CT burden means the load connected to CT secondary. The load may be of the resistive type or of the inductive type. However, the load connected to CT is largely resistive type. The load may have some part of the inductive part, but it is negligible.
The CT burden ratings as per IEC standard are 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA, 20 VA, 30 VA, 45 VA & 60 VA. This means a CT with a rating of 15 VA can deliver the 15 VA load. For example, a CT with CTR 200/5, connected to 0.5 Ohms, then the secondary voltage is (V=IR=5 X 0.5 = 2.5 Volts).
CT burden = CT secondary Voltage X CT secondary current
= 2.5 X 5 = 7.5 VA
The CT burden ratings as per ANSI/IEEE standard are B-0.1, B-0.2, B-0.5, B-1.0, B-2.0, and B-4.0. Here, the second figure shows the burden resistance.
The burden is the total resistance connected to the secondary. The current transformer has its widespread application in metering & protection. The CT is connected to the metering or protection relay through a pair of connecting wires.
The resistance of the connected wire and the meter input impedance contributes to resistance in the CT secondary circuit. The resistance of the connecting lead wires is more if the distance between the CT and the meter/relay is more. That is why it is important to reduce the burden. We can reduce the resistance of lead wires by installing the meter near CT, if possible, & selecting larger gauge wire.
CT Burden Formula
= CT secondary Resistance + Lead Wires resistance + meter/relay resistance
The actual CT burden must not increase beyond the rated CT burden. The excessive burden changes the operating point of the CT on the magnetization curve, and thus the knee point voltage shifts. Thus, the accuracy in the measurement greatly affects if it operates beyond its maximum rated burden.
The reliability of the protection/ metering system entirely depends on the operation of the CT below the saturation point on the B-H curve. Therefore, the right selection of burden is very important,