Current Transformer Secondary Burden Calculation

Introduction

In medium voltage protection systems, even a perfectly specified Current Transformer can fail to deliver reliable fault signals if the secondary burden is miscalculated. Secondary burden — the total impedance connected to the CT secondary terminals — directly determines whether your CT maintains accuracy during fault conditions, or saturates and sends corrupted signals to your protection relays. For electrical engineers designing MV protection schemes and procurement managers sourcing CTs for industrial substations or power grid feeders, an incorrect burden calculation is one of the most common yet most consequential specification errors in the field. This guide provides a structured, engineering-grade methodology for calculating CT secondary burden, covering every resistance component in the secondary loop, and translating that calculation into a correct CT specification per IEC 61869-2.

Table of Contents

• What Is CT Secondary Burden and What Does It Include?
• How Do You Calculate Total Secondary Burden Step by Step?
• How Does Secondary Burden Affect CT Selection for MV Protection?
• What Are the Most Common Burden Calculation Errors in Protection Circuits?

What Is CT Secondary Burden and What Does It Include?

CT secondary burden is the total impedance (expressed in VA or Ω) presented to the CT secondary winding by all connected devices and conductors in the secondary loop. It is not simply the relay coil impedance — it is the sum of every resistive and reactive element the secondary current must drive through.

Per IEC 61869-2¹, the rated burden (Sₙ) of a protection CT is defined at rated secondary current (typically 1A or 5A) and rated power factor (usually cos φ = 0.8). The CT must maintain its accuracy class up to this burden value. Exceed it, and the effective ALF drops — potentially below your system fault level requirement.

Components of CT Secondary Burden

The total secondary burden comprises four distinct elements:

• Relay Burden (S_relay): The VA consumption of all connected protection relays — overcurrent, earth fault, differential, distance. Modern numerical protection relays² typically consume 0.1–0.5VA per phase; electromechanical relays can consume 3–10VA
• Cable Burden (R_cable): Resistance of the secondary wiring between CT terminals and relay panel — often the largest single burden component in field installations
• Terminal Block and Connection Resistance (R_terminal): Small but non-negligible in long secondary chains; typically 0.01–0.05Ω per terminal block pair
• CT Secondary Winding Resistance (R_ct): Internal winding resistance of the CT itself — not part of the external burden but critical for ALF calculation; measured at 75°C per IEC standard

Key Technical Specifications to Confirm

• Rated Secondary Current: 1A or 5A — this choice dramatically affects cable burden (5A secondary produces 25× more cable voltage drop than 1A for same resistance)
• Insulation System: Epoxy resin cast, rated 12kV / 24kV / 36kV per IEC 61869
• Accuracy Class: 5P or 10P for protection circuits
• Rated Burden Range: Standard values — 2.5VA, 5VA, 10VA, 15VA, 30VA
• Operating Temperature: Class E (120°C) or Class F (155°C) — affects Rct correction factor

How Do You Calculate Total Secondary Burden Step by Step?

A rigorous secondary burden calculation follows a four-step process. Each step must be completed before CT specification is finalized — skipping any step introduces risk of under-specification.

Step 1: Determine Relay Burden

Obtain VA consumption from relay manufacturer datasheets for each connected device:

$S_{relay} = \sum_{i=1}^{n} S_{relay,i}$

Convert VA to resistance at rated secondary current:

$R_{relay} = \frac{S_{relay}}{I_{2n}^2}$

Example: Numerical overcurrent relay = 0.3VA, earth fault relay = 0.2VA, total = 0.5VA
At I₂ₙ = 5A: $R_{relay} = \frac{0.5}{25} = 0.02 , \Omega$
At I₂ₙ = 1A: $R_{relay} = \frac{0.5}{1} = 0.5 , \Omega$

Step 2: Calculate Cable Resistance

This is the most critical calculation step, especially for installations where CTs are located far from relay panels:

$R_{cable} = \frac{2 \times L \times \rho}{A}$

Where:

• $L$ = one-way cable length (meters)
• $\rho$ = resistivity of copper³ = 0.0175 Ω·mm²/m (at 20°C)
• $A$ = cable cross-sectional area (mm²)
• Factor 2 accounts for both outgoing and return conductors

Temperature correction to 75°C:

$R_{cable,75} = R_{cable,20} \times [1 + 0.00393 \times (75 – 20)]$

$R_{cable,75} = R_{cable,20} \times 1.216$

Example: 30m cable run, 2.5mm² copper:
$R_{cable,20} = \frac{2 \times 30 \times 0.0175}{2.5} = 0.42 , \Omega$
$R_{cable,75} = 0.42 \times 1.216 = 0.511 , \Omega$

Step 3: Add Terminal and Connection Resistance

For a typical secondary circuit with 6 terminal block pairs:

$R_{terminal} = 6 \times 0.03 = 0.18 , \Omega$

Step 4: Sum Total External Burden

$R_{burden,total} = R_{relay} + R_{cable,75} + R_{terminal}$

$R_{burden,total} = 0.02 + 0.511 + 0.018 = 0.549 , \Omega$

Convert to VA at rated secondary current:

$S_{burden,total} = R_{burden,total} \times I_{2n}^2 = 0.549 \times 25 = 13.7 , VA$

→ Specify CT rated burden ≥ 15VA (next standard value above 13.7VA)

Burden Comparison: 1A vs 5A Secondary

Parameter	1A Secondary	5A Secondary
Cable Resistance Impact	Low (I² effect minimal)	High (25× more VA loss)
Relay Burden (VA→Ω)	Higher Ω per VA	Lower Ω per VA
Recommended Cable Run	Up to 100m practical	Keep under 30m ideally
Standard Burden Rating	2.5VA–15VA typical	10VA–30VA typical
Core Size	Smaller	Larger
Application	Remote installations, long cable runs	Local panel installations

The key takeaway: For CT installations more than 20 meters from the relay panel, 1A secondary is strongly preferred — cable burden at 5A secondary can consume the entire rated VA budget before the relay even receives a signal.

Client Case — Power Grid EPC Contractor, 33kV Substation:
An EPC contractor in South Asia specified 5A secondary CTs for a 33kV outdoor substation where CT marshalling boxes were located 45 meters from the main relay panel. Initial burden calculation (relay only) showed 8VA — well within the 15VA rated burden. However, Bepto’s application engineer recalculated including cable resistance: 45m × 2.5mm² copper at 75°C added 1.23Ω = 30.7VA to the burden. Total burden exceeded 38VA — more than double the CT rating. The specification was revised to 1A secondary CTs with 15VA burden rating, resolving the issue before manufacturing. This single calculation prevented a complete protection system failure on a live grid feeder.

How Does Secondary Burden Affect CT Selection for MV Protection?

Once total secondary burden is calculated, it directly drives three CT specification parameters: rated burden class, accuracy class selection, and the verification of actual ALF against system fault level requirements.

Step 1: Select Rated Burden Class

Always select the next standard burden value above your calculated total burden:

• Calculated burden = 13.7VA → Specify 15VA
• Calculated burden = 22VA → Specify 30VA
• Never specify a CT with rated burden equal to calculated burden — this leaves zero margin

Step 2: Verify Actual ALF Against Fault Level

With rated burden selected, verify the actual ALF using:

$ALF_{actual} = ALF_{rated} \times \frac{R_{ct} + R_{burden,rated}}{R_{ct} + R_{burden,actual}}$

Ensure: $ALF_{actual} \geq \frac{I_{sc,max}}{I_{1n}} \times 1.1$

Step 3: Application-Specific Burden Recommendations

• Industrial MV Distribution (6–12kV): 5A secondary, 15VA, Class 5P20 — short cable runs in compact MCC panels
• Power Grid Substation (33–36kV): 1A secondary, 15VA, Class 5P30 — long cable runs to remote relay rooms
• Solar Farm MV Collection (33kV): 1A secondary, 10VA, Class 10P10 — lower fault levels, cost-optimized
• Urban Ring Main Unit (12kV): 1A secondary, 5VA, Class 5P20 — compact epoxy-cast CT, space-constrained
• Marine / Offshore Platform: 1A secondary, 10VA, Class 5P20, IP67 epoxy encapsulation — corrosive environment

Reliability Impact of Correct Burden Specification

• ✅ CT operates within linear region during fault → relay receives accurate fault current signal
• ✅ Protection relay trips within correct time-current characteristic
• ✅ Differential protection maintains stability on through-faults
• ✅ System reliability and uptime preserved across full fault level range
• ❌ Over-burdened CT saturates → relay under-reads fault current → delayed or failed trip
• ❌ Under-specified burden rating → effective ALF reduced → protection blind spot at high fault multiples

What Are the Most Common Burden Calculation Errors in Protection Circuits?

Installation and Verification Checklist

1. Measure actual cable length — use as-built drawings, not design estimates; field routing adds 15–25% to calculated length
2. Obtain relay burden from current datasheet — not from memory or previous project specs; relay models vary significantly
3. Apply temperature correction to Rct and cable resistance — always calculate at 75°C, not ambient
4. Account for all terminal blocks — especially in marshalling kiosks with multiple intermediate terminal strips
5. Verify with burden meter during commissioning — measure actual secondary loop impedance before energization
6. Check for parallel relay connections — multiple relays on same CT secondary reduce total burden but require individual verification

Common Errors That Cause Protection Failures

• Using relay nameplate VA without temperature correction — electromechanical relay coil resistance increases significantly at operating temperature
• Ignoring return conductor resistance — the factor of 2 in the cable formula is frequently omitted, halving the calculated cable burden
• Assuming numerical relay burden equals electromechanical relay burden — numerical relays consume 10–50× less VA; over-specifying burden wastes cost, but under-specifying for legacy relay replacements causes errors
• Failing to recalculate burden after relay panel relocation — cable length changes during construction are common and must trigger burden recalculation
• Specifying CT burden based on relay room distance only — forgetting intermediate junction boxes, marshalling kiosks, and test terminal blocks

Client Case — Procurement Manager, Industrial Petrochemical Plant:
A procurement manager at a petrochemical facility in the Middle East ordered replacement CTs based on the original 1995 project specification — 5A secondary, 15VA, Class 5P20. The relay panel had been relocated during a 2018 plant expansion, extending cable runs from 12m to 38m. Nobody recalculated the burden. After CT replacement, the overcurrent protection on a 11kV motor feeder failed to trip during a phase-to-phase fault, causing motor winding damage. Post-incident analysis revealed the actual burden was 28.4VA — nearly double the 15VA CT rating. Bepto now provides free burden calculation review as part of CT replacement consultation, ensuring specification accuracy before any order is placed.

Conclusion

CT secondary burden calculation is not a formality — it is a foundational engineering step that determines whether your entire MV protection scheme functions correctly under fault conditions. By systematically accounting for relay burden, cable resistance at operating temperature, terminal block resistance, and verifying the result against CT rated burden and ALF requirements, engineers ensure that Current Transformers deliver accurate, reliable signals when the power system needs protection most. For medium voltage power distribution, substations, and industrial installations, correct burden specification is the bedrock of protection reliability.

FAQs About CT Secondary Burden Calculation

1. Official international standard for current transformer performance and accuracy criteria. ↩

2. Modern digital devices with significantly lower VA consumption compared to legacy electromechanical models. ↩

3. Standard physical constant used to calculate voltage drop and power loss in secondary cabling. ↩

4. Technical parameter that determines the CT’s ability to maintain accuracy during high fault currents. ↩