What are buried and elevated transformers?
Series isolation transformers installed below ground or above it, and how the two positions differ in protection, access and risk.
A series isolation transformer can sit below ground or above it. A buried unit is installed in a base can, a transformer housing or a pit. An elevated unit is mounted above ground, for example in an enclosure, a fixture stem or a support. Either way its primary is in the series loop and its secondary feeds the fitting.
1Buried: protected, but wet
Below ground the unit is shielded from weather and impact, and nothing stands up to be hit. The price is water: a pit can flood, seals can fail and frost can heave a housing. Reaching it means opening the pit, and buried cables must be located before anyone digs.
2Elevated: easy to reach, exposed
Above ground the unit is easier to reach and keeps the connections out of the pit water. It is exposed to sun, wind, vibration and impact. On the operational area, equipment and supports must be frangible where required and mounted as low as practicable, so the enclosure and its support must meet that requirement.
3The circuit is live either way
Position changes the hazards, not the electrics. The primary is part of a live series loop, so the circuit must be switched off, isolated, locked off and proved dead before any work, and discharged or earthed as the site procedure requires, whether that means a wet pit or working at height.
4Match the whole unit
A buried and an elevated unit do the same job. A replacement must still have the same current ratio and a suitable wattage rating, and the position is set by the design, not chosen on site.
| Point | Buried | Elevated |
|---|---|---|
| Position | Below ground, in a base can, housing or pit | Above ground, in an enclosure, stem or support |
| Protection | From weather and impact | From pit water and flooding |
| Main risk | Water ingress, flooding, frost | Sun, wind, vibration, impact |
| Access | Open the pit; locate before digging | Reach the enclosure; height work on structures |
| Special point | Seals and drainage | Frangibility where required |
Frequently Asked Questions
Q1What is a buried isolation transformer?
It is a series isolation transformer installed below ground, in a base can, a transformer housing or a pit, with its primary in the series loop and its secondary lead running up to the fitting. It is protected from weather and impact, but exposed to water, flooding and frost.
Q2What is an elevated isolation transformer?
It is a series isolation transformer mounted above ground, for example in an enclosure, a fixture stem or a support, where the fitting stands above ground. It is easier to reach, but exposed to sun, wind, vibration and impact.
Q3Does the position change what the transformer does?
No. A buried and an elevated unit do the same electrical job: the primary is in the series loop and the secondary feeds the fitting. The current ratio and wattage rating still come from the nameplate and must suit the circuit and the fixture.
Q4Why does frangibility matter for elevated units?
Equipment and supports on the operational area are required to be frangible where specified, and mounted as low as practicable, so that they break away rather than damage an aircraft. An elevated transformer and its enclosure must fit that requirement, so check ICAO Annex 14 and the authority's requirement.
Q5What are the main hazards when working on each?
For a buried unit they are a wet or flooded pit and buried cables that must be located before digging. For an elevated unit they are working at height and an exposed enclosure. In both cases the circuit must be isolated, locked off, proved dead and discharged or earthed as the site procedure requires, because the primary is part of a live series loop.
Q6Who decides whether a unit is buried or elevated?
The design, which depends on the fixture type, the site conditions and the authority's requirement. A transformer should not be relocated on site without the designer's agreement, because cable lengths, sealing and frangibility all depend on its position.
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