Where 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 metersThree Phase Meters, direct connected. 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 benchThree Phase Test Bench CL3000, 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".