Get the current transformer ratio wrong and every reading that follows is worthless. The panel ammeter shows the wrong figure, the energy meter bills the wrong amount, and the overcurrent relay either nuisance-trips or sits still through a fault.
Current transformers (CTs) sit behind nearly every metering and protection circuit on a board, so reading the ratio off the nameplate and matching it to the job is bread-and-butter work for anyone working on switchgear and instrumentation.
Three numbers decide whether a CT does its job on the board: the ratio, the accuracy class and the burden.
What the CT Ratio Actually Tells You
A current transformer steps a large primary current down to a small secondary current that your instruments can handle safely. The current transformer ratio is the link between the two: rated primary current over rated secondary current. Take a 400/5 CT. It gives 5 A out of the secondary when 400 A flows through the primary, which is a ratio of 80 to 1.
Read the figures as current ratings, not just a division sum. A 100/5 CT and a 20/1 CT both reduce to 20:1 on paper, yet they are designed for different primary currents and different secondary circuits, and you cannot swap one for the other. That is why the primary rating is chosen to suit the load you expect to measure: the CT is matched to the circuit, not the circuit to the CT.
Primary and Secondary Ratings
Two figures appear on every CT nameplate, and each does a different job.
- Secondary Rating: In practice almost always 5 A or 1 A, so meters, relays and test gear are built around those two figures.
- Primary Rating: Set by the load you need to measure. Panel CTs cover primaries from tens of amps up to several thousand, in steps such as 100/5, 200/5, 400/5 and 800/5.
Do not run a CT continuously beyond its rated primary current unless the nameplate allows it. Many CTs carry a rated continuous thermal current above rated, commonly 1.2 times, and metering classes are specified up to 120% of rated current, so a small margin is fine. Push well past the thermal rating and the CT overheats. At high overcurrents, a metering CT is also designed to saturate (its instrument security factor, such as FS5) so the meter is protected, which means its output no longer follows the true ratio.
Choosing Between a 1 A and a 5 A Secondary
- 5 A Secondary: The long-standing default. It suits most switchboard instruments sitting close to the CT.
- 1 A Secondary: The better choice for long cable runs. The burden from cable resistance rises with the square of the secondary current, so for the same cable the volt-amps lost at 1 A are one twenty-fifth of the loss at 5 A.
The rule of thumb is simple. Keep 5 A for short runs to the meter, and move to 1 A when the meter or relay is mounted well away from the CT.
Accuracy Classes and What They Mean on the Board
Accuracy class tells you how far the secondary is allowed to drift from the exact current transformer ratio. Classes divide into two camps with opposite priorities: metering and protection.
Metering Classes
Metering CTs are built for accuracy around normal running current, where billing and monitoring happen. The number in the class is the maximum ratio (current) error, in per cent, at rated current. Each class also has a limit on phase displacement, which matters for energy and power factor readings.
- Classes 0.1, 0.2, 0.5 and 1: Class 1 allows up to 1% ratio error at rated current, and Class 0.5 allows up to 0.5%. The limits widen at lower currents.
- Extended S Classes (0.2S, 0.5S): Accuracy is specified from 1% up to 120% of rated current, with tighter limits at low load, so revenue meters stay accurate on light overnight loads.
Protection Classes
Protection CTs have the opposite brief. They do not need pinpoint accuracy at running current. They need to stay reasonably accurate during a fault, when current jumps to many times normal, so the relay sees something close to the true current and operates at the right moment.
The common protection classes are 5P and 10P:
- 5P: Up to 1% ratio error at rated current, with composite error held to 5% at the accuracy limit primary current.
- 10P: Up to 3% ratio error at rated current, with composite error held to 10% at that limit.
The number after the P is the accuracy limit factor. A 5P10 CT holds that 5% composite error out to ten times rated primary current, and 5P20 holds it to twenty times, both at rated burden. Match the limit factor to the fault current the relay has to see. For differential and other unit protection schemes, Class PX CTs are specified instead by knee point voltage and winding resistance.
Burden, and Why It Makes or Breaks the Accuracy
Burden is the load on the secondary, and it is the figure people tend to skip over. The rated burden, the VA figure on the nameplate, has to cover everything in the external secondary circuit:
- Meter or Relay Input: The VA drawn by the connected instrument or protection relay.
- Lead Resistance: The cable out to the device and back, which grows with the run length.
- Terminal Connections: The small resistance added at each joint and test link.
Rated burden is quoted in volt-amps, with common standard values of 2.5, 5, 10, 15 and 30 VA. It can also be expressed in ohms. When you move on to knee point and saturation checks, you add the CT’s own secondary winding resistance to that external figure to get the total burden.
A CT only holds its stated accuracy class up to its rated burden. Load it past that and the errors climb. Cable is the usual cause. Because the loss scales with the square of the current, at a 5 A secondary even half an ohm of loop resistance adds 12.5 VA. A long run on undersized cable can push a 15 VA CT over its limit before the meter is even connected.
Going too big can backfire too. Fit a 30 VA metering CT on a 3 VA load and its accuracy is no longer guaranteed, and the meter is less well protected during overcurrents. For metering classes, the standard holds a CT to its class across 25% to 100% of rated burden, so size the CT to keep the real load inside that window rather than at either extreme.
The Safety Rule You Do Not Break
Never open-circuit the secondary of a current transformer while the primary is live. With current in the primary and nothing closing the secondary circuit, the core is driven hard into saturation and the CT tries to force its current through an open circuit. That produces dangerously high voltage peaks across the open terminals, enough to injure you and damage the CT’s insulation.
Wherever possible, make the circuit dead and follow safe isolation procedures before working on the CT, meter or relay. Where work on a live CT secondary circuit is genuinely justified, short the secondary first using the shorting link or test block. Short it, then disconnect. No shortcuts on this one.
Where the UK Standards Fit In
In the UK, current transformers for metering and protection are made to BS EN 61869-2, the UK adoption of IEC 61869-2 published by the British Standards Institution. It defines the accuracy classes, error limits and burden ratings behind everything above, and it replaced the older BS EN 60044-1.
The National Physical Laboratory, the UK’s national measurement institute, calibrates current transformers for current error and phase angle error against BS EN 61869-2:2012, generally for secondaries rated at 1 A and 5 A. For a revenue or test CT that needs traceable accuracy, that is the reference point.
Other UK rules apply around the CT. BS 7671, the IET Wiring Regulations, covers the electrical installation the CT forms part of, while the switchgear assembly it sits in is typically built to BS EN 61439. On the billing side, the energy meter itself must be approved under the Measuring Instruments Regulations 2016, and for settlement metering the Balancing and Settlement Code Codes of Practice set the CT accuracy classes required.
Reading a CT comes down to those three numbers working together. The ratio sets what the secondary gives you, the accuracy class sets how far it can wander, and the burden decides whether it keeps its word once the meter and cable are connected. Match all three to the circuit and the current transformer ratio does exactly what the nameplate promises.
For further electrical learning, explore Tradefox’s interactive online programmes. Work through guided, risk-free simulations, build your understanding step by step, and revisit each activity at your own pace whenever you need a refresher.
Calculators for this job
Free Tradefox calculators that go with this guide:
- CT effective accuracy limit factor – See how the real burden changes a protection CT’s ALF.
- CT knee point voltage – Check the knee point a protection scheme needs.
- Transformer current ratio – Primary to secondary current for a given ratio.
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