Choosing a Current Transformer (CT) for Power Analyzers: 5-Criteria Guide

Lucas Moreau Electrical Engineer · Low-voltage installation specialist · 15 years of experience

Key Takeaway

Choosing a current sensor (current transformer or CT) for a power analyzer depends on five criteria: rated primary current, accuracy class, output power (VA), construction type (solid core, split core, or flexible), and compatibility with the analyzer. For a Socomec DIRIS analyzer, choose a class 0.5 CT (or 0.2 for billing), a transformation ratio matching the circuit's nominal current (e.g., 200/5 A, 600/5 A), and sufficient VA rating to supply all instruments connected to the secondary.

What Is the Role of a Current Sensor?

A current sensor — most commonly a current transformer (CT) — is a device that proportionally reduces a high current to a low current (typically 5 A or 1 A on the secondary) usable by a power analyzer, energy meter, or protection relay.

A CT is essential whenever the circuit current exceeds the analyzer's direct input capacity (typically 63 A). Without a CT, it is impossible to connect a power analyzer to a 200 A, 600 A, or higher feeder.

Key point The CT does not measure — it transforms the current. The power analyzer interprets the signal and calculates electrical quantities (P, Q, S, cos φ, THD, harmonics). Final accuracy depends on both the CT class and the analyzer's own accuracy.

5 Criteria for Choosing a Current Sensor

1Rated Primary Current (Ipn)

The transformation ratio must match the circuit's nominal current. Choose a CT whose primary current equals or slightly exceeds the normal operating current. Common ratios: 100/5 A, 200/5 A, 400/5 A, 600/5 A, 800/5 A, 1000/5 A, 1600/5 A.

An oversized CT (e.g., 1000/5 A on a 150 A circuit) operates below its optimal range and loses accuracy. An undersized CT risks saturation and damage.

2Accuracy Class

The class defines the maximum allowable error on the transformation ratio:

Current transformer accuracy classes
Class	Max. error	Application
Class 0.2 / 0.2S	± 0.2%	Billing, MID metering, high precision
Class 0.5 / 0.5S	± 0.5%	Power analyzers, monitoring, EMS — the standard
Class 1	± 1%	Protection, indicative monitoring
Class 3	± 3%	Low-accuracy applications

"S" classes (0.2S, 0.5S) maintain accuracy even at low load (1 to 20% of nominal current), which is critical when the circuit frequently operates at partial load.

3Output Power (VA)

The output power (or "burden") expresses the maximum load the CT can supply on the secondary without exceeding its class error. It must cover the consumption of all connected instruments (power analyzer, meter, relay) plus cable losses.

For a power analyzer alone, 2.5 to 5 VA is generally sufficient. If multiple instruments are connected in series, add up their consumption and include cable resistance (approximately 0.1 VA per meter of 2.5 mm² cable).

4Construction Type

Current transformer types
Type	Installation	Advantage	Limitation
Solid core (wound)	Cable passes through the opening	Best accuracy, lower cost	Requires cable disconnection
Split core (opening)	Clips around the cable	Installation without power interruption	Slightly less accurate, more expensive
Flexible (Rogowski)	Flexible loop around the cable	Very easy to install, large diameters	mV/A signal requiring compatible input

Split-core CTs are particularly suited for commissioning on existing installations without power interruption. Rogowski coils are ideal for very large diameter cables or busbars.

5Compatibility with the Power Analyzer

Verify that the CT is compatible with the analyzer in terms of:

Secondary current: 5 A (standard) or 1 A (long distances between CT and analyzer).
Signal type: current (classic CT) or voltage (Rogowski coil) — the analyzer must have the corresponding input.
Number of channels: multi-channel analyzers (Socomec DIRIS Digiware) accept multiple CTs on a single device.
Connectors: screw terminals, spring terminals, or plug-in connectors depending on the model.

Sizing Example

Consider a three-phase 400 V feeder protected by a 250 A circuit breaker, supplying an industrial process consuming an average of 180 A. The analyzer is a Socomec DIRIS A40 with 5 A inputs:

Ipn: 250/5 A (matched to circuit breaker rating and operating current).
Class: 0.5 (EMS monitoring) or 0.2 (if MID billing).
Power: 5 VA (DIRIS A40 consumes approximately 0.5 VA per phase + cable margin).
Type: Solid core for new installations, split core for retrofit without power interruption.
Quantity: 3 CTs (one per phase) — 3 × CT 250/5 A class 0.5 – 5 VA.

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Frequently Asked Questions — Current Sensors

How do I choose the transformation ratio for a CT?

The CT's rated primary current should equal or slightly exceed the circuit's nominal current. For example, for a feeder protected by a 250 A breaker, choose a 250/5 A CT. An oversized CT loses accuracy at low loads; an undersized CT risks saturation.

Which accuracy class should I choose for a CT?

Class 0.5 is the standard for power analyzers and monitoring (EMS, ISO 50001). Class 0.2 is required for billing metering (MID meters). "S" classes (0.5S, 0.2S) maintain accuracy even at low load (1-20% of nominal current), which is important for circuits with variable load.

What is the difference between solid core, split core, and flexible (Rogowski) CTs?

Solid core CTs offer the best accuracy and lowest cost but require cable disconnection for installation. Split-core CTs clip around the cable without power interruption — ideal for existing installations. Rogowski coils (flexible) wrap around large-diameter cables or busbars but require a compatible input on the analyzer.

How do I calculate the required VA rating for a CT?

Add up the consumption of all instruments connected to the CT secondary (power analyzer: 0.2 to 1 VA, meter: 0.5 to 2 VA, relay: 1 to 5 VA) and include cable losses (approximately 0.1 VA per meter of 2.5 mm² cable). For a single analyzer with short cable, 2.5 to 5 VA is sufficient.

Do I need one CT per phase for three-phase measurement?

Yes, for complete three-phase measurement, you need 3 identical CTs (one per phase). Some analyzers allow "2 CT + calculation" measurement on balanced networks, but 3-CT measurement is recommended for accuracy and load imbalance detection.

Which CTs are compatible with Socomec DIRIS analyzers?

Socomec DIRIS analyzers (A40, A-20, Digiware) accept standard CTs with 5 A or 1 A output. The DIRIS Digiware range also accepts Rogowski coils with mV/A signal. Socomec offers its own CTs (192T range) in solid and split-core versions, from 50/5 A to 4000/5 A, in classes 0.5 and 1.

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Key Takeaways

Choose a CT with primary current matching the circuit's nominal current (neither oversized nor undersized).
Class 0.5 for monitoring and EMS; class 0.2 for MID billing.
"S" classes (0.5S, 0.2S) guarantee accuracy even at partial load.
Solid core for new installations, split core for existing installations without power interruption.
VA rating must cover all instrument consumption + cable losses.
3 CTs per three-phase measurement, with 5 A (standard) or 1 A (long distances) secondary output.
Optim-Elec offers Socomec CTs (192T range) and compatible DIRIS analyzers.