The minimum current and the starting current of an electricity meter are two distinct technical parameters. Their definitions, functions, and test standards are completely different, and they should not be confused.

1. Definition and Function of Minimum Current (Imin)

The minimum current is the lowest current value at which the meter is designed to operate continuously and maintain its specified accuracy over the long term. It is the lower threshold of the current range within which the meter's error meets the accuracy requirements.

Core attributes

It is a fundamental operating parameter of the meter, corresponding to the rated maximum current (Imax). Together, they define the effective operating current range of the meter: Imin ∼ Imax.

For example, a common single‑phase meter specification might be 0.25‑0.5(60) A. Here, 0.25 A is the minimum current – meaning that within the current range of 0.25 A to 60 A, the meter's measurement error must meet the national standard.

Key requirement

At the minimum current, the meter's accuracy must reach the class marked on the nameplate (e.g., Class 1, Class 2), and the meter must operate stably over the long term without losing its metering capability due to the small current.

2. Definition and Function of Starting Current (Ist)

The starting current is the lowest current value at which the meter begins to continuously rotate (or produce pulse outputs). It is a key indicator of the meter's sensitivity.

Core attributes

It is a sensitivity parameter, used to test the meter's ability to respond to very small load currents. When the load current reaches or exceeds the starting current, the meter's disc (for electromechanical types) will rotate continuously, or the metering chip of a smart meter will produce continuous pulses. If the current falls below this value, the meter may stop measuring.

Key requirement

The starting current is much smaller than the minimum current and is subject to strict national standard limits.

For example, for a Class 1 meter rated 5(60) A, the national standard requires the starting current to be no greater than 0.004 Ib (where Ib is the basic current). That means the starting current for this meter is:

0.004 × 5 A = 0.02 A

Converted to power at 220 V:

0.02 A × 220 V = 4.4 W

This means that when the total standby power of household appliances exceeds 4.4 W, the meter will begin to register consumption.

3. Why Is the Starting Current Not Equal to the Minimum Current?

In simple terms:

• The starting current is the "lowest current that can be detected".

• The minimum current is the "lowest current that can be measured accurately".

When the current falls between the starting current and the minimum current, the meter can record energy, but the measurement error may exceed the accuracy class requirements. This is a "non‑accurate measuring zone".

Only when the current reaches or exceeds the minimum current does the meter measure with the accuracy stated on its nameplate. This is the meter's core operating range.

A simple analogy

Think of a car:

• The starting current is like the minimum throttle that makes the wheels turn.

• The minimum current is like the minimum throttle needed for the speedometer to read accurately.

Just because the wheels are turning does not mean the speedometer is giving an accurate reading.