Together with tender documents and meter specifications you will find various subscripts related to current.

- current
**I**without subscript

This is the actual current flowing trough the energy meter. - starting current
**I**_{st}

This is the lowest value of current at which the meter should register electrical energy at unity power factor and, for poly-phase meters, with balanced load. - minimum current
**I**_{min}

This is the lowest value of current at which the meter is specified to meet the accuracy requirements. - transitional current
**I**_{tr}

At this value of current and above the meter must to lie within the smallest maximum permissible error corresponding to the accuracy class of the meter. - basic current
**I**_{b}

This term is used in IEC standards for direct connected meters. All accuracy values are related to**I**_{b}*Note:***I**is replaced by_{b}**I**in the Edition 2 of the IEC62052-11:2020_{n} - nominal current
**I**_{n}

This is the same like**I**but for transformer operated meters_{b} - reference current
**I**_{ref}

The term is only used in EN 50470-1. It is the reference current (for direct connected meters**I**= 10 x_{ref}**I**=_{tr}**I**according to EN 62052-11, 3.5.1.2; for CT-connected meters_{b}**I**= 20 x_{ref}**I**=_{tr}**I**_{n}

- maximum current
**I**_{max}

This is the highest value of current at which the meter is specified to meet the accuracy requirements.

Now let’s look more in detail:

**Starting current I _{st} according to IEC**

The starting current is a fraction of the basic current or nominal current. See the multiplication factors below:

IEC 62053-11 direct connected | class 1: 0.004 Ib class 2: 0.005 Ib |

IEC 62053-11 transformer connected | class 0.5: 0.002 In class 1: 0.002 In class 2: 0.003 In |

IEC 62053-21 direct connected | class 1: 0.004 Ib class 2: 0.005 Ib |

IEC 62053-22 transformer connected | class 0.2S: 0.001 In class 0.5S: 0.001 In |

IEC 62053-23 direct connected | class 2: 0.005 Ib class 3: 0.01 Ib |

IEC 62053-23 transformer connected | class 2: 0.003 In class 3: 0.005 In |

**PASS/FAIL criteria:**

The meter has to start and continue recording energy. Means, we need to receive at least two pulses from the meter within a certain period of time.

**How to calculate the time?**

We have the nominal voltage, the number of elements, the starting current and the meter constant. Now we can calculate:

This is the duration for one pulse for a meter with zero error. So, first we double the time because we need to receive two pulses. Then, the IEC does not specify any accuracy for starting test. So we need to consider the meter error. Best practice is to add 20 % to your calculated time for two pulses. This will be in most cases sufficient.

If the meter fails with this time-out calculation you are allowed to extend it. The standard setting of CLOU test benches is 120 %.

From the formula you can see that a higher meter constant is preferable because it saves testing time.