Current Transformers and Voltage Transformers: Accuracy Classes, Burden Calculation and Selection Guide

The Two Roles of Instrument Transformers

Current transformers (CTs) and voltage transformers (VTs) exist to solve a single practical problem: the primary circuit carries currents and voltages that no meter, relay or operator can safely or accurately interface with directly. Instrument transformers scale those quantities down to standard, isolated levels that protection and metering equipment can use.

The two devices work in opposite ways, and understanding that difference prevents most commissioning errors.

• A CT is connected in series with the primary circuit. Its primary current is determined by the network, and its secondary produces a proportional current, usually 5 A or 1 A. Because the secondary supplies a low-impedance burden, the CT behaves with its secondary effectively short-circuited.
• A VT is connected in parallel with the primary circuit. Its secondary produces a proportional voltage, typically 100 V, 110 V, or their divided-by-root-three equivalents. Its secondary may be left open because it is functionally a small power transformer.

Two non-negotiable wiring rules follow. The CT secondary circuit must remain closed whenever primary current flows — opening it drives the core into deep saturation and induces a hazardous voltage across the open terminals, which can destroy the transformer and endanger personnel. The VT primary is fused; the secondary is not.

Accuracy Classes and Why Metering Cores Must Not Be Used for Protection

Accuracy classes are separated into metering and protection families, and the separation exists for a physical reason.

Current transformer classes

The designation 5P20 means the CT holds composite error below 5 percent at up to 20 times rated current. The figure 20 is the accuracy limit factor (ALF). Above the ALF the core saturates and the secondary no longer represents the primary, which is exactly why the relay engineer must state the minimum ALF each protection function requires.

The suffix "S" in classes such as 0.2S and 0.5S indicates that accuracy is maintained at very low primary current, down to a few percent of rated value. This matters greatly for renewable generation, where output swings widely and a conventional metering core would fall outside its accurate range during low-output periods.

A metering core must never be used for protection: it saturates deliberately at moderate overcurrent, so a protection relay fed from it will fail to operate when it is needed most. Equally, a protection core is inaccurate at normal load and must not be used for billing. Where both duties exist, specify a multi-core CT — one metering core and one protection core in the same housing — or install separate CTs.

Voltage transformer classes

Metering VTs use classes 0.2, 0.5 and 1.0. Protection VTs use classes 3P and 6P, where consistent transformation during a fault matters far more than absolute precision at normal voltage. Inductive VTs for metering must hold ratio accuracy within a narrow burden range, while capacitive voltage transformers intended for protection must reproduce voltage faithfully during transients. The two types have different transient responses, and a specification that does not record which winding feeds which device will eventually produce a unit optimised for only one of the two.

Burden: The Parameter Most Often Underestimated

Burden is the total impedance presented to the secondary terminals by everything connected: relay coils, meter coils, transducers, test-block contacts, terminal resistance, and the connecting cable itself. It is expressed in volt-amperes at a stated power factor for VTs, and commonly in VA or ohms for CTs.

Burden cannot be estimated from nameplate ratings. A worked CT example for a 5 A secondary:

If the CT is rated 10 VA, this selection is acceptable. If it is rated 5 VA and the actual burden is 8 VA, the CT operates outside its tested accuracy band. The practical remedies are a larger conductor, a shorter cable route, or a higher-VA CT. A frequently encountered field problem follows exactly this pattern: protective cores rated at a modest VA, installed with long cable runs that push real burden well beyond the rating.

Where secondary cable runs exceed about 30 metres, a 1 A secondary reduces voltage drop and wiring losses significantly compared with 5 A, and is often the more economical overall solution.

A Practical Selection Sequence

1. Define the application first. Revenue metering, internal metering, overcurrent protection, differential protection, and motor protection each imply a different class. Modern switchgear generally uses multi-core CTs so that a protection-grade saturation event cannot disturb a revenue measurement.
2. Determine the ratio. For metering, normal operating current should fall between 50 and 80 percent of the CT primary rating; sustained operation below 20 percent pushes the device outside its accurate range. For protection, the CT must reproduce the maximum fault current without saturating inside the accuracy limit factor.
3. Calculate the burden for every winding separately, from actual cable impedance and device input impedance at the operating frequency.
4. Select the accuracy class for each winding, and state it separately in the specification.
5. Verify short-circuit withstand. Confirm both the dynamic current rating and the thermal current rating against the fault study. A CT that saturates gracefully is still useless if electromagnetic forces damage its windings.
6. Check the environment. Indoor or outdoor service, ambient temperature range, altitude, seismic zone, and pollution level all affect the external insulation and therefore the physical dimensions of the unit.

Standards-wise, the IEC 61869 series is the general framework, with class-specific parts for CTs, inductive VTs, capacitive VTs, and combined or electronic instrument transformers. IEC 61869-1 governs type, routine and acceptance testing of the manufactured product; it does not decide what ratio, class or burden a project needs. Those remain engineering inputs that must be supplied in the data sheet. For high-voltage units, IEC 61869-20 addresses safety requirements and the documentation that must accompany transport, storage and handling of insulating media.

Installation and Commissioning

• Single-point earthing. Each secondary circuit, and each VT neutral, must be earthed at exactly one point. A second earth path creates a circulating current that corrupts measurements and can cause protection misoperation.
• Polarity discipline. Primary and secondary polarity markings must be respected. A reversed CT polarity in a differential or directional scheme will cause the relay to operate incorrectly, and the error is invisible during routine load checks.
• Test terminals. Use make-before-break test switches or a calibrated shorting block whenever a meter or relay is disconnected. Short and earth the CT secondary before touching any lead.
• Commissioning tests. Perform ratio verification, polarity test, insulation resistance test, and a burden measurement on each core. Confirm the CT secondary loops are closed and correctly terminated before primary current is applied — this is a formal hold point, not a formality.
• Documentation. Record which winding feeds which device at the specification stage. Many re-wiring problems at commissioning originate from a supplier who received an ambiguous requirement and optimised for the wrong duty.

Frequently Asked Questions

What happens if a CT secondary is left open? With primary current flowing, the entire primary ampere-turns are forced into the magnetising branch, the core saturates deeply, and a peak voltage of several hundred to several thousand volts appears across the open terminals. This is both an equipment and a personnel hazard.

Can one CT serve both metering and protection? Not on a single core. Use a multi-core CT with separate metering and protection cores, or install two CTs.

Does a VT secondary have to stay closed? No. A VT is functionally a small power transformer, so an open secondary simply means no load current flows. Its primary is fused for protection.

Why specify 0.2S instead of 0.5? Only when the metering point has a commercial or regulatory consequence that justifies the cost. Specifying 0.2S where 0.5 would suffice adds expense without benefit; specifying 0.5 for a revenue meter can expose the operator to settlement penalties.