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Entry type: FAQ Entry ID: 85477190, Entry date: 10/28/2015

How do you select a current transformer for the ET 200SP Energy Meter and how do you connect it to the module?

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Select current transformer for the ET 200SP Energy Meter and connect it to the module
The AI Energy Meter is designed for measuring the current for the connection to the toroid (at connections IL1, IL2 and IL3). Certain requirements have to be met to obtain correct results when measuring and not to overload or damage the current transformer. This entry will help you with this.

Choice of current transformer
Transformer types of the following accuracy classes can be connected to the AI Energy Meter:

  • 3
  • 1
  • 0.5

The minimum load power of the transformers to be used is specified in the technical data of the AI Energy Meter.

Maximum length of the connection cable
In order not to overload or damage the current converter, you must ensure that the load Zn (in VA) specified in the data sheet of the current transformer is not exceeded. To avoid this, the total load impedance (consisting of the resistance of the connection cable and the internal resistance of the AI Energy Meter (see Fig. 01)) must be less than a specific resistance value (depending on Zn and Imax).

Fig. 01

The maximum value for the resistance of the connection cable is derived from the following formula:

Fig. 02

RL= line resistance in ohms
ZN= rated load impedance of current transformer in VA
I= secondary current of the current transformer
Rload= resistance in the Energy Meter = 25 mΩ

You can calculate the maximum length of the connection cable from the maximum line resistance in ohms. Refer here to the data sheet of the connection cable used.

The length of the connection cable (forward and backward route) must not exceed 200 meters.

Example 1
Current transformer 500/5A
maximum primary current in the application: 400A -> maximum secondary current: 4A
load in the AI Energy Meter including connection resistance: R = 25 mΩ
rated load impedance of the current transformer ZN: 5VA

Fig. 03

Therefore the maximum line resistance between the toroid and the ET 200SP terminal must not exceed 292.5mΩ.

Example 2
Current transformer 200/1A
maximum primary current in the application: 160A -> maximum secondary current: 0.8A
load in the AI Energy Meter including connection resistance: R = 25 mΩ
rated load impedance of the current transformer ZN: 1.25VA

Fig. 04

Therefore the maximum line resistance between the toroid and the ET 200SP terminal must not exceed 1.92Ω.

Line resistance
Table 2 includes typical resistance values of copper wires for quick estimation . The resistance values of copper wires are calculated with the following formula:

Fig. 05

R = line resistance in ohms
ρCU = specific electrical resistance of copper = 0.0178 Ωmm2/m
I = line length in m
A = line cross-section in mm2


Table 1

You must add the cable lengths for the forward and backward paths when connecting the toroid.

The tables below show current transformers that can be connected to the AI Energy Meter for measurement data acquisition.

Current transformers of the type 4NC5

Current transformerArticle numberAccuracy class
Donut-type current transformer 50/1A 2.5VA 4NC5112-0BC203
Donut-type current transformer 50/5A 2.5VA4NC5112-2BC203
Donut-type current transformer 60/1A 2.5VA4NC5113-0BC203
Donut-type current transformer 60/5A 2.5VA4NC5113-2BC203
Donut-type current transformer 75/1A 2.5VA4NC5115-0BC203
Donut-type current transformer 75/5A 2.5VA4NC5115-2BC203
Donut-type current transformer 100/1A 2.5VA4NC5117-0CC201
Donut-type current transformer 100/5A 2.5VA4NC5117-2CC201
Donut-type current transformer 150/1A 2,5VA4NC5121-0CC201
Donut-type current transformer 150/5A 2,5VA4NC5121-2CC201
Donut-type current transformer 200/1A 5VA



Donut-type current transformer 200/5A 5VA



Donut-type current transformer 250/1A 5VA



Donut-type current transformer 250/5A 5VA



Donut-type current transformer 400/1A 5VA



Donut-type current transformer 400/5A 5VA



Donut-type current transformer 500/1A 5VA4NC5326-0CE201
Donut-type current transformer 500/5A 5VA4NC5326-2CE201
Donut-type current transformer 600/1A 5VA4NC5327-0CE201
Donut-type current transformer 600/5A 5VA4NC5327-2CE201
Donut-type current transformer 750/1A 5VA4NC5328-0CE201
Donut-type current transformer 750/5A 5VA4NC5328-2CE201
Donut-type current transformer 1000/1A 10VA


Donut-type current transformer 1000/5A 10VA4NC5431-2CH201
Donut-type current transformer 1250/1A 10VA4NC5433-0CH201
Donut-type current transformer 1250/5A 10VA4NC5433-2CH201
Donut-type current transformer 1500/1A 10VA4NC5434-0CH201
Donut-type current transformer 1500/5A 10VA4NC5434-2CH201

Table 2

Current transformer of the type 7KT12

Current transformerArticle numberAccuracy class
Current transformer AC 3x60/5A7KT12001
Current transformer AC 3x100/5A7KT12011
Current transformer AC 3x150/5A7KT12021

Tabelle 3

Current transformer of the type 3NJ69x0-3Bxxx

Current transformer
Article number
Accuracy class
Current transformer 50/1A 1VA3NJ6920-3BB111
Current transformer 50/5A 1VA3NJ6920-3BB211
Current transformer 100/1A 2.5VA3NJ6920-3BD111
Current transformer 100/1A 1.5VA3NJ6920-3BD120.5
Current transformer 100/1A calibrated 1.5VA3NJ6920-3BD130.5
Current transformer 100/5A 2.5VA3NJ6920-3BD211
Current transformer 100/5A 1.5VA3NJ6920-3BD220.5
Current transformer 100/5A calibrated 1.5VA3NJ6920-3BD230.5
Current transformer 150/1A 2.5VA3NJ6920-3BE111
Current transformer 150/1A 1.5VA3NJ6920-3BE120.5
Current transformer 150/1A 1VA3NJ6920-3BE130.5
Current transformer 150/5A 2.5VA3NJ6920-3BE211
Current transformer 150/5A 1.5VA3NJ6920-3BE220.5
Current transformer 150/5A calibrated 1.5VA3NJ6920-3BE230.5
Current transformer 200/1A 2.5VA3NJ6930-3BF111
Current transformer 200/1A 5VA3NJ6930-3BF120.5
Current transformer 200/5A 2.5VA3NJ6930-3BF211
Current transformer 200/5A 2.5VA3NJ6930-3BF220.5
Current transformer 300/1A 5VA3NJ6940-3BH111
Current transformer 300/1A 5VA3NJ6940-3BH120.5
Current transformer 300/1A calibrated 5VA3NJ6940-3BH130.5
Current transformer 300/5A 5VA3NJ6940-3BH211
Current transformer 300/5A 5VA3NJ6940-3BH220.5
Current transformer 300/5A calibrated 5VA3NJ6940-3BH230.5
Current transformer 400/1A 5VA3NJ6940-3BJ111
Current transformer 400/1A 5VA3NJ6940-3BJ120.5
Current transformer 400/1A calibrated 5VA3NJ6940-3BJ130.5
Current transformer 400/5A 5VA3NJ6940-3BJ211
Current transformer 400/5A 5VA3NJ6940-3BJ220.5
Current transformer 400/5A calibrated 5VA3NJ6940-3BJ230.5
Current transformer 500/1A 5VA3NJ6940-3BK111
Current transformer 500/1A 5VA3NJ6940-3BK120.5
Current transformer 500/1A calibrated 5VA3NJ6940-3BK130.5
Current transformer 500/5A 5VA3NJ6940-3BK211
Current transformer 500/5A 5VA3NJ6940-3BK220.5
Current transformer 500/5A calibrated 5VA3NJ6940-3BK230.5
Current transformer 600/1A 5VA3NJ6940-3BL111
Current transformer 600/1A 5VA3NJ6940-3BL120.5
Current transformer 600/1A calibrated 5VA3NJ6940-3BL130.5
Current transformer 600/5A 5VA3NJ6940-3BL211
Current transformer 600/5A 5VA3NJ6940-3BL220.5
Current transformer 600/5A calibrated 5VA3NJ6940-3BL230.5

Table 4

Further Information
More information about the AI Energy Meter is available in the follwing manuals:

Additional Keywords
Instrument transformer, Total resistance, Zn