Ask a protection engineer why a distance relay did not trip instantly for a fault on its own line, and the answer is usually the same: the fault was in Zone 2, and Zone 2 is designed to wait.
A distance relay does not measure distance directly. It measures voltage and current at the relay location and calculates the impedance to the fault:
Z = V / I
Because line impedance is roughly proportional to length, that impedance is a proxy for distance. If the calculated impedance is less than the relay's reach setting, the relay decides the fault is inside its zone. That simple idea — compare a measured impedance with a set reach — is the basis of distance protection, ANSI device 21, used in IEEE/ANSI practice and, in a similar form, under IEC standards.
Zone 1: fast, but deliberately short
Zone 1 is the relay's first line of defence for its own line. It is usually set to about 80 to 85 percent of the protected line's impedance, and it trips with no intentional time delay.
Why not 100 percent? Because no measurement is perfect. Current and voltage transformers have errors, the line impedance data are never exact, and fault resistance and infeed from the far end can make a fault look closer or farther away than it really is. If Zone 1 reached all the way to the far bus, a fault just beyond that bus could look as if it were on this line. So Zone 1 stops short on purpose, leaving a 15 to 20 percent gap at the far end. That gap is not a mistake; it is the price of being certain. Something else has to cover it, and that something is Zone 2.
Zone 2: the patient backup
Zone 2 reaches farther, typically to about 120 percent of the protected line — the whole line plus about 20 percent into the next line. It covers the far end that Zone 1 leaves unprotected, and it also provides backup if the relay at the far end fails to clear a fault on the next line.
But Zone 2 trips only after a time delay, commonly about 0.3 to 0.5 seconds. That delay is the whole point of this article.
Imagine a fault on the next line, just beyond the far bus. Both relays can see it: the far relay sees it in its Zone 1 and trips instantly, and our relay sees it in its Zone 2. If our relay also tripped instantly, both lines would be lost for a fault on only one of them. By waiting, Zone 2 gives the correct relay time to do its job first. Only if that relay or its breaker fails — if the fault is still there after the delay — does Zone 2 step in. In protection language, that is selectivity: the relay nearest the fault operates first, and the more distant relay acts as backup.
Zone 3: backup for the backup
Zone 3 reaches still farther, often to about 150 percent of the protected line or more, with a longer delay of about one second. It is remote backup for faults on lines beyond the next one, and it must be set carefully so heavy load is not mistaken for a distant fault.
A worked example
Take a 50 km line with a series impedance of 0.35 ohms per kilometre, in primary ohms:
Z_line = 0.35 x 50 = 17.5 ohms
Zone 1 reach = 0.80 x 17.5 = 14.0 ohms, instantaneous
Zone 2 reach = 1.20 x 17.5 = 21.0 ohms, after about 0.4 seconds
Now consider three faults.
Fault A, 30 km from the relay: Z = 0.35 x 30 = 10.5 ohms. That is less than 14.0 ohms, so Zone 1 sees it and trips instantly.
Fault B, 45 km from the relay: Z = 0.35 x 45 = 15.75 ohms. That is beyond Zone 1 but inside Zone 2, so the relay waits 0.4 seconds. If the downstream relay clears the fault first, ours resets. If the fault is still there, Zone 2 trips.
Fault C, beyond the Zone 2 reach: Z is greater than 21.0 ohms, so neither Zone 1 nor Zone 2 operates. That fault belongs to Zone 3, or to another relay altogether.
The formula basis, in one place
Z_measured = V / I
Z1 = 0.80 to 0.85 x Z_line, with no intentional delay
Z2 = about 1.20 x Z_line, with a delay of about 0.3 to 0.5 seconds
Z3 = about 1.50 x Z_line or more, with a delay of about one second
Those are typical starting points, not universal rules. The exact reaches and delays come from a coordination study for the specific network, and they must be checked against the actual line data, the instrument transformer ratios, and infeed conditions.
Why the wait matters in practice
The wait is what makes distance protection selective instead of merely fast. Without it, every relay that could see a fault would trip, and a single fault would take out several lines. With it, the nearest relay clears the fault, the healthy lines stay in service, and the backup is still there if the first attempt fails. When you review a distance scheme, do not only check the reaches. Check the timers too, and check that Zone 2 at one end coordinates with Zone 1 at the other: the delay must be long enough for the downstream breaker to open, with margin to spare.
If you want to see how the related protection calculations are laid out, with the formula and a worked example, browse the GridCircuits calculators.
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