What are secondary shorting and overvoltage events?
The two ways a series transformer is stressed from outside its normal duty. A short makes the lamp go out but cannot overload a constant-current loop, while an overvoltage tests the insulation.
A series isolation transformer is built for one duty: pass the circuit current to a lamp. Two things can push it outside that duty. The secondary can short, and the loop can see an overvoltage. They look different on the airfield and stress the unit in different ways.
1A short does not overload the loop
The CCR supplies a fixed current, not a fixed voltage. A shorted secondary cannot make it push more, so the loop current stays at its set value and the primary is not overloaded. The secondary voltage collapses towards zero and the lamp goes out, and the CCR output voltage falls a little because the load is lighter.
2The heat goes to the fault
The secondary still carries its full current, now through the short, so the power is released at the fault point and in the secondary leads and connector. A poor, high-resistance short can run hot or arc, which is why damaged or wet leads and connectors are inspected after a lamp-out.
3Overvoltage tests the insulation
A surge on the primary loop, a high voltage when the current is interrupted, and the peaky voltage of an open secondary all land on the insulation between the windings, to the core and to earth. A single event may pass unnoticed and leave a weak spot that fails later.
4Protect it, then prove it
Protection is layered: the transformer's insulation and potting, sound earthing and bonding, surge protection on the CCR and the circuit, and the CCR's open-circuit protection. After a short or a surge, isolate and prove the circuit dead, then check insulation resistance and continuity before it goes back in service.
| Event | Typical cause | Secondary voltage | Loop current | Main effect |
|---|---|---|---|---|
| Secondary shorted | Lamp or lead short, damaged connector | Close to zero | Unchanged, held by the CCR | Lamp out, local heating |
| Secondary open | Failed lamp, loose or open lead | High and peaky | Unchanged, held by the CCR | Core saturates, treat as live |
| Surge on the loop | Lightning or switching event | Short high spike | Disturbed briefly | Insulation stress, possible hidden damage |
Frequently Asked Questions
Q1What happens when the secondary of a series isolation transformer is shorted?
The loop current stays at its set value, because the CCR holds it constant, so a short is not an overcurrent on the primary. The secondary voltage collapses towards zero and the lamp goes out. The heat is produced at the fault point and in the secondary leads, which carry the full secondary current.
Q2Is a shorted secondary the same as an open secondary?
No. Both put the lamp out, but an open secondary drives the core into saturation and can show a high, peaky voltage at its terminals, while a shorted secondary shows almost no voltage and keeps the core flux small. The risk with a short is local heating and damage at the fault, not a high terminal voltage.
Q3Why is a short not a heavy fault like on a mains supply?
A series circuit is fed with a fixed current, not a fixed voltage. Shorting a load cannot make the CCR push more current through it, so the fault current is limited to the circuit current. The CCR output voltage simply falls a little because the load is lighter.
Q4What overvoltage events can reach an isolation transformer?
Typical ones are a lightning or switching surge travelling along the primary loop, a high voltage when the circuit current is interrupted or the loop opens, and the peaky voltage of an open secondary. The insulation between the windings, to the core and to earth takes the stress.
Q5What protects the transformer from overvoltage?
The insulation system and the potting, a sound earthing and bonding arrangement, surge protection devices on the CCR and the circuit, and the open-circuit protection of the CCR. How much protection is fitted and how it is rated depends on the design and the specification.
Q6What should I do after a short or a surge on a circuit?
Isolate the circuit and prove it dead under the site procedures, then inspect the transformer, its leads and connectors for heat damage and water, and check insulation resistance and winding continuity before the circuit goes back in service. Replace any unit that fails the checks.
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