A main-tie-main scheme, also called a secondary-selective arrangement, is a way of building switchgear so that a facility survives the loss of one power source without going dark. Two independent sources each feed one half of a split bus through its own main breaker, and a tie breaker sits between the two bus halves, normally open. When one source fails, its main breaker opens and the tie breaker closes, so the surviving source picks up both halves of the bus. It is the redundant power backbone that lets a critical facility keep running through the loss of a transformer, feeder, or utility supply.
Main-Tie-Main Scheme in one line: A main-tie-main scheme is a switchgear arrangement with two power sources, each feeding half of a bus through a main breaker, joined by a normally-open tie breaker. If one source is lost, its main opens and the tie closes so the remaining source supplies the whole bus. This gives a facility a redundant, automatically transferring power supply at the switchgear level.
The arrangement is built around a bus divided into two sections. Each section has its own incoming source, brought in through a main breaker, and under normal conditions both mains are closed while the tie breaker between the sections stays open. In that normal state the two halves run independently, each source carrying its own half of the load, and the open tie keeps the two sources from ever being connected together directly. Every load on the bus is therefore fed, but from whichever source is on its side.
The value of the scheme appears when a source is lost. If one transformer, feeder, or utility supply fails, the main breaker on that section opens to isolate the dead source, leaving one half of the bus unpowered. The tie breaker then closes, connecting the two bus halves so the surviving source feeds across the tie and picks up the load that the failed source was carrying. The facility rides through the loss with only a brief interruption on the affected half rather than an indefinite outage.
The whole scheme depends on an interlock among the three breakers, and understanding that interlock is central. The two mains and the tie form a set where, in normal open-transition operation, only two of the three may be closed at once. This prevents the two sources from being paralleled through a closed tie unless the design specifically allows synchronized paralleling. The interlock is what guarantees the scheme transfers safely, opening the failed source before or as it closes the tie, rather than tying two mismatched sources together.
The transfer can be automatic, and on critical facilities it usually is. A control scheme senses the loss of a source, typically through undervoltage and other protection, opens the failed main, and closes the tie according to the interlock logic. Operators can also do this manually, but automating it removes the delay and the risk of human error during an upset, so the bus is restored in seconds without anyone present. The same logic reverses when the failed source returns, transferring the load back and reopening the tie so the system settles to its normal split.
Most main-tie-main schemes transfer in open transition, meaning the failed source is disconnected before the tie connects the two halves. There is a momentary break during which the affected half is dead, which loads must be able to tolerate. A closed-transition or paralleling scheme can make the transfer bumpless by briefly connecting both sources, but that requires synchronizing the sources and permitting them to parallel, which is a more complex and tightly controlled operation. Which transition a facility uses is a deliberate design choice driven by how much the loads can tolerate an interruption.
Capacity planning is what makes the redundancy real. Because a single source has to carry the entire bus after a transfer, each source and its transformer must be sized to handle the combined load, not just its own half. If the sources are only sized for their normal split, closing the tie onto a fully loaded surviving source would overload it, and the scheme's redundancy would be illusory. Properly designed, each source has the headroom to take the whole bus, which is the cost of the resilience the scheme provides.
Main-tie-main is a standard backbone for critical oilfield facilities where losing power means lost production, safety exposure, or environmental risk: gas plants, compressor stations, central processing facilities, and similar sites. It provides source redundancy at the switchgear level, complementing the device-level redundancy that transfer switches provide for individual loads and downstream equipment. Where a transfer switch backs up a specific piece of equipment, main-tie-main backs up an entire bus, so the two are often used together in the same facility at different scales.
Because the scheme spends most of its life in the normal state and only proves itself during a rare source loss, knowing its exact configuration at any moment is important. Which mains are closed, whether the tie is open or closed, which source is currently feeding each half, and whether the last transfer completed cleanly are all things an operator needs to see, especially since a bus running temporarily on a single source is more vulnerable and may need attention before the second source is restored.
A cloud SCADA platform such as Merobix reads the states of the two mains and the tie, the source voltages, the bus loading, and the protection trips from the switchgear's controllers, and trends them. That lets operators confirm from anywhere that the scheme is in its normal split, get an immediate alarm when a source is lost and the tie closes, and verify that the surviving source is carrying the combined load within its capacity. On remote and unmanned oilfield sites, that visibility means a transfer event is reported the moment it happens, with the configuration and loading in hand, rather than being discovered later as a facility running on borrowed capacity.
It describes a switchgear arrangement with a bus split into two sections, each fed by its own main breaker from a separate source, with a tie breaker between the sections. Normally the two mains are closed and the tie is open, so each source feeds its own half. When one source fails, its main opens and the tie closes so the surviving source feeds the whole bus, giving the facility redundant power.
A main-tie-main scheme provides source redundancy for an entire switchgear bus using two mains and a tie breaker, while an automatic transfer switch transfers a specific load or piece of equipment between two sources at the device level. Main-tie-main operates at the bus level within the switchgear; a transfer switch operates at the load level. Facilities often use both, at different scales, in the same system.
Because after a transfer the surviving source has to carry the entire bus, not just its own half. If the sources were sized only for their normal split, closing the tie onto a fully loaded remaining source would overload it, and the intended redundancy would fail exactly when it is needed. Sizing each source and transformer for the combined load is what makes the scheme genuinely redundant.
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