Switch onto fault (SOTF) protection: how it works

What switch onto fault logic protects

There is one case that line protection handles poorly: closing the breaker onto a line that is already faulted. The classic example is a line returning from maintenance with its earthing switches still closed, or with portable grounds forgotten somewhere along it. On closing, a bolted three-phase fault appears, often close to the relay.

SOTF logic protects the line, the breaker and the system at that moment: it clears the fault without delay instead of waiting for zone 2 or a backup element. It avoids damage to the breaker and the line, long voltage dips on the bus and the operation of remote backup protection that would take out more circuits.

• ANSI/IEEE C37.2: SOTF has no device number of its own; on drawings it appears by its acronym next to the distance or overcurrent function.
• IEC 61850: each manufacturer models it in its own way, as a separate logical node or inside the distance function block, and its trip is published through PTRC.
• IEC 60617: no dedicated symbol.

Where it is applied

Operating principle

The logic has two parts: detecting that a close took place and accelerating an element during a time window.

Close detection. Relays use one or more of these criteria:

• Manual close command received by the relay or by the bay controller.
• Breaker position: a change from open to closed according to its auxiliary contacts.
• Dead line: the line has been without voltage and current for a set time, and then current appears. This criterion does not depend on position wiring and also detects a close from the remote end.

Accelerated element. During the window, the relay trips without delay on one of these elements:

• A high-set non-directional overcurrent, which does not depend on voltage.
• The extended zone or zone 2 of the distance protection, without its time delay.
• A non-directional distance element, so that the lack of polarizing voltage does not block it.

Distance protection 21 uses voltage memory to determine direction. If the VT is on the line side, there is no voltage to remember before the close. That is the technical reason for SOTF.

Why zone 1 is not enough. A fault with maintenance grounds is three-phase and bolted. If it is close to the relay, the measured voltage is practically zero and, without memory, the directional element cannot decide. If it is at the far end of the line, it may fall outside the zone 1 reach and wait for the zone 2 time. In both cases the line and the bus carry the fault longer than necessary. SOTF covers both cases with an element that depends on neither direction nor reach.

The characteristic

There is no curve. The characteristic is a time window: SOTF is armed by the close detection and stays active for a few hundred milliseconds, or for the time set. If the accelerated element picks up inside the window, the trip is instantaneous. Once the window expires, the line returns to its normal protection.

Setting criteria

• Enabling criterion: whichever is most reliable in that bay. Dead-line detection covers cases that the manual command does not see.
• Window duration: long enough for the fault to develop and the element to pick up, but short enough that it is not left active in normal operation.
• Accelerated element: if it is an overcurrent, above the inrush and capacitive charging current of the line, and below the minimum fault current on closing.
• Coordination with reclosing: decide whether the same logic acts after a reclose or whether a separate acceleration is used.

The criteria for each element are explained in protection setting criteria.

Interaction with other functions

• Reclosing 79: a reclose onto a permanent fault is cleared by the trip-on-reclose logic. A SOTF trip usually blocks any further attempt. See automatic reclosing 79.
• VT supervision (60): if the loss of voltage comes from a blown fuse, supervision must tell it apart from a dead line; otherwise SOTF could be armed for no reason.
• Breaker failure 50BF: initiated by the SOTF trip like any other trip.
• Stub protection: in breaker-and-a-half arrangements, with the line disconnector open, the section between breakers is protected by separate overcurrent or differential logic.
• Interlocks: the root cause is usually a closed earthing switch. Line and bus earthing switch interlocks are the first barrier; SOTF is the second.

How each manufacturer implements it

• Siemens SIPROTEC (DIGSI): close detection and instantaneous high-current trip on closing, with a choice of accelerated elements.
• SEL (AcSELerator QuickSet): switch-onto-fault logic with a window and elements assignable in the trip equations.
• Hitachi Energy and ABB Relion (PCM600): automatic switch-onto-fault function block based on voltage and current (ZCVPSOF in the 650 series, for example).
• GE Vernova Multilin (EnerVista): line pickup function with accelerated zones.
• Schneider MiCOM: switch-on-to-fault and trip-on-reclose logic in the distance relays.

IEC 61850 logical node

Manufacturers do not model this logic in the same way. Each represents it with its own logical node or as part of the distance function block, and the resulting trip is published through PTRC, which carries it by GOOSE or by a hardwired output to XCBR. For integration, check in each relay’s ICD file which node publishes the SOTF trip, because that is the signal the recorder and the SCADA must identify.

How it is tested

The complete sequence is simulated with secondary injection. In general the test verifies:

• Each enabling criterion: manual command, position change and dead line.
• The window duration: a fault inside it trips without delay; a fault outside it trips with the normal time.
• The accelerated element: a three-phase fault with near-zero voltage, and that normal energization does not trip.
• Blocking of the reclose and initiation of 50BF.

Typical mistakes

• Enabling criterion not wired: the logic depends on a close command that never reaches the relay.
• Directional accelerated element, which does not operate with zero voltage.
• Low-set overcurrent that trips when energizing long lines or lines with transformers.
• Window too long, leaving the line with instantaneous tripping for faults that should be coordinated.
• Confusing this logic with the reclosing logic and testing only one of the two.

Did your line trip on closing, or do you need to review its switch onto fault logic? At Tiza Group we review events and settings remotely, worldwide. See our remote relay configuration service or contact us.

Related guides

• Distance protection 21
• Automatic reclosing 79
• Dead line and live bus closing
• Main 1 and main 2 line protection

References

• IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines: line protection and switch onto fault logic.
• IEEE C37.104-2022, Guide for Automatic Reclosing on AC Distribution and Transmission Lines: reclosing and trip on reclose.
• IEC 60255-121, Functional requirements for distance protection: the distance element.
• IEC 61850-7-4, Compatible logical node classes and data object classes: PTRC logical node.
• G. Ziegler, Numerical Distance Protection: Principles and Applications.

Frequently asked questions