The dead zone is the section between a circuit breaker and its current transformer: a fault there can stay fed even after the bus or line protection has opened its breaker. The stub is the section left between the two breakers of a breaker-and-a-half or ring bus bay when the line disconnect switch is open. Both need dedicated logic to be cleared quickly.
What the dead zone is
Every protection zone ends at a current transformer (CT). Zones overlap around each circuit breaker so that no point is left unprotected. This guide, part of substation protection schemes, covers the two places where that overlap is not enough.
The problem appears when the CT is on one side only of the breaker, which is common with live-tank breakers and separate CTs, or in switchgear with a single CT set. A fault between the breaker and the CT falls inside one zone but electrically on the side of the other. The protection that sees it opens its breaker, and the fault is still fed from the other side. That section is the dead zone (also called end zone or end fault).
What the stub is
In breaker-and-a-half or ring bus arrangements, each line connects between two breakers, and the line VT is usually on the line side of the disconnect switch. When the line is taken out of service with its disconnect open and both breakers are closed to complete the diameter or the ring, an energized section remains between the breakers and the disconnect: the stub. The line protection loses its voltage and no longer protects it, and the bus protection does not cover it either. That is why stub protection is added to the functions of the line’s main 1 and main 2 scheme.
Which protections are involved and how they are coordinated
For the dead zone:
- Breaker open and current present logic: if the breaker is open and its CT still measures fault current, the fault is in the dead zone.
- The action depends on where the CT is: trip the corresponding bus zone, or send a direct transfer trip to the remote line end.
- Breaker failure 50BF as backup.
For the stub:
- Stub overcurrent on the sum of the two breaker currents, enabled by the open auxiliary contact of the line disconnect.
- Or a stub differential between the CTs of both breakers and the line CT, where one exists.
- Trip of both breakers of the bay.
| Aspect | Dead zone | Stub |
|---|---|---|
| Where it is | Between the breaker and its CT, when the CT is on one side only | Between the two breakers and the open line disconnect, in breaker-and-a-half or ring bus |
| How it is detected | Breaker open and current present in its CT | Stub overcurrent or differential, enabled by the open disconnect |
| What it trips | The corresponding bus zone, or the remote end through transfer trip | Both breakers of the bay |
Logic and sequence
Fault in the dead zone of a line with the CT on the bus side:
- The line protection sees the fault as its own and opens the breaker.
- The bus protection does not see it: the CT places it outside the bus.
- The fault is still fed from the bus. The breaker is open and its CT measures current.
- The dead zone logic trips the bus after a short delay that confirms the opening.
Fault in the stub:
- The line disconnect is open; the stub logic is enabled.
- A fault between the breakers and the disconnect produces current in the sum of both CTs.
- Stub overcurrent trips both breakers of the bay.
- With the disconnect closed, the function is blocked and the line protection takes the section back.
Variants by voltage level
- Extra-high voltage: breaker-and-a-half and ring bus, with stub and dead zone functions built into the line and bus relays.
- High voltage: double bus with CTs on one side; dead zone in the bus couplers, which often lack CTs on both sides. See busbar protection schemes.
- Medium voltage: switchgear with CTs on one side of the breaker; the dead zone is covered by the incomer backup, 50BF or arc flash detection.
Design criteria
- CT location: with CTs on both sides of the breaker and crossed zones, the dead zone disappears; with CTs on one side only, the logic is needed.
- Reliable position of the breaker and disconnects: supervised auxiliary contacts.
- Time delay long enough to cover the difference between the auxiliary contact change and the actual current interruption in a normal opening, and to make sure the measured current is not just the CT demagnetization tail.
- Transfer trip available when the action must reach the remote end.
- Coordination with 50BF to avoid duplicated actions.
IEEE C37.234 covers how CT and breaker location affects bus protection and includes the breaker-and-a-half arrangement and the stub among its topics; IEEE C37.113 covers line protection.
How each manufacturer implements it
Line, bus and breaker failure relays from Siemens SIPROTEC (DIGSI), SEL (AcSELerator QuickSet), Hitachi Energy Relion 670 (PCM600), GE Vernova Multilin UR (EnerVista) and MiCOM include stub, dead zone or end fault functions, sometimes as a dedicated element and sometimes as programmable logic built from overcurrent elements and positions. Names vary by brand: for example, the Multilin B90 includes it as end fault protection and the MiCOM P746 as dead zone overcurrent. How they are enabled also changes from model to model.
Why it fails or misoperates
- Stub trip with the line in service: disconnect auxiliary contact reporting open when it is closed.
- Stub function does not operate: function not enabled, auxiliary contact not wired, current below the pickup.
- Dead zone fault cleared only by remote backup: logic missing or disabled.
- Bus trip from the dead zone logic during a normal opening: delay too short compared with the current interruption time or the auxiliary contact change.
Start by checking the breaker and disconnect positions in the event record and the current in each CT after the opening.
How it is tested
It is tested by secondary injection while simulating breaker and disconnect positions: a fault with the breaker open and current present, a fault in the stub with the disconnect open, and blocking with the disconnect closed. It is completed with functional tests of the real positions and with end-to-end tests when the action is a transfer trip.
Do you have a breaker-and-a-half or ring bus bay without stub protection, or a trip nobody can explain? At Tiza Group we review and configure this logic remotely. See remote relay configuration or contact us.
References
- IEEE C37.234-2021, Guide for Protective Relay Applications to Power System Buses.
- IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines.
- IEEE C37.119-2025, Guide for Power System Circuit Breaker Failure Protection.
- CIGRE TB 140 (1999), Reliable fault clearance and back-up protection.
- J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.
Frequently asked questions
Why can a fault between the breaker and the CT remain uncleared?
Because the protection that sees it trips its own breaker, but the fault sits on the other side and is still fed from another source. If no other zone sees it as its own fault, only breaker failure protection or remote backup clears it, with a longer delay.
When is stub protection enabled?
When the line disconnect switch of a breaker-and-a-half or ring bus bay is open and the breakers remain closed. It is enabled by the disconnect auxiliary contact and protects the section between the breakers and the disconnect.
Does 50BF cover the dead zone?
It can, because current keeps flowing after the breaker opens and 50BF eventually trips. But it does so with its own delay and breaker failure logic, even though the breaker did open. A dedicated dead zone logic clears faster and with the correct action.