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A current transformer (CT) is an instrument transformer whose secondary current is, within its accuracy limits, proportional to the primary current and differs in phase by approximately zero degrees for a correctly connected connection.
Overview
A current transformer (CT) is an instrument transformer whose secondary current is, within its accuracy limits, proportional to the primary current and differs in phase by approximately zero degrees for a correctly connected connection. It reduces a high primary current (tens to thousands of amps) to a low, standardised secondary value — usually 5 A or 1 A — that can be safely fed to ammeters, energy meters, transducers and protective relays, while galvanically isolating those instruments from the primary circuit. CTs are built as ring/toroidal (window) types where the primary conductor or busbar passes through the core, bar-primary types with a built-in primary bar, or wound-primary types for low primary currents; the secondary is many turns wound on a toroidal core to minimise leakage-flux error. Buyers distinguish measuring CTs (accuracy classes 0.1 to 3, optimised for accuracy at normal load and saturating quickly above it to protect meters) from protection CTs (classes 5P/10P/PS, which must stay accurate up to a high accuracy limit factor so relays operate correctly during faults). A buyer chooses ACE for application-matched ratio, accuracy class and burden, resin-cast or tape-wound insulation suited to indoor LT panels, and compliance with IS 2705 / IEC 61869 (formerly IEC 60044). Standard parameter set and typical ranges for the component class (not ACE-specific figures): primary current ~5 A up to 4000 A and beyond; secondary 5 A or 1 A; rated frequency 50/60 Hz; rated burden ~2.5 to 30 VA; accuracy classes 0.1/0.2/0.2S/0.5/0.5S/1.0/3.0 (metering) and 5P10 to 10P20 / PS (protection); rated short-time thermal current (Ith) and dynamic current (Idyn) for fault withstand; LT insulation level 0.72/3 kV class for low-voltage units.
Specifications
| Transformation ratio | primary 5 A up to 4000 A (and higher on request) : secondary 5 A or 1 A |
|---|---|
| Rated secondary current | 5 A (standard) or 1 A |
| Rated frequency | 50 Hz / 60 Hz |
| Accuracy class — metering | 0.1, 0.2, 0.2S, 0.5, 0.5S, 1.0, 3.0 (per IEC 61869-2 / IS 2705) |
| Accuracy class — protection | 5P10, 10P10, 5P20, 10P20, PS (class with accuracy limit factor) |
| Rated burden | ~2.5–30 VA (accuracy held between 25%–100% of rated VA) |
| Construction | ring/toroidal (window), bar-primary, or wound-primary; resin-cast or tape-wound |
| Insulation / voltage class | LT 0.72 kV / 3 kV class; rated insulation and power-frequency withstand per standard |
| Rated short-time thermal current (Ith, 1 s) and rated dynamic current (Idyn) for fault withstand | |
| Installation | indoor LT panel / switchboard mount; outdoor variants available |
| Standards | IS 2705 (Parts 1–4), IEC 61869-1/-2 (formerly IEC 60044-1); BIS certified where applicable |
Applications
- Current metering and energy measurement in LT panels and switchboards
- Feeding ammeters, multifunction meters and transducers
- Protection-relay circuits (overcurrent, earth-fault, differential)
- Panel-builder and switchgear OEM supply
- Industrial power distribution and motor control centres (MCC/PCC)
- Utility and substation metering / protection
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