The techniques of metering can be classified depending upon the voltage at which consumers are fed as:

• LT (Low Tension) Metering, and

• HT (High Tension) Metering.

We describe these techniques in the following subsections.

LT Metering

The following types of meters are commonly used for LT consumers:

Whole Current Meters

i)Single phase meters

ii) 3-phase 4 wire meters

iii) 3-phase 3 wire meters

CT Operated Meters

iv) 3-phase 4 wire meters with CT and MD

v) 3-phase 3 wire meters with CT and MD

We now briefly describe these meters.

i)Single phase meters

Single phase meters are generally rated for 240 V AC supply; the current ratings are 5/10, 10/20, 2.5/10 or 5/20 A and are for direct connection to the mains. They start registration with small load of about 6 W and record up to 4800 W.

The main feature of a single-phase meter (electromechanical induction type) is its simplicity, compactness and robust design. It comprises a potential coil and a current coil. The potential coil is fitted on the middle limb of an E shaped electro-magnet and connected across the supply mains.

Similarly, the current coil consisting of a few turns of heavy gauge copper wire is wound on two limbs of a U shaped electro-magnet. The two fluxes,produced by the voltage (pressure or potential) coil and current coil, set up a mechanical torque on the non-magnetic aluminium disc (which is located between the 2 coils) causing it to rotate.A brake magnet of C shaped alloy steel is provided to control the movement or rotation of the disc set up by two fluxes. The disc rotates through the narrow air gap of the C shaped magnet and eddy currents are set up, which interact with the field and exert braking effect.

ii) 3-phase 4 wire meters

3-phase 4 wire meters (10/20, 20/50 A) are used for agricultural and industrial consumers. For loads up to 50 A, the meters are directly connected to the supply. For greater loads, it is preferable to provide CTs.They are rated for 415 V.In a 3-phase 4 wire meter, 3 pairs of coils of each type, i.e., potential coil and current coil are provided for each of the three phases. Each of the 3 elements (coil pairs) of the meter produces equal torque, i.e., one-third of the total torque under all conditions of varying power factor.

iii) 3-phase 3 wire meter

A 3-phase 3 wire meter has 2 elements of each type instead of 3 elements as in the case of 3-phase 4 wire meter. The basic principle of operation is the same as that for 4 wire meter. The torque produced by 2 elements is equal to each other when power factor (P.F.) is unity. At other power factors, the torques produced by the 2 individual elements are not equal and are of varying proportion. B phase element produces more torque at lagging power factors, while the R phase element produces more torque at leading power factor. This particular feature can be made use of to find out the P.F. of the load current of the consumer for the purpose of checking at site. Power factors less than 0.5 are indicated by the reverse running of the meter with only the R element in operation.

Usually, 3-phase 3 wire meters are not used in recording consumption of LT power consumers for the simple reason that the lighting load has to be properly connected on one of