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Current Transformer Secondary Wiring & Burden Calculation Guide: Circuit Design, Voltage Drop & Compliance per IEC 61869-2
Meta Description: Comprehensive guide on current transformer secondary wiring design and burden calculation. Covers cable sizing, voltage drop analysis, burden matching, multi-point grounding hazards, terminal connection standards, and compliance with IEC 61869-2 and IEEE C57.13. Includes practical calculation examples for metering and protection circuits.
1. Introduction
The secondary circuit of a current transformer (CT) is the critical link between the primary power system and the protective relays or metering instruments. Poor secondary wiring design can lead to:
- Excessive burden → CT saturation, protection maloperation
- Voltage drop → Metering errors, relay under-reach
- Open circuits → Dangerous high voltages, equipment damage
- Multi-point grounding → Circulating currents, false tripping
This guide provides a systematic approach to CT secondary circuit design, burden calculation, cable sizing, and wiring best practices, aligned with IEC 61869-2:2012, IEEE C57.13-2016, and IEC 60364 standards.
2. CT Secondary Circuit Fundamentals
2.1 Equivalent Circuit
The CT secondary circuit can be modeled as:
┌─────────────────────────────────────┐
│ │
│ ┌─────┐ R_ct Z_cable Z_relay│
Primary│──┤ CT ├───/\/\/\───/\/\/\───┤│││││──┤
│ └─────┘ │ │ └───┘ │
│ │ │ │
│ I_s I_s I_s
│ │ │ │
│ ┌─────────────────────────────────┐│
│ │ Excitation ││
│ │ Impedance (Z_e) ││
│ └─────────────────────────────────┘│
│ │
└─────────────────────────────────────┘
Key Parameters:
– R_ct = CT secondary winding resistance
– Z_cable = Cable impedance (resistance + reactance)
– Z_relay = Relay/meter burden impedance
– I_s = Secondary current (5A or 1A)
2.2 Total Burden Definition
The total burden seen by the CT secondary is:
Z_total = R_ct + Z_cable + Z_relay
In VA terms:
VA_total = I_s² × |Z_total|
Acceptance Criteria:
VA_total ≤ VA_rated (CT nameplate burden)
3. Burden Calculation Methodology
3.1 Step-by-Step Calculation
Step 1: Identify All Burden Components
| Component | Typical Burden (VA) | Power Factor | Reference |
|---|---|---|---|
| Overcurrent Relay | 0.1-0.5 VA @ 5A | 0.9-1.0 | Relay datasheet |
| Distance Relay | 0.5-2.0 VA @ 5A | 0.8-0.9 | Relay datasheet |
| Differential Relay | 0.5-3.0 VA @ 5A | 0.8-0.9 | Relay datasheet |
| Digital Meter | 0.1-0.3 VA @ 5A | 1.0 | Meter datasheet |
| Analog Ammeter | 1.0-5.0 VA @ 5A | 1.0 | Meter datasheet |
| Power Transducer | 0.5-1.5 VA @ 5A | 0.8 | Transducer datasheet |
| SCADA Input | 0.1-0.2 VA @ 5A | 1.0 | RTU datasheet |
Step 2: Calculate Cable Impedance
Cable Resistance:
R_cable = (2 × L × ρ) / A
Where:
– L = One-way cable length (m)
– ρ = Resistivity of copper = 0.0175 Ω·mm²/m at 20°C
– A = Cable cross-sectional area (mm²)
– Factor of 2 accounts for go-and-return conductors
Cable Reactance:
For control cables, reactance is typically negligible (0.05-0.1 Ω/km). For long runs (>100m), include:
X_cable ≈ 0.08 Ω/km × L (km)
Cable Impedance:
Z_cable = √(R_cable² + X_cable²)
Step 3: Calculate CT Winding Resistance
Obtain from CT nameplate or factory test report:
R_ct = V_knee / I_excitation_at_knee (approximately)
Or measure directly with micro-ohmmeter.
Step 4: Calculate Total Burden
Z_total = R_ct + Z_cable + ΣZ_relay
VA_total = I_s² × |Z_total|
3.2 Calculation Example
System Parameters:
– CT: 1000/5A, 5P20, rated burden 15 VA, R_ct = 0.8 Ω
– Cable: 150m one-way, Cu, 2.5 mm²
– Relay: Numerical overcurrent relay, 0.3 VA @ 5A, PF=1.0
Calculation:
1. Cable Resistance:
R_cable = (2 × 150 × 0.0175) / 2.5 = 2.1 Ω
2. Total Resistance:
R_total = R_ct + R_cable = 0.8 + 2.1 = 2.9 Ω
3. Relay Impedance:
Z_relay = VA_relay / I_s² = 0.3 / 25 = 0.012 Ω
4. Total Impedance:
Z_total ≈ 2.9 + 0.012 = 2.912 Ω
5. Total Burden:
VA_total = 5² × 2.912 = 72.8 VA
6. Result:
72.8 VA >> 15 VA (Rated) → CT SATURATION LIKELY!
Solution:
– Use 1A secondary CT → VA_total = 1² × 2.912 = 2.9 VA ✓
– Or increase cable size to 6 mm² → R_cable = 0.875 Ω → VA_total = 5² × (0.8+0.875+0.012) = 41.3 VA (still high)
– Or reduce cable length by relocating relay panel
4. Cable Sizing Guidelines
4.1 Standard Cable Sizes
| Cross-Section (mm²) | Resistance (Ω/km) @ 20°C | Typical Application |
|---|---|---|
| 1.0 | 17.5 | Low-burden digital relays, short runs |
| 1.5 | 11.7 | General protection, medium runs |
| 2.5 | 7.0 | Standard protection, long runs |
| 4.0 | 4.4 | High-burden applications, very long runs |
| 6.0 | 2.9 | Critical protection, extreme runs |
4.2 Minimum Cable Size Requirements
| Standard | Minimum Size | Application |
|---|---|---|
| IEC 60364-5-54 | 1.5 mm² Cu | General control circuits |
| IEEE 80 | 2.5 mm² Cu | Substation control cables |
| IEC 61869-2 | Per burden calculation | CT secondary circuits |
| Local Utility Spec | Often 2.5-4.0 mm² | Varies by utility |
4.3 Cable Selection Decision Tree
Calculate required cable size based on burden:
│
├── If A_calc ≤ 1.5 mm² → Use 1.5 mm² (minimum per IEC)
├── If 1.5 < A_calc ≤ 2.5 mm² → Use 2.5 mm²
├── If 2.5 < A_calc ≤ 4.0 mm² → Use 4.0 mm²
└── If A_calc > 4.0 mm² → Consider 1A secondary CT
5. Secondary Circuit Wiring Practices
5.1 Wiring Diagram Standards
Common CT Secondary Wiring Configurations:
5.1.1 Single-Phase CT (3-wire)
P1 ───────────────────────── P2 (Primary)
│
S1 ────┬──── Relay ────┬──── Ammeter ────┐
│ │ │
└──── Ground ───┘ │
│
S2 ──────────────────────────────────────┘
│
└──── Ground (Single point!)
5.1.2 Differential Protection (3-phase CTs)
CT-A S1 ───┐
CT-B S1 ───┼── Relay (Differential) ─── Ground
CT-C S1 ───┘
CT-A S2 ───┐
CT-B S2 ───┼── Common Return ─────────── Ground
CT-C S2 ───┘
5.1.3 Star Connection (3-phase Metering)
CT-A S1 ─── Phase A ───┐
CT-B S1 ─── Phase B ───┼── Relay/Meter
CT-C S1 ─── Phase C ───┘
CT-A S2 ───┐
CT-B S2 ───┼── Star Point ─── Ground (Single point)
CT-C S2 ───┘
5.2 Terminal Connection Standards
5.2.1 Terminal Block Requirements
| Parameter | Requirement | Standard |
|---|---|---|
| Type | Test-links / Disconnect terminals | IEC 60947-7-1 |
| Rating | ≥ CT secondary current (5A or 1A) | – |
| Shorting Capability | Must allow safe CT shorting | IEEE C57.13 |
| Material | Copper or brass, tin-plated | – |
| Wire Range | 0.5-4.0 mm² | – |
5.2.2 Test Terminal Functions
┌─────────────────────────────────┐
│ ┌───┐ ┌───┐ ┌───┐ │
│ │ C │────│ T │────│ M │ │
│ └───┘ └───┘ └───┘ │
│ CT Test Meter/Relay │
│ │
│ C = CT connection │
│ T = Test link (shorting) │
│ M = Meter/Relay connection │
└─────────────────────────────────┘
Operation Sequence for Testing:
1. Close test link (short CT secondary)
2. Disconnect meter/relay
3. Perform test
4. Reconnect meter/relay
5. Open test link
5.3 Cable Routing and Separation
| Parameter | Requirement | Reason |
|---|---|---|
| CT cable separation from AC power | ≥ 150 mm | Reduce electromagnetic interference |
| CT cable separation from DC control | ≥ 50 mm | Prevent interference |
| CT cable in tray | Dedicated layer or divider | Prevent cross-interference |
| CT cable bending radius | ≥ 6× cable diameter | Prevent conductor damage |
| CT cable support spacing | ≤ 300 mm horizontal, ≤ 400 mm vertical | Prevent mechanical stress |
6. Grounding Requirements
6.1 Single-Point Grounding Principle
CRITICAL RULE: CT secondary circuits must be grounded at ONE point only.
Correct: Incorrect:
CT S1 ─── Relay ───┐ CT S1 ─── Relay ───┐
│ │
├── Ground (at relay panel) ├── Ground 1
│ │
CT S2 ─────────────┘ ├── Ground 2
│
Circulating current!
6.2 Grounding Location
| Application | Grounding Point | Reason |
|---|---|---|
| Protection CTs | Relay panel (protection room) | Safety, reference potential |
| Metering CTs | Relay panel or metering panel | Consistent reference |
| Busbar Protection | One CT only (per zone) | Prevent circulating currents |
| Differential Protection | Relay panel (star point) | Common reference for all CTs |
6.3 Multi-Point Grounding Hazards
Causes:
– Accidental ground at CT terminal box AND relay panel
– Cable damage causing ground fault
– Incorrect wiring during installation
Effects:
Fault Current in Primary System
│
├── Ground 1 (CT location) ───┐
│ │
├── Ground 2 (Relay panel) ───┤
│ │
└── Circulating Current (I_circ) flows in CT secondary!
│
├── Relay sees false current → False tripping!
└── CT saturates → Protection failure!
Detection:
– Measure current in ground wire with clamp meter
– Check for voltage between ground points
– Insulation resistance test to detect accidental grounds
7. Voltage Drop Analysis
7.1 CT Secondary Voltage Calculation
The voltage developed across the CT secondary is:
V_s = I_s × Z_total
Maximum Allowable Voltage:
– Limited by CT insulation class
– Typically < 500V for indoor CTs
– Typically < 1000V for outdoor CTs
7.2 Relay Voltage Requirements
| Relay Type | Minimum Operating Voltage | Typical Burden |
|---|---|---|
| Overcurrent | 0.1-0.5 V @ 5A | 0.1-0.5 VA |
| Distance | 1.0-5.0 V @ 5A | 0.5-2.0 VA |
| Differential | 0.5-2.0 V @ 5A | 0.5-3.0 VA |
| Directional | 1.0-5.0 V @ 5A | 1.0-5.0 VA |
7.3 Voltage Drop in Long Cables
Problem: Long cable runs cause significant voltage drop, reducing voltage available at relay terminals.
V_ct = I_s × (R_ct + R_cable + Z_relay)
V_relay = I_s × Z_relay
Voltage Drop Percentage:
% Drop = (V_ct - V_relay) / V_ct × 100%
= (R_ct + R_cable) / (R_ct + R_cable + Z_relay) × 100%
Example:
R_ct = 0.8 Ω
R_cable = 2.1 Ω (150m, 2.5mm²)
Z_relay = 0.012 Ω
% Drop = (0.8 + 2.1) / (0.8 + 2.1 + 0.012) × 100% = 99.4%
Result: 99.4% of CT voltage is dropped in cable and CT winding!
Only 0.6% reaches the relay → Relay may not operate!
Solution: Use 1A secondary CT → Cable current reduced by 5× → Cable burden reduced by 25×.
8. 1A vs. 5A Secondary Current Selection
8.1 Comparison
| Parameter | 5A Secondary | 1A Secondary |
|---|---|---|
| Cable Burden | I²R = 25R | I²R = 1R (25× lower) |
| Max Cable Length | Short (typically < 50m) | Long (typically < 250m) |
| CT Core Size | Smaller | Larger (more turns) |
| Open Circuit Voltage | Lower | Higher (dangerous) |
| Relay Compatibility | Universal | Check relay input rating |
| Cost | Lower CT cost, higher cable cost | Higher CT cost, lower cable cost |
8.2 Selection Guidelines
| Application | Recommended Secondary Current |
|---|---|
| Short cable runs (< 30m) | 5A |
| Medium cable runs (30-100m) | 5A or 1A (calculate burden) |
| Long cable runs (> 100m) | 1A |
| Digital substations (IEC 61850) | 1A (typically) |
| High-burden applications | 1A |
8.3 Cable Length Comparison
For 15 VA burden, R_ct = 0.5 Ω:
| Secondary Current | Max Cable Resistance | Max Cable Length (2.5mm²) | Max Cable Length (4.0mm²) |
|---|---|---|---|
| 5A | 1.0 Ω | 71 m | 114 m |
| 1A | 14.5 Ω | 1035 m | 1657 m |
9. Special Wiring Considerations
9.1 Summation CTs
Application: Combine three-phase currents into a single residual current for earth-fault protection.
CT-A S1 ───┐
CT-B S1 ───┼── Summation CT Primary ─── Relay
CT-C S1 ───┘
All S2 ───── Common Return ─── Ground
Burden Calculation:
– Summation CT burden = 3 × individual CT burden (approximately)
– Verify summation CT rating matches total burden
9.2 CTs with Multiple Secondary Windings
Common Configurations:
| Winding 1 | Winding 2 | Application |
|---|---|---|
| 0.5 (Metering) | 5P20 (Protection) | Separate metering and protection |
| 5P20 (Protection) | 5P20 (Protection) | Redundant protection systems |
| 0.2S (Revenue) | 0.5 (General) | Dual metering |
Wiring Rules:
– Each winding must be grounded separately
– Windings must not be paralleled
– Each winding has independent burden calculation
9.3 CT Ratio Change Wiring
Tapped CTs:
S1 ───┬── 200A tap
├── 400A tap
└── 600A tap (full winding)
S2 ─────────────────
Rules:
– Use only one tap at a time
– Unused taps must be left open (NOT shorted)
– Verify ratio matches relay settings
10. Testing and Commissioning of Secondary Circuits
10.1 Pre-Energization Tests
| Test | Method | Acceptance Criteria |
|---|---|---|
| Continuity Test | Low-resistance ohmmeter | Resistance ≈ calculated cable resistance |
| Insulation Test | 500V or 1000V Megger | > 1 MΩ (all conductors to ground) |
| Polarity Test | Battery and voltmeter | Correct polarity at all terminals |
| Shorting Test | Verify test terminals | CT can be safely shorted |
| Grounding Check | Verify single-point ground | Only one ground point exists |
10.2 Energization Tests
| Test | Method | Acceptance Criteria |
|---|---|---|
| Secondary Current Check | Clamp meter on secondary cable | Matches primary current / ratio |
| Phase Sequence Check | Phase sequence relay or meter | Correct phase sequence |
| Burden Measurement | Measure voltage and current | VA ≤ rated burden |
| Vector Check | Measure phase angles under load | Angles match system power factor |
10.3 Common Wiring Errors
| Error | Symptom | Detection Method |
|---|---|---|
| Open circuit | No secondary current, high voltage | Visual inspection, continuity test |
| Reversed polarity | Reverse power flow, relay maloperation | Polarity test, vector check |
| Multi-point ground | Circulating current, false tripping | Clamp meter on ground wire |
| Wrong tap | Incorrect ratio | Ratio test |
| High burden | CT saturation, relay under-reach | Burden measurement |
11. Standards and References
11.1 IEC Standards
| Standard | Title | Relevant Sections |
|---|---|---|
| IEC 61869-2 | Current Transformers | §6.2 (Tests), §7 (Rating) |
| IEC 60364-5-54 | Electrical Installations – Protection against Electric Shock | §543 (Grounding) |
| IEC 60228 | Conductors of Insulated Cables | §2 (Resistance) |
| IEC 60947-7-1 | Terminal Blocks | General requirements |
11.2 IEEE Standards
| Standard | Title | Relevant Sections |
|---|---|---|
| IEEE C57.13 | Instrument Transformers | §3.4 (Burden), §5.2 (Wiring) |
| IEEE 80 | Substation Grounding | §6 (Grounding practices) |
| IEEE 400 | Shielded Power Cable Testing | §4 (Insulation testing) |
12. Engineering FAQ
Q1: How do I calculate the maximum cable length for a 5A CT secondary?
A:
1. Determine CT rated burden (VA_rated) and winding resistance (R_ct)
2. Determine relay burden (VA_relay)
3. Calculate available cable burden: VA_cable = VA_rated – VA_relay – I_s²×R_ct
4. Calculate max cable resistance: R_cable_max = VA_cable / I_s²
5. Calculate max length: L_max = (R_cable_max × A) / (2 × ρ)
Q2: Can I use 1.5 mm² cable for CT secondary circuits?
A: Per IEC 60364-5-54, 1.5 mm² is the minimum for control circuits. However, for CT secondaries:
– 1.5 mm² is acceptable only for short runs (< 20m) with low burden (digital relays)
– 2.5 mm² is the practical minimum for most protection applications
– 4.0 mm² is recommended for long runs or high-burden applications
Q3: What happens if the CT burden exceeds the rated value?
A: Excessive burden causes:
– CT saturation → Secondary current waveform clipped
– Ratio error → Metering inaccuracy, relay under-reach
– Phase error → Directional relay maloperation
– Overheating → Insulation degradation
– High voltage → Insulation stress, safety hazard
Q4: Why must CT secondaries be grounded at only one point?
A: Multi-point grounding creates a ground loop. During ground faults, earth potential rise causes circulating current in the CT secondary circuit. This current:
– Adds to or subtracts from the true secondary current
– Causes relay maloperation (false trip or failure to trip)
– Can saturate the CT core
Q5: How do I verify correct CT wiring after installation?
A: Perform these tests:
1. Continuity test: Verify all connections
2. Insulation test: Verify no ground faults
3. Polarity test: Verify correct polarity
4. Ratio test: Verify transformation ratio
5. Vector check: Measure phase angles under load
6. Primary injection test: Verify relay operation with actual fault current
13. Conclusion
Proper CT secondary wiring and burden calculation are essential for accurate metering and reliable protection. The secondary circuit is often overlooked in design, leading to CT saturation, relay maloperation, and safety hazards.
Key design principles:
– Calculate burden rigorously: Include CT winding, cable, and all relay burdens
– Select appropriate cable size: 2.5 mm² minimum, 4.0 mm² for long runs
– Consider 1A secondary: For cable runs > 100m or high-burden applications
– Ground at one point only: Prevent circulating currents and false tripping
– Use test terminals: Enable safe testing and maintenance
– Verify with commissioning tests: Continuity, insulation, polarity, vector check
Design checklist:
☐ Burden calculation completed (CT + cable + relay)
☐ Cable size selected (≥ calculated, ≥ 2.5 mm²)
☐ Secondary current selected (5A or 1A)
☐ Single-point grounding location identified
☐ Test terminals specified
☐ Wiring diagram reviewed and approved
☐ Commissioning test procedure prepared
Technical Reference: IEC 61869-2:2012, IEEE C57.13-2016, IEC 60364-5-54, IEEE 80-2013
Product Reference: Duomatech LZZBJ9 series (cast-resin CTs), LJWD series (oil-immersed CTs) — all designed for standard secondary burdens