CT Connection Diagram: Installation, Polarity, and Safety Procedures
This is a free printable ct connection diagram: download the diagram as SVG or open it and print to paper or PDF.
Current transformers step down high AC currents to standard measurement ranges (typically 5 amps). Proper connection, polarity, and grounding ensures accurate metering and prevents dangerous open-circuit overvoltage conditions.
Diagram checks and scope
- Components
- 5
- Wired connections
- 7
- Automated check
- Automated pin, route, source, and topology checks passed; this is not a safety certification.
A current transformer is a toroidal transformer with primary winding consisting of one or more passes of the main power conductor through the core window. The secondary winding (200-300 turns) produces a proportional current output. Standard CT ratios include 100:5, 200:5, 400:5, 600:5 (meaning 100-amp primary produces 5-amp secondary). The core material is typically silicon steel or ferrite optimized for 50-60 Hz power frequencies. Primary terminals connect the main power conductor by passing it through the core window—not by terminal connection. Secondary terminals provide output to ammeters, power analyzers, or protective relays. Polarity markings (P1/P2 on primary, S1/S2 on secondary) indicate phase relationship. P1 and S1 always correspond positively, making them reference points for polarity verification. The secondary output impedance must be calculated including ammeter burden, relay burden, and cable resistance. Maximum rated burden (typically 2-20 VA depending on CT class) must not be exceeded or core saturation occurs, causing measurement errors. Opening a CT secondary creates dangerously high voltage (potential 1000+ volts AC) that damages instruments and risks personnel injury. Secondary terminals must be permanently jumpered or shorted during operation and maintenance. Single-point grounding at the CT case prevents circulating current loops that degrade accuracy. Class 0.2 CTs (billing accuracy) maintain better than 0.2% ratio error; Class 1 and 3 CTs (protective relays) accept higher error rates. Cable connecting CT secondary should be twisted pair or shielded to minimize noise induction into control circuits.
How to wire ct connection diagram
- Calculate maximum expected primary current from system load analysis
- Select CT ratio producing 5A secondary at maximum primary current (divide primary by 5)
- Calculate total secondary burden: ammeter coil resistance + relay coil impedance + cable resistance
- Verify total burden is within CT rated burden (typically 2-20 VA)
- Install CT core around power conductor by clamping or bolting halves together
- Connect power conductor through core window ensuring complete passage with proper polarity
- Install shorting block or jumper across secondary terminals
- Connect secondary output terminals to meters or relays with twisted-pair or shielded cable
- Ground secondary at one point only near the CT case with heavy gauge wire
- Verify all connections are clean and corrosion-free
- Test with voltmeter across secondary terminals before removing shorting block
Frequently asked questions
What is the primary winding on a current transformer?
The primary consists of the main power conductor itself—you loop it through the CT window one or more times. The secondary winding (200-300 turns of fine wire) is wound on the toroidal core and provides the 5-amp output.
What is CT burden and why do I calculate it?
Burden is total load (ohms) connected to the secondary: ammeter coil resistance, relay coil impedance, and cable resistance combined. If total burden exceeds the CT rating, core saturation causes ratio errors and measurement inaccuracy. Always calculate burden before selecting a CT.
What voltage develops if a CT secondary opens during operation?
The open secondary prevents secondary current flow, causing the core to saturate and develop dangerously high voltage (potential 500-1500V AC) that destroys connected instruments and creates shock hazard. Always short the secondary or install a shorting block.
How do I know if I have the CT ratio right for my application?
Your maximum expected primary current divided by CT ratio should produce 5 amps secondary output. For 150-amp primary maximum, select a 150:5 ratio (30:1). Never undersize the ratio—causes permanent saturation.
Where should I ground the CT secondary?
Ground at one point only—typically at the CT core case or near the transformer. Single-point grounding prevents circulating currents through multiple ground paths that degrade measurement accuracy.
Sources and verification
Review status: Not independently reviewed. Automated topology checks confirm stored terminals and routes, not the correctness of manufacturer pin assignments, ratings, regional codes, or installation decisions. Verify those claims against the current primary documentation before use.
- IEC 61082-1:2014 — Rules for electrotechnical documents — International Electrotechnical Commission. Supports: General diagram, drawing, table, and reference-designation presentation conventions. Checked 2026-07-15.
Related diagrams
- 3 phase 4 wire energy meter connection diagram with ct
- 3 phase ct connection diagram
- 3 phase energy meter connection diagram with ct
- ct and pt connection diagram
- ct meter connection diagram
- lt ct meter connection diagram