In the DCB (directional comparison blocking) scheme, each end trips when its overreaching element sees a forward fault and no blocking signal arrives within a coordination time. The end that sees the fault behind it sends the block. Internal faults do not need the channel, so DCB keeps tripping even if the signal is lost.
What the DCB scheme is
The DCB (directional comparison blocking) scheme reverses the logic of permissive schemes. The signal does not allow the trip: it prevents it.
- Each end has a forward overreaching element (a zone of distance protection 21 or a directional ground element) that detects faults on the whole line and somewhat beyond.
- Each end has a reverse element that detects external faults behind it and sends a block to the other end.
- An end trips if its overreaching element sees the fault and receives no block during the coordination timer.
It is a device 85 logic (ANSI/IEEE C37.2), modeled in IEC 61850 with logical node PSCH. The comparison with the other schemes is in the guide on teleprotection schemes.
How it works: logic and signals
Internal fault: both ends see the fault forward. Neither sees a reverse fault, nobody sends a block and both trip when their coordination timer expires. The channel plays no part.
External fault behind B: A’s overreaching element sees the fault and starts its coordination timer. B sees the fault in reverse and sends a block. The block reaches A before the timer expires and A does not trip. A keeps its time-delayed zone 2 as backup.
| Condition | End A | End B | Result |
|---|---|---|---|
| Internal fault | Sees the fault forward | Sees the fault forward and sends no block | Both trip when the coordination timer expires |
| External fault behind B | Its overreaching element starts the timer | Sees the fault in reverse and sends a block | A does not trip; its time-delayed zone 2 remains |
| External fault with the channel lost | Its overreaching element sees the fault and gets no block | Sends a block that never arrives | A trips incorrectly |
In classic schemes with on-off power line carrier, the signal is started by nondirectional fault detectors and stopped when the forward element confirms an internal fault. An SEL technical paper describes this two-bit method (carrier start and carrier stop) as a way to allow nondirectional starting without relying on the reverse element staying picked up the whole time.
The golden rule of DCB: the reverse element at each end must see every fault that the overreaching element at the opposite end sees, with margin in reach, sensitivity and speed. Otherwise, the remote end trips for an external fault.
Channel and timing
DCB asks the channel for the shortest transmission time and high dependability: the block must always arrive, and before the coordination timer expires. Channel security matters less, because a false signal only blocks, which at worst delays a trip until zone 2.
The coordination timer delays every trip of the scheme. It is calculated from the operating time of the remote reverse element, the channel transmission time and a margin, according to the study and the manufacturer.
Brief interruptions of the block during an external fault (so-called carrier holes) are handled with integrating timers or by extending the received signal.
If the channel is lost: DCB keeps tripping for internal faults, but any external fault inside the overreach causes a false trip. It loses security, not dependability.
Advantages and limitations
Advantages:
- Clears internal faults even if the channel fails, which suits channels that run over the line itself, such as power line carrier.
- Works with a weak or open end: that end sends no block and the strong end trips on its own.
- The channel can be simplex at each end and on-off.
Limitations:
- Slower than a permissive scheme because of the coordination timer.
- Security depends on the channel: a channel down with no alarm turns every nearby external fault into a false trip.
- Requires strict coordination between the reverse elements and the overreaching elements.
- On parallel lines it needs current reversal guard.
Where it is used
DCB is traditional on high and extra-high voltage lines with power line carrier, especially in utilities with long experience on that channel. It is also applied with optical fiber in the shield wire (OPGW): that fiber shares the line corridor and can be damaged by the same fault, and DCB does not need it to trip internal faults. With supervised digital channels, many utilities have moved to the POTT scheme or to line differential, although DCB remains in use where the utility’s practice prefers it.
How each manufacturer implements it
GE Vernova (Multilin, EnerVista) offers it as a directional comparison blocking scheme in relays such as the D60. SEL (AcSELerator QuickSet) implements it with separate start and stop bits and a coordination timer. Siemens SIPROTEC 4 7SA (DIGSI) includes it as the blocking mode of its distance teleprotection. Hitachi Energy Relion 670 (PCM600) configures it as the blocking mode of its scheme communication logic. MiCOM P44x relays (GE Vernova MiCOM Agile and Schneider Electric Easergy MiCOM) include it among their blocking aided schemes. What changes is which elements start the signal, how it is stopped and how carrier holes are handled.
Why it fails or misoperates
- Trip for an external fault: channel out of service, transmitter that did not start, coordination timer shorter than the actual channel time, reverse element too short or too slow.
- Trip on a parallel line after the adjacent fault clears: no current reversal guard.
- Internal fault cleared with a delay: carrier that did not stop (forward element did not confirm) and blocked its own end or the remote one.
Start with the records from both ends: whether a block was sent, when it was received and how much of the coordination timer was left.
How it is tested
At each end, secondary injection verifies the overreaching element, the reverse element, the signal start and stop, and the coordination timer. The complete scheme is validated with a GPS-synchronized end-to-end test, measuring the actual channel time and checking that no external fault produces a trip.
Did your DCB trip for a fault that was not on the line? Our team reviews the events, reaches and coordination 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.
- CIGRE, Technical Brochure 192, Protection using telecommunications (2001).
- Blackburn and Domin, Protective Relaying: Principles and Applications.
Frequently asked questions
What is the DCB coordination timer for?
It gives the remote end time to detect a reverse fault and for its block to cross the channel. It is set from the operating time of the remote reverse element and the actual channel transmission time, and it delays every trip of the scheme.
Why did a DCB scheme trip for an external fault?
Because the block did not arrive in time: channel out of service or with dropouts, coordination timer too short, or a remote reverse element with less reach or sensitivity than the local overreaching element.
Why is DCB associated with power line carrier?
Because power line carrier travels over the protected line and an internal fault can attenuate it. Since DCB does not need the channel to trip internal faults, that problem does not affect it; and for external faults the healthy line carries the signal well.