The main-tie-main interlock allows at most two of the three circuit breakers closed: both mains with the tie open, or one main and the tie. It is implemented with key interlocks, a mechanical interlock between breakers or electrical logic in the relays. Automatic transfer is built on top of it and must be blocked on a bus fault, and, if the design allows it, a momentary parallel with synchronism check and a time limit.
Main-tie-main interlocking is the logic of the main-tie-main arrangement. In the secondary-selective arrangement each transformer feeds its section of the switchgear through its main breaker (52M1 and 52M2), and a normally open tie breaker (52T) connects the two sections. The rule that defines the whole scheme is simple: at most two of the three breakers closed.
It is a medium- and low-voltage arrangement, very common in industry, and its logic is as important as that of a high-voltage substation: it protects the switchgear from a short-circuit level it was not designed for.
The two-out-of-three rule
| State | Main 1 | Tie | Main 2 | Use |
|---|---|---|---|---|
| Normal | closed | open | closed | each transformer with its section |
| Transformer 1 out | open | closed | closed | transformer 2 feeds both sections |
| Transformer 2 out | closed | closed | open | transformer 1 feeds both sections |
| Forbidden | closed | closed | closed | transformers in parallel |
Typical logic: each main closes only if the tie or the other main is open, and with the earthing switch on its transformer’s LV terminals open; the tie closes only if one of the two mains is open.
The full table for this arrangement, with the CILO equations and the list of test cases, can be generated with our free interlocking table tool.
Three ways to implement it
- Key interlocks. Each breaker has a lock, with fewer keys in circulation than breakers, so the breaker left without a key cannot close. It is the classic low-voltage solution and depends on no auxiliary supply.
- Mechanical interlock by cables or rods between the breakers, offered by low-voltage breaker manufacturers for this arrangement: it allows any two of the three to close.
- Electrical logic in the relays or in a controller: auxiliary contacts of each breaker in the closing circuit of the others, or the same condition by GOOSE. It is the only one that allows automatic transfer.
The first two prevent the parallel even if the control fails, but force manual operation. A lineup can combine one of them with electrical logic.
Automatic transfer
With electrical logic, the scheme can transfer on its own: if a section loses voltage, it opens its main and closes the tie. The transfer has its own permissives:
- Is the main of the affected section open?
- NoWait for it to open: with it closed there would be three breakers closed
- YesDid the main open on overcurrent or on a bus fault?
- YesBlocked: the tie would close onto the fault
- NoHas the residual voltage of the section decayed, or is it in phase with the healthy source?
- YesClose the tie
- NoWait: the motors are still generating an out-of-phase voltage
Fault blocking is usually done with a lockout relay 86 that the main overcurrent operates and that is reset by hand. The residual voltage criterion protects the motors, which keep spinning after the outage and generate a voltage that drifts out of phase with the healthy source. See automatic source transfer.
Retransfer, when the lost source returns, can be manual or automatic. If it is automatic and must be done without an outage, it goes through a parallel.
Momentary parallel or closed transition
In a closed transition the third breaker is closed with both sources live and in synchronism, and the outgoing one is opened right after. During that instant the two-out-of-three rule is broken on purpose, and the interlocking must allow it only under conditions:
- synchronism check between the two sections;
- a time limit on the parallel and a protection that opens a breaker if it lasts too long;
- a short-circuit study confirming that the equipment withstands the contribution of both transformers during that instant.
If the lineup has keys or a two-out-of-three mechanical interlock, closed transition is not possible without changing that interlock. See open vs closed transition transfer.
Isolation and earthing
In low-voltage switchgear, isolation is provided by the withdrawn position of the main breaker, and earthing is done with portable equipment. To work on a transformer it must be isolated on both sides: on the medium-voltage side by its disconnector or switch; on the low-voltage side by the withdrawn main. The tie can back-feed voltage to the transformer section, and an open main is not isolation.
Bay level and station level
A main-tie-main has no independent bays: the condition of each breaker depends on the other two, so the whole logic is collective. With hardwired logic, each breaker carries auxiliary contacts of the other two in its closing circuit. With protective relays that also do the control of each panel, positions are shared by relay-to-relay messages or by wiring, and each close permission is computed in its own relay. Automatic transfer also needs the voltage of both sections and the fault state of each main, so it is usually concentrated in one of the relays or in a dedicated controller. If the lineup has more than two sections, such as a main-tie-tie-main, the principle is the same but the number of allowed combinations grows, and they are best written in a table before being programmed.
Typical mistakes
- Tie without the mains condition. Closing by command is enabled «for testing» and the transformers end up in parallel.
- Transfer without fault blocking. The tie closes onto a faulted bus and the fault also takes out the other section.
- Fast closing onto motors. The transfer closes before the residual voltage decays.
- Duplicate keys. An extra key in circulation defeats the interlock.
- Closed transition without a time limit.
How it is tested
Each combination of the three breakers is tested, attempting to close the third one from each control location. Then the transfer is tested with the loss of each source, with a simulated fault on each bus and with motor load, followed by the retransfer. If there are keys, the test confirms that the number of keys in circulation matches the design.
We can help with your interlocking logic
At Tiza Group we design main-tie-main logic as part of secondary engineering: interlocking, automatic transfer and blocking, and we program it in the protective relays through remote configuration, for industry and substations. Commissioning on site is done in Colombia, or elsewhere on request.
Related guides
- Medium-voltage automatic transfer: logic and blocking
- Fast bus transfer for motor buses: three methods
- Interlocking in medium-voltage switchgear and RMUs
- Primary-selective and MV loop interlocking
- Low-voltage breaker and tie interlocking
References
- IEEE Std 141 (Red Book): electric power distribution for industrial plants.
- IEEE Std 242 (Buff Book): protection and coordination of industrial and commercial power systems.
- ANSI/NEMA C50.41: polyphase induction motors for power generating stations (motor bus transfer and reclosing).
- Schneider Electric, FAQ «How do key interlocks work?» (FA337357).
- Schneider Electric, MasterPact MTZ user guide, mechanical interlock for main-tie-main.
- IEC 61850-7-4: logical nodes CILO and CSWI.
Frequently asked questions
Why can't both mains and the tie be closed at the same time?
Because the two transformers would be in parallel. The short-circuit current in the switchgear rises, roughly doubling with two identical transformers, and can exceed the rating of the breakers and the buses. A fault in one transformer would also be fed by the other through the tie.
Which is better, key interlocks or electrical interlocking?
They do different jobs. Keys and mechanical interlocks do not depend on auxiliary supply or on a relay, but they do not allow automatic transfer. Electrical logic does, but it depends on the contacts and the relay working. A switchgear lineup can combine both: mechanical as backup, electrical for automation.
Can a main-tie-main transfer be done without an outage?
Only with closed transition: the tie is closed with both sources live and synchronized, and the outgoing main is opened right after. During that instant three breakers are closed, so the design, the short-circuit level and the interlocking must allow for it, with a time limit and a protection that opens if the parallel lasts too long.