Voltage transformer supervision, known as fuse failure or loss of potential (ANSI 60, voltage or current balance relay), detects that the relay has stopped receiving a correct voltage because of an open fuse or miniature circuit breaker, and not because of a fault on the network. It then blocks the functions that depend on voltage, such as distance or undervoltage, and can enable an emergency overcurrent element. CT supervision does the equivalent for an open current circuit.
What 60 supervision protects
VT supervision does not protect a piece of power equipment: it protects the other protection functions from a false measurement. If the fuse or the miniature circuit breaker of one phase of the voltage transformer (VT) opens, the relay sees that voltage drop as if there were a close-in fault. Distance protection can trip with no fault; undervoltage can take out a whole plant; a directional element can lose its sense of direction.
Supervision prevents those false trips, alarms so that someone repairs the circuit and, if configured, leaves active a replacement protection that does not need voltage. CT supervision does the same with an open current circuit, which is also a hazard to people because of the voltage that appears on the secondary.
- ANSI/IEEE C37.2: 60, voltage or current balance relay. Relays also call it VTS, LOP or fuse failure. The same number designates capacitor bank unbalance protection, which also compares quantities that should be equal.
- IEC 61850: there is no specific protection node in common use; each manufacturer uses its own and publishes a blocking or alarm signal.
- IEC 60617: no widely used symbol.
Where it is applied
| Equipment | LV | MV | HV and EHV | Industry | What supervision protects |
|---|---|---|---|---|---|
| Line | No | Lines with directional elements | Always, with 21 and 67N | Service entrances | Prevents distance trips |
| Transformer | No | With 27 or 51V | With backup 21 or 24 | Plant transformers | Blocks voltage functions |
| Generator | No | Yes | Yes | Cogeneration | Blocks 40, 32, 51V and 21 |
| Buses and plant | Motor and interconnection relays | Yes | Yes | Undervoltage load shedding | Prevents load shedding caused by a fuse |
| CT circuits | No | Differentials and 87N | Differentials | Motors with 87M | Alarm and blocking of the differential |
Measuring principle
Voltage supervision. Relays combine several criteria:
- Sequence components: a blown fuse produces negative-sequence (V2) or zero-sequence (V0) voltage without the negative-sequence (I2) or zero-sequence (I0) current that a real unbalanced fault would have. It is the main criterion for the loss of one or two phases. You can practice with those phasors in the free symmetrical components tool.
- Three-phase loss: all three voltages drop with no change in current, or with the breaker closed and load current present.
- Miniature circuit breaker contact: the signal from the MCB on the VT secondary, wired to a relay input, is the most direct and fastest method.
CT supervision. An open current circuit produces residual (or negative-sequence) current without residual voltage. The relay alarms and can block the differential or the ground overcurrent.
Why looking at the voltage is not enough. A close-in three-phase fault also makes all three voltages drop. The difference is in the current: with the fault, the current rises or changes suddenly; with the loss of the VT, the current stays the same. That is why three-phase detection relies on the change in current, on the breaker position or on the MCB contact, and not only on the absence of voltage. Something similar happens with the CT: an unbalanced load produces negative-sequence current without any open circuit, and supervision must tell it apart using the voltage or by comparing with another set of CTs.
The characteristic
It is logic, not a curve, and it is drawn with gates:
- AND: high V2 and low I2, or high V0 and low I0, for a short time.
- OR: that condition or the miniature circuit breaker contact or the three-phase loss.
- Output: blocking of the voltage-dependent functions and an alarm, sealed in until the voltage returns and stays normal.
- Measurement. V2, V0, I2, I0, phase voltages and MCB contact
- Detection. Unbalanced voltage without unbalanced current, or MCB open
- Confirmation. Short time to rule out transients
- Blocking. 21, 27, 67, 32, 40 and 51V, depending on the configuration
- Alarm and emergency. Warning to the operator and replacement overcurrent if enabled
Setting criteria
- Sequence thresholds: the voltage must exceed the normal network unbalance and the current must stay below what the minimum unbalanced fault would produce.
- Fast blocking: it must act before the function it protects trips, especially distance zone 1.
- Wiring of the MCB contact to a relay input, so that detection does not depend on sequence components alone.
- Functions blocked and emergency functions: defined with the study and the owner’s philosophy.
Interaction with other functions
- Distance 21 and directional 67 and 67N: blocked or switched to non-directional. See distance protection 21 and directional overcurrent 67.
- Undervoltage 27 and load shedding: blocked so that load is not shed because of a fuse. See undervoltage protection 27.
- Generator 40, 32 and 51V: blocked; without voltage, the 51V would behave as a very sensitive overcurrent element.
- Emergency overcurrent: enabled only while the loss of voltage lasts.
- Switch onto fault: a dead line and a blown fuse must be told apart, or the closing logic would be enabled for no reason.
The alarm in service and how to diagnose it are covered in loss-of-potential alarm.
How each manufacturer implements it
- Siemens SIPROTEC (DIGSI): measuring voltage supervision with sequence criteria and an MCB input.
- SEL (AcSELerator QuickSet): loss-of-potential logic that supervises the distance and directional elements.
- Hitachi Energy and ABB Relion (PCM600): fuse failure and current circuit supervision blocks with the manufacturer’s own names.
- GE Vernova Multilin (EnerVista): VT fuse failure element that blocks the assigned elements.
- Schneider MiCOM and Easergy: VT and CT supervision with function blocking.
IEC 61850 logical node
There is no standardized protection node in general use for this supervision. Manufacturers model it with their own nodes and publish the detection as an alarm and blocking signal. That signal is used inside the relay and can be published by GOOSE to other IEDs, for example to a bay controller that blocks load shedding. Check in the ICD file which node publishes it before mapping it to SCADA.
How it is tested
With secondary injection, the following are generally verified:
- Detection with the loss of one, two and three phases, with and without load current.
- That a real unbalanced fault is not taken as a fuse failure.
- Effective blocking of each assigned function, and the blocking time against the zone 1 time.
- The MCB input and the reset when the voltage returns.
- CT supervision with a residual current and no residual voltage.
Typical mistakes
- VT miniature circuit breaker with no auxiliary contact wired to the relay.
- Incomplete list of blocked functions, for example the 27 of a load shedding scheme.
- Low current threshold, which prevents detecting the fuse with an unbalanced load.
- Distance tests with supervision disabled that is never enabled again.
- Not repeating the three-phase loss test after changing the closing logic.
Did your relay block or trip on fuse failure and it is not clear why? At Tiza Group we review the events and settings remotely, worldwide. See our remote relay configuration service or contact us.
Related guides
References
- IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations: number 60, voltage or current balance relay.
- IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines: loss of potential in line protection.
- IEEE C37.102-2023, Guide for AC Generator Protection: blocking of generator functions.
- IEC 61869-3, Instrument transformers – Part 3: Additional requirements for inductive voltage transformers, and IEEE C57.13, Standard Requirements for Instrument Transformers.
- IEC 61850-7-4, Compatible logical node classes and data object classes.
Frequently asked questions
How does the relay tell a blown fuse from a network fault?
A network fault produces voltage and current changes at the same time. A blown fuse changes the voltage without the current changing. That is why the classic logic requires high negative- or zero-sequence voltage and, at the same time, low negative- or zero-sequence current, plus the VT miniature circuit breaker contact when it is wired.
Which functions does fuse failure block?
Those that trip or lose their direction without voltage: distance 21, undervoltage 27, directional 67 and 67N, power 32, loss of field 40 and voltage-restrained overcurrent 51V, among others. The exact list is defined in each relay.
Why is ANSI number 60 called a balance relay?
Because C37.2 defines it as a voltage or current balance relay: it operates on a difference between two circuits. Old schemes compared two sets of voltage transformers; today the number is also used for numerical VT and CT supervision and for capacitor bank unbalance.