The Current Transformer

Current Transformer Ratios & Applications

The Current Transformer: Complete Guide to CTs, Ratios & Applications

In electrical power systems, measuring and monitoring high currents is essential for metering, protection, and control. However, directly connecting measuring instruments to high-voltage, high-current circuits is impractical and dangerous. The solution is the Current Transformer (CT), a specialized instrument transformer designed to step down high primary currents to standardized, safe secondary levels.

Current transformers are the eyes of the power system. They provide isolated, proportional current signals to:

Energy meters for billing and monitoring
Protective relays for fault detection and isolation
Ammeters for operator indication
Control systems for automation and regulation
Power quality analyzers for harmonic monitoring

Unlike power transformers that transfer energy, current transformers are signal transformers designed for accuracy and safety. Understanding CT operation, ratios, accuracy classes, and safety requirements is critical for electrical engineers working with power systems, protection schemes, and instrumentation.

This comprehensive guide will explore current transformer construction, operating principles, ratio calculations, accuracy classes, burden considerations, saturation effects, and critical safety precautions.

What is a current transformer?
A current transformer (CT) is an instrument transformer that steps down high primary currents to standardized, safe secondary currents (typically 5A or 1A) for metering and protection. The secondary current is proportional to the primary current and is electrically isolated from the high-voltage primary circuit.

Construction and Operating Principle

Basic Construction

A current transformer consists of:

1. Primary Winding:

Usually a single turn (bar-type or window-type CT)
Connected in series with the line carrying the current to be measured
Carries the full line current
Low voltage rating (insulated for system voltage)

2. Secondary Winding:

Many turns of fine wire
Connected to measuring instruments or protective relays
Standardized output: 5A or 1A (depending on standard)
Must never be open-circuited when primary is energized

3. Magnetic Core:

Made of high-permeability silicon steel or nickel-iron alloy
Provides low-reluctance path for magnetic flux
Designed for high accuracy at rated current

4. Insulation:

Separates primary and secondary windings
Withstands system voltage
Oil-immersed or dry-type construction

Types of Current Transformers

1. Bar-Type CT:

Primary is a solid copper or aluminum bar passing through the core
Single-turn primary
Common in switchgear and panel boards
Easy to install

2. Window-Type (Ring-Type) CT:

Toroidal core with no primary winding
The line conductor itself acts as the primary turn
Can be installed around existing conductors
Very common for retrofits and monitoring

3. Wound-Type CT:

Primary winding is wound on the core
Multiple primary turns
Used for low-ratio applications
More expensive, less common

4. Bushing-Type CT:

Mounted on transformer or circuit breaker bushings
Uses the bushing conductor as primary
Integrated design

Operating Principle

The current transformer operates on the same basic principle as a conventional transformer:

Primary Current ($I_p$) creates a magnetic flux in the core.

Secondary Current ($I_s$) is induced to oppose this flux (Lenz’s Law).

Ampere-Turn Balance:
In an ideal CT:
$I_p \times N_p = I_s \times N_s$

Where:

$I_p$ = Primary current
$N_p$ = Number of primary turns (usually 1)
$I_s$ = Secondary current
$N_s$ = Number of secondary turns

Since $N_p$ is typically 1:
$I_s = I_p \times \frac{N_p}{N_s} = \frac{I_p}{N_s}$

The secondary current is inversely proportional to the number of secondary turns.

Why must a CT secondary never be open-circuited?
When a CT secondary is open-circuited while primary current flows, there is no secondary current to oppose the primary flux. This causes the core to saturate, inducing dangerously high voltages (thousands of volts) across the open secondary terminals. This can cause insulation failure, arcing, fire, and severe electric shock hazards.

Current Transformer Ratio and Calculations

CT Ratio Definition

The CT ratio is the ratio of primary current to secondary current at rated conditions:

$\text{CT Ratio} = \frac{I_{p(rated)}}{I_{s(rated)}}$

Standard CT Ratios:

100:5, 200:5, 400:5, 600:5, 800:5, 1000:5, 1200:5, 1600:5, 2000:5, 3000:5, 4000:5
Also available with 1A secondary: 100:1, 200:1, 400:1, etc.

Example:
A 400:5 CT means:

Rated primary current: 400A
Rated secondary current: 5A
Ratio: 400/5 = 80:1

Calculating Secondary Current

For a given primary current:

$I_s = I_p \times \frac{I_{s(rated)}}{I_{p(rated)}} = \frac{I_p}{\text{CT Ratio}}$

Example:
CT Ratio: 600:5 (ratio = 120)
Primary current: 300A

$I_s = \frac{300}{120} = 2.5\text{A}$

Calculating Primary Current from Secondary

When measuring with a CT:

$I_p = I_s \times \frac{I_{p(rated)}}{I_{s(rated)}} = I_s \times \text{CT Ratio}$

Example:
CT Ratio: 1000:5 (ratio = 200)
Measured secondary current: 3.2A

$I_p = 3.2 \times 200 = 640\text{A}$

Multi-Ratio CTs

Some CTs have multiple taps on the secondary winding, allowing different ratios from a single CT:

Example:
A CT with secondary taps at 50%, 75%, and 100%:

Full winding: 400:5
75% tap: 300:5
50% tap: 200:5

This provides flexibility for different load conditions or future expansion.

CT Accuracy Classes

Current transformers are classified by their accuracy, which defines how closely the secondary current matches the ideal proportional value.

Metering CTs

Metering CTs are designed for high accuracy at normal load currents (typically 5-120% of rated current). They are used for revenue metering and precision measurements.

Accuracy Class Designations (IEEE/ANSI):

0.1, 0.2, 0.3, 0.6, 1.2: The number indicates the maximum percentage ratio error at rated current.

Example:

Class 0.3 CT: Maximum error of ±0.3% at rated current
Used for revenue metering (billing)

Standard Accuracy Classes:

0.15S, 0.3S, 0.6S: “S” class CTs maintain accuracy over a wider range (1-120% of rated current)
Used for high-precision metering

Protection CTs

Protection CTs are designed to maintain accuracy during fault conditions when currents can be 10-20 times the rated value. Accuracy at normal load is less critical.

Accuracy Class Designations (IEEE/ANSI):

C (Calculated) Class: CTs with low leakage flux, accuracy can be calculated
T (Tested) Class: CTs with higher leakage flux, must be tested

Common Classes:

C100, C200, C400, C800: The number indicates the secondary terminal voltage the CT can deliver at 20 times rated secondary current without exceeding 10% ratio error.

Example:

C200 CT: Can deliver 200V to the burden at 100A secondary current (20 × 5A) without exceeding 10% error.

IEC Standard Classes:

5P10, 5P20, 10P10, 10P20:
First number (5 or 10): Maximum composite error percentage
P: Protection
Second number (10 or 20): Accuracy Limit Factor (ALF)

Example:

5P20: Maximum 5% error at 20 times rated current

What is the difference between metering and protection CTs?
Metering CTs are designed for high accuracy at normal load currents (5-120% of rated) and are used for billing and measurement. Protection CTs are designed to maintain accuracy during fault conditions (up to 20× rated current) and are used for protective relaying. Metering CTs saturate at high currents to protect instruments; protection CTs must NOT saturate during faults.

CT Burden and Saturation

CT Burden

Burden is the total impedance (resistance and reactance) connected to the CT secondary, including:

Resistance of connecting wires
Impedance of meters, relays, and instruments
Contact resistance of terminals

Burden is expressed in:

Volt-Amperes (VA) at rated secondary current
Ohms (Ω) impedance

Standard Burden Ratings (IEEE):

B-0.1, B-0.2, B-0.5, B-1.0, B-2.0, B-4.0 (in ohms)
Or: 2.5 VA, 5 VA, 10 VA, 15 VA, 20 VA, 30 VA

Calculating Burden:

$S_{burden} = I_s^2 \times Z_{burden}$

Where:

$S_{burden}$ = Burden in VA
$I_s$ = Secondary current (typically 5A)
$Z_{burden}$ = Total secondary impedance in ohms

Example:
CT secondary current: 5A
Total burden impedance: 0.4Ω

$S_{burden} = 5^2 \times 0.4 = 25 \times 0.4 = 10 \text{ VA}$

Effects of Excessive Burden

If the actual burden exceeds the CT’s rated burden:

Accuracy decreases: Ratio error increases
Phase angle error increases: Critical for power measurements
CT may saturate: Especially during faults
Protective relays may misoperate: False trips or failure to trip

CT Saturation

Saturation occurs when the CT core cannot support additional magnetic flux. This happens when:

Primary current is too high (severe fault)
Burden is too high (excessive impedance)
DC offset in fault current (asymmetrical faults)
Remanent flux in the core (residual magnetism)

Effects of Saturation:

Secondary current becomes distorted (flattened waveform)
Ratio error increases dramatically
Protective relays may not operate correctly
Metering becomes inaccurate

Preventing Saturation:

Use CTs with higher accuracy class (C400 vs C200)
Reduce burden (shorter wires, lower impedance devices)
Use CTs with larger core cross-section
Use air-gap cores for DC offset tolerance

CT Wiring and Connections

Polarity Markings

CTs have polarity markings to ensure correct connection:

Standard Markings:

H1, H2: Primary terminals
X1, X2: Secondary terminals

Polarity Convention:

When primary current enters H1, secondary current leaves X1
X1 is the “polarity” or “dot” terminal
Critical for:
Differential protection
Power and energy metering
Directional relaying

Polarity Test:
A simple DC test can verify polarity:

Connect a DC battery and switch to primary
Connect a DC voltmeter to secondary
Close the switch momentarily
If voltmeter kicks positive, polarity is correct (H1 to X1)

CT Secondary Connections

Never Open-Circuit a CT Secondary!

When a CT secondary is open while primary current flows:

No opposing secondary current
Core flux increases dramatically
Core saturates
Very high voltage induced (thousands of volts)
DANGER: Electric shock, insulation failure, arcing, fire

Safety Rules:

Always short-circuit the secondary before disconnecting instruments
Use shorting terminals or test switches
Never leave secondary open when primary is energized
Ground one side of the secondary (usually X2) for safety

Grounding Requirements

One Point Grounding:

CT secondary must be grounded at one point only
Prevents multiple ground loops
Usually grounded at the relay/meter panel
Prevents high voltage buildup

Multiple CTs:

If multiple CTs are connected (e.g., differential protection), ground at a single common point
Prevents circulating currents

Practical Applications

1. Revenue Metering

CTs are essential for billing high-current loads:

Residential services > 200A
Commercial and industrial services
Utility substations

Typical Setup:

CT ratio: 400:5 or 800:5
Accuracy class: 0.3S or 0.6S
Connected to watt-hour meters
Burden: < 2.5 VA

2. Protective Relaying

CTs provide current signals to protective relays:

Overcurrent Protection:

Detects excessive current
Trips circuit breaker
CT class: C100 or C200

Differential Protection:

Compares current entering and leaving a zone
Detects internal faults
Requires matched CTs with same ratio and accuracy
CT class: C200 or C400

Distance Protection:

Measures impedance to fault
Requires accurate current and voltage
CT class: C400 or higher

3. Ammeters and Indication

CTs provide scaled current for panel meters:

Analog ammeters (0-5A scale)
Digital meters
SCADA systems

Typical Setup:

CT ratio matched to ammeter scale
Accuracy class: 1.2 or B-0.5
Low burden requirement

4. Power Quality Monitoring

CTs provide current signals for:

Harmonic analysis
Power factor measurement
Event recording
Fault analysis

Requirements:

Wide bandwidth (for harmonics)
High accuracy
Low phase angle error

5. Ground Fault Detection

Zero-Sequence CTs (Core Balance CTs):

All three phase conductors pass through a single CT window
Under normal conditions, sum of currents = 0
During ground fault, unbalanced current flows
CT detects ground fault current
Very sensitive (can detect < 1A ground fault)

Practical Examples and Calculations

Example 1: CT Ratio Selection

Problem: A 480V industrial feeder has a maximum load current of 320A. Select an appropriate CT ratio for metering and calculate the secondary current at full load.

Solution:

Given:

Maximum load: 320A
Standard CT ratios available: 200:5, 300:5, 400:5, 600:5

Selection Criteria:

CT rated current should be ≥ maximum load current
For metering, optimal operation is 50-100% of rated current
Choose next standard size above maximum load

Selection:

320A load requires CT rated ≥ 320A
Choose 400:5 CT

Verification:

At full load (320A): $320/400 = 80\%$ of rated ✓
Good metering range

Secondary Current at Full Load:
CT Ratio = 400:5 = 80:1

$I_s = \frac{I_p}{\text{Ratio}} = \frac{320}{80} = 4.0\text{A}$

Result: Use 400:5 CT. At 320A load, secondary current is 4.0A.

Example 2: CT Burden Calculation

Problem: A 600:5 CT is connected to a relay with the following burden:

Relay impedance: 0.2Ω
Wire length: 50 feet (one way) of #12 AWG copper
Contact resistance: 0.05Ω

Calculate the total burden in VA and determine if it exceeds the CT rating of 10 VA.

Solution:

Given:

CT secondary current: 5A (rated)
Relay impedance: 0.2Ω
Wire: #12 AWG, 50 ft one way (100 ft total loop)
Contact resistance: 0.05Ω
CT rating: 10 VA

Step 1: Calculate Wire Resistance

12 AWG resistance: 1.588 Ω/1000 ft (at 75°C)

For 100 ft loop:
$R_{wire} = \frac{1.588 \times 100}{1000} = 0.1588 \Omega$

Step 2: Calculate Total Burden Impedance

$Z_{total} = R_{relay} + R_{wire} + R_{contact}$
$Z_{total} = 0.2 + 0.1588 + 0.05 = 0.4088 \Omega$

Step 3: Calculate Burden in VA

$S_{burden} = I_s^2 \times Z_{total}$
$S_{burden} = 5^2 \times 0.4088$
$S_{burden} = 25 \times 0.4088 = 10.22 \text{ VA}$

Step 4: Compare to CT Rating

Actual burden: 10.22 VA
CT rating: 10 VA

Result: The burden (10.22 VA) slightly exceeds the CT rating (10 VA).

Recommendations:

Use larger wire (#10 AWG) to reduce resistance
Or use a CT with higher burden rating (15 VA)
Or reduce wire length if possible

Example 3: CT Saturation Check

Problem: A C200 CT with a 400:5 ratio is supplying a fault current of 8000A primary. The total secondary burden is 2.0Ω. Will the CT saturate?

Solution:

Given:

CT class: C200
CT ratio: 400:5 = 80:1
Primary fault current: 8000A
Secondary burden: 2.0Ω

Step 1: Calculate Secondary Current

$I_s = \frac{I_p}{\text{Ratio}} = \frac{8000}{80} = 100\text{A}$

(This is 20 times the rated 5A secondary current)

Step 2: Calculate Required Secondary Voltage

$V_s = I_s \times Z_{burden}$
$V_s = 100 \times 2.0 = 200\text{V}$

Step 3: Check Against CT Class

C200 CT can deliver 200V at 20 times rated current (100A) without exceeding 10% error.

Required voltage: 200V
CT capability: 200V

Result: The CT is operating at its limit. It should NOT saturate, but it will be at the edge of the 10% accuracy limit. For better performance, use a C400 CT or reduce the burden.

Conclusion

Current transformers are essential components in electrical power systems, providing safe, isolated, and proportional current signals for metering, protection, and control. Understanding CT operation, ratios, accuracy classes, and safety requirements is critical for electrical engineers.

Key takeaways from this guide:

Construction: CTs have a single-turn (or few-turn) primary and multi-turn secondary, with standardized 5A or 1A secondary output.
Ratio: CT ratio = Primary rated current / Secondary rated current. Secondary current is proportional to primary current.
Accuracy Classes:

Metering CTs (0.1, 0.2, 0.3S): High accuracy at normal loads
Protection CTs (C100, C200, 5P20): Maintain accuracy during faults

Burden: Total impedance connected to CT secondary. Must not exceed CT rating to maintain accuracy.
Safety: NEVER open-circuit a CT secondary when primary is energized. Always short-circuit before disconnecting. Ground one point only.
Saturation: Occurs when core flux exceeds capacity. Causes distortion and relay misoperation. Prevented by proper CT selection and burden management.
Applications: Revenue metering, protective relaying, ammeters, power quality monitoring, ground fault detection.
Polarity: Critical for differential protection and power metering. Must be verified during installation.

Mastering current transformer principles enables engineers to design accurate metering systems, reliable protection schemes, and safe electrical installations. Whether selecting CTs for a new substation or troubleshooting a protection relay, understanding CT behavior is fundamental to power system engineering.