Common Isolating Transformer Issues and How to Detect Them

One of the earliest indicators of isolating transformer deterioration is an abnormal increase in primary voltage during testing. This can point toward several possible conditions, including:

• Increased losses from shorted turns or winding damage
• Core deterioration or overheating damage
• Increased impedance, or a load mismatch between transformer rating and fitting
• Poor or high-resistance primary connections

When the transformer condition worsens, the Constant Current Regulator (CCR) compensates by increasing voltage to maintain constant current in the circuit. This is why rising primary voltage should never be ignored during routine maintenance — a gradual increase can be an early indication that a transformer is no longer operating under normal conditions.

Compare like with like: log the CCR output voltage at the same brightness step and rated current, and test individual units at the manufacturer’s test conditions. Insulation degradation and moisture ingress mainly show up as falling insulation resistance or leakage to earth, not as a higher primary voltage, so they need the insulation tests described below.

If one light appears dimmer or brighter than adjacent fittings in the same circuit, the issue may not always be the lamp. The isolating transformer may be operating inefficiently, developing partial winding damage, or producing unstable secondary output.

In AGL systems, uniformity is extremely important — small intensity variations can become highly visible to pilots during low-visibility operations. When a fitting behaves differently from its neighbours, check the simple causes first — lens soiling, lamp or LED module, secondary connector — and then consider the transformer. Under Annex 14 maintenance provisions, a light whose main-beam average intensity falls below half the specified value is treated as unserviceable, so persistent dimming is a compliance matter as well as a reliability one (check the current edition for the exact rule).

Healthy isolating transformers generate heat during operation. However, abnormal heating is a warning sign. Possible causes include overloading, internal shorted turns, poor secondary connections, and age-related insulation breakdown.

Field technicians should pay particular attention to transformers that run noticeably hotter than nearby units under similar loading — assessed by thermal imaging or on de-energised, isolated equipment, never by touching live series-circuit equipment. Thermal irregularities can indicate internal deterioration long before total failure occurs — a transformer that consistently runs abnormally hot should receive further investigation, not be dismissed as normal operating heat.

Routine testing can reveal secondary voltage or current values outside acceptable limits. Such abnormalities may indicate efficiency loss, winding damage, load mismatch, or partial internal faults.

Secondary abnormalities directly affect lamp performance and circuit stability. Ignoring these readings increases the probability of sudden outages during airport operations — secondary-side measurements should be a routine part of transformer condition monitoring.

Watch for an open secondary (failed lamp, broken lead or unplugged connector). An isolating transformer keeps the series circuit intact when the secondary opens, but the secondary voltage and heating rise, so it should be corrected promptly and within the manufacturer’s limits.

Preventive testing is not only about compliance — it is about identifying degradation before operational failure occurs. Routine transformer testing can help detect:

• Insulation deterioration
• Efficiency reduction
• Load mismatch between transformer rating and fitting
• Leakage current issues
• Thermal stress conditions

Periodic testing also helps maintenance teams identify transformers approaching end-of-life conditions before they impact runway availability. The objective should be to identify trends and warning signs early rather than waiting for a complete component failure. Insulation resistance is tested between primary and secondary and from each winding to earth, only on an isolated, locked-out circuit, with results trended against earlier readings and the manufacturer’s limits rather than judged from a single number.

A transformer fault is not always limited to one fitting. In a series AGL circuit, an open primary winding or a developing insulation fault can affect the whole circuit, leading to:

Runway lighting failures can directly affect aerodrome operations and, in low visibility, safety. This makes early detection particularly important in AGL applications — a transformer may be physically small, but its failure can have consequences far beyond the individual component.

The traditional approach of replacing a transformer only after failure is reactive. A more reliable approach is to monitor the transformer throughout its operating life:

The earlier deterioration is identified, the greater the opportunity to plan maintenance before an unexpected outage occurs.

Series isolating transformers are specified in FAA AC 150/5345-47 (L-830/L-831) and IEC 61823; maintenance of visual aids is covered by ICAO Annex 14 Volume I and the Aerodrome Design Manual (Doc 9157) Part 5. Always work to the manufacturer’s test procedure and acceptance limits, and check current editions and your regulator’s requirements.