Relays and meters cannot be connected directly to a 132 kV line carrying thousands of amperes. Instrument transformers — the current transformer (CT) and the potential transformer (PT) — stand between the power circuit and the instruments. This post explains what they are for, and why a power system needs two different kinds of CT: one for metering and one for protection.
Objectives of CT and PT
(1) Scale reduction and standardisation. A CT steps a high transmission current (hundreds or thousands of amperes) down to a standardised low value — 1 A or 5 A — and a PT steps a high voltage (kV) down to 110 V line-to-line. These standard low values suit ordinary low-cost relays and energy meters, so one meter design works on any system voltage.
(2) Galvanic isolation and safety. The secondary circuit is electrically isolated from the high-voltage primary. Control panels, protective relays and human operators are therefore kept away from hazardous primary voltages.
Why two different CTs?
A metering CT and a protection CT do opposite jobs, so their cores are built differently:
| Feature | Metering CT | Protection CT |
|---|---|---|
| Job | Accurate measurement of normal load current for billing | Faithful reproduction of fault current for relays |
| Accuracy class | Very high — 0.1 to 0.5 | Moderate — e.g. Class 5P20 |
| Working range | Normal load only — 5% to 120% of rated current | Linear during severe faults — 10 to 30× rated current |
| Core material | High-permeability nickel-iron (permalloy / mumetal) | Grain-oriented silicon steel, larger cross-section |
| Saturation behaviour | Low saturation flux density — Instrument Security Factor, ISF < 5. The core saturates early on purpose, so the secondary current cannot rise proportionally during a fault and delicate meter coils are protected from thermal burnout | High saturation knee-point voltage — Accuracy Limit Factor, ALF = 10 to 20. The core does not saturate during faults, so the relay receives an accurate replica of the fault current |
Figure 2.1: Metering CT vs protection CT — saturation behaviour. The metering CT saturates early (ISF < 5) to protect instruments; the protection CT stays linear to 10–30× rated current (ALF 10–20, e.g. 5P20). (Schematic, not to scale.)
The two behaviours are deliberate opposites. A metering CT that stayed linear during a fault would drive a huge secondary current through a delicate energy meter and burn its coils out. A protection CT that saturated during a fault would feed the relay a flattened, wrong current — the relay would under-measure the fault and might fail to trip. One CT cannot do both jobs, so both types are installed side by side.
Key points to remember
- CTs step current down to 1 A or 5 A and PTs step voltage down to 110 V, and both give galvanic isolation from the HV primary.
- Metering CT: accuracy class 0.1–0.5, range 5–120%, nickel-iron core, ISF < 5 — saturates early to protect meters.
- Protection CT: linear at 10–30× rated current, grain-oriented silicon steel, ALF 10–20 (e.g. Class 5P20) — must not saturate during faults.
- Early saturation is a feature in a metering CT and a failure in a protection CT.
Based on Power System Protection & Switchgear, Complete Examination Solution, Exam 07 May 2026, Q2(a).