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Current Transformers


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Hi there,


Basic question on current transformers:


Say if I had a 100:5 Amp CT and 80 Amps runs through the primary side, does that just mean I'll have 4 Amps on the secondary? Also, is 100 Amps the maximum allowable to run through the primary side in this case?



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Hello mml


Yes, that is correct provided that the impedance or burden on the output of the CT is low enough. If the impedance is too high, then the secondary current will be limited by the secondary impedance.


The CT will have a VA rating, typically 5VA and that gives you the maximum voltage output at rated current. If the CT is 100/5 and it is 5VA, then the maximum output voltage at 5 amps is 1 volt. Therefore the resistance of the secondary should be less than 0.2 ohms.


You can have overcurrent on CTs, some are designed for this. As the current is increased, the effective primary voltage increases and so the flux in the iron also increases. As the flux int eh iron increases, you reach a point where the induction of the iron core begins to reduce and the ratio will begin to change. This will effect the output current which will begin to read low.


Best regards,

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