I am reading this datasheet for an energy meter, to start understanding how this market works.

A friend of mine wants to make a test in a small office building to see if it pays having one of these and he asked for my help, and finally I have an excuse to get to know this kind of devices.

I am leaning towards that meter, good price (around 170eur ~ 200USD), but I don't know what CT clamp to buy for it.

I want one clamp for 100A, 200A and around 600A or more.

But looking at the datasheet:

enter image description here

I am not able to tell which CTs to buy. So far the meters I've studied they have something like primary coil 100A -> 5A secondary or primary coil 100A -> 1A secondary, primary coil 100A -> 0.333A secondary, primary coil 500A -> 10A secondary, and so on. But this one is different...

For this meter I am not being able to interpret what's the maximum secondary coil current it can handle, and consequently nor the primary.

Can you help me with some examples covering their ratios (1.0 to 99.9/ 100 to 999 / 1.00k to 6.00k etc.?


PS: I know the dangers of high voltages and current, and no, I am not going to handle the installation of these devices, nor touch electric boards, etc.


It looks like straightforward but poorly formatted.

Because their display cannot do half digits e.g. 0 to 1.999 they have spec'd it 1 to 99.9 range.

So you would choose this range in programming the meter for use of a 100A CT.

The next range includes both 200A and 600A.

The burden value selected is such that the meter is rated as; "Current Impedance 5(6)A = < 0.3VA"

So whatever CT ratio you choose the output is 5A or (6A)

see page 2 of spec for EM2172D DS ENG 190210

It looks like it may have been translated to english with somewhat amateur format. But I would never compl;)aint about it.

  • \$\begingroup\$ It is strange because when I search for Gavazzi CTV3X200A333MV CT (which outputs 333mV -- dropbox.com/s/5q3catgk2yl1b1n/CTV3X200A333MV.pdf?dl=0), I get this meter related (the EM21 72D). Are you sure it only reads CT complying with 5A secondary? I cannot understand this meter datasheet at all. \$\endgroup\$
    – nemewsys
    Jun 28 '17 at 23:34
  • \$\begingroup\$ The EM21 the CT turns ratio is 1:600 (that is: 3000A:5A) where the CTV3x200A333mV is Current ratio 200A/333mV Turns Ratio Np:Ns=1:3000 \$\endgroup\$ Jun 28 '17 at 23:48
  • \$\begingroup\$ The load or "burden" resistor is built into the CTV3x200A333MV so that it is calibrated for excitation current losses so intead of getting 200A/3000= 0.666A into an external load R, you get only 333mV with the internal load R and secondary winding resistance losses and primary core excitation current losses included. so you dont have to add a load R. \$\endgroup\$ Jun 29 '17 at 0:10
  • \$\begingroup\$ "The EM21 the CT turns ratio is 1:600 (that is: 3000A:5A)" what? \$\endgroup\$
    – nemewsys
    Jun 29 '17 at 0:25
  • \$\begingroup\$ "CTV3x200A333mV is Current ratio 200A/333mV Turns Ratio Np:Ns=1:3000" So, can this CT be used with EM21? This is so confusing for the wrong reasons. \$\endgroup\$
    – nemewsys
    Jun 29 '17 at 0:25

Pretty much world wide, the standard for current transformers is to provide a 5 amp secondary for any selected primary current. Transformers with a 1 amp secondary are not uncommon, but the linked data sheet for the energy meter indicates that use with a 1 amp current transformer does not conform to the listed standard.

The top left corner of page 6 of the linked data sheet shows a graph of accuracy. The best accuracy is for operation between 0.25 and 6 amps or 5% to 120% of the rated primary current. Operation below 1% is shown as a dotted line. I would interpret that as "undefined," "not recommended," or "not guaranteed." If you buy a 600 amp CT, the meter might not be accurate below 6 amps. In addition, the current transformer itself may have a lower limit on recommended accurate current measurement.

Measurement up to 120% of rated current is accurate, but there does not seem to be a description of the consequences of exceeding 120% of the rating. For motor starting inrush currents etc., it would have to be safe to have currents of 500% of rated or more for a few seconds, but measurement accuracy is undefined.

You probably need to read the related standards.

You need to understand what kind power system you are measuring: 3-phase, 4-wire; 3-phase, 3-wire; 2-phase, 3-wire; or 1-phase, 2-wire. Note that 2-phase, 3-wire is either nonexistent or very nearly nonexistent anywhere in the world. Perhaps that is supposed to be 1-phase, 3-wire. That is used for virtually every domestic, retail and office premises in North America.

It would be a good idea to learn enough to have a reasonable understanding of most of the linked data sheet.


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