What are dual feeders and incoming supply redundancy?
Two independent incoming supplies and a bus tie, so losing one feeder costs a short transfer instead of the whole airfield substation.
One utility feed is a single point of failure for the whole airfield substation: if it drops, every CCR on the bus loses its input. Dual feeders remove that. Two independent incoming supplies, each with its own breaker, end on two bus sections joined by a normally open bus tie. Each CCR is fed from one bus, so in normal running feeder A supplies bus A and feeder B supplies bus B.
Lose one feeder and its incoming breaker opens, the tie closes, and the surviving feeder carries both buses. There is a short break while the breakers change over, then the lights come back. That only works if the survivor can take everything, so each feeder is sized for the full load. If both feeders go, the standby generator is the next layer. Here “incoming” means the utility side; the outgoing feeders to each CCR are on the single-line diagram.
Pair this with interleaved circuits: put CCR A of an interleaved pair on bus A and CCR B on bus B, and a dead bus thins the row of lights instead of blanking it, even if the transfer fails.
Two sources, two buses, one tie. Losing one feeder costs a short transfer.
One source, one bus. Losing it drops every CCR until the generator is on line.
| Situation | Supply state | Effect on the AGL |
|---|---|---|
| Both feeders healthy | Each bus on its own feeder, tie open | Normal; every CCR energised |
| One feeder lost, tie closes | Surviving feeder carries both buses | Short break during transfer, then normal |
| One feeder lost, tie fails to close | Bus on the lost feeder stays dead | Only CCRs on the healthy bus keep running |
| Surviving feeder too small | Overload after the tie closes | Load shed or second feeder trips |
| Both feeders lost | Generator starts, tie closed | Dark until the generator is on line |
| Common-mode fault (shared trench or upstream source) | Both feeders lost together | Dual feeders give no protection |
| Transfer control supply fails | Scheme cannot operate | No transfer, though one feeder is healthy |
Frequently Asked Questions
Q1What are dual feeders in an airfield substation?
Dual feeders are two separate incoming supplies to the airfield substation, each with its own incoming breaker, so the airfield keeps its power when one is lost. The low-voltage switchgear is split into two bus sections joined by a bus tie, which is normally open. Each CCR is fed from one bus, so in normal running feeder A supplies bus A and feeder B supplies bus B.
Q2Why not one feeder and a standby generator?
A generator covers a total loss of supply, but only after its start-up delay and only while the engine and fuel hold out. A second feeder covers the loss of one incoming supply with just the short transfer, so the generator is called on less often. Many airfields use all three layers: two feeders, a bus tie and a standby generator.
Q3Does each feeder have to carry the full load?
Yes, or the arrangement protects very little. If feeder B can carry only half the load, losing feeder A means shedding load or tripping feeder B as well. Each feeder, its incoming breaker and its transformer should be sized for the whole AGL load plus the other loads on the substation, checked at the worst case, such as every circuit at full brightness.
Q4Does the pilot see a break when it transfers?
Usually a brief one. In a dead-bus (open transition) transfer the incoming breaker opens and the tie closes, so supply is interrupted for a moment and the CCRs may drop out and restart. A closed-transition design avoids the break but briefly parallels two sources, which needs the utility's agreement and matching protection. How long a break is acceptable depends on the facility and runway category; see the generator changeover entry.
Q5What defeats dual feeders?
Common-mode failures. Both feeders from the same upstream substation or transformer, both cables in one trench or duct, a single DC or control supply for the transfer scheme, one protection relay, or one bus where both feeders end. If one event can remove both, they are two feeders on paper and one in practice. Independence should be checked from the utility source all the way to the bus.
Q6Are dual feeders required?
ICAO Annex 14 sets secondary power supply requirements by facility and runway category but does not say how to meet them. Two feeders is common design practice where a single supply is not enough, but whether it is required, and with what transfer time, depends on the standard and the operation. See ICAO Annex 14 Vol I Chapter 8, Doc 9157 Part 5 and FAA AC 150/5340-30, and confirm against your design basis.
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