Summary Highlights
- Read a CT ratio without confusing rated current with the actual operating current.
- Choose the measurement or protection core using its class, burden and required operating conditions.
- Compare 1 A and 5 A secondary circuits using the actual instrument inputs and complete lead loop.
- Distinguish window, bar and wound-primary construction from metering or protection duty.
- Keep CT secondary work within the approved equipment scheme; a small rated current is not a touch-safety rating.
Article Details
A current transformer (CT) is an instrument transformer that supplies current information to meters, monitoring instruments or protection relays. In a conventional electromagnetic CT, primary current produces magnetic flux and a corresponding secondary current. The purpose is a usable measurement signal with specified accuracy and insulation, rather than the power-delivery duty of a distribution transformer. The CT supplies the signal; a protection relay and switching equipment perform their separate protection functions.
What does a CT measure, and where does it fit?
A CT responds to the current through its identified primary path. Depending on the design, that path is a conductor through a window, an integral bar or a wound primary. The secondary feeds the instrument circuit specified for that core. A conventional current-output CT, a low-power sensor and a Rogowski coil do not have interchangeable outputs or connection requirements; identify the actual device before selecting an input or applying a test method.
Current and voltage transformers often supply the same meter or relay, but represent different quantities. A CT is associated with the current path; a VT/PT monitors a voltage across its primary connections. Their ratings use different units, and their secondary-circuit risks must not be confused.
How to read a 100/5 A current-transformer ratio
The marking 100/5 A states rated primary and secondary currents. Their nominal ratio is 100 ÷ 5 = 20. It does not mean that the secondary continuously delivers 5 A, or that 5 A is its maximum possible current. Actual output follows primary current approximately within the equipment’s specified operating conditions; accuracy, burden and saturation affect the real relationship.
| Hypothetical primary current | Ideal secondary current for 100/5 A | What the example establishes |
|---|---|---|
| 20 A | 1 A | Rated ratio applied to one operating point |
| 40 A | 2 A | 40 ÷ 20; an arithmetic example |
| 100 A | 5 A | The rated-current relationship |
These values are illustrative calculations, not measurements from a Pow-Sys project and not proof of any accuracy class. A meter configured with the wrong ratio can display an incorrect primary value even if its secondary reading is consistent. Tapped CTs require the ratio and terminal pair actually in use to be recorded; another available tap is not automatically the active ratio.
Metering and protection cores have different jobs
Metering selection concerns the required measurement accuracy over the specified current and burden range. Protection selection also considers fault current, saturation and the demands of the particular relay scheme. Two cores in one housing may therefore have different ratings. An accurate metering core is not automatically suitable for a protection function, and a protection designation alone does not establish billing accuracy.
| Nameplate item | What it describes | What it does not establish |
|---|---|---|
| 100/5 A | Rated current relationship for the identified terminals | Every operating current, accuracy or thermal capability |
| Metering class, for example 0.5 | An accuracy designation under its applicable conditions | A constant error at every current and connected burden |
| Protection class and ALF, for example 5P10 | A protection-class designation with rated accuracy limit factor 10 | A universal knee-point voltage or proof against every transient |
| Rated burden in VA | The specified external secondary-load basis | The actual connected load or an unlimited permissible load |
| FS where specified | Instrument security-factor characteristic for the metering core | Touch safety or a guarantee that a connected meter cannot be damaged |
Read class, rated burden, frequency and the applicable manufacturer data together. For protection, actual lead and device burden, system fault levels and the relay’s required performance matter. The FS of a metering core and the ALF of a protection core describe different characteristics. Neither should be selected by a blanket “higher is always better” rule.
1 A versus 5 A: calculate the lead contribution
The CT output must match the connected instrument inputs. For an assumed purely resistive lead loop, its power contribution is I²R. At the same assumed 0.2 Ω complete loop resistance, 1 A produces 0.2 W and 5 A produces 5 W. For that resistive part, the corresponding apparent-power contribution is numerically 0.2 VA and 5 VA. This explains the 25-fold difference at those rated currents; it does not prove that every 1 A installation is preferable.
The calculation excludes the meter or relay load, contact effects and any reactive contribution. Use the full outgoing-and-return path where that represents the actual circuit. A measured DC resistance alone does not describe every AC burden. There is no universal cable-distance boundary for choosing 1 A or 5 A: input compatibility, conductor size, complete load, installation conditions and the required CT performance decide the choice.
Window, bar and wound-primary construction
| Construction | Physical distinction | Selection question |
|---|---|---|
| Window or ring | The primary conductor passes through an opening | Does the conductor arrangement fit the specified window and insulation conditions? |
| Bar | An integral primary bar forms part of the equipment | Are primary connections, dimensions and withstand ratings compatible with the installation? |
| Wound primary | The device has a dedicated primary winding | Are the rated current, terminals and insulation suitable for the intended circuit? |
Construction and duty are separate classifications: a product photograph does not establish whether a core is for metering or protection. Check mounting, insulation level, ambient and installation conditions, short-time and dynamic withstand, terminal identification and the actual drawing. Do not derive a CT’s ratio or class from its external size.
Secondary safety and polarity are part of the design
Opening a conventional CT secondary while primary current flows can create hazardous voltage and damage insulation. A 1 A or 5 A marking is not permission to touch the circuit. Any secondary intervention belongs to qualified personnel following the approved equipment and installation procedure, which must prevent an unintended open secondary while the primary is energized.
Polarity marks relate identified primary and secondary terminals. Their relationship matters to directional measurements and protection, so record it alongside phase assignment and the actual wiring. Unused taps and separate unused cores are different cases; do not apply a generic shorting or grounding instruction to both. Follow the exact manufacturer scheme and approved installation drawing.
What information is needed to select or check an existing CT?
Provide the readable nameplate and terminal diagram, the active ratio/core, connected meter or relay model, intended metering or protection function, cable route and size, and the installation’s operating and fault-current information. Record known wiring changes and the reason for the assessment. If the question is whether an existing circuit works correctly, CT testing and maintenance must distinguish device characterization from the current path and functions actually tested.
For an MV/HV testing enquiry, describe the equipment, required outage and measurement or protection concern. The agreed scope should state deliverables and any excluded functions. A CT photograph, nominal ratio or successful individual test is not evidence that every connected protection function has been verified.
Technical references and their scope
The GE instrument-transformer application guide explains construction, ratio, polarity and burden using predominantly US examples. ABB’s 2004 CT application note illustrates fault-current, burden and relay requirements for named relay families. These are technical references; their example ratings and settings are not universal acceptance values for an installation.
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- What Is a Protection Relay? How It Works and Types
- What Is a Transformer Substation? What Does It Do, How Does It Work and Which Sections Does It Consist Of?
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Frequently Asked Questions
What is a current transformer?
A CT supplies current information to meters or protection relays through a specified secondary output. In a conventional electromagnetic CT, primary current produces a corresponding secondary current. Identify its ratio, class, burden, construction and insulation conditions; an output rating of 1 A or 5 A does not make the circuit safe to touch.
What does a current transformer do?
It represents current in the identified primary path so a suitable meter, monitor or protection relay can use that signal. A protection CT does not itself interrupt a fault: relay logic and switching equipment have separate roles. Correct operation depends on the CT, connected load, wiring and the required function.
How does a current transformer work?
A conventional CT uses electromagnetic induction. Its nominal current ratio relates rated primary and secondary currents, while actual output also depends on the operating point, burden and magnetic behavior. Window, integral-bar and wound-primary designs implement the primary path differently. Low-power sensors require their own specified output and test method.
What does a 100/5 current transformer mean?
100/5 A denotes rated primary current 100 A and rated secondary current 5 A, a nominal ratio of 20. In a hypothetical ideal example, 40 A primary corresponds to 2 A secondary. It is not a fixed 5 A output or maximum-current statement, and the calculation does not verify accuracy.
Why is a current transformer connected in series?
Its identified primary path must carry the current being represented. In a window CT the conductor passes through the opening; a bar or wound-primary device has a different physical primary arrangement. This current-path function differs from a VT’s voltage-measurement connection. Installation follows the actual equipment drawing.
Should 1 A or 5 A be selected for a current transformer?
Match the instrument inputs and assess the complete secondary load. For the same resistive lead loop, I²R at 5 A is 25 times its value at 1 A, but device burden, conductor size and required performance also matter. No fixed cable length determines the answer for every installation.
What should be considered when selecting a current transformer?
Check purpose, active ratio/core, rated secondary current, class and burden, frequency, insulation and withstand ratings, mounting and environmental conditions. Protection selection additionally considers system fault levels and the relay scheme. A physically compatible CT or a suitable nominal ratio alone does not establish the required performance.
Why must the secondary of a current transformer not be left open?
If a conventional CT secondary opens while primary current flows, hazardous voltage can develop and insulation can be damaged. The rated secondary current is not a touch-safety rating. Qualified personnel must use the approved equipment and installation procedure to prevent an unintended open secondary; this article is not a switching sequence.
What is the difference between a current transformer and a voltage transformer?
A CT represents current in its primary path; a VT/PT represents voltage across its primary connections. Their outputs, classes and applicable tests differ. Opening a conventional CT secondary with primary current flowing is hazardous; short-circuiting a VT secondary is hazardous. Neither secondary is inherently safe to touch.