How does an isolation transformer work?
The loop current drives the primary, a magnetic core carries the energy across, and the secondary feeds the fitting. With no electrical connection between the windings, a failed lamp does not break the loop.
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Normal operation. The constant loop current flows through the primary and sets up a changing magnetic field in the core. That field induces a current in the secondary, which lights the fitting. The two windings are never joined by a wire.
A series isolation transformer connects a light fitting to the constant-current loop without any electrical connection between them. The loop current flows through the primary winding, a magnetic core carries the energy across, and the secondary winding supplies the fitting at a low voltage.
Because the windings are isolated, a failed lamp or a fault on the fitting side does not open the loop or take down the other lights, and the loop’s high voltage is kept away from the fitting. The current ratio is set by the transformer design (6.6 A : 6.6 A is common for lamp fittings, but other ratios exist, so the current is not always the same on both sides), while the voltage on the secondary follows the load. Isolation does not make the circuit safe to touch: the primary is part of a high-voltage loop and an open secondary can show a higher voltage than normal, so treat every lead as live until the circuit is isolated and proved dead under site procedures.
Frequently Asked Questions
Q1What is an isolation transformer in AGL?
A transformer that couples a light fitting to the series circuit without any electrical connection between them. The loop current flows through the primary winding, a magnetic core carries the energy across, and the secondary winding supplies the fitting.
Q2Why are isolation transformers used on series circuits?
They keep the high-voltage loop away from the fitting, so the lamp or LED unit runs from a low-voltage secondary. They also mean a failed lamp or a fault on the fitting side does not open the loop and take down every other light on the circuit.
Q3How does energy get across if the windings are not connected?
The changing current in the primary sets up a changing magnetic field in the core. That field links the secondary winding and induces a current in it. The energy passes through the core, not through any wire joining the two windings.
Q4What happens if the lamp fails and the secondary goes open-circuit?
The primary still carries the loop current, so the rest of the circuit keeps running. With no load the core can saturate: the voltage across the primary rises, the waveform distorts, and the secondary terminals can show a high, peaked voltage. Treat it as live, and have the unit replaced or repaired under site procedures. Do not open or unplug a secondary on an energised circuit.
Q5Does isolation make the circuit safe to touch?
No. The primary side is part of a high-voltage loop and the secondary can show a hazardous voltage if it goes open. Treat every lead as live until the circuit has been isolated and proved dead under site procedures.
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