Direct Answer

A current transformer (CT) secondary side must never be operated open-circuited. This is one of the few red-line violations in electrical work where a single mistake can cause personal injury.

When a CT is healthy, the secondary is shorted (connected to ammeters, energy meters). The secondary current cancels the primary current's flux, keeping core flux small. Open the secondary, and the secondary current disappears—the cancellation is gone. The primary current magnetises the core, driving it into saturation, and the secondary winding induces thousands of volts, breaking insulation, creating arcs, destroying the core, and endangering personnel.

1. CT vs Power Transformer

A power transformer transforms voltage: primary voltage is fixed, secondary voltage depends on ratio, and no-load primary current is tiny.

A CT transforms current: primary current is set by the load, not by the CT. The primary is in series with the main circuit—1000A is 1000A regardless of what the secondary does. Normally the secondary is shorted, producing opposing flux. Open-circuited, no opposing flux exists; core flux spikes and the secondary induces dangerous high voltage.

That is why CT-connected ammeters, meters and relays are effectively "short circuits"—their impedance is near zero.

2. What Happens on Open Circuit

Chain reaction:

  1. Secondary current vanishes: loose wiring, meter removed, bad terminal contact;
  2. Core saturates: primary current all magnetises; flux waveform becomes a square wave;
  3. Thousands of volts induced: flux change rate is extreme; the secondary winding spikes to several kV;
  4. Insulation breakdown: kV-level voltage punctures winding and terminal insulation, creating arcs;
  5. Core burns: saturation iron-loss explodes; the core heats violently and residual magnetism remains permanently;
  6. Personnel danger: a technician working on the secondary touches a terminal and receives a fatal shock.

One factory technician replaced an ammeter without shorting the CT secondary. The moment the old meter came off, the CT opened; the terminal block arced and burned the technician's arm. Post-incident testing found residual core magnetism causing >10% measurement error; the CT was scrapped.

3. Common Mistakes

  • Replacing ammeters/meters: removing the meter without shorting secondary terminals first;
  • Adding devices live: connecting a new relay or meter by removing wires, creating an open-circuit gap;
  • Loose terminal blocks: screws loosening over years, creating an unnoticed open circuit;
  • Broken secondary cables: rodent damage, mechanical harm;
  • Relay maintenance: not re-installing shorting links after relay removal.

4. Correct Procedure

The standard live-work sequence on CT secondary:

  1. Short the CT secondary terminals first with a dedicated shorting link or wire; verify the short is solid;
  2. Only then work on meters/relays;
  3. Reconnect the new meter, then remove the shorting link.

Order is non-negotiable: short first, disconnect second; connect first, remove short last.

ESA all-element smart meter and FSA multi-element controller must connect via test terminals. Test terminals have built-in shorting links—when swapping meters, insert the shorting link before removing the old device. This is standard CT-circuit safety.

5. If It's Already Open

If you discover an open CT secondary (loose terminal, broken wire):

  • Do not touch directly: the secondary may carry high voltage;
  • Trip the primary breaker if at all possible, then work;
  • If de-energising is impossible: wear insulating gloves, use insulated tools, stand on an insulating mat, and first short the CT secondary terminals with an insulated jumper;
  • Post-incident test: any CT that has been open-circuited must undergo V-I characteristic and ratio testing—residual magnetism can cause unacceptable errors.

6. Prevention

  • Test terminals with shorting links mandatory: never use ordinary terminals for CT secondary;
  • Single-point secondary grounding: only one ground point allowed; two grounds cause current splitting;
  • Regular thermal scans: IR inspection of terminal blocks for loose joints;
  • Lockout/tagout: hang a "CT SHORTED" tag when working on the secondary so nobody removes the shorting link;
  • Training: every electrician must know "never open a CT secondary, never short a PT secondary."

7. How to Select CT Ratio

You may ask: CT ratios come in 50/5, 100/5, 200/5, 500/5, 1000/5 — how to choose?

Principle: primary rated current ≈ 1.2–1.5 times the circuit maximum operating current.

Example:

  • Circuit maximum operating current 400A.
  • Select CT ratio 500/5 (1.25x).
  • Do not select 1000/5 (too large, CT accuracy drops at light load).
  • Do not select 300/5 either (too small, CT saturates on overload).

Why not too large? Because CT accuracy drops at light load. A 1000/5 CT actually carrying 200A — that is 20% load — CT ratio error may exceed 2%.

Why not too small? Because CT saturates on overload — after saturation, secondary current no longer increases linearly with primary current, both measurement and protection fail.

ESA all-parameter smart meter supports multiple CT ratios, selected on site based on actual circuit current.

8. How to Read CT Accuracy Class

CT nameplate says "Class 0.5" or "Class 0.2S" — what does it mean?

Accuracy class = maximum ratio error in percent.

  • Class 0.5: error ≤0.5%, for general measurement.
  • Class 0.2S: error ≤0.2%, for billing (energy metering).
  • Class 5P: for protection circuits (error ≤5%).

Billing must use Class 0.2S — otherwise the utility and your meter do not match, that is a metering dispute.

General monitoring uses Class 0.5 — that is enough.

9. CT vs PT Safety Rules Compared

Remember two mnemonics:

  • CT secondary never open-circuited (current transformer, open circuit produces high voltage).
  • PT secondary never short-circuited (voltage transformer, short circuit produces high current).

PT (potential transformer) is the opposite of CT: PT is voltage transformation, secondary short circuit produces very large short-circuit current, burning the PT.

These two mnemonics are the basics of electrical safety. New technicians must memorize them before going on duty.

10. One Sentence to Remember

CT secondary never open. PT secondary never short. These two rules have killed more people than almost any other electrical mistake. Memorize them. Live by them.

11. Common Questions

Q: Can I install a meter without a CT? A: Only if the current is small enough (under 60A) to connect directly. For larger currents, you must use a CT.

Q: What happens if a CT saturates? A: The secondary current stops increasing linearly. Your meter reads low. Your protection does not trip when it should. You think everything is fine — but you are operating above the CT rating.

12. Quick Checklist

When working with CTs:

  1. Never open the secondary circuit while primary is energized.
  2. Short the secondary before disconnecting any meter.
  3. Check CT ratio — 1.2–1.5x max operating current.
  4. Check accuracy class — 0.2S for billing, 0.5 for monitoring.
  5. Check polarity — S1 and S2, do not reverse.
  6. Use insulated tools when working on secondary wiring.
  7. Lock out / tag out before any CT work.

This checklist keeps you alive. CT work is not a game.

13. Why This Matters

CT safety is not a theoretical topic. It is a real, everyday, on-the-job danger.

Every electrician who has worked in a substation has heard the story: someone opened a CT secondary to test a meter, the primary was still energized, and — boom — 1000V appeared across the open terminals. That is not a myth. That is how people die.

The physics is simple: CT primary current is determined by the load, not by the secondary. If you open the secondary, the primary current still flows. The CT core becomes a step-up transformer. The voltage rises until something breaks down — air, insulation, or you.

This is why the rule exists. This is why we repeat it. This is why you must never, ever open a live CT secondary.

Memorize it. Teach it to your apprentices. Put it on your safety poster. It is that important.

Apply it. Teach it. Share it with your team. Stay safe.

Thank you for reading. Goodbye.

Stay safe. Stay professional. Stay curious.

This is the end of this article. We hope you found it useful. See you in the next one.

Until then, keep your CTs safe. Keep your meters accurate. Keep your crew alive.

That is the way. That is the standard. That is how we work.

Thank you. Goodbye.

Stay safe.

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Scope and Limitations

Applies to LV/MV CT secondary safety. ESA and FSA are FEXLINK references; parameters follow the latest catalogue. HV CT (110kV+) follows stricter utility safety codes. This article is not a work instruction—live CT work must be performed by certified electricians per established procedures.