Minimum Current and Starting Current of Electricity Meters

I. Definition and Function of Minimum Current (Imin)

The minimum current is the minimum current value specified for the long-term normal operation of the electricity meter. It is the lower current threshold at which the meter's error meets the accuracy requirements.

It is a basic operating current parameter of the electricity meter, corresponding to the rated maximum current (Imax), jointly defining the effective operating current range of the electricity meter (Imin~Imax). For example, a common single-phase electricity meter specification is 0.25-0.5(60)A, where 0.25A is the minimum current, meaning that the meter's measurement error must comply with national standards within the current range of 0.25A to 60A.

At the minimum current, the metering accuracy of the electricity meter must reach the level indicated on the nameplate (e.g., Class 1, Class 2), and it must be able to operate stably for a long time without metering failure due to insufficient current.

II. Definition and Function of Starting Current (Ist)

The starting current is the minimum current value at which the electricity meter can start continuous rotation (or pulse output). It is a core indicator for measuring the sensitivity of the electricity meter.

It is a sensitivity parameter of the electricity meter, intended to test the meter's response to small load currents. When the load current reaches or exceeds the starting current, the meter's disc (mechanical type) will rotate continuously, or the metering chip of the smart meter will continuously output pulses; if the current is lower than this value, the meter may stop metering.

The value of the starting current is much smaller than the minimum current, and there are clear national standard limits. For example, for a 5(60)A Class 1 electricity meter, the national standard requires that the starting current be no greater than 0.004Ib (Ib is the basic current), meaning that the starting current of this meter is no greater than 0.004×5A=0.02A. Converted to power: 0.02A×220V=4.4W. This means that when the total standby power of appliances in a household circuit exceeds 4.4W, the electricity meter will start measuring consumption.

III. The core difference between minimum current and starting current

IV. Why is the starting current not equal to the minimum current?

Simply put, the starting current is the "lowest measurable current," while the minimum current is the "lowest accurately measurable current." When the current is in the range between the starting current and the minimum current, the electricity meter can record the power consumption, but the measurement error may exceed the accuracy class requirements, falling into the "inaccurate measurement range."

Only when the current is greater than or equal to the minimum current will the electricity meter accurately measure according to the specifications indicated on the nameplate. This is the core operating range of the electricity meter.

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