What is a ring and loop cable arrangement?
A cable run that returns to its source, so one faulted section can be cut out and the substations beyond it fed the other way round.
A radial cable feed is one path from the source to the loads, so a cable fault darkens everything beyond it until the cable is repaired. A ring cable leaves the source, passes through each substation or CCR room, and returns to the source, so every point can be fed from either direction. Switches at both ends of each section let a faulted section be isolated while the rest is supplied the other way.
Rings are usually run open, with one normally open point so power flows one way round. When a section faults, its switches open and the open point closes, and the loads beyond the fault come back. For that to work every cable and switch must be rated for the reversed load, not just the normal one, and the two directions must not share one trench, or a single dig takes out both.
A series lighting circuit is also a loop, out through the lights and back to the CCR, but it is one path: a break anywhere opens it, as the open circuit entry shows. Ring redundancy belongs to the power cables feeding the substations, alongside dual feeders and network redundancy for control.
Two paths round. A fault is cut out and the rest re-fed.
One path. Everything beyond the fault is dark until repaired.
| Situation | Ring state | Effect |
|---|---|---|
| Ring healthy | Run open at one point | Every substation fed, power flowing one way |
| Fault on one section | Section isolated, open point closes | Loads re-fed the other way after a short interruption |
| Cable sized for normal load only | Reversed load too high | Overload or trip after reconfiguration |
| Isolating switch fails to open | Fault not isolated | Wider outage than the fault needs |
| Both directions in one trench | One dig cuts both paths | Ring gives no protection |
| Second fault before the first is repaired | Ring already open | Part of the ring dark |
| Radial feed, same fault | Single path | Everything beyond the fault is dark |
Frequently Asked Questions
Q1What is a ring cable arrangement?
A ring is a distribution cable run that leaves a source, passes through each substation or CCR room, and returns to the same source or to a second one. Every point can then be fed from either direction. Switches at both ends of each cable section let a faulted section be isolated while the rest of the ring is supplied the other way round.
Q2How is a ring different from a radial feed?
A radial feed is a single path from the source out to the loads, so a cable fault leaves everything beyond it dark until the cable is repaired. A ring gives a second path, so after a fault the faulted section is isolated and the loads beyond it are re-fed the other way. There is usually a short interruption while the switching is done, unless the ring is automated.
Q3Is a series lighting circuit already a loop?
Yes. A series circuit runs from the CCR out through the lights and back to the CCR, so it is a loop in the electrical sense. But it is one path: a break anywhere opens the whole circuit, as the open circuit entry explains. A ring arrangement is about the power cables that feed substations and CCR rooms, where a second path is possible, not about the series loop itself.
Q4Why is a ring usually run open?
A ring normally has one open point, so power flows one way round and protection stays simple. When a section faults, that section is isolated and the open point is closed to feed the loads from the other side. A closed ring keeps both paths live and can restore faster, but needs protection that can tell which section has faulted, which is more complex.
Q5What defeats a ring?
Both directions sharing one trench or duct, so a single dig removes both. Cables or switches sized only for their normal load, which overload when the load reverses. A switch that fails to open, so the fault is not isolated. And a second fault before the first is repaired, since a ring is built to survive one fault, not two.
Q6Is a ring arrangement required?
It is design practice, not a rule for every airfield. Whether a ring is needed, and how fast the supply must be restored, depends on the standard, the size of the airfield 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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