What is Burden on Current Transformer (CT)

Burden on CT

Q. Define CT burden

• Impedance of secondary circuit is expressed in ohms and power factor. It can be expressed as apparent power and rated secondary current at specified power factor.
• This power factor is not the power factor of the secondary load.
• The circuit connected the secondary winding is termed as burden of the current to transformer. If term load is used, it refers to the primary current magnitude.
• Burden is expressed preferably in terms of impedance of the circuit connected to the secondary and its resistance and reactance.
• The British method is to specify the burden on the CT in volt-amperes at rated secondary current at specified power factor.
• e.g. calculate the VA output required for a CT of 5 A rated secondary current when burden consists of relay requiring 10 VÀ at 5 A plus loop lead resistance 0.1 ohm. Calculate the VA rating required for CT.
• Volt ampere required to compensate loop lead resistance

                = I²R
= 5² x 0.1 = 2.5 VA

• Relay requires 10 VA
• Hence total VÀ output required is

                = 10 + 2.5 = 12.5 VA
    Hence CT of 15 VA and secondary current 5 A may be used.