What is the insulation system?
The insulation system is the layers that keep the windings apart from each other and from the core. Its main job is to hold the loop’s high voltage on the primary side, away from the fitting.
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Normal. Each layer does its job: the coating and turn insulation keep the turns apart, the barrier keeps the loop’s high voltage on the primary side, and the core and case insulation keep every winding off the core. The secondary sees only a low voltage, and its current circulates opposite to the primary’s, as the moving dots show.
The insulation system of a series isolation transformer is the set of insulating layers between its turns, its windings, its core and its case. The most important is the barrier between the primary and secondary. The secondary carries only a low voltage (a few volts up to a few tens of volts, depending on the lamp), but the primary is in a series loop that can be at thousands of volts to earth, so the insulation is rated for the whole loop, not for the winding voltage.
Insulation usually wears out slowly. Moisture through a damaged lead or housing, heat and age lower its resistance, and the transformer may keep working while it weakens, so the fault is found by insulation-resistance testing or a falling trend. If the barrier breaks down, the loop’s potential can reach the secondary side and the fitting, and any path from there to earth then shows as an earth fault, and the fitting may still light. The insulation cannot be repaired in the field, so a suspect unit is replaced. Isolate and prove the circuit dead under site procedures before touching any lead.
Frequently Asked Questions
Q1What is the insulation system in an AGL isolation transformer?
The set of insulating layers that keep the conductors apart: a coating on each wire, insulation between turns and layers, a barrier between the primary and secondary windings, and insulation to the core and case. Together they keep the loop’s high voltage off the secondary, the leads and the fitting.
Q2Why must the insulation be rated far above the secondary voltage?
The secondary carries only a low voltage, but the primary is part of a series loop that can sit at thousands of volts relative to earth. The insulation has to withstand the voltage of the whole loop, not the low voltage across the winding. The insulation class is shown on the nameplate.
Q3What damages the insulation system?
Mainly moisture getting in through a damaged lead, connector or housing, and heat from overload or poor cooling. Age and physical damage also wear it down. Weak insulation often shows up first as a falling insulation-resistance reading, long before it fails.
Q4How do you tell the insulation is weakening?
With an insulation-resistance test: the circuit is isolated and disconnected from the CCR, and a DC test voltage is applied under site procedures. A low reading, or one that falls from test to test, points to moisture, damage or ageing. An earth-fault alarm can also be the first sign.
Q5What happens if the insulation breaks down between primary and secondary?
The loop’s potential can reach the secondary side and the fitting, and any path from there to earth shows up as an earth fault. The fitting may still light, so a breakdown is not always obvious. Treat every lead as live, isolate under site procedures, and replace the transformer, since the insulation cannot be repaired in the field.
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