What is transformer replacement?
Taking out a failed isolation transformer and fitting a correct new one in the same position, then sealing, testing and recording it before the circuit is restored. The primary is part of the series loop, so the order of work matters.
A transformer is replaced when inspection, testing or a fault shows it cannot be relied on: a failed winding, insulation resistance that stays low, a cracked or heat-damaged housing, or leads that cannot be repaired. The new unit must carry the circuit exactly as the old one did, in the same position, so the job is about the right part, the right connections and proof before the circuit goes live again.
1Make it safe, then label everything
Isolate and lock out the circuit and prove it dead, as your site procedure requires. A live series loop must never be opened. Before anything is disconnected, photograph and tag every lead so each goes back where it came from. Work on the airfield only with the right permission and under the site’s safety rules.
2Remove the old unit, fit the right new one
Lift the old transformer out by its body, never by its leads. Check the pit and the connectors while the unit is out; damaged connectors are replaced, not reused. Fit the part the manufacturer or specification calls for, with the right current ratio, power rating and leads or connectors for that position. A different rating changes the load on the circuit. Seat it on its support, make the connections to the tags, seal the connectors on clean, dry parts and refit the earth bond where the design has one.
- Tag and photograph before disconnecting
- Fit the specified type and rating
- Support the unit and refit any earth bond
- Lift or hang a transformer by its leads
- Substitute a part that only seems to fit
- Close the pit before testing
3Test, restore and record
Before the pit is closed, test insulation resistance and winding continuity. When the circuit is restored, confirm the secondary is connected to the fitting, because a series transformer energised with its secondary open can produce a dangerous voltage. Then check that the fitting lights and that the CCR output and alarms are normal. Record the new unit, its location, the date and the results, and update the as-built records where needed.
| Stage | What to do |
|---|---|
| Reason for replacement | Defect from inspection, test or fault, noted and photographed |
| Circuit | Isolated, locked out and proved dead |
| Old unit | Leads tagged and photographed; lifted by the body |
| Replacement | Specified type, current ratio, power rating and leads |
| Mounting | Seated on its support, not hanging by the leads |
| Connections | On the tagged leads; connectors clean, dry and sealed |
| Earth bond | Where the design has one: refitted, tight and undamaged |
| Tests | Insulation resistance and continuity before closing the pit |
| After restoring | Fitting lit; CCR output and alarms normal |
| Records | Unit, location, date, readings; update as-built records |
Frequently Asked Questions
Q1When is an isolation transformer replaced?
When inspection, testing or a fault shows it can no longer be relied on, for example a failed winding, low insulation resistance that stays low, a cracked or heat-damaged housing, or damaged leads that cannot be repaired.
Q2Can a transformer be replaced with the circuit live?
No. Its primary is part of the series loop, so the circuit must be isolated, locked out and proved dead first. A live series loop must never be opened.
Q3Does the new transformer have to match the old one?
Use the type and rating the manufacturer or project specification calls for, with the right current ratio, power rating and leads or connectors for that position. A different rating changes the load on the circuit.
Q4Why label the old transformer's leads before removing it?
So each lead goes back to the right connection on the new unit. Record or photograph the connections before anything is disconnected.
Q5What should be checked after the new transformer is fitted?
Test insulation resistance and winding continuity before the pit is closed, then once the circuit is restored check that the fitting lights and the CCR output and alarms are normal. Record the unit, the date and the results.
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