Instrument transformers are basically the transformers like normal purpose transformers, instrument transformers have also to carry electrical loads. So, like other general purpose transformers instrument transformer (current transformers and potential transformers) must have a certain load carrying capacity. But this load is not similar to that of a normal transformer. Hence, to distinguish the load of an instrument transformer from that of general purpose transformers we use the word burden instead of load. Like load we express burden also in VA.
In normal power transformer the volt ampere rating is specified depending on the allowable temperature rise. But in current transformer or voltage transformer the burden is the VA rating above. Which the errors of the instrument transformer raise above the tolerable specified error limit.
For example suppose a current transformer has accuracy class 0.1 and burden of the instrument transformer is 15KV. That means when we connect any burden to the secondary of the instrument transformer above 15KV may cause error beyond the limits of errors specified for 0.1 accuracy class. So, to keep the error within tolerance limit we need to operate the instrument transformer below 15VA.
The total burden of an instrument transformer may be due to connected voltmeter, watt meters, power factor meters, frequency meters, energy meters and other type of relays and wiring systems.
The metering system consists of ammeters, voltmeters, watt meters, power factor maters, energy meters etc. Each of these instruments has its own VA consumption rating. For example, a standard ammeter has the VA rating of 3VA. Likewise a standard voltmeter normally has high VA as its consumption rating. Again watt meters, power factor meters and energy meters have VA rating of high VA each.
We normally express the burden of different relays in VA. But for selecting appropriate current transformers for differential and distance protections, we need to know the ohmic values of these relays. Again the relays are generally of 5A or 1A ratings. So, if we divide the VA rating of a relay with the square of its current rating we get the ohmic burden of the relay circuit. For example, suppose a 1A relay has VA rating of 6VA. Then the ohmic burden of these relay will be 6/12 or 6 ohms.
Burden of Control Cable
The data sheet gives the value of the resistance per unit length of each conductor in a control cable. So we have to work out the VA rating of the control cable. We do that by multiplying the resistance of the entire used conductors with square of the secondary ampere rating of the instrument transformer.