Instrument Transformer Burden Calculation Guide for MV Protection Systems

Introduction

Burden calculation is one of the most frequently misunderstood — and most consequential — engineering tasks in medium voltage protection system design. Every device connected to a CT or VT secondary circuit adds impedance, and when the total burden exceeds the transformer’s rated VA, accuracy degrades, cores saturate, and protection relays receive distorted signals that can cause dangerous misoperations.

The direct answer: instrument transformer burden is the total Volt-Amp load imposed on the secondary circuit, and it must always remain within the transformer’s rated burden to guarantee accuracy class compliance and reliable fault detection.

For electrical engineers and EPC contractors specifying MV switchgear, getting burden wrong is not a minor calibration issue — it is a system-level reliability failure waiting to happen. This guide walks through the complete burden calculation methodology, common pitfalls, and selection criteria to ensure your CT and VT installations perform exactly as designed.

Table of Contents

• What Is Instrument Transformer Burden and How Is It Defined?
• How Do You Calculate CT and VT Burden Step by Step?
• How Does Burden Affect CT Accuracy Class and Protection Performance?
• What Are the Most Common Burden Calculation Mistakes in MV Systems?

What Is Instrument Transformer Burden and How Is It Defined?

Burden is the total external impedance — expressed in Volt-Amps (VA) or Ohms (Ω) — connected to the secondary terminals of an instrument transformer. It represents the sum of all loads the transformer must drive while maintaining its rated accuracy. For a CT, this includes every device and conductor in the secondary loop. For a VT, it includes all connected measuring and protection equipment in parallel.

Understanding burden begins with understanding the two ways it is expressed:

• VA Burden: Total apparent power consumed by the secondary circuit at rated secondary current or voltage
• Impedance Burden (Ω): Total resistance and reactance of the secondary circuit, used in detailed calculations

Key technical parameters governing CT burden per IEC 61869-2¹:

• Rated Burden: The maximum VA the CT can supply while maintaining stated accuracy class (e.g., 15VA, 30VA)
• Rated secondary current²: Standard values of 1A or 5A — burden impedance scales with the square of this value
• Accuracy Class: 0.2, 0.5 for metering; 5P, 10P for protection — each has a defined burden range
• Power Factor of Burden: Typically 0.8 lagging for protection class; 1.0 for resistive loads
• Rated Accuracy Limit Factor (ALF³): Inversely proportional to actual burden — increases as burden decreases
• Insulation Level: 12kV / 24kV / 36kV class for MV applications
• Thermal Continuous Current Rating: ≥1.2× rated primary current
• Creepage Distance: ≥25mm/kV for standard indoor environments (IEC 60815)

A critical but often overlooked point: burden is not fixed by the relay alone. Secondary cable resistance, terminal contact resistance, and the combined impedance of all series-connected devices all contribute. Ignoring cable burden is the single most common cause of accuracy class violations in field installations.

How Do You Calculate CT and VT Burden Step by Step?

Burden calculation follows a structured process. Here is the complete methodology used for MV protection and metering CT circuits.

Step 1: List All Secondary Circuit Devices

Identify every device connected in the CT secondary loop:

• Protection relay (distance, overcurrent, differential)
• Energy meter or power quality analyzer
• Transducer or transmitter
• Ammeter (if applicable)
• Interposing CT (if applicable)

Step 2: Obtain VA or Impedance Rating for Each Device

Each device manufacturer provides a burden rating at rated secondary current. Convert all values to impedance (Ω) using:

$Z = \frac{VA}{I_s^2}$

Where $I_s$ is the rated secondary current (1A or 5A).

Example — 5A secondary circuit:

Device	Rated Burden (VA)	Impedance (Ω)
Distance Protection Relay	1.0 VA	0.040 Ω
Overcurrent Relay	0.5 VA	0.020 Ω
Energy Meter	1.5 VA	0.060 Ω
Secondary Cable (2× 30m, 2.5mm²)	—	0.432 Ω
Terminal Contact Resistance	—	0.010 Ω
Total Burden	—	0.562 Ω

Convert total impedance back to VA: $VA_{total} = Z_{total} \times I_s^2 = 0.562 \times 25 = 14.05\ VA$

Step 3: Calculate Cable Burden

Cable resistance is calculated as:

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

Where:

• $L$ = one-way cable length (meters)
• $rho$= resistivity of copper =$0.0172\ \Omega \cdot mm^2/m$
• $A$ = cable cross-sectional area (mm²)

For 30m one-way run with 2.5mm² copper: $R_{cable} = \frac{2 \times 30 \times 0.0172}{2.5} = 0.413\ \Omega$

Step 4: Verify Against Rated Burden

Total calculated burden must satisfy: $VA_{actual} \leq VA_{rated}$

If actual burden exceeds rated burden, options include:

• Increase cable cross-section (reduces resistance burden)
• Specify higher rated burden CT
• Reduce number of series-connected devices
• Switch from 5A to 1A secondary (reduces cable burden by factor of 25)

Step 5: Verify Effective ALF

Actual ALF changes with burden. The relationship per IEC 61869-2 is:

$ALF_{actual} = ALF_{rated} \times \frac{VA_{rated} + VA_{internal}}{VA_{actual} + VA_{internal}}$

Where $VA_{internal}$ is the CT’s own internal winding burden (from datasheet). This step is critical for distance protection⁴ and differential protection applications.

CT vs VT Burden Calculation Comparison

Parameter	CT Burden Calculation	VT Burden Calculation
Circuit Topology	Series loop	Parallel connection
Burden Expression	VA or Ω (series impedance)	VA or Ω (parallel impedance)
Cable Impact	High — series resistance adds directly	Low — parallel loads dominate
Secondary Standard	1A or 5A	100V or 110V
Key Risk	Core saturation from excess burden	Voltage drop and accuracy loss
Governing Standard	IEC 61869-2	IEC 61869-3

Customer Case — Burden Miscalculation in a 33kV Feeder Protection Panel:
A procurement manager at an EPC firm in North Africa reached out after their newly commissioned 33kV feeder protection system showed persistent accuracy errors on energy metering — readings were consistently 3–4% low. Investigation revealed the secondary cable run was 45 meters (longer than the original design assumed at 20 meters), adding 0.62Ω of unaccounted resistance burden. The installed CT was rated 15VA but actual burden reached 22VA, pushing the CT outside its 0.5 accuracy class range. Bepto supplied replacement 30VA-rated CTs with matched specifications, and metering accuracy returned to within 0.2% — well within billing-grade requirements.

How Does Burden Affect CT Accuracy Class and Protection Performance?

The relationship between burden and CT performance is not linear — it is a threshold effect. Within rated burden, the CT maintains its stated accuracy class. Beyond rated burden, errors compound rapidly, and under fault conditions, core saturation⁵ occurs earlier than the ALF specification assumes.

For distance protection specifically, this has direct operational consequences:

• Under-burden: Effective ALF increases — generally beneficial, but relay input impedance must still be met
• At rated burden: CT performs exactly per accuracy class specification
• Over-burden (110–150% rated): Composite error exceeds class limit; metering reads incorrectly
• Severe over-burden (>150% rated): Core saturates during fault conditions; protection relay receives clipped waveform; impedance calculation fails; distance relay may not trip Zone 1

Impact on Protection Reliability by Burden Level

Burden Level	Metering Accuracy	Protection CT Behavior	Distance Relay Response
<80% Rated	Within class	ALF effectively higher	Reliable Zone 1 trip
80–100% Rated	Within class	Per specification	Reliable Zone 1 trip
100–130% Rated	Marginal error	Reduced effective ALF	Possible Zone 1 delay
>150% Rated	Significant error	Early saturation	Misoperation risk

The practical recommendation for protection-critical applications: design to 75–80% of rated burden maximum, preserving margin for future relay additions or cable re-routing that increases resistance.

Customer Case — Protection Misoperation Traced to Excess Burden:
A power utility contractor in Southeast Asia reported that a 22kV overhead line distance relay was consistently failing to clear close-in faults within Zone 1 time, defaulting to Zone 2 (400ms delay). Detailed commissioning analysis revealed the CT secondary circuit included three relays, a transducer, and a 38-meter cable run — total burden of 28VA against a 15VA-rated CT. The CT was saturating at approximately 8× rated current, well below the 5P20 specification’s implied 20× capability at rated burden. Replacing with Bepto 5P20 30VA CTs resolved the Zone 1 timing issue completely.

What Are the Most Common Burden Calculation Mistakes in MV Systems?

Installation and Commissioning Checklist

1. Measure actual cable length — never use design drawing estimates for burden calculation
2. Measure conductor resistance with a low-resistance ohmmeter before energization
3. Verify each relay’s actual input burden from manufacturer datasheet — not catalog summaries
4. Calculate total burden at rated secondary current before specifying CT VA rating
5. Perform secondary injection test to verify CT ratio, polarity, and accuracy at commissioning
6. Document as-built burden for future maintenance reference

Common Mistakes That Compromise Reliability

• Ignoring cable burden: In 5A secondary circuits, a 30m cable run can contribute 8–15VA — often exceeding relay burden
• Mixing 1A and 5A devices: Connecting a 5A-rated relay to a 1A CT secondary causes severe over-burden and potential relay damage
• Assuming relay burden equals total burden: Forgetting meters, transducers, and terminal resistance is extremely common
• Not recalculating ALF after burden changes: Adding a relay during a system upgrade without rechecking effective ALF is a hidden protection risk
• Using VT burden calculation method for CTs: Series vs. parallel topology — the calculation approach is fundamentally different
• Neglecting temperature effects: Copper resistance increases approximately 0.4% per °C — in high-ambient installations, cable burden at 60°C is measurably higher than at 20°C

Conclusion

Accurate burden calculation is not an optional engineering refinement — it is a fundamental requirement for instrument transformer accuracy class compliance and protection system reliability in medium voltage power distribution. The core takeaway: always calculate total secondary burden including cable resistance, verify effective ALF for protection applications, and design to a maximum of 75–80% of rated CT burden to maintain reliable fault detection. At Bepto Electric, every CT we supply includes full datasheet burden specifications and internal winding resistance values — giving your engineering team everything needed to perform accurate burden calculations from day one.

FAQs About Instrument Transformer Burden Calculation

1. Learn more about the international standard governing current transformer requirements ↩

2. Understand how selecting secondary output levels impacts system burden ↩

3. Identify how saturation limits affect the accuracy of protection transformers ↩

4. Explore how calculated impedance identifies fault locations in distribution lines ↩

5. Prevent signal distortion caused by transformer core magnetic limitations ↩