Why do high voltages develop across the leads of an open CT secondary?

Current Transformers are transformers that produce proportional current in the secondary. The open-circuit voltage of a CT has absolutely nothing to do with the system voltage of the primary. For a given core secondary winding design, the secondary open-circuit voltage is the same, whether the CT is on a 400V circuit or an 110kV system. The voltage developed on the secondary of an open-circuited CT is a function of the current flowing in the primary and the inductance properties of the secondary windings. If the secondary is open-circuited, that is seen as infinite impedance, so all of the primary current is forced through the magnetizing branch. The voltage seen on the secondary is this primary current times the magnetizing impedance.

Why Should Current Transformer (CT) Secondary Be Short-Circuited?

Why Should Current Transformer (CT) Secondary Be Short-Circuited?

Current Transformers (CTs) are vital components in power systems, used to measure high currents by stepping them down to manageable levels for meters, relays, and protection devices. But there’s a critical safety rule when dealing with CTs:

Never leave the CT secondary open-circuited when current is flowing through the primary.

⚠️ Why is This Dangerous?

When a CT’s secondary circuit is open while the primary is energized, the transformer cannot deliver current to a load (i.e., burden). As a result, the magnetic flux in the core increases rapidly. This leads to a dangerously high voltage being induced in the secondary winding — often in the kilovolt range.

These high voltages can cause:

• Severe electric shock to personnel

• Breakdown of insulation in the CT

• Damage to connected equipment

• Arcing across terminal strips or insulation feedthroughs (especially in oil-filled CTs)

🔌 Why Short-Circuiting Helps

By short-circuiting the secondary terminals when a burden (like a meter or relay) is not connected, the CT’s current has a low-resistance path to flow through. This:

• Prevents the dangerous buildup of voltage

• Protects the CT from insulation failure

• Ensures safe operation during maintenance or testing

✅ Best Practices

• Always short the CT secondary if it’s not connected to a load.

• Use shorting links or terminal blocks designed for CT circuits.

• Never disconnect a CT under load without shorting it first.

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In summary:
CT secondaries must be short-circuited when not in use to avoid dangerously high voltages and protect both equipment and personnel.

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