The burden of a current transformer can be defined as the total impedance in ohms on the secondary output terminals. The total burden is a combination of impedance offered by watt-hour meter coils, relay current coils, contact resistance, terminal blocks, wire resistance and test switches used in the secondary loop.

Each current transformer has a secondary burden when connected in a relay or metering circuit. CT's are expected to provide the secondary output current based upon their accuracy class.

If a current transformer is not properly sized based upon secondary loop burden, it may result in a decrease in CT secondary current. Burden testing is important to verify that CT is supplying current to a circuit that does not exceed its burden rating.

The burden test is also useful in ensuring that the CTs are:

Not energized with shorting devices installed (if used for metering or protection)

Not left with an open circuit when not used

Connected with a single ground point

All connections are tight

Measure burden by injecting the rated secondary current of the CT from its terminals towards load side by isolating the CT secondary with all connected load, and observe the voltage drop across the injection points - and at every point of the circuit to ground.

This method is time consuming, but only requires a voltage source, a resistance, and a voltmeter. Measuring the voltage drop at the source combined with ohms law will give us the burden impedance. Analyzing the voltage drop patterns throughout the circuit confirms the wiring is correct.

Current transformer burdens are typically expressed in VA. The burden test is performed to verify that the CT is capable of supplying a known current into a known burden while maintaining its stated accuracy. A burden test is typically performed at full rated secondary current value (ex. 5A or 1A).