Teleprotection schemes let the relays at the two ends of a transmission line share a signal over a communication channel so they can clear a fault anywhere on the line at high speed. They fall into two families that differ in what the signal means. In a permissive scheme, a relay sends a signal that permits the far end to trip - the far end trips only when it receives that permission along with seeing the fault itself. In a blocking scheme, a relay sends a signal that tells the far end not to trip, and the far end trips fast unless it receives that hold-off. The difference determines how each scheme behaves when the channel fails.
Permissive vs Blocking Schemes in one line: Permissive schemes (POTT and PUTT) send a trip-permit signal, so a relay trips only when it both sees a fault and receives permission from the far end. Blocking schemes (DCB) send a hold-off signal, so a relay trips fast unless it receives a block. Permissive favors security against false trips; blocking favors dependability of tripping. Their opposite channel-failure behavior is the key distinction.
Distance protection on a transmission line is fast for faults in the first zone, which reaches to about eighty to ninety percent of the line, but for faults in the last stretch near the remote end the local relay only sees them in a time-delayed second zone. So a fault near one end is cleared instantly by the near relay but only after a delay by the far relay, and slow clearing at one end stresses equipment and can hurt stability. The whole line cannot be cleared instantaneously from both ends by distance elements acting alone.
Teleprotection removes that delay by letting the two relays exchange a signal that conveys what each end sees. If the near relay knows the far relay also detects the fault as being on the line, the two can agree it is an internal fault and both trip at high speed, even for a fault the local relay would otherwise place in its delayed zone. This communication-aided tripping gives the full line high-speed protection from both terminals, which is why pilot schemes are standard on important transmission lines.
What differs between schemes is the logic of that signal - specifically whether the received signal is a condition to allow tripping or a condition to prevent it. That single design choice splits teleprotection into the permissive and blocking families and drives everything about how each behaves, especially when the communication channel is not available.
In a permissive scheme the signal is a trip permit. Each relay uses a directional element looking down the line, and when it sees a fault in the forward direction it sends a permissive signal to the other end. A relay trips at high speed only when two conditions coincide: its own element sees a forward fault, and it has received the permissive signal from the far end confirming that the far relay also sees the fault as internal. Requiring both an internal decision and permission is what makes the scheme secure - it will not trip on a signal alone or a local pickup alone.
The permissive overreaching transfer trip scheme, POTT, uses overreaching zones at each end. Each relay's forward element is set to reach beyond the remote terminal, so both ends see any internal fault, and each sends a permit when its overreaching element operates. Because both overreaching elements operate for any fault on the line, both permits are sent and both ends trip fast for the whole line. For an external fault beyond one end, that end's element does not see it as forward, so no permit is sent and the scheme correctly restrains. The permissive underreaching transfer trip scheme, PUTT, instead keys the permit from an underreaching zone-one element that only picks up for faults it is certain are internal, which is simple and secure but sends a permit only for the portion of the line its zone one covers.
The defining behavior of permissive schemes is what happens on channel failure. Because a trip at the remote end for a far-end fault depends on receiving the permit, a lost channel means that permit does not arrive, so the scheme cannot provide high-speed tripping for those faults and falls back to time-delayed distance backup. A permissive scheme therefore leans toward security: a spurious received signal will not cause a trip by itself, and channel loss makes the scheme less willing to trip rather than more. The trade is a loss of dependability for high-speed clearing when the channel is down.
A directional comparison blocking scheme, DCB, inverts the logic: the signal is a hold-off, not a permit. Each relay has a forward overreaching element that wants to trip fast for a line fault, and a separate reverse-looking element. When a relay's reverse element sees a fault behind it, it sends a blocking signal to the far end, meaning that fault is external to the protected line so do not trip on it. Each relay trips at high speed for a forward fault unless it receives that block, after only a short coordination delay allowing time for a possible block to arrive.
This gives blocking schemes the opposite channel-failure behavior. Because tripping is the default and only a received block prevents it, a channel failure that stops the block from arriving does not stop tripping - the relay trips anyway once its short wait expires. So a blocking scheme keeps high-speed tripping available even with the channel down, favoring dependability, at the cost that a channel failure at the wrong moment could allow a trip for an external fault that should have been blocked. This is the classic security-versus-dependability trade-off: permissive schemes are secure against false trips but depend on the channel to trip for far-end faults, while blocking schemes trip dependably even without the channel but rely on the channel to prevent tripping on external faults.
Where each fits depends on the channel and the consequences. Blocking schemes historically suited power-line-carrier channels, where the signal travels on the protected line itself and a nearby internal fault could attenuate it - a hold-off philosophy tolerates a missing signal better in that case. Permissive schemes suit independent, reliable channels such as fiber or dedicated links where a permit can be trusted to arrive, and are chosen where security against false tripping is paramount.
Many utilities standardize on POTT over fiber for its balance of speed and security, while blocking remains common where dependable tripping without a guaranteed channel is the priority. The right choice is a deliberate decision about which failure mode - failing to trip or tripping wrongly - is least acceptable on that particular line.
In practice the decision is made per line and per channel type, weighing how reliable the available communication path is against the consequences of a false trip or a failure to trip. A line whose loss would threaten stability may justify a scheme and channel chosen for dependability, while a line where an unnecessary trip is especially costly may favor the security of a permissive approach over a trusted fiber link.
It is what the transmitted signal means. In a permissive scheme (POTT or PUTT) the signal is a permit, and a relay trips at high speed only when it both sees a fault and receives that permit. In a blocking scheme (DCB) the signal is a hold-off, and a relay trips fast unless it receives a block. The difference shows most clearly on channel failure, where permissive loses high-speed tripping for far-end faults while blocking keeps tripping.
Security means not tripping when you should not; dependability means tripping when you should. Permissive schemes are secure because they need both a local fault decision and a received permit, but they depend on the channel to trip for far-end faults, so channel loss reduces dependability. Blocking schemes trip dependably even with the channel down because tripping is the default, but they rely on the channel to prevent tripping on external faults, so channel loss can reduce security. Each scheme optimizes one at the other's expense.
Both are permissive transfer trip schemes, but they differ in which element keys the permit. POTT uses overreaching elements at each end that reach past the remote terminal, so both ends send a permit for any internal fault, covering the whole line. PUTT keys the permit from an underreaching zone-one element that only operates for faults it is certain are internal, which is simpler but sends a permit only for the portion of the line that zone one already covers.
Primary references from the standards bodies and regulators that define this topic:
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