Current transformer polarity test (dot marking, P1/P2 vs S1/S2) — why a reversed CT can trip a healthy circuit or hide a real fault

The guides on CT protection class and knee-point voltage and CT accuracy class and burden for kWh metering both assume the current transformer (CT) is connected with the correct orientation between its primary and secondary windings. This article covers what "correct orientation" actually means, and why getting it wrong is a subtle fault that a simple continuity or ratio check will not catch: CT polarity.

What the P1/P2 and S1/S2 markings mean

A CT's primary terminals are marked P1 and P2, and its secondary terminals S1 and S2 (sometimes shown with a dot symbol • next to P1 and S1 instead). The polarity convention states that at the instant primary current flows into P1, secondary current flows out of S1 (into the external burden or measuring circuit) — the two marked terminals are, in that sense, "in phase" with each other.

This is not a cosmetic labelling detail: it determines whether the secondary current's phase relationship to the primary current (and to other CTs' secondary currents in the same measurement) is the one the connected relay or meter assumes.

Why a reversed CT causes a problem, even though the CT itself is fine

A CT wired with S1/S2 swapped relative to the diagram still measures current correctly in magnitude — a simple ratio test with a single CT in isolation will not reveal the reversal. The problem only appears once that CT's secondary current is combined, in phase, with a reference — another CT, a voltage signal, or a relay's assumed current direction:

Differential protection (see the guide on transformer differential protection, 87T) sums the secondary currents of CTs at both ends of the protected zone; for a healthy, through-flowing load current those currents should very nearly cancel. If one CT is reversed, they add instead of cancel, and the relay sees a large, permanent "differential" current even on normal load — a false trip risk (or, depending on the exact configuration, a masked real fault).
A directional relay (67/67N) uses the phase relationship between a CT's secondary current and a voltage reference to decide whether a fault lies in front of or behind the relay. A reversed CT flips that sense front-to-back, so the relay can operate for a fault in the wrong direction, or fail to operate for one in the right direction.
A kWh meter on a net-metering or bidirectional installation (for example with local generation) reads energy flow direction from the phase relationship between voltage and CT current. A reversed CT can make imported energy read as exported, or vice versa.

How to actually test polarity

Because a reversed CT is invisible to a simple ratio check, polarity needs its own dedicated test:

Primary injection with phase check: inject a known primary current and verify not only the secondary current magnitude, but also its phase relationship to the reference (voltage, or the other CTs in a differential zone), using a modern secondary injection test set with a polarity/phase test function.
DC "kick test": momentarily connect a small DC source (such as a battery) across the primary, in the polarity direction marked P1→P2, and observe the direction of the resulting deflection on a galvanometer or multimeter connected across the secondary S1/S2 — a standard, low-tech commissioning check that does not require load current to be present.
Nameplate and wiring-diagram cross-check: confirm the physical P1/S1 markings on the installed CT match the polarity assumed in the protection or metering scheme's wiring diagram — simple, but easy to skip when a CT is replaced.

Note: the exact test procedure and acceptance criteria depend on the specific test equipment and the protection or metering scheme in question; this article covers why the test is needed and the principle behind it, not one manufacturer's test-set manual.

Practical relevance

Polarity should be verified whenever a CT is newly installed, or whenever an existing CT is replaced — particularly with a different model or manufacturer, since the physical position of the dot marking on the CT body is not standardised, only its meaning is. A CT that "looks" correctly wired by cable colour alone gives no assurance that the polarity marking itself was respected during connection.

Common mistakes

Wiring a CT by cable colour convention rather than by checking the actual P1/S1 markings — colour conventions are not a substitute for verifying the manufacturer's polarity marking on the specific CT installed.
Assuming a "symmetric-looking" CT installation is automatically correct without a dedicated polarity test — a reversed CT gives no visible symptom under normal balanced load.
Skipping a polarity check when replacing a failed CT with a different model — the physical placement of the dot marking is not standardized across manufacturers, only its electrical meaning is.
Relying only on a resistive ratio test at commissioning, without any test that includes a genuine directional or differential check — a ratio test alone cannot reveal a reversed polarity.