Current Transformer (CT) Selection: Ratio, Accuracy Class, Burden and ALF
current transformer selection: To select a current transformer, first round the primary rating up to a standard ratio of roughly 1.25 times the continuous load current, then choose the accuracy class by application (0.5S for metering, 5P for protection), and finally verify that the total burden including cabling stays below the CT rated output.
Contents
A current transformer converts the hundreds of amperes flowing through a switchboard into a 5 A or 1 A signal that instruments and relays can handle safely. When the ratio, class or burden is chosen carelessly, the power analyser reads incorrectly, the overcurrent relay fails to trip in time, and dangerous voltages can appear in the secondary circuit. This article walks through ratio, accuracy class, burden and saturation in a practical way, following the framework of IEC 61869-2.
What a Current Transformer Does Inside a Switchboard
A current transformer magnetically couples the primary conductor to a secondary winding, producing a small current that is proportional to the primary one. Low voltage assemblies typically use ring or split-core types slipped over a busbar or cable, while medium voltage panels use resin-cast support types. Analysers, ammeters, protection relays and meters are therefore fed with galvanic separation from the main power path. That separation reduces instrument cost and improves operator safety at the same time.
Choosing the Ratio and Primary Rating
The primary rating should sit slightly above the continuous operating current of the circuit. A practical rule is to round the calculated load current multiplied by 1.20 to 1.30 up to the nearest standard value such as 100, 150, 200, 250, 400, 630, 800, 1000, 1250, 1600, 2000, 2500 or 3200 A. An oversized ratio destroys measurement resolution at partial load and produces visible analyser error, while an undersized ratio increases the risk of saturation during normal operation. For long secondary runs, a 1 A secondary is preferable because burden grows with the square of the secondary current.
Metering Classes Versus Protection Classes
Metering classes such as 0.2S, 0.5S, 0.5 and 1 aim for very low error within the normal operating range and deliberately saturate under fault conditions, which protects the connected instrument. Protection classes such as 5P and 10P do the opposite: they preserve the ratio well above rated current, and the number in 5P10 denotes the accuracy limit factor. Sensitive schemes such as differential protection (ANSI 87) need PX or class X cores where knee point voltage and secondary resistance are specified. Never feed metering and protection from the same core; specify a multi-core CT instead.
Calculating the Burden Including Cable Resistance
| Accuracy class | Typical use | Point to watch |
|---|---|---|
| 0.2S | Revenue metering | Accurate even at low load |
| 0.5S / 0.5 | Panel analyser, energy monitoring | Error grows fast if burden is exceeded |
| 1 | Indicating ammeter | Economical, not for critical metering |
| 5P10 / 5P20 | Overcurrent protection (50/51) | ALF must cover the fault current |
| 10P10 | Simple backup protection | Lower accuracy weakens selectivity |
| PX / class X | Differential protection (87) | Knee point voltage and Rct specified |
Burden is the apparent power absorbed by the total secondary impedance at rated current, expressed in VA. It includes not only the instruments but also the go-and-return cable resistance and terminal contact resistance: with a 5 A secondary, 20 metres of 2.5 mm² loop adds roughly 3.5 VA. The total must stay below the rated output of the CT, yet an extremely low burden is not ideal either, because some classes lose accuracy when severely underloaded. The table below compares classes for everyday selection work.
ALF, Saturation and Secondary Circuit Safety
The accuracy limit factor defines the multiple of rated primary current up to which the ratio is maintained, so the selected ALF must cover the maximum fault current the relay has to see. The effective ALF rises when the connected burden is lower than the rated burden, which is why the calculation must use the real load. The single most important safety rule is that an energised secondary circuit must never be left open: the core saturates and kilovolt-level peaks appear across the terminals. Test terminal blocks with shorting links are therefore mandatory, and the secondary must be shorted before any maintenance work.
Key takeaways: current transformer selection
- Match the ratio to the load — Round roughly 1.25 times the continuous current up to the nearest standard ratio.
- Separate metering from protection — Use a multi-core CT rather than feeding both functions from one core.
- Include cabling in the burden — A 1 A secondary dramatically reduces cable-related VA on long runs.
- Never open an energised secondary — Keep shorting links in the test terminal block and short the secondary before maintenance.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
What happens if the CT ratio is far larger than the actual load?
The secondary current stays very small in the normal operating range and falls outside the error band guaranteed by the accuracy class. The analyser may under-report consumption and the power factor reading becomes unstable. The ratio should be chosen so that the circuit normally operates between about a quarter and the full primary rating.
Should I specify a 5 A or a 1 A secondary?
For short runs inside the same enclosure, 5 A is common and economical. When the instrument sits in a remote mimic panel or a control room, a 1 A secondary is far better because cable burden increases with the square of the current, allowing much longer distances with the same conductor size.
Which accuracy class is sufficient for a protection relay?
For standard overcurrent and earth fault protection, 5P10 or 5P20 is adequate in most installations. If busbar or transformer differential protection is applied, class PX cores with defined saturation behaviour are required, otherwise external faults can cause unwanted tripping.
Related articles
- Voltage Transformer (VT) Selection: Ratio, Accuracy Class and Ferroresonance
- Energy Monitoring in Switchgear: Meters, Modbus and SCADA Integration
- Protection Relays and ANSI Codes: 50/51, 27/59 and 87 Functions
- All technical articles
Devpan calculates CT ratios, accuracy classes and burden budgets for your low and medium voltage switchboards and delivers them as part of a verified design. Contact us to plan your metering and protection chain together. Related Devpan solution: electrical panel design services.
