WHERE IS THE BASIC CURRENT?

By IEC62052-11 Edition 2 definitions it's gone.
In the last 100 years we had a lovely I_b for direct connected meters. Now the name has changed to nominal current I_n.
Besides this, the new designed meters will show also the minimum current.

For directly connected meters the labelling will be like this:
I_min – I_n (I_max) or practically 0.5 – 10(40) A for a meter with a minimum current of 0.5 A, nominal current of 10 A and maximum current of 40 A.

The IEC defines four different current subscripts:

• starting current I_st
• minimum current I_min
• nominal current I_n
• maximum current I_max

Which accuracy error is allowed between I_st and I_min?

The IEC doesn't specify the error in this region. When we look into the OIML R46 recommendations, we find a calculation formula for a maximum permissible error. For a class B meter it's ± 1.5 * I_min/I.
Converting this to IEC, we have a meter with accuracy class 1 and the current I is our nominal current I_n.
A meter with a minimum current of 250 mA, a nominal current of 5 A and starting current of 20 mA will be allowed to have an accuracy error of ±18.75% on I_st.
Calculation examples:
Starting current I_st: 1.5 * 250 mA / 20 mA = ±18.75%
Current 150 mA: 1.5 * 250 mA / 150 mA = ±2.5%
Minimum current I_min: 1.5 * 250 mA / 250 mA = ±1.5%

Keep in mind that this is only a recommendation for MID meters. The IEC doesn't give allowed error values.

If you have an idle test bench, maybe you'd like to make an accuracy test on starting current. Write a comment and share your test results with us.

Thank you for reading and "Happy Testing".