Question

Analyze TWO effects of relay burden characteristics if different types and sizes of
wires are used in the system.

Answer 1

In a current transformer the external load connected to the secondary coil is termed as burden. The types of meter, relays, size and length of the wire connected to the secondary side of the instrument together with the resistance of interconnecting leads usually makes up its burden. Relay burden is usually expressed in terms of impedance of the load components. Thus if the type and size of the wires used are varied, this will result in a variation of its impedance thereby creating impact on the CT accuracy.

1. The formula for resistance vs AWG (American wire gauge) number is given by:

Most of the field wirings are done with AWG#10 which is 1 ohm per 1000 feet. If the relay is at a larger distance from CT a heavy gauge CT cable must be used to ensure good performance. Since the lead resistance is directly proportional to the distance, the relay should be placed close to the CT. For CT leads AWG#12 is considered to be the minimum wire size irrespective of the distance between relay and CT.

2. Area or thickness of the lead also effects its resistance. Increasing the area of the lead can reduce lead resistance. A reduced AWG# or multiple runs of the CT wires can provide a solution to this. However reducing the connected burdens by using static or numerical relays and by increasing the CT tap to a higher value so that higher saturation voltage and lower secondary fault current is obtained, can allow use of thinner CT wires for the particular requirement.