, the cheapest interface option is a “Pulsed Output”. A pulsed output gives you one pulse per watt-hour or kilowatt-hour. These pulses are easily counted to determine the amount of energy consumed during the counting period – typically every half-hour.
Modern meters are however capable of the telling us much, much more! For little additional cost, an energy meter can give us information such as the local line voltage (volts), how evenly the energy consumption is distributed across the electrical phases into our building (current per phase) and how efficiently the loads we have use their energy (Power Factor), etc. To access this information, your meter will require a “Modbus interface” or less commonly an “M-Bus” interface.
To count pulses or read data via an interface, you will also need to consider the “Energy Logger” the hardware device that will collect the data and send it to your software or service that will display the information for you.
If you already have a logger, then you will need to check the interface types it supports before making a final choice. If you don’t, then thankfully there are lots of loggers to choose from, many of which support multiple interface types. We may (possibly) do a similar article to this one on logger selection in the future, in the meantime, if you need a hand just ask.
Direct connect or Current Transformers?
Will the meter replace an existing meter or is it a new ‘secondary’ or ‘sub-meter’? This is key to answering this one.
A “direct connect” meter, like your utility meter, is fitted in-line with your electrical supply; the electricity you use directly flows through the meter. Direct connect meters can be cheaper BUT you must turn off the power to fit it. As a result, direct connect meters are generally only used to replace an existing direct connect meter or are installed in new electrical installation before power is first applied.
For all other uses, such as secondary metering (i.e. metering in parallel with a directly connected meter that doesn’t have an interface to enable automated reading) or for sub-metering smaller loads, the use of clip-on Current Transformers (CTs) is the easiest option as they can usually be fitted without a power-down.
Current rating
We’re nearly there, just 2 more questions to ask …
All direct connect meters have a current rating and where you use CTs, the CTs themselves have a current rating. To ensure accuracy, the current rating of your meter or CTs should closely match the current rating of the load or loads you wish to monitor?
The easiest way to determine this is to look at the size of the circuit breaker that is used to power the loads you wish to monitor. If the load is powered by a 32 Amp circuit breaker, the meter/CT rating should be a minimum of 32 Amps. If in doubt, it’s always better to go slightly larger – a 50 Amp CT will accurately monitor a 32 Amp load.
If you can’t find or access the circuit breaker, the next best thing to do is look at the energy rating of the load you wish to monitor. This is easy if you are monitoring a single load, as each item of equipment will have its own “rating plate” attached but if you are monitoring lots of small loads, such is in a specific area of an office building, this is more involved. Remember: if in doubt, go slightly higher than you think you need.
1 phase or 3?
The final specification, is not really a choice as it is determined for you by the electrical infrastructure of the building or the load being monitored. Most small loads – those that plug into a 13 Amp socket will be single-phase loads. Larger loads which usually have their own dedicated power switch in the wall, may well be 3-phase.
Where you are monitoring power at the distribution board, the incoming supply will usually be a 3-phase supply but the outgoing supply may be single or 3-phase supplies. If in doubt ask a qualified electrician or the equipment manufacturer.
Phew…we made it!
So that’s it, we made it. Sorry if this was overly techie but unfortunately it needed to be to cover the topic adequately. Hopefully now, you’ll have the confidence to say …
“I need a 3-phase electricity meter with 400 Amp CT’s and a Modbus interface” or “just install a couple of 32 Amp directly connected meters which I can read manually”
If so then I’m happy we could help!
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