In the POTT (permissive overreaching transfer trip) scheme, each end sends a permissive signal when its overreaching element sees a forward fault, and trips without delay if its overreaching element is picked up and it also receives the permissive signal from the other end. Only an internal fault is seen forward by both ends, which makes the scheme selective over 100 % of the line.
What the POTT scheme is
The POTT (permissive overreaching transfer trip) scheme is one of the most widely used teleprotection schemes at high and extra-high voltage. Unlike PUTT, the signal is not sent by zone 1 but by an element that overreaches the line in the forward direction:
- Each end sends a permissive signal when its overreaching element sees a forward fault.
- Each end trips without delay if its overreaching element is picked up and it receives the permissive signal from the other end.
The idea is simple: an internal fault is forward for both ends; an external fault is behind one of them, which sends no permissive signal. It is a device 85 logic (ANSI/IEEE C37.2), modeled in IEC 61850 with logical node PSCH, working on top of distance protection 21 and directional ground elements. Its place among the other schemes is covered in the guide on teleprotection schemes.
How it works: logic and signals
The overreaching element is usually distance zone 2 or a zone dedicated to the scheme, set well beyond 100 % of the line. Many utilities add a directional ground element (67N) or a negative-sequence directional element to detect high-resistance faults that distance protection does not see.
Sequence for an internal fault anywhere on the line:
- The overreaching elements at A and B pick up; both send a permissive signal.
- A receives the permissive from B; since its overreaching element is picked up, it trips.
- B receives the permissive from A and trips.
For an external fault behind B, A’s overreaching element picks up and sends a permissive signal, but B sees the fault in reverse, sends nothing and blocks its own trip. A receives no permissive and waits for its zone 2 delay.
Modern implementations also use reverse elements that block the send and the local trip when the fault is behind. An SEL technical paper calls this variant a “hybrid scheme”, because it combines the permissive signal with a local block, and stresses that the reverse element must be at least as sensitive as the overreaching element at the remote end.
Channel and timing
As in PUTT, a false command alone does not trip. The channel must provide short transmission time and good dependability: a lost or late permissive turns accelerated clearing into zone 2 clearing. If the channel is lost, POTT stops accelerating but does not overtrip.
Since the permissive signal is needed in both directions at the same time, the channel must be duplex. With power line carrier on the same line, an internal fault can attenuate exactly the signal that is needed; that is why the DCUB unblocking variant is often used with that channel.
Advantages and limitations
Advantages:
- Works on short lines, where zone 1 is unreliable.
- With directional ground elements it detects resistive faults along the whole line.
- Secure against loss of channel and against spurious commands.
Limitations and the logic it requires:
- Weak-infeed end or end with an open breaker: that end does not see the fault and sends no permissive, so the strong end cannot accelerate. This is solved with echo and weak-infeed logic.
- Parallel lines: when a fault on the adjacent line clears, the current can reverse on the healthy line and cause a false trip. This is solved with current reversal guard.
- It depends on the channel being available: without it, the end zone is cleared with a delay.
Where it is used
POTT is the preferred scheme on many high and extra-high voltage lines with a dedicated digital channel (optical fiber, a multiplexer with IEEE C37.94, radio), on short lines, on series-compensated lines (where zone 1 must be shortened a lot and relying on underreach is not advisable; there the directional elements must be studied for the voltage or current inversion the capacitor can cause) and on lines with renewable generation at one end, always with echo. It is also the base scheme of distance relays with multi-bit channels, which allow separate permissive signals per phase or per element type.
How each manufacturer implements it
GE Vernova (Multilin, EnerVista) separates POTT and hybrid POTT as distinct schemes in relays such as the D60. SEL (AcSELerator QuickSet) offers it as a selectable scheme with its current reversal and echo logic; the SEL-421 adds two-channel (POTT2) and per-phase (POTT3) variants. Siemens SIPROTEC 4 7SA (DIGSI) includes it in its distance teleprotection and handles echo, weak-infeed tripping and the transient blocking against current reversal separately. Hitachi Energy Relion 670 (PCM600) separates the scheme communication logic block (ZCPSCH) from the current reversal and weak-end infeed block (ZCRWPSCH). MiCOM P44x relays (GE Vernova MiCOM Agile and Schneider Electric Easergy MiCOM) call it POR, with open-breaker echo, weak-infeed tripping and current reversal guard. Zone names, timers and the way distance is combined with directional ground elements differ by brand.
Why it fails or misoperates
- False trip on a parallel line: current reversal guard disabled, or with times shorter than the channel reset delay.
- Trip for an external fault: the reverse element at one end shorter or less sensitive than the overreaching element at the other.
- Internal fault cleared in zone 2: weak-infeed end without echo, remote breaker open, channel in alarm or permissive with excessive delay.
- Trips on power swings or heavy load: overreaching element encroaching on the load region without power swing blocking.
Start by comparing the records from both ends: which element keyed the send, when it was received and what each relay saw in reverse.
How it is tested
At each end, secondary injection verifies the overreaching element, the reverse elements, the send, the trip on receipt, the echo and the current reversal guard. The complete scheme is validated with a GPS-synchronized end-to-end test, including internal faults, external faults behind each end and cases with one breaker open.
Did your POTT trip for a fault on the adjacent line, or fail to accelerate an internal fault? Our team reviews the events and settings at both ends remotely. See remote relay configuration or contact us.
References
- IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines.
- IEC 60834-1, Teleprotection equipment of power systems – Performance and testing – Part 1: Command systems.
- IEC 61850-7-4, logical node models (PSCH).
- Kasztenny, Mynam, Fischer and Guzmán, Permissive or Blocking Pilot Protection Schemes? How to Have It Both Ways, SEL, Western Protective Relay Conference, 2020.
- WECC Relay Work Group, Application Guide for Echo Keying Logic on Permissive Overreaching Transfer Trip Schemes, 2021.
- Blackburn and Domin, Protective Relaying: Principles and Applications.
Frequently asked questions
Which elements key the permissive signal in POTT?
Forward overreaching elements: usually distance zone 2 or a zone dedicated to the scheme, and often directional ground or negative-sequence elements for resistive faults.
Why does POTT need echo and current reversal guard?
Echo solves the ends that cannot send a permissive signal because their breaker is open or their infeed is weak. Current reversal guard prevents a false trip on parallel lines when the current direction changes as an external fault clears.
What happens to POTT if the channel goes down?
It does not overtrip: it stops accelerating and the line is left with instantaneous zone 1 and time-delayed zone 2. To keep dependability for faults that disrupt the channel, the unblocking variant (DCUB) is used.