HOW TO SELECT AN INSTRUMENT TRANSFORMER 
   

Selecting a Current Transformer

Permissible Current in a Busbar | Accuracy of Current Transformers | Important Warning for CT's


Determining the transformer ratio Ip/Is

• For the transformer primary current, select the standard value immediately higher than the current to be measured.
ie : current to be measured 1124 A. Select Ip = 1250 A
• For the secondary current, choose 1 A or 5 A depending on the instrument or relay, and on the distance between the transformer and the instrument it is feeding:
- 5A secondary is used when instruments or relays are close to the transformer, ie less than 10m (30ft).
- 1A secondary is preferably selected when the distance between the current transformer and the instrument transformer or the relay is above 10m (30ft). Joule losses by wire resistance are 25 times higher with 5A than with 1A.


Determining the transformer output

• In order to define the transformer output in VA, sum up the burdens absorbed by the equipments connected to the transformer, and the lead burden.
• lt is recommended to select the output rating as close as possible to, but higher than the calculated burden, since the characteristics regarding accuracy and safety factor might otherwise be modified.


Defining the accuracy class as per the application

Standards = Class 0.1 or 0.2
Tariff metering= Class 0.5 - 0.5S - 0.2 - 0.2S
Industrial division metering = Class 1
Measure display = Class 1 or 3
Protection = Class 5P or 10P

• Remember that instrument errors add up to the errors of the transformers feeding them.
• This also applies when several transformers are mounted in tandem.
• In the case of protection class 5P or 10P, selects the accuracy limit factor, generally 5, 10, 15, 20, or 30.


Defining the transformer type with focus on fitting

• When the primary current is very low, a few amps to a few tens of amps, a wound primary transformer is preferably used.
• Window type transformers, where the primary conductor is a cable or a bar passing through the transformer, are used when the currents to be measured are above 150A. Their performances are reduced when the primary current is quite low (between 40 and 150A).
• It is neither practial nor economic to use them below 40A. The model is them selected depending on the type of primary conductor, cable or busbar, and of its cross-section.

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