Current Transformer (CT) Sizing: P1-P2 / S1-S2 Wiring, Burden, and Safety

IEC 61869-2 current transformer polarity (P1-P2 / K-L and S1-S2), open-secondary hazard prevention, 5 A vs 1 A VA cable burden calculation, and PFC relay placement.

A current transformer (CT) steps high primary busbar or feeder currents down to a standardized, isolated 5 A or 1 A secondary signal under IEC / EN 61869-2 for energy meters, network analyzers, and reactive power factor correction (PFC) controllers.

> Related Category: Compare window-type, wound, and split-core CTs alongside instrumentation in Current Transformers, Reactive Power Control Relays, and Power Network Analyzers.

P1–P2 (K–L) and S1–S2 (k–l) polarity and PFC placement

Power meters and PFC relays measure both current magnitude and phase angle relative to phase voltage:

For reactive power control relays, install the CT upstream of both the plant loads and the capacitor bank so the controller sees the net compensated current; match each CT (`L1, L2, L3`) strictly to its own voltage sensing phase, and verify busbar window dimensions before ordering.

> Critical Safety Rule — Never Open-Circuit an Energized CT Secondary!
> Disconnecting `S1–S2` while primary current is flowing drives the magnetic core into deep saturation and induces lethal kilovolt peak voltages across the open terminals, destroying insulation and endangering personnel. Always short `S1–S2` via a dedicated CT shorting terminal block before removing a meter.

Ratio sizing, accuracy class (`0.5` vs `0.5S`), and `/5 A` vs `/1 A` VA burden

Choose a primary rating 20%–30% above maximum continuous feeder current, and verify total secondary burden (`VA_meter + VA_cable ≤ VA_CT`):

`P_cable (VA) = I_sec² × (2 × L) / (56 × S)`

Over a 20 m run (`40 m` loop) of 2.5 mm² copper wire:

During commissioning, verify true primary-to-secondary ratio non-intrusively with a clamp meter.

Frequently asked questions

Why should one CT secondary terminal (S2) be earthed?

Earthing S2 protects instruments and operators from dangerous high voltages if primary-to-secondary insulation ever breaks down.

What happens if P1–P2 or S1–S2 is wired backwards?

The current vector flips by 180°, causing power analyzers to report negative active power (export) and PFC controllers to misjudge inductive vs capacitive steps.

When are split-core CTs preferred over solid-core CTs?

Split-core CTs hinge open around existing insulated cables or busbars, enabling retrofit energy monitoring without shutting down switchgear or unbolting copper bars.

How does looping the primary cable twice through the CT window change the ratio?

Two primary passes halve the effective primary rating—for example, a 100/5 A CT with two primary turns operates as a 50/5 A CT.