What is frangible mounting at installation?
Fitting an elevated fixture so it stands in normal use but breaks away cleanly if struck — the right coupling at its designed break point, nothing added to stiffen it, and a lead that lets go.
Frangible mounting at installation is fitting an elevated fixture on a frangible coupling that is strong enough for wind and normal use but breaks at its designed break point if an aircraft or vehicle strikes it. The fixture then separates and the base can and foundation stay in the ground. It is done as part of fixture installation, at the height set in fixture elevation.
The installer’s job is mainly to avoid defeating the design. Extra bolts, grout or concrete around the coupling, a bracket or guard bridging it, rigid conduit, or a longer or stiffer part can stop it breaking where it should. Use the manufacturer’s coupling as supplied, tightened to the specified value, and leave the lead slack with a connector that can release. Replace any coupling that has been struck, bent or cracked. For the sign version of this idea, see Frangible Mounting. Work only on a circuit that has been isolated, locked out and proved dead.
- Isolate. Switch off, lock out and prove the circuit dead before any work.
- Check the part. Confirm it is the correct coupling and fixture and that it is undamaged.
- Set the base. Level the base can at the design height and clean the threads and faces.
- Fit the coupling. Position it so the break point is where the design requires and tighten to the specified torque.
- Connect. Make and seal the connector as specified, leaving the lead slack below the break point.
- Verify and record. Check nothing rigid bridges the coupling, clear soil and grout from around it, then log the result.
Frequently Asked Questions
Q1What is frangible mounting at installation?
Fitting an elevated fixture on a mounting that is designed to break away cleanly if an aircraft or vehicle strikes it. The mounting is strong enough to stand in wind and normal use, but it gives way at its designed break point so the fixture does little damage and the base stays in the ground.
Q2Why must a frangible coupling not be strengthened or modified?
Its strength is designed so that it breaks under impact. Extra bolts, grout or concrete around it, a bracket or guard bridging it, or a longer or stiffer replacement part can stop it breaking at the designed point. Fit the manufacturer’s parts as supplied and do not modify them.
Q3What must be considered about the cable and connector?
A rigid lead can hold the fixture to the ground after the coupling has broken. Leave the lead slack below the break point and use the connector arrangement the manufacturer specifies so the fixture can separate without tearing the base. Keep the connector sealed and clear of the break point. Any earth bond to the fixture should also be a flexible lead that can part, so it does not hold the fixture back.
Q4What should be checked after a frangible fixture is installed?
Check that the coupling is the correct part and undamaged, sits at the designed break point and is tightened to the specified value. Check that nothing rigid bridges or stiffens it, that soil, grass or grout is not built up around it, and that the fixture is at its design height. Record the result.
Q5Can a frangible coupling be reused after it has been struck?
A coupling that has been struck, bent or cracked should be replaced rather than reused, because its break strength can no longer be relied on. Inspect the base can, the connector and the lead for damage too, and follow the manufacturer’s data. Work only on a circuit that has been isolated, locked out and proved dead.
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