Reverse interlocking is a busbar protection scheme for medium-voltage switchgear that uses the existing overcurrent relays. When a feeder sees a fault, it sends a blocking signal to the incomer relay; if the incomer sees the fault and receives no block, the fault is on the bus and it trips after a short delay.
What reverse interlocking is
On a medium-voltage bus with one incomer and several radial feeders, a bus fault can only be fed through the incomer. Time grading forces the incomer relay to wait until every feeder has had the chance to clear, so a bus fault is cleared with the longest delay in the switchboard, exactly where the fault energy and the arc flash risk are highest.
Reverse interlocking (also called a blocking scheme or zone-interlocked scheme) solves this with the overcurrent relays that are already there. Each feeder that sees fault current blocks a fast element of the incomer. If the incomer sees a fault and nobody blocks it, the fault lies between the incomer and the feeders: on the bus.
IEEE C37.234-2021 includes zone-interlocked blocking schemes among the bus protection methods, alongside the differential. It is one of the options described in busbar protection schemes, within the substation protection schemes.
The protection functions and how they are coordinated
- Incomer relay: overcurrent with a fast pickup element used for the bus, plus its normal time-delayed curve as backup.
- Each feeder relay: overcurrent with an instantaneous pickup element that, on detecting a fault, issues the blocking signal to the incomer. The rest of the radial feeder protection scheme does not change.
- Bus coupler, if any: it acts as a feeder towards the neighbouring section and as an incomer for its own.
- Feeder breaker failure: if the feeder breaker does not open, the block must be removed or 50BF must trip the incomer.
- Arc flash detection, often as a complement.
Each relay takes its own CT and trips its own breaker. No relay trips another panel’s breaker, except the incomer, which trips its own breaker for the bus fault.
Logic and sequence
Fault on a feeder:
- The feeder relay picks up and sends a block to the incomer.
- The incomer picks up, but its fast element is blocked.
- The feeder clears on its curve. The incomer resets without tripping.
Fault on the bus:
- The incomer picks up. No feeder sees fault current, so nobody blocks.
- The incomer’s short delay expires and it trips its breaker.
- The bus is cleared much earlier than with the time-delayed curve.
| Condition | Feeder relay | Incomer relay | Result |
|---|---|---|---|
| Fault on a feeder | Picks up and sends a block | Picks up with the fast element blocked | The feeder clears on its curve and the incomer resets |
| Fault on the bus | Sees no fault current and does not block | Picks up unblocked and its short delay expires | The incomer trips much earlier than with the time-delayed curve |
The short delay exists to give the block time to arrive: it includes the pickup time of the feeder relay, its output time and the signal transmission time.
Variants
- Hard-wired: feeder output contact to an incomer binary input, in parallel or in series depending on the chosen logic.
- IEC 61850 GOOSE: each feeder publishes a blocking message that the incomer subscribes to; message quality and arrival are supervised. See IEC 61850 GOOSE.
- With a bus coupler and two incomers: blocking per section, conditioned on the coupler position.
- With generation on feeders: directional elements so that only current flowing out of the bus blocks.
- Industry: the same scheme in plant switchboards with motor control centres.
Design criteria
- Incomer delay: longer than the worst-case arrival time of the block; it depends on the relay, the outputs and the medium.
- Pickups: the feeder blocking element must be as sensitive as or more sensitive than the incomer fast element, so that the incomer never sees a fault the feeder did not see.
- Inrush and motor starting currents, which can block or pick up elements unnecessarily.
- Supervision of the signal and behaviour on its loss.
- Coordination with the time-delayed curve according to the protection coordination study.
How each manufacturer implements it
The feeder relays of Siemens SIPROTEC (DIGSI), SEL (AcSELerator QuickSet), ABB Relion 615/620 (PCM600), GE Vernova Multilin (EnerVista) and Schneider Easergy / MiCOM (Easergy Studio, MiCOM S1) support this scheme with programmable logic and blocking inputs. Some offer it as a predefined application and others are built with free logic; what changes is how the block is assigned to an element, how GOOSE is supervised and how 50BF is handled. In switchboards with relays of several brands, the signal must be interpreted the same way by all of them.
Why it fails or misoperates
- Incomer trips for a feeder fault: the block did not arrive (loose wire, GOOSE not subscribed, wrong VLAN), a less sensitive feeder pickup, or a delay that is too short.
- No trip for a bus fault: block stuck on by a feeder that picked up on inrush or by a welded contact.
- Wrong behaviour with the coupler closed or with two incomers in parallel.
- Feeder with generation blocking on reverse current.
Check first the event records of the incomer and of all feeders with a common time reference: who picked up, who sent a block and when it arrived.
How it is tested
Each relay is tested by secondary injection and then the complete scheme: a simulated fault on each feeder (the incomer must stay blocked) and a bus fault (the incomer trips with its short delay). With GOOSE, publication, subscription and behaviour on loss of the message are also verified.
Do you want to implement reverse interlocking in your switchgear or find out why the incomer tripped? At Tiza Group we configure the relays and the GOOSE messaging remotely. See our remote relay configuration service or contact us.
References
- IEEE C37.234-2021, Guide for Protective Relay Applications to Power System Buses.
- IEEE C37.230-2020, Guide for Protective Relay Applications to Distribution Lines.
- IEC 61850-8-1, Communication networks and systems for power utility automation – Part 8-1: Specific communication service mapping (SCSM) – Mappings to MMS (ISO 9506-1 and ISO 9506-2) and to ISO/IEC 8802-3 (GOOSE).
- J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.
Frequently asked questions
Does reverse interlocking replace a bus differential?
Not entirely. It is simpler and cheaper, and it clears bus faults much faster than time grading, but it is somewhat slower than a differential and depends on the blocking signal arriving. On critical or high-voltage buses the differential is preferred.
What happens if the blocking signal wire is cut?
If the feeder cannot block, a fault on that feeder can make the incomer trip fast and take out the whole bus. That is why the signal should be supervised: with GOOSE the message itself is supervised, and with hard wiring supervision logic or a normally closed contact is used, depending on the design.
Can it be used with distributed generation connected to the bus?
Yes, but it has to be reviewed. If a feeder with generation feeds current into the bus, its relay must use directional elements so that it does not block or trip in the wrong direction.