Electric Meters

Introduction 

After the successful invention of alternating current in the late 1880s, electricity became an essential commodity around the world. It is pertinent to note that the electric current was charged as per the number of lamps in the respective premises. Later, scientists discovered that electricity consumption was not only based on the number of lamps but also the hours of usage. So, a device was invented for measuring the flow of current. The device was named as an electric meter. This article will explain the features of the electric meter and its types.

An electric meter

An electric meter is a device used to measure the amount of electricity consumed by a consumer. They are also known as electricity meters or energy meters. Electric meters are installed in the consumer’s premises by the electricity distribution utilities. The customers are charged by the utility based on electricity consumed. Generally electricity is measured in kWh (kilowatt hour), which is denoted as one unit.

Types of meters

Based on make and working conditions, different types of electric meters are available as follows

1. Electromechanical meters
2. Digital meters
3. Bi-directional meter
4. Smart meters

Electromechanical Meter

Electromechanical meters, which were more common earlier, measure electricity consumption using a combination of electrical and mechanical parts. A rotating non-magnetic metallic disc is attached internally, which rotates in a clockwise direction depending upon the electricity consumption. Electricity consumed is measured using the rate of rotation. It requires about 2 watts of power to rotate the disc which is not recorded by the meter. Electromechanical meters have mechanical counter dials which show the electricity consumed. For every ten rotations of the right dial, the nearest left dial will spin once. Also each electromechanical meter has a unique number that is used to identify it.

Image 1: Electromechanical meter 

Disadvantage of electromechanical meters 

1. As it contains moving parts, like the counter dial and spinning disk, it is prone to wear and tear, which affects the accuracy;
2. Local pollution -  dirt and dust will accumulate and cause damage; and
3. Electricity consumption below 15w is not recorded since the power of low magnitude is not enough to rotate the disc.

Digital meter

A digital meter is a type of electric meter that is currently being used in India. The older version of the digital meters use the mechanical system. It measures the electricity consumed mechanically - such as an electromechanical meter - and converts it into a digital signal using ADC (Analog to Digital Converter). The accuracy of this older version of the digital meter is the same as that of the electromechanical meter. 

Image 2: Digital meter 

The recent version of digital meters uses CT (Current Transformer) sensors which measures the amount of energy consumed. This is deemed more accurate as compared to the mechanical system. The readings are shown either using the LED or LCD display. This version of digital meters does not have any mechanical parts, hence the probability of it getting damaged is also comparatively low. It also provides information like date, time, voltage, ampere, instantaneous and maximum rate of usage demand, kWh, kVAh, and power factor. The digital meter also has a push button to navigate between the above mentioned details. All these data are stored in non-volatile memory (can retain content even when power is off) for 45 days to 6 months which varies depending on the make of the meters. This type of meter also has a unique identification number. The electromechanical and digital meters are unidirectional meters.

Bi-directional meter

Bi-directional meter is similar to digital meters where digital meters can measure only the electricity consumed from the grid whereas the bi-directional meter can measure the flow of current in two directions, energy consumed and exported to the grid. For example, in the case of consumers having rooftop solar a bi-directional meter is used to measure electricity drawn from the grid and the solar output. 

In addition, a bi-directional meter displays information like date, time, kWh imported from the grid, kWh exported to the grid, net unit of kWh (difference between import and export), imported kVAh, exported kVAh, frequency and power factor.  

Image 3: Bi-directional meter 

Smart meters

Smart meters are the latest electric metering system where it can track and record the electricity usage of premises and communicate it directly with the utility without human interference. Smart meters have a two-way communication system whereas all the other meters are one way communication metering systems. The smart meter has either a pre-installed Radio Frequency (RF) circuit or General Packet Radio Service (GPRS) on the meter for communication. This enables instantaneous transfer of data from each meter to the utility and vice-versa at a particular interval. 

GPRS Communication: When GPRS is pre-installed, it communicates directly with the electricity provider. However, GPRS requires internet connectivity hence the data has to be transmitted by the telecom network. The disadvantages in installing GPRS in all the meters is connectivity issues and high cost.

RF sensors:  RF communication is similar to the mobile phone networks. Data from a set of meters in a community will be transmitted to the Data Collection Unit (DCU), which acts like a mobile tower that transmits data to the electricity provider. Here only the DCU has the GPRS system, whereas meters have only RF sensors hence the cost of the meters is less when compared to the meters which have GPRS.

Smart meters record the power usage of each house instantaneously, which helps the utility monitor the electricity demand. Smart meters also help arriving at a load curve to track consumption over short and long periods of time. 

Generally, the electric meter reading is taken manually by an accessor hence increasing the chance of human error. In case of smart meters, data can be automatically sent to the service provider, thereby ensuring accuracy of the electric bill.

Image 4: Smart meter 

Smart meters send notification to the electricity provider incase of any kind of tampering. Further, customers can set a data alert for excessive consumption. 

Prepaid metering system is also possible while using the smart meter. Prepaid metering is similar to prepaid mobile recharging where a consumer has to recharge first to make a call,  likewise a consumer can pay in advance to use electricity. A consumer can keep track of energy used and therefore prevent energy wastage.