Never open-circuit a current transformer secondary — the hazard of an interrupted CT circuit and the use of a shorting block

The guide on CT protection class and knee-point voltage and the guide on CT measuring class and burden for kWh metering both briefly note that a current transformer (CT) must never be operated open-circuited (unloaded). This article goes deeper into why that is the case, and the standard practice for safely avoiding it.

Why an open CT secondary is dangerous: the counter-mmf collapses

Unlike a voltage or power transformer, a current transformer is primarily driven by the primary current, not by a primary voltage: the primary conductor carries the installation's or network's operating current, regardless of what is connected on the secondary side. With a closed secondary circuit, the secondary current produces a magnetomotive force (mmf) that is almost exactly opposed to the mmf of the primary current, keeping the net magnetic flux in the core small. If the secondary circuit is interrupted, this counter-mmf disappears entirely, while the primary current — which is imposed by the network or the installation, not by the CT itself — simply keeps flowing. The full primary mmf then drives the core deep into saturation, causing a strongly distorted, peaked flux change. Because the induced secondary voltage is proportional to the rate of change of flux over time (dΦ/dt), this produces short but extremely high voltage spikes — on the order of hundreds to thousands of volts, depending on the turns ratio and the primary current at the moment of interruption.

The consequences: electrocution hazard, insulation breakdown and overheating

These voltage spikes are not merely a metering problem, but a direct safety hazard:

Electrocution hazard: anyone touching the open secondary terminals (for example while disconnecting a meter or relay) can be exposed to a potentially lethal voltage. Cross-reference: this is the mirror image of the hazard covered in the VT guide, where it is instead a short circuit on the secondary side that is dangerous.
Insulation breakdown: the voltage spike can exceed the insulation strength of the CT winding or the connected wiring, risking internal flashover, damage or even fire.
Core overheating: the deep saturation causes significant iron losses in the core, which can lead to noticeable heating of the CT if the secondary circuit remains open for a prolonged period.

The standard solution: shorting block or test switch

Because in many cases the primary current cannot practically be interrupted before work is carried out on the secondary CT circuit (the primary conductor is often simply part of an in-service installation or network connection), standard practice is to short-circuit the secondary circuit first, before disconnecting any connected device (kWh meter, ammeter, relay):

A shorting block is a terminal block with built-in shorting links that are engaged with a screw or lever, so that the CT secondary current continues to flow through the link while the terminals to the connected device are removed.
A test switch is a switch specifically designed for this purpose, with make-before-break shorting contacts: when operated, the short circuit is established first, before the circuit to the connected device is broken — so that the CT secondary current is never interrupted, not even briefly during the switching action itself.

Why this requires a different approach from ordinary LOTO

The guide on the 5 LOTO steps covers de-energising, locking out and verifying a dead installation before work begins. On a CT secondary circuit, the primary-side operating current often cannot be (easily) switched off without interrupting the entire supply — shorting the secondary side is therefore an additional, specific safety measure for this type of circuit, not a replacement for the ordinary LOTO procedure covering the rest of the installation.

Note: after completing the work, the shorting link must be correctly removed or switched back to the normal operating position; a permanently short-circuited CT secondary does not itself pose an immediate hazard, but renders the connected metering or protection completely inoperative until this is restored.

Practical relevance

Before disconnecting a kWh meter, ammeter or relay connected via a current transformer, it must always first be checked whether a shorting block or test switch is present and whether it has actually been operated (shorted) before any terminal is removed — never assume the circuit is already safe without explicitly verifying this.

Common mistakes

Disconnecting a connected meter or relay from a CT secondary without first operating a shorting block or test switch — this can immediately produce a dangerous voltage spike.
Assuming a test switch automatically shorts as soon as it is operated, without verifying the make-before-break action — an incorrectly operated or defective switch can open the circuit first instead.
Confusing the CT rule ("never leave open") with the VT rule ("never short-circuit") — the two measuring transformers have opposite safety hazards on the secondary side.
Forgetting to remove the shorting link after completing the work — this permanently disables the metering or protection until it is noticed and restored.