Burden of CT:

 Burden of CT:

The total load connected to the CT secondary terminal is called “burden”.

This is the value of the impedance of the secondary circuit (the impedance of all the devices

connected to it), expressed in ohms and power-factor or volt-amperes, at the rated

secondary current and frequency.

Precaution measures:

It should be noted that the secondary of the CT should not be kept open while the primary is

energized. If the secondary is left open, the secondary current becomes zero i.e. secondary

demagnetising ampere-turn disappears. Actually under normal operating condition in an

ordinary transformer the secondary ampere turns (demagnetising ampere-turn) opposes

the primary ampere turns (magnetising ampere-turn).In case of CT primary ampere-turn is

the fixed quantity. Hence, the full primary ampere-turn now become active produce very

large magnetic flux in the core as there is no counter secondary mmf. This results more core

losses and thereby increases the heat of the core. In addition, this causes to induce high

EMFs on secondary side leads to damage the insulation and an electric shock to the

attending personnel. Apart from the above, when power is removed for the circuit, this high

magnetising force leave behind an appreciable amount of residual magnetism in the core

which causes the ratio and phase angle error to differ considerably from the before.

Hence it is very important that the secondary must be connected in series with low

resistance current coils of the instruments or simply shorted. And also, for simplify the

insulation problem of the instrument and makes it safe to handle one end of the secondary

side must be connected to the ground. In practice, CTs are provided with a short circuit

switch at the secondary terminals.

Connection diagram for measuring the Current, Voltage, Power and
Protection Relays (like over current, under voltage, distance and differential)
using Current Transformer (CT) and Voltage Transformer (VT).