Protection & Controls
Main-Tie-Main Systems: Design and Automatic Transfer Logic
A main-tie-main lineup splits the bus so that two sources each carry part of the load and either can pick up the whole load when the other is lost. The value is in the automatic transfer logic, and this article explains how source loss is detected, how the transfer is sequenced, and which interlocks keep the scheme from doing harm.
8 min read · Updated 2026-09 · Apex Power Distribution Engineering
The arrangement and its normal state
A main-tie-main (MTM) lineup has two incoming main breakers, a tie breaker and a bus split into two halves. In the normal state both mains are closed and the tie is open, so each source carries the load on its half of the bus. Each source, transformer and main is sized to carry the entire load, because that is what happens after a transfer.
The sources can be two utility feeders, two transformers from the same feeder, or a utility and an on-site generation bus. A less common variant runs with the tie normally closed and one main open, which turns the second source into a standby rather than a shared supply.
Detecting loss of source
Automatic transfer is initiated by undervoltage relays (device 27) monitoring each source on the line side of its main breaker, using voltage transformers ahead of the breaker so the relay still sees the source after the main opens. Three-phase sensing catches single-phase loss that a single-phase relay would miss. Some schemes add overvoltage (59), underfrequency (81) and phase-sequence (47) supervision so a badly distorted source is treated as lost.
Dropout settings and delays are chosen to ride through voltage sags and upstream utility reclosing without transferring unnecessarily; a dropout in the range of 70 to 80 percent of nominal with a delay of a fraction of a second to several seconds is common. The scheme must also distinguish a lost source from a bus fault. If the main was tripped by overcurrent protection (50/51) or a lockout relay (86), the transfer is blocked, because closing the tie would connect the healthy source to the fault.
The transfer sequence and its timers
A typical open-transition sequence runs as follows. The 27 relay on source 1 drops out and its timer runs. When the timer expires, the logic trips main 1 and confirms it is open through the 52b contact. It then checks that source 2 is healthy, that main 2 is closed and that no lockout is asserted on the dead bus. Only then does it close the tie, restoring bus 1 from source 2.
For buses with significant motor load, a residual voltage check or an additional delay is added before the tie closes, because motors coasting down keep the dead bus energized for a second or more and an out-of-phase reconnection can damage shafts and couplings. Every step has a supervisory timer; if a breaker fails to open or close within its window, the sequence stops and alarms rather than continuing blindly.
Interlocks: only two of three closed
The defining interlock of an MTM scheme prevents all three breakers from being closed at the same time unless paralleling is intended. Electrically, the close circuit of each breaker is wired through auxiliary contacts of the other two so that a close command is blocked when both others are closed. The same rule is programmed in the transfer relay or PLC, and many owners add Kirk-key mechanical interlocks as a backup that does not depend on control power or logic.
Paralleling two utility feeders doubles the available fault current on the bus, can circulate current between feeders and is usually prohibited by the utility. Where a momentary parallel is allowed for closed transition, it is supervised by a synchronism-check relay (25) that confirms voltage, frequency and phase angle are within limits before the third breaker is permitted to close.
Open versus closed transition and return to normal
Open transition is break-before-make: the dead bus sees an outage lasting from the 27 delay through the breaker operations, typically a few seconds in total. Closed transition is make-before-break: with sync-check supervision, the tie closes first and the affected main opens a fraction of a second later, so the load never sees an interruption. It is used for planned transfers and return to normal, and it requires utility approval and a fault study that covers the paralleled condition.
Return to normal, or retransfer, can be automatic or manual. Automatic return waits for the restored source to remain healthy for a set time, often several minutes, and then either opens the tie and closes the main (a second outage) or performs a closed-transition return if permitted. Many facilities choose manual return so that operators pick the time of the second disturbance.
Operating modes and coordination with other systems
The scheme should offer at least three modes selectable at the switchgear or HMI: automatic, manual and maintenance. Maintenance mode disables automatic transfer so a breaker can be racked out or tested without the logic reacting to the resulting voltage change.
MTM transfer must be coordinated with the rest of the facility. Generator-backed automatic transfer switches downstream should have time delays long enough not to start generators for a transfer the MTM will complete in seconds. UPS ride-through must cover the open-transition gap. A complete functional test with simulated loss of each source is the only way to prove the logic, and it belongs in commissioning and in the periodic maintenance plan.
Key takeaways
- Normal MTM state is both mains closed and the tie open, with every source and main sized to carry the full load after a transfer.
- Loss of source is detected by line-side undervoltage relays (27) with delays that ride through sags and reclosing; overcurrent trips and lockouts block transfer.
- The two-of-three interlock, implemented electrically, in logic and often mechanically, prevents unintended paralleling.
- Open transition accepts a short outage; closed transition needs sync-check supervision, utility approval and a paralleled fault study.
- Provide automatic, manual and maintenance modes, report status to SCADA, and functionally test the scheme with simulated source loss.