CT and PT Connection Diagram
This is a free printable ct and pt connection diagram: download the diagram as SVG or open it and print to paper or PDF.
Current transformers must never have their secondary left open-circuit; voltage transformers must never have their secondary short-circuited — these are the two critical rules governing CT and PT wiring for metering and protection.
Current transformers (CTs) and potential (voltage) transformers (PTs, also called VTs) are instrument transformers that scale down high primary currents and voltages to levels safe for measuring instruments, energy meters, and protection relays — typically 1 A or 5 A for CT secondaries and 110 V or 115 V line-to-line for PT secondaries.
**Current Transformer (CT)**
A CT primary winding is connected in series with the circuit being measured. The primary is often just a single bus bar pass-through (one turn). The secondary winding, wound around the same core, produces a proportional current — for example, a 500/5 A CT produces 5 A on its secondary when the primary carries 500 A.
The cardinal rule of CT operation: the secondary must never be left open-circuit while current flows in the primary. When the secondary is open, the core flux is no longer limited by the secondary current's magnetomotive force. The core saturates and the resulting high rate of flux change induces an extremely high voltage across the open terminals — potentially thousands of volts. This voltage can destroy the CT, flash over insulation, and is lethal to anyone touching the secondary terminals. If a meter or relay must be removed from a CT secondary circuit while the primary is live, the secondary must be short-circuited first using the CT's shorting bar or a dedicated test block.
**Polarity and burden**
CT connections observe polarity marking (usually a dot or P1/P2 on primary, S1/S2 or K/L on secondary). For metering and directional protection, correct polarity is critical. The secondary burden (total impedance of connected instruments and cables) must not exceed the CT's rated burden — excessive burden causes core saturation and measurement error.
**Potential Transformer (PT / VT)**
A PT primary connects in parallel (phase-to-phase or phase-to-earth) with the circuit being monitored. The secondary is typically 110 V or 115 V, and instruments connect across it. Because the PT is a voltage source on its secondary, a short circuit on the secondary is equivalent to a short-circuit fault. It will draw destructive current and thermally destroy the transformer instantly. PT secondary circuits must be protected by appropriately rated miniature circuit breakers or fuses directly at the PT secondary terminal.
**Earthing**
One terminal of each CT secondary (typically S2 or L) and one terminal of each PT secondary should be earthed at one point only in the circuit, to prevent shock hazards from insulation faults and to stabilise the circuit reference potential.
**Applications**
CTs and PTs are used in energy metering, kWh billing, power factor correction, over-current relays, differential protection, earth-fault relays, and synchronising circuits.
This is a generic educational reference. CT and PT installations at medium and high voltage are work on or near live equipment and must only be performed by suitably qualified electrical engineers and licensed electricians in accordance with IEC 61869-2 (CTs), IEC 61869-3 (PTs), IEC 60364, and applicable local electrical safety regulations.
How to wire ct and pt connection diagram
- Determine measurement or protection requirements Establish the primary current or voltage range, required accuracy class, burden of connected instruments, and whether the application is metering (accuracy at normal current) or protection (accuracy at fault current). This determines CT and PT class and ratio selection.
- Select CT and PT with appropriate ratio and class Choose a CT ratio so that normal operating current utilises 80–100% of the CT's primary rating for best accuracy. For a PT, choose a ratio that produces the standard secondary voltage (110 V L-L or 63.5 V phase-to-earth) from the system voltage. Verify the rated burden matches or exceeds the planned instrument load plus cable resistance.
- Install the CT with secondary shorted Before making any secondary connections, confirm the CT's secondary shorting link is engaged. Install the CT onto the primary conductor (bus bar through the window, or series winding connected). Route the secondary wiring but do not connect to instruments yet.
- Wire CT secondary to test block or terminal block Connect CT secondary terminals S1 and S2 (or K and L, per the CT marking) to the protection or metering terminal block. Use appropriate wire gauge to minimise lead resistance and therefore burden. Observe polarity — S1 is typically the polarity-marked terminal, corresponding to P1 on the primary.
- Connect instruments and earth the secondary circuit Connect protection relays or meters to the terminal block. Earth the S2 (or L) terminal at one point only. Only remove the CT secondary shorting link after all instruments are connected and wiring is verified.
- Install PT secondary fuses or MCBs, then connect to instruments Fit appropriately rated fuses or MCBs on all PT secondary conductors before making any other connections. Connect PT secondary to instruments, observing polarity. Earth one phase (or the neutral point on a star secondary) at a single location.
- Commission and verify — ratio check and polarity test With the circuit energised at rated voltage and with known load current: verify PT secondary voltage matches the expected ratio. Inject known primary current (or use load current) and verify CT secondary current matches the ratio. For protection applications, perform polarity checks and vector group verification with a CT primary injection set if required.
Specifications
| Standard CT secondary rated current | 1 A or 5 A (IEC 61869-2) |
|---|---|
| Standard PT secondary rated voltage | 100 V, 110 V, or 115 V line-to-line (IEC 61869-3) |
| CT metering accuracy classes (IEC) | 0.1, 0.2, 0.5, 1, 3, 5 |
| CT protection accuracy classes (IEC) | 5P, 10P (e.g., 5P10 = 5% error at 10× rated current) |
| PT accuracy classes (IEC) | 0.1, 0.2, 0.5, 1, 3, 3P, 6P |
| Minimum secondary earth wire size | 2.5 mm² copper (IEC 61869 recommendation) |
| Open-circuit secondary voltage (CT, indicative) | Can exceed 1 kV — exact value depends on primary current, core design, and open-circuit inductance; sufficient to cause death |
Safety warnings
- A CT secondary must NEVER be left open-circuit while current flows in the primary. Open-circuit CT secondaries generate potentially lethal high voltage. Always short the CT secondary (using the test block shorting bar or a temporary short link) before disconnecting any instrument or relay. This is a fundamental safety rule with no exceptions.
- A PT secondary must NEVER be short-circuited. Fit suitably rated fuses or MCBs on all PT secondary conductors immediately at the PT secondary terminals. Do not rely on upstream protection — the PT's own secondary must be fused.
- CT and PT primary circuits in medium and high voltage (MV/HV) switchgear are on or near live conductors at potentially lethal voltages. Only suitably authorised, trained, and licensed electrical engineers may access these areas. Strict electrical safety rules (permit-to-work systems, isolation verification, personal protective equipment) are mandatory.
- Do not assume a circuit is dead because an upstream switch is open. CT primaries carry induced voltages from adjacent live conductors in switchgear. Always verify dead with appropriate voltage-detection equipment rated for the system voltage.
- All CT and PT installations must comply with IEC 61869-2, IEC 61869-3, IEC 60364, and applicable local electrical safety legislation and utility company metering codes.
Tools needed
- CT primary injection set (for commissioning protection CTs)
- Ratio and polarity test set
- Calibrated clamp-on ammeter (for secondary current verification)
- Calibrated voltmeter (for PT secondary voltage verification)
- Insulated screwdrivers and terminal torque screwdriver
- Temporary CT shorting link or crocodile clip leads (insulated)
- Continuity and insulation resistance tester (megger)
- Phase rotation meter (for three-phase PT installations)
Common mistakes
- Removing a meter or relay from a CT secondary without first engaging the test block shorting bar — the most dangerous and most common CT handling error.
- Installing a PT without secondary fuses, leaving the PT winding unprotected against secondary faults in instrument wiring.
- Earthing both S1 and S2 of a CT secondary, creating a short circuit across the CT output that will circulate current through the earth system and corrupt the measurement.
- Connecting CTs with incorrect polarity in a three-phase metering scheme, causing power and energy readings to show incorrect values (the error may not be obvious without checking all three phases).
- Using undersized secondary wiring for CT circuits over long cable runs, adding resistance burden that causes core saturation at high current and significant measurement error.
- Using a metering-class CT (which saturates at fault current) in a protection relay circuit, causing the relay to fail to operate during high-fault-current events.
Troubleshooting
- Energy meter reads significantly lower than expected power consumption
- Cause: CT core saturation due to excessive burden, incorrect CT ratio programmed in the meter, or one CT polarity reversed causing partial cancellation in the metering system Fix: Verify CT secondary burden: measure lead resistance and calculate total VA against the CT's rated burden. Verify CT ratio matches the meter's programmed ratio. Perform a polarity check using the meter's power factor reading — a reversed CT gives an apparent negative power factor on that phase.
- Protection relay fails to operate during a fault test
- Cause: CT has insufficient accuracy at fault current (wrong class), CT is saturating, or the relay pickup setting is above the actual injected current Fix: Verify the CT is protection class (e.g., 5P20) and check that the maximum fault current does not exceed the instrument's limit factor. Perform a primary injection test to verify CT output at fault-level current. Review relay setting calculations against CT ratio and system fault levels.
- PT fuse blows repeatedly on energisation
- Cause: PT primary magnetising inrush current exceeding fuse rating, a fault in the PT secondary circuit, or an undersized fuse for the transformer's magnetising current Fix: Check PT secondary wiring for insulation faults with a megger. If wiring is healthy, the fuse may be undersized for magnetising inrush — consult the PT manufacturer's recommended fuse size and use a slow-blow fuse or MCB with appropriate tripping characteristics.
Frequently asked questions
What happens if a CT secondary is left open-circuit?
With no load on the secondary to oppose the primary magnetomotive force, the core flux rises without limit until saturation. The rapidly changing flux in the saturated core induces very high voltage spikes on the secondary terminals — potentially several kilovolts. These spikes can destroy the CT's insulation, damage connected equipment, and deliver a fatal electric shock. Always short the secondary before removing any connected instrument.
What happens if a PT secondary is short-circuited?
A PT secondary short-circuit is equivalent to a secondary fault on a power transformer. The transformer attempts to maintain the secondary voltage against zero impedance, drawing fault-level current from the primary. This current destroys the PT's windings within seconds. Protect PT secondary circuits with suitably rated fuses or MCBs installed as close to the PT secondary terminals as possible.
Why is only one point of the CT secondary circuit earthed?
Earthing at multiple points creates a loop. Any potential difference between earth points circulates a current through the loop, which superimposes on the measurement signal and introduces error. A single-point earth establishes a stable reference without creating a circulating current path. The unconnected terminal floats relative to earth — which is acceptable at secondary (low) voltage levels.
What is CT burden and why does it matter?
CT burden is the total VA load imposed on the CT secondary — the sum of instrument impedances multiplied by secondary current squared, plus cable resistance. If the burden exceeds the CT's rated value, the CT core saturates at high primary current levels. A saturated CT produces a distorted, non-proportional secondary current, causing meters to under-read and protection relays to either fail to operate or operate at the wrong current level.
What is the difference between a metering class and protection class CT?
Metering class CTs (e.g., class 0.5, 1) maintain high accuracy at normal load currents but are designed to saturate at fault currents to protect sensitive meters from overvoltage. Protection class CTs (e.g., 5P10, 5P20) maintain accuracy up to a defined multiple of rated current (10× or 20× for the examples given) so that protection relays receive a faithful signal during fault conditions.
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