Teleprotection schemes (ANSI 85) — permissive and blocking with distance protection

The guide on distance protection (21) — impedance protection and zones covers how a single distance relay measures the impedance to a fault and, based on that, trips in preset zones with an increasing time delay: zone 1 typically covers only 80-90% of the line length instantaneously, to avoid overreach from measurement uncertainty, while the remaining part of the line is tripped with a delay via zone 2. This article covers how that inherent delay on the last part of the line is avoided by having the relays at either end of the line "talk" to each other over a communication channel — referred to in practice by ANSI code 85 (pilot or teleprotection relay).

The problem: zone 1 never fully covers the line

A fault that falls just outside the reach of zone 1 (in the last 10-20% of the line) is only tripped by the local relay after the zone 2 delay, even though the relay at the other end of the line may well measure that same fault instantaneously in its own zone 1. Without communication between the two relays, the fault persists for the full zone 2 time, even though relay information is available at one end of the line that would allow a faster trip. Teleprotection solves this by exchanging that information between the two relays over a communication channel (fibre optic, a dedicated pilot cable, or a signal over the high-voltage line itself).

Permissive overreach transfer trip (POTT)

With POTT, each relay sets its instantaneous zone to reach beyond the end of its own line (overreach) — enough to cover the full line length with margin. As soon as a relay detects a fault within that overreach zone, it sends a permissive signal to the relay at the other end. Only when a relay both detects a fault in its own overreach zone and receives the permissive signal from the other end does it trip instantaneously. This double condition prevents a relay from tripping incorrectly based solely on its own, further- reaching (and therefore more sensitive to external faults) zone setting.

Permissive underreach transfer trip (PUTT)

With PUTT, each relay operates with its normal, underreaching zone 1 (which deliberately does not cover the last 10-20% of the line to avoid overreach). As soon as a relay's zone 1 picks up, that relay knows with certainty that the fault lies within its own line — an underreaching zone can by definition not pick up for a fault beyond its own line. The relay therefore sends a permissive signal to the other end, which is thereby given permission to trip its own, overreaching zone 2 instantaneously instead of waiting for the normal zone 2 delay.

Note: both POTT and PUTT require the relay itself to also detect a fault before it acts on the received signal — the communication signal alone is never sufficient to trip. This makes both schemes inherently more secure against a fault in the communication channel that, without this requirement, could lead to an unwanted, unfounded trip.

Blocking schemes: the reverse logic

With a blocking scheme, the logic works the other way around: each relay uses an overreaching zone that looks backward, outside its own line (a "reverse-looking" element). If a relay detects a fault in that reverse direction, it sends a blocking signal to the other end, which prevents an instantaneous trip there — after all, the fault lies outside its own line, as seen from that relay. If the blocking signal is absent, the receiving relay is permitted to trip its own overreaching, forward-looking zone instantaneously. A blocking scheme is inherently less dependent on the availability of the communication channel than a permissive scheme: if the communication itself fails, the blocking signal is simply absent, and the relay still trips — possibly slightly less selectively, but not more slowly — whereas a communication failure in a permissive scheme means fast tripping is no longer possible at all.

Practical relevance

When assessing the protection concept of a high-voltage line, it is important to check not only the zone settings of the distance relays individually, but also to verify which teleprotection scheme (POTT, PUTT, or blocking) is used, whether the communication channel has sufficient availability and speed for the chosen scheme, and what the protection behaviour is if that channel fails — a permissive scheme and a blocking scheme respond fundamentally differently to the same communication fault.

Common mistakes

Applying a permissive scheme (POTT/PUTT) on a line with an unreliable or frequently failing communication channel, without realising that the fast, full-line-length trip is then lost and the fault falls back to the delayed zone 2 time.
Not correctly setting the reverse-looking element in a blocking scheme, so that a fault outside the own line is not recognised as a reason to send the blocking signal.
Forgetting that with both POTT and PUTT the relay must always also detect a fault itself before acting on the received signal, and treating this as a redundant extra condition rather than an essential security margin.
Not factoring the teleprotection scheme into the assessment of the selectivity of the entire protection chain, while the scheme directly determines how fast a fault over the full line length is tripped.