CT Accuracy Limiting Factor Calculation Guide

Introduction

In medium voltage power distribution systems, a Current Transformer (CT) doesn’t just measure current — it must maintain measurement integrity even when fault currents surge to 10, 20, or even 30 times the rated value. That’s where the Accuracy Limiting Factor (ALF) becomes mission-critical. The ALF defines the maximum multiple of rated primary current up to which a CT maintains its rated accuracy class, directly determining whether your protection relay receives a trustworthy signal during a fault event. For electrical engineers designing protection schemes, and for procurement managers specifying CTs for substation or industrial MV panels, misunderstanding or miscalculating ALF leads to relay misoperation, equipment damage, and costly downtime. This guide breaks down the ALF calculation methodology, the key parameters involved, and how to select the right CT for your protection reliability requirements.

Table of Contents

• What Is the CT Accuracy Limiting Factor and Why Does It Matter?
• How Is ALF Calculated? Core Formula and Parameters Explained
• How to Select the Right ALF for Your Application?
• What Are the Common Mistakes in ALF Specification and Installation?

What Is the CT Accuracy Limiting Factor and Why Does It Matter?

The Accuracy Limiting Factor (ALF) is a dimensionless parameter defined under IEC 61869-2¹ that specifies the highest multiple of rated primary current at which the CT composite error² does not exceed the prescribed limit for its accuracy class. In simpler terms: it tells you how far into a fault condition your CT can still be trusted.

For protection-class CTs (Class 5P and 10P per IEC standard), the composite error at ALF must not exceed 5% or 10% respectively. Beyond the ALF threshold, the CT core saturates, the secondary current becomes distorted, and protection relays may fail to trip — or worse, trip incorrectly.

Key Technical Parameters Defined

• Rated Primary Current (I₁ₙ): Nominal operating current, e.g., 400A, 600A, 1200A
• Rated Burden (Sₙ): The rated VA load the CT is designed to drive, e.g., 15VA, 30VA
• Accuracy Class: 5P or 10P for protection CTs; defines allowable composite error
• ALF (Accuracy Limiting Factor): Typically 5, 10, 20, or 30 — stamped on nameplate
• Instrument Security Factor (FS): Relevant for measuring CTs; opposite concept to ALF
• Core Material: Cold-rolled grain-oriented silicon steel³ (CRGO) — determines saturation behavior
• Insulation System: Epoxy resin cast, rated for 12kV / 24kV / 36kV per IEC 60044 / IEC 61869
• Thermal Rating: Class E (120°C) or Class F (155°C) depending on installation environment

A CT with ALF = 20 and rated current 400A will maintain accuracy up to 8,000A primary fault current — a specification that must align with your system’s prospective short-circuit current.

How Is ALF Calculated? Core Formula and Parameters Explained?

The ALF is not a fixed physical constant — it shifts depending on the actual connected burden versus the rated burden. This is the most misunderstood aspect of CT specification in MV protection systems.

The Core ALF Formula (IEC 61869-2)

The Actual ALF under real operating burden is calculated as:

$ALF_{actual} = ALF_{rated} \times \frac{R_{ct} + R_{burden_rated}}{R_{ct} + R_{burden_actual}}$

Where:

• $ALF_{rated}$ = nameplate ALF value
• $R_{ct}$ = secondary winding resistance (Ω) — measured at 75°C
• $R_{burden_rated}$ = resistance equivalent of rated burden at rated secondary current
• $R_{burden_actual}$ = actual connected burden resistance (relay + lead resistance)

Burden Resistance Conversion

For a CT with rated burden Sₙ = 15VA at I₂ₙ = 5A:

$R_{burden_rated} = \frac{S_n}{I_{2n}^2} = \frac{15}{25} = 0.6 , \Omega$

If actual connected burden (relay coil + cable) = 0.3Ω, then:

$ALF_{actual} = 20 \times \frac{0.4 + 0.6}{0.4 + 0.3} = 20 \times \frac{1.0}{0.7} \approx 28.6$

This means a lower actual burden increases the effective ALF — a critical insight for engineers who under-burden their CTs.

Comparison: Protection CT Classes

Parameter	Class 5P	Class 10P
Composite Error at ALF	≤ 5%	≤ 10%
Phase Displacement Limit	±60 min	Not specified
Typical ALF Range	10–30	5–20
Application	Differential / Distance Protection	Overcurrent / Earth Fault
Core Size	Larger (lower saturation)	Compact
Cost	Higher	Lower

Client Case — EPC Contractor, Southeast Asia Substation Project:
A contractor specified Class 10P20 CTs for a 24kV feeder protection scheme using numerical distance relays. During commissioning, relay engineers discovered the actual burden (including 40-meter cable runs) was only 35% of rated burden — pushing the effective ALF to nearly 34. The CT was technically over-performing, but the original relay coordination⁴ calculations based on ALF=20 had to be revised. Bepto’s technical team provided recalculated ALF curves and updated relay coordination data, preventing a full protection study re-run. Lesson: always calculate actual ALF, not just nameplate ALF.

How to Select the Right ALF for Your Application?

Selecting ALF is a system-level decision, not just a CT nameplate choice. Here’s a structured approach used in real MV protection engineering projects.

Step 1: Define System Fault Level

• Obtain the maximum prospective short-circuit current (Isc) at the CT installation point
• Calculate required ALF: $ALF_{required} = \frac{I_{sc}}{I_{1n}}$
• Example: Isc = 16kA, I₁ₙ = 800A → ALF required = 20

Step 2: Determine Actual Burden

• Measure relay burden (VA or Ω from relay datasheet)
• Calculate cable resistance: $R_{cable} = \frac{2 \times L \times \rho}{A}$ (copper, 0.0175 Ω·mm²/m)
• Sum all series impedances in the secondary loop

Step 3: Calculate Actual ALF and Verify Margin

• Apply the ALF formula above
• Ensure ALF_actual ≥ ALF_required × 1.1 (10% safety margin recommended)
• If margin insufficient: increase CT rated burden class or select higher nameplate ALF

Step 4: Match Standards and Environmental Ratings

• IEC 61869-2 for protection CT performance
• IP65 minimum for indoor MV switchgear environments
• IP67 or IP68 for outdoor or coastal installations (salt fog per IEC 60068-2-52)
• Insulation voltage: confirm 12kV / 24kV / 36kV class matches system Um

Application-Specific ALF Recommendations

• Industrial MV Distribution (6–12kV): Class 5P20, 15VA — for motor protection and feeder overcurrent
• Power Grid Substation (33–36kV): Class 5P30, 30VA — for distance and differential protection
• Solar Farm MV Collection: Class 10P10, 10VA — lower fault levels, cost-optimized
• Marine / Offshore Platform: Class 5P20 with epoxy encapsulation, IP67, anti-vibration mounting
• Urban Underground Substation: Compact epoxy-cast CT, Class 5P20, space-optimized core design

What Are the Common Mistakes in ALF Specification and Installation?

Installation & Commissioning Checklist

1. Verify nameplate data — confirm ALF, accuracy class, rated burden, and Rct before installation
2. Measure actual secondary burden — use a burden meter or calculate from relay + cable data
3. Recalculate actual ALF — never assume nameplate ALF equals operating ALF
4. Perform polarity check — incorrect CT polarity causes differential relay maloperation
5. Conduct secondary injection test⁵ — verify relay pickup at calculated fault multiples
6. Check open-circuit protection — never open CT secondary under energized primary conditions

Common Specification Errors to Avoid

• Undersizing ALF for high fault-level feeders — CT saturates during fault, relay fails to trip within required time
• Ignoring cable resistance in burden calculation — especially critical for CTs located far from relay panels (>20m runs)
• Mixing 5A and 1A secondary CTs in the same protection scheme — causes severe burden mismatch
• Specifying measuring-class CT (Class 0.5 or 1.0) for protection circuits — these have high FS (instrument security factor) designed to saturate early, opposite of what protection needs
• Neglecting temperature correction for Rct — winding resistance increases ~20% from 20°C to 75°C, affecting actual ALF

Client Case — Procurement Manager, Industrial Plant Expansion:
A procurement manager sourced CTs from a low-cost supplier without verifying Rct values. The supplier-stated Rct was 0.3Ω; actual measured value was 0.72Ω. This shifted the actual ALF from the calculated 22 down to 14 — below the required fault level multiple. The protection engineer caught this during FAT (Factory Acceptance Testing), but it caused a 3-week delivery delay for replacement units. Bepto provides full test reports including Rct measurement, excitation curves, and composite error verification with every CT shipment.

Conclusion

Getting the ALF right is the difference between a protection system that operates correctly during a fault and one that fails at the worst possible moment. For medium voltage power distribution, protection reliability depends on accurate ALF calculation using real burden values — not just nameplate data. Whether you’re designing a substation protection scheme, specifying CTs for an industrial MV panel, or reviewing a solar farm collection system, applying the IEC 61869-2 ALF methodology ensures your Current Transformers perform when it counts most.

FAQs About CT Accuracy Limiting Factor

1. Detailed technical requirements for instrument transformers under the International Electrotechnical Commission. ↩

2. Understanding the mathematical definition of total current transformer error according to IEC standards. ↩

3. Exploring the magnetic saturation characteristics and grain orientation of electrical steel cores. ↩

4. Learning how to coordinate protective devices to minimize system downtime during fault events. ↩

5. Step-by-step procedures for verifying protection relay functionality and CT integrity on-site. ↩