A circuit breaker closes only if several independent conditions, called permissives, are met at the same time: the mechanism has stored energy, the interrupting medium is at sufficient pressure or density, no lockout relay 86 is operated, the anti-pump is clear, the interlock allows it, there is synchronism when it ties two sources, and the command comes from the level with authority. Some belong to the breaker and others to the control system, and a single missing one is enough to prevent the close.
What a close permissive is
A permissive is a condition that must be present for a command to reach the closing coil. Closing a breaker does not depend on a single condition but on a chain of them, like contacts in series: if one is missing, the command does not get through. IEEE C37.2 assigns number 69 to the permissive control device, although on drawings each permissive usually appears with the number of its function.
Opening and closing are not symmetrical. Protection must always be able to open the breaker, which is why the trip circuit has few conditions. Closing, on the other hand, should only happen if the breaker can complete the operation and, if needed, open again immediately, and if connecting both sides is safe for the network and for people.
The chain of permissives
| Permissive | What it checks | Owner |
|---|---|---|
| Springs charged | The mechanism has energy to close and, in many designs, for the following open | Breaker |
| Pressure or density of the interrupting medium | SF6 or compressed air above the lockout stage; in hydraulic mechanisms, the oil pressure | Breaker |
| Anti-pump clear | The previous close command has been removed, so the breaker does not close and trip in a cycle on a fault | Breaker |
| Pole discrepancy | All three poles are in the same position | Breaker and protection |
| Lockout relay 86 reset | No protection that requires inspection before closing is active | Protection |
| Interlock | Disconnectors and earthing switches in the position that makes the operation safe | Control system |
| Synchronism or dead voltage | The voltages on both sides allow the close | Control or protection system |
| Control authority | The command comes from the level selected by the local/remote switch | Control system |
| Circuit supervision | The close circuit and its supply are healthy | Breaker and control |
- 01Close commandComes from the level with control authority
- 02System permissivesInterlock, synchronism and lockout relay 86 reset
- 03Breaker permissivesSprings charged, sufficient pressure or density, poles in agreement
- 04Anti-pumpClear: the previous command has gone
- 05Closing coilWith every permissive present, the breaker closes
Breaker permissives
They live in the breaker control cabinet and are wired directly into the close circuit:
- Springs charged. The operating mechanism stores energy in springs charged by a motor. A limit switch enables closing only with the springs charged.
- Pressure or density. The SF6 density monitor has stages: the first gives an alarm; the second blocks operation, because below that density the breaker does not guarantee its interrupting capability. The values of each stage are set by the breaker manufacturer.
- Anti-pump. If the close command is maintained and the breaker closes onto a fault, the protection opens it and the maintained command would close it again and again. The anti-pump circuit cuts the closing coil until the command is removed.
- Pole discrepancy. On breakers with independent pole operation, the pole discrepancy logic opens the breaker if the poles end up in different positions and blocks a new close until it is checked.
System permissives
They are calculated in the bay controller or in the protective relays and reach the close circuit as a contact or as the logic that enables the command:
- Lockout relay 86. After a trip by protections such as transformer or bus differential, the lockout relay 86 prevents closing until someone resets it, to force a review of the cause before re-energizing.
- Interlock. In IEC 61850, the EnaCls data object of the breaker’s CILO node must be active. It normally requires the bay earthing switches to be open and the disconnectors not to be in an intermediate position.
- Synchronism. If the breaker ties two energized parts of the network, the synchronism check 25 gives the permission only with voltage, frequency and angle differences within its windows, or with the dead voltage condition enabled.
- Control authority. The command must come from the level selected by the local/remote switch.
Automatic reclosing has its own permissives (for example, that the breaker is ready for a full cycle and that the reclosing relay 79 is not blocked), on top of the ones above.
Why a missing permissive looks like a failure
The symptom that reaches maintenance is “the breaker will not close”. In most cases the breaker is fine and is obeying a missing permissive. The diagnosis time depends on each permissive having its own signal: a springs alarm, a density lockout signal, the 86 flag, the command rejection cause. If several permissives are wired in series without indicating which one opened, the operator only sees that it does not close.
- Does the missing signal come from the breaker cabinet (springs, pressure, anti-pump, discrepancy)?
- YesPrimary equipment: check the mechanism, the interrupting medium or the close circuit
- NoDoes it come from a protection (86) or from the synchronism check?
- YesProtection: identify the function that operated or the voltage condition before resetting
- NoControl system: check the interlock, the position of the devices and the control authority
What goes wrong in practice
- Permissives without individual indication, which lengthen the diagnosis.
- Test jumpers across a permissive, forgotten in the close circuit.
- 86 reset out of habit without analyzing the cause of the trip.
- Springs-charged contact out of adjustment, which enables closing before the spring is fully charged.
- Logic that treats an invalid position as allowed in the breaker interlock.
How it is verified
During commissioning, each permissive is removed one at a time to check that it blocks the close from every control level, and that its signal reaches the HMI and the control center. The anti-pump is checked with a maintained command, and the pressure or density lockout at its actual stage. Pressure values and times are those of the manufacturer and the project.
Do you need to review the close logic of your breakers or integrate their permissives into the controller? At Tiza Group we configure the close logic, the interlocks and the permissive indication in controllers and relays remotely through our remote relay configuration service, for substations and industry at medium, high and extra-high voltage. Site testing is done in Colombia, or elsewhere on request. Contact us.
Related guides
- Lockout relay 86: what it is and how it works
- Synchronism check 25 (RSYN): what it is and how it works
- Control levels and the local/remote switch (43)
- Dead line and live bus closing: 25 modes
- Voltage selection for synchronism check in double bus
- Select before operate (SBO): how the command works
References
- IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations (25, 43, 63, 69, 86).
- IEC 62271-100, High-voltage switchgear and controlgear – Alternating-current circuit-breakers.
- IEC 61850-7-4, Compatible logical node classes and data object classes (CILO, CSWI, XCBR, RSYN).
Frequently asked questions
What is the difference between a permissive and an interlock?
Every interlock is a permissive, but not the other way round. The interlock looks at the position of other devices. The other permissives look at the state of the breaker itself (springs, pressure, anti-pump), at the protections (86) or at the network (synchronism).
Do close permissives affect opening?
Opening by protection must not depend on permissives, except for the low pressure or density lockout of the interrupting medium below the stage at which the breaker can no longer interrupt safely. In that case the design defines whether the breaker is blocked or opens preventively.
What ANSI number do permissives have?
IEEE C37.2 assigns 69 to the permissive control device. In practice, each permissive keeps the number of its function: 25 for synchronism, 63 for the pressure switch, 86 for the lockout relay and 43 for the control selector switch.