Protection zones
The key idea
A protection scheme cuts the network into zones. Each zone has its own relay and its own breaker. The zones overlap at the breakers, so every part of the network sits inside at least one zone. Every fault then runs a fixed script. The relay of the faulted zone acts first. Each layer behind it waits longer, and it disconnects more of the network when it acts.
The idea
A relay does not watch the network. It watches a zone. A zone is a defined stretch of equipment, and the breakers that the relay can open bound it. Current transformers at those breakers measure the current in the zone. Draw the zones for a scheme, then check one thing first. Every cable, busbar and transformer in the drawing must sit inside at least one zone.
That requirement is why zones overlap. Imagine that Zone 1 ended exactly where Zone 2 began. The boundary would be a single line on the drawing, and the breaker on that line would belong to neither zone. No relay would then clear a fault at that point. So engineers cross the zones over each other at the breaker. They assign the current transformers on each side of the breaker to different zones, and the breaker itself sits inside both zones.
A fault in the overlap trips both zones, and that costs a slightly wider outage. The trade is deliberate: a small overlap is better than a small stretch with no protection.
Once the zones exist, every fault follows an ordered script. Primary protection is the first layer. The relay of the faulted zone sees the fault, times out, and trips its own breaker. The clearing time is the relay operating time plus the interrupting time of the breaker. Nothing else in the network has to move.
Backup is the second layer. The upstream relay sees the same fault, because its zone reaches past the boundary. A grading margin makes that relay act more slowly on purpose. If the primary relay never acts, the backup clears the fault later. It opens a breaker further upstream, so healthy equipment goes off with the faulted equipment.
The TCC curves page covers that grading margin on inverse curves. The model below uses definite times, so the margin here is a flat step rather than a gap between two curves.
Breaker failure is a different failure, and it needs its own answer. Here the relay works correctly and sends the trip command, but the breaker does not open. A breaker-failure timer starts with the trip command. It checks whether current still flows after the normal opening time of the breaker. When the timer expires, it trips the breakers around the stuck one. This layer clears the fault sooner than graded backup, because its clock starts at the trip command and not at the fault.
Try it
Clear a fault on feeder 2 in the ordinary way. Then fail the relay, and then jam the breaker. Read the clearing times from the sequence: 0.30 s, 0.70 s, 0.55 s.
cleared: 0.30 s by B2 · off supply: Feeder 2 and the load
0.00 s · Fault · fault
The fault is on Feeder 2. It sits inside Zone 2, and inside the backup reach of Zone 1.
0.00 s · R2 · relay picks up
R2 picks up. Feeder 2 is its own zone, so R2 is the primary relay for this fault.
0.20 s · R2 · trip command
R2 trips B2 after its 0.20 s operating time.
0.30 s · B2 · breaker opens
B2 opens 0.10 s after the trip command, and the current stops.
0.30 s · B2 · fault cleared
B2 clears the fault at 0.30 s. Feeder 2 and the load are off, and Feeder 1 stays live.
Fixed model: relay 0.20 s, breaker 0.10 s, backup graded 0.40 s behind the primary. The breaker-failure timer runs 0.25 s from the trip command. The zones overlap at B2, so a relay watches every stretch of the chain. The backup waits on purpose, because the primary needs time to finish first. When the backup does act, it disconnects more of the network.
Why it matters
- A gap between zones is unprotected equipment. The overlap is deliberate, and it is not an accident of the drawing. A fault between two current transformers must still sit inside a zone. The cheapest way to guarantee that is to put the breaker inside both zones.
- Every backup operation is a bigger outage. Backup does not only clear late. It clears from further upstream, so a fault on one feeder can also disconnect the feeders beside it. That is the real cost of a relay that fails to operate. For that reason commissioning tests check the primary relay rather than assume it works.
- A stuck breaker is not a slow relay. If graded backup has to cover breaker failure, the fault stays on the network for the full margin. A dedicated timer reduces that time to a fraction of the margin. It is the only layer that acts when the relay is correct and the switchgear is not.
- The deliverable is a sequence, not a setting. A protection study answers three things for each credible fault: what happens, in what order, and what the network loses. On meshed and ring networks the answer also depends on direction. That is the subject of directional overcurrent and of the network topology.
The math, if you want itOptional — the page reads completely without it
The backup relay must wait for the whole primary sequence before it acts:
graded backup time
tbackup = tprimary + tbreaker + tmargin
The widget uses 0.20 s of relay time, 0.10 s of breaker time, and 0.30 s for relay overshoot, timing tolerance and safety. Those numbers give a 0.40 s step, so the backup relay operates at 0.60 s. The TCC curves page measures the same margin on inverse curves. Here both relays use definite time, so the step is a single number instead of a gap that changes with current.
The breaker-failure timer sits inside that budget. It must be longer than the breaker time plus the reset time of the current check. A shorter timer would operate during healthy operations:
breaker-failure timer
tBF > tbreaker + treset · tclear = tprimary + tBF + tbreaker
The widget numbers give 0.20 + 0.25 + 0.10 = 0.55 s. The graded backup would clear the same stuck breaker at 0.70 s.
See it in Phasor
In Phasor the zones come from the single-line diagram itself. Each protective device and its current transformers bound one zone. The study reports which equipment falls inside which zone, and where the overlaps are. It also names the stretches that have only one layer of protection. Apply a fault at any point, and you get the sequence of operation. The sequence lists primary, backup and breaker failure, each with its time and the load it disconnects.