Instrument Transformer Burden Measurement: Purpose and Evaluation

What it is

Burden measurement determines the load that the secondary circuit places on an instrument transformer. For a current transformer the burden is the impedance of everything in the secondary loop: the leads out to the relays and meters, the device coils or electronic inputs, the terminal blocks and the connections. For a voltage transformer, including a coupling-capacitor voltage transformer, the burden is the load drawn by the meters, relays, indicating instruments and wiring across the secondary, usually expressed as apparent power at a stated power factor. The measurement is made in place, on the circuit as installed, because the real burden depends on the actual length and size of the wiring and the actual devices connected. For current circuits it is taken at the terminal block where the current transformer secondary is shorted, and for voltage circuits across the secondary at the point where the connected devices are supplied. The result is compared with the rated burden on the nameplate, which is the maximum load the transformer is designed to carry while staying within its stated accuracy class.

Why it is done

An instrument transformer is accurate and dependable only within its rated burden. If the connected burden is too high on a current transformer, the core must develop more secondary voltage to push the current through, so it saturates at a lower fault current than expected, and the relay receives a distorted signal exactly when it matters most. On a voltage transformer, excess load increases the ratio and phase errors, which degrades metering accuracy and can disturb impedance and directional relays. A burden that is too low can also matter, because some classes of transformer are specified to perform within a band. Burden problems arise from field changes: a relay replaced by a model with a different input, extra meters added, wiring extended or run in a smaller conductor size, or a connection with high resistance. Measuring the burden confirms that the as-built circuit matches the design assumptions and catches high-resistance connections that would also show up as heat.

The principle

A current transformer is a current source, and the voltage it must develop at its secondary terminals equals the secondary current multiplied by the total impedance in the loop. That impedance is the burden. The core can support only a limited voltage before it saturates, so a higher burden means a lower current at which saturation begins. For a voltage transformer the relationship is reversed: the transformer is a voltage source, and the load draws current through the winding impedance, causing a voltage drop that appears as ratio error and phase error. The burden is therefore a real physical quantity with a resistive and a reactive part. It can be measured by driving a known current through the loop and reading the resulting voltage, or by reading voltage, current and power in the loop in service, and then calculating impedance or apparent power. Standards express the rated burdens in designated classes, with a stated power factor, so the measured value is compared on a like-for-like basis with the nameplate.

How it is performed

For a current circuit, the current transformer is isolated from the primary and the circuit is de-energized under the owner's safety procedures, and the secondary is kept closed whenever primary current could flow. A low-voltage alternating current of convenient magnitude is injected into the secondary loop at the shorting terminal block, with the transformer side shorted or disconnected as the procedure directs, and the voltage drop across the loop is measured with an accurate voltmeter. From the current and voltage the impedance, and the equivalent burden at the standard current, are calculated. Some analyzers measure burden directly and report impedance, power factor and apparent power. For a voltage circuit, the connected load is measured across the secondary with the circuit energized by a safe source, or the individual burdens of the connected devices are determined from their published data and the wiring is accounted for. All devices that are normally connected are in place during the measurement, and the temporary test connections are removed and the shorting block restored at the end. The instrument, its calibration status and the lead resistance of the test connections are taken into account, since low impedance readings are easily distorted by the test leads themselves.

What is recorded

The record states which circuit and which transformer were measured, the phase, and the point at which the burden was taken. It holds the measured burden in the units the instrument reported, together with the power factor or the resistive and reactive parts where they were obtained, and the nameplate rated burden against which it is compared. The instrument and its calibration status, the date and the condition of the circuit are noted, along with which devices were connected at the time. If any connection was found loose or any lead was corrected, that as-found condition is recorded, because it explains a change between this result and an earlier one.

How results are judged

The central comparison is between the measured burden and the rated burden marked on the transformer nameplate. The connected load should sit within the rating, so that the transformer remains within its accuracy class and the core has adequate margin before saturation. Readings on the three phases of the same circuit are compared with each other, since one phase that differs from the other two often indicates a poor connection, an extra device or a wiring difference. Comparison with earlier results shows creep from added load or deteriorating terminations. For protection circuits the burden figure feeds into the saturation check made by the relay engineer. The definitions of standard burdens, accuracy classes and the limits that go with them are found in IEEE C57.13-2016 and IEEE C57.13.1-2017, while ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 call for burden measurement as part of the instrument transformer test sequence. Refer to your licensed copies of the standards for the applicable values and criteria.

Variations

At acceptance, burden is measured on every current and voltage circuit as part of the instrument transformer sequence and establishes the baseline for the installation. In maintenance, it is repeated when devices or wiring have been changed, when accuracy or protection performance is in question, and at intervals chosen by the owner. Current circuits are measured at the shorting block, while voltage circuits, including those fed by coupling-capacitor voltage transformers, are measured across the secondary and the accuracy depends more on the connected devices and the wiring length. Electronic relays and meters present a much lower burden than the electromechanical ones they replace, so older circuits may carry far more margin than they used to, while circuits crowded with additional devices may have little left. High-accuracy transformers used for revenue metering are examined more closely because their accuracy class is tied to a narrow burden range.