Burdens of Current and Potential Transformers

Author: E.E. Kimberly

The load on a current transformer or a potential transformer is called the burden. Potential transformers are usually designed to operate with commercially acceptable accuracy with a burden of two potential instruments, such as a voltmeter and a wattmeter. Greater burden should not be imposed without specific knowledge of the ultimate safe burden and the resultant ratio and phase-angle errors of the transformer when so burdened.

Current transformers are usually designed to operate with commercially acceptable accuracy with burdens consisting of an ammeter, a wattmeter, and an overload relay trip coil of a circuit breaker. Some current transformers of inferior quality are intended for trip-coil burden only or ammeter burden only, and are not acceptably accurate for additional burden or for measurement of power. Transformers of inferior quality, or others of excellent quality that are over-burdened, permit excessive phase-angle errors when used with wattmeters, and should not be so used.