What is open-secondary behaviour and core saturation?
What a series isolation transformer does when its lamp circuit opens. The loop stays closed, but with nothing to oppose the primary the core is driven into saturation, and the secondary terminals must be treated as live.
In a series circuit the CCR holds the current constant, and every light sits behind an isolation transformer so that a failed lamp does not break the loop. The failed lamp leaves the transformer with an open secondary, while its primary is still carrying the full circuit current. That is the condition this entry describes.
1The balance is lost
With a working lamp, the secondary current opposes the primary, so only a small magnetising current is needed to build the flux in the core. When the secondary opens, nothing opposes the primary. The whole loop current now acts as magnetising current.
2The core saturates
A core can carry only so much flux. Once it reaches the knee of its magnetisation curve, more current adds very little flux, and the flux flat-tops instead of following a smooth sine wave. This is core saturation, and it is how a series transformer copes with a current that is held constant whatever the secondary does.
3The voltage turns peaky
The secondary voltage follows the rate of change of the flux. With a flat-topped flux, the flux changes quickly only as it passes through zero, so the voltage becomes narrow, high peaks instead of a sine wave. The terminals can show a higher voltage than normal, and an ordinary RMS meter may not show how high the peaks are. Treat an open secondary as live.
4The loop stays closed, the core runs hot
The primary stays in the loop, so the other lights stay on. This is not an open circuit, which is what an open primary winding would cause. A saturated core does carry extra core loss and heat, though, so a failed lamp should be found and corrected and not left for the transformer to tolerate.
| Condition | Secondary current | Core | Secondary terminals | Result |
|---|---|---|---|---|
| Loaded (lamp lit) | Opposes the primary | Small flux swing, below the knee | Low sine voltage | Normal operation |
| Secondary open | None | Driven up to the knee | Higher voltage than normal | Lamp out, loop still closed |
| Open and saturated | None | Flux flat-tops | Narrow, high voltage peaks | Core heating, treat as live |
Frequently Asked Questions
Q1What is open-secondary behaviour?
It is what an isolation transformer does when its secondary circuit is open, usually because the lamp has failed or the secondary leads or connector are disconnected. The series loop stays closed through the primary, but nothing on the secondary side is drawing current any more.
Q2Why does an open secondary saturate the core?
Normally the secondary current opposes the primary, so only a small magnetising current builds the flux. With the secondary open nothing opposes the primary, so the whole loop current acts as magnetising current. That drives the flux up to the knee of the core’s magnetisation curve, where the core saturates and the flux flat-tops.
Q3Why is the voltage on an open secondary a hazard?
A saturated core produces a distorted, peaky voltage, with narrow high peaks as the flux passes through zero. The terminals can show a higher voltage than normal, and an ordinary RMS reading may not show how high the peaks are. Treat an open secondary as live.
Q4Does an open secondary break the series circuit?
No. The primary winding stays in the loop and keeps carrying the circuit current, so the other lights stay on. This is different from an open primary winding, which breaks the loop and puts the whole circuit out with an open-circuit alarm at the CCR.
Q5Can a transformer run with an open secondary for long?
Series isolation transformers are designed to tolerate it, but a saturated core runs with extra core loss and heating. How much and for how long depends on the design, so check the manufacturer’s data and the specification, and do not leave a failed lamp as it is.
Q6What should I do when I find an open secondary?
Do not touch the secondary leads or connector on the assumption that they are safe. Isolate the circuit and prove it dead under the site procedures before handling anything, then find the cause, such as a failed lamp or a loose or damaged connector, and restore the secondary circuit.
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