Current Transformers (CTs) and Potential Transformers (PTs) — together known as instrument transformers — step down high currents and voltages to standardized, safe levels for metering instruments and protection relays. Getting the specification wrong is a common (and costly) mistake, since CTs and PTs directly determine measurement accuracy and protection reliability.
Current Transformers (CT)
A CT steps down primary current (e.g. 800A) to a standard secondary current, almost always 1A or 5A, for connection to meters and relays. Key specifications to get right:
- Ratio: Primary/secondary current ratio (e.g. 800/1A) — sized to the actual expected load current, not just the cable or breaker rating, for best accuracy.
- Accuracy Class: Metering CTs typically use class 0.2S, 0.5, or 0.5S (tighter accuracy, especially at low current, for billing-grade metering); protection CTs use class 5P or 10P with a defined Accuracy Limit Factor (ALF) to remain accurate up to several times rated current during a fault.
- Burden (VA rating): The load the CT secondary can drive (meters, relays, wiring resistance) without losing accuracy — oversizing burden capacity wastes accuracy at normal load; undersizing causes saturation.
- Knee-Point Voltage (for protection CTs): Especially important for differential protection schemes, to ensure the CT doesn't saturate during high fault currents.
Potential Transformers (PT/VT)
A PT (also called a Voltage Transformer, VT) steps down high system voltage to a standard secondary, typically 110V or 110/√3 V, for metering and protection. Key specifications:
- Ratio: Matched to system voltage (e.g. 11000/110V for an 11kV system).
- Accuracy Class: Similar classes to CTs (0.2, 0.5, 3P) depending on metering vs protection use.
- Connection: Single-phase (phase-to-earth) or three-phase; star or open-delta configurations depending on whether earth fault detection is also needed from the same PT.
- Burden (VA): Same principle as CT burden — match to the actual connected load of meters/relays.
Metering vs Protection — Don't Mix Them
A common mistake is using a single CT/PT core for both metering and protection. Metering cores are designed to saturate at low overcurrent to protect connected meters from fault-level currents — which means they become inaccurate exactly when a protection relay needs accurate current data during a fault. Always specify separate metering and protection cores (most CTs are available as dual- or triple-core units for exactly this reason).
TEKTOW stocks CTs and PTs across a wide range of ratios and accuracy classes. Request a quote with your system voltage, current, and whether it's for metering or protection use.