Directional Comparison Unblocking (DCUB) Scheme

The DCUB Scheme, like POTT Scheme, uses two frequencies to maintain system protection. When the system is operating under normal condition, the transceiver emits the “Guard” frequency. When a fault appears in the system, the distance relays shift the carrier to “Trip” frequency. This is where the similarities between POTT and DCUB schemes end.

Unlike POTT, the DCUB scheme permits the breakers to trip when the guard signal from the remote end relay is lost. Ofcouse, there are certain conditions that need to be met before this happens. Lets understand them using the figure below.

Directional Comparison UnBlocking Scheme
Directional Comparison UnBlocking Scheme

Trip Scenario No. 1:  Fault in the zone of protection (F1)

  • Relays at CB1 and CB2 will see a loss of guard frequency and obtain the permissive “trip” frequency
  • Relays see zone 2 distance elements pick-up
  • Reverse looking zone 4 elements do not pick-up (if configured)

Both relays see a fault using their forward looking zone 2 elements, transmit the permissive trip signal to each other and ultimately trip their associated breakers.

Trip Scenario No. 2: Fault outside the zone of protection (F2)

  • Relay at CB1 sees the fault using zone 2 elements
  • Relay at CB1 shifts carrier from Guard to Trip frequency, providing the permissive trip signal to relay at CB2
  • Relay at CB2 does not see a fault in it’s zone
  • Relay at CB2 maintains guard frequency towards CB2

CB1 relay sees the fault using its zone 2 element, however, CB2 relay does not. A lack of trip signal from the CB2 relay prevents the CB1 relay from tripping. Thus, both relays are restrained from tripping.

Trip Scenario No. 3: Fault in the zone of protection (F1) but the guard/trip signal is shorted out by the fault

  • Both relays see a loss of guard
  • Associated permissive trip signal is not available either
  • Fault is picked up by the zone 2 elements of both relays
  • Reverse looking zones (if configured) do not pick up fault

Both relays initiate a timer. In this time duration the relays are permitted to trip the breaker provided their zone 2 elements still see the fault. After the time elapses, tripping is blocked.

It is this last extra bit that makes this scheme viable on a Power Line Carrier set-up.

(Updated wording on scenario 3 12/21/2017)

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Scenario 2: CB2 does not see a fault and sends gaurd frequency towards CB1 (NOT CB2). More over reverse looking element Zone 4 picks up to block trip.


After further investigation, I think you’re right Dave. In case of channel loss, a 150 ms window is
open when permissive signal is bypassed and Z2 is allowed to trip high speed. His notes are incorrect above. Thanks for pointing this out.


There’s actually always a timer, he only mentions it in the 3rd scenario because it’s more relevant in that scenario. To answer your question, his wording is correct. Both relays will trip their end’s breaker after the timer expires “IF” zone 2 elements still detect a fault.

Dave Felix

For scenario #3, I am thinking that the relays are permitted to trip during the 150 milliseconds. After the timer elapses, tripping is blocked. Am I misinterpreting or should the wording be changed? Thanks!

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