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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