CT vs PT Instrument Transformer: Current and Voltage Transformer Selection for Switchgear

CT and PT Are Small Components with Big Project Impact

Instrument transformers connect the power system to protection relays, meters, and control equipment. A current transformer measures current. A potential transformer, also called a voltage transformer, measures voltage. In a switchgear project, wrong CT or PT selection can create metering errors, relay misoperation, failed utility acceptance, or commissioning delays.

This guide explains the practical difference for procurement teams, EPC contractors, and panel builders preparing medium-voltage or low-voltage switchgear quotations.

What a CT Does

A current transformer steps high primary current down to a safe secondary current, commonly 5A or 1A, for meters and protection relays. It is selected by primary current ratio, accuracy class, protection class, burden, insulation level, and installation type.

LZZBJ9 current transformers are commonly used in indoor medium-voltage switchgear for metering and protection circuits.

What a PT or VT Does

A potential transformer, also called a voltage transformer, steps medium voltage down to a safe secondary voltage for meters, relays, and voltage monitoring. PT selection depends on voltage ratio, accuracy class, burden, insulation level, connection method, fuse protection, and anti-ferroresonance requirements.

JDZ voltage transformers are often used in MV switchgear metering cabinets, PT cabinets, and protection systems.

CT vs PT Comparison

Item	CT	PT / VT
Measures	Current	Voltage
Common secondary	5A or 1A	100V, 110V, or project-specific voltage
Main use	Metering, protection, current sensing	Voltage measurement, metering, protection reference
Key rating	Current ratio, accuracy, burden, protection class	Voltage ratio, accuracy, burden, insulation level
Safety note	Do not open-circuit CT secondary when energized	Protect against secondary short circuit and resonance risks

Selection Data Buyers Should Provide

• System voltage: 3.6kV, 7.2kV, 12kV, 24kV, 40.5kV, or project-specific insulation level.
• CT ratio: primary current and secondary current, for example 600/5A or 1000/1A.
• PT ratio: primary voltage and secondary voltage, including line-to-line or line-to-ground connection.
• Accuracy class: metering accuracy and protection class should be separated when required.
• Burden: confirm relay, meter, and cable load in VA.
• Installation type: indoor epoxy cast resin, busbar type, cable type, cabinet-mounted, or outdoor type.
• Standards: confirm IEC 61869 or project-specific utility standard.

Common Procurement Mistakes

• Using one CT core for both metering and protection when separate classes are required.
• Quoting CT ratio without burden and accuracy class.
• Ignoring relay requirements, especially for differential protection or sensitive earth fault protection.
• For PTs, missing fuse, earthing, connection group, and resonance protection requirements.
• Not confirming physical dimensions with the switchgear cabinet design.

RFQ Checklist

1. Switchgear model, rated voltage, insulation level, and installation location.
2. For CT: ratio, secondary current, number of cores, accuracy class, burden, and protection class.
3. For PT: voltage ratio, connection method, accuracy class, burden, and fuse requirements.
4. Relay and meter model or protection schematic when available.
5. Required standards, routine test reports, drawings, packing method, and destination country.

Ziyao Power supplies instrument transformers for switchgear manufacturers, EPC projects, utilities, and industrial power systems. Send your relay and metering requirements and our team will help confirm CT/PT configuration before production.