In short. A main-tie-main arrangement has two incomers and a tie breaker, with only two of the three closed at a time. If one source is lost, the tie closes to feed both sections from the remaining source. The logic must stop the tie closing onto a fault and prevent unplanned paralleling of sources.
Arrangement
Sources A and B feed bus sections 1 and 2. Normally incomers A and B are closed and the tie is open. If A is lost, incomer A opens, the tie closes, and B feeds both sections. The reverse applies for B.
Transfer logic
| Step | Condition |
|---|---|
| Detect loss of source A | Undervoltage with time delay |
| Check no fault on section 1 | No protection operation, no lockout |
| Open incomer A | Confirm open by feedback |
| Check source B healthy and capacity available | Voltage, loading |
| Close tie | Interlocks satisfied |
| Optional load shedding | If B cannot carry both sections |
Fault lockout
If section 1 lost supply because of a bus fault, closing the tie would put the fault on the healthy source. Block auto transfer when bus protection or incomer overcurrent has operated. See coordination and selectivity.
Interlocks
- Never allow all three breakers closed unless a deliberate paralleling mode with sync check exists.
- Use key interlocks or hard-wired logic in addition to software. See interlocking.
Closed and open transition
Open transition has a brief outage on one section. Closed transition parallels sources momentarily, needing sync and checks of fault level. See closed transition.
Motor residual voltage and restart
When sections transfer, motors may still hold voltage. Use a delay or residual voltage check, and a staged restart. See restart control.
With generators
A generator may serve as a third source on one section. Include it in the logic, with transfer from utility to generator and the generator’s limited capacity. See utility-DG transfer.
Illustrative example
Source A is lost at a plant with two transformers. The tie closes in about a second and section 1 loads restart in stages. In a second test, a simulated bus fault on section 1 operates protection, and the logic correctly blocks tie closing, so source B stays healthy on section 2.
The arrangement
Two incomers feed two bus sections joined by a tie breaker. Normally the tie is open and each incomer supplies its own section. If one incomer is lost, the tie closes and the remaining incomer supplies both.
| Event | Automatic action |
|---|---|
| Incomer 1 voltage lost, incomer 2 healthy | Open incomer 1, close tie |
| Incomer 2 voltage lost, incomer 1 healthy | Open incomer 2, close tie |
| Fault on a bus section | Block transfer and alarm |
| Source restored | Manual or automatic return after delay |
Critical safeguards
- Lockout on fault. If an incomer tripped on a bus fault, closing the tie onto the same fault would extend the outage. Block transfer after protection trips.
- Capacity check. The surviving incomer must carry both sections. Shed non-essential loads first if needed.
- Interlock. All three breakers cannot be closed together unless parallel operation is designed.
- Motor residual voltage. Allow decay or use residual voltage check before closing.
Generator in the scheme
A generator may serve as an alternative source on one or both sections. The scheme should define the order of preference: other incomer first, or generator first.
Example timings (illustrative)
Detect loss 0.5 to 2 s; open incomer 100 ms; close tie after 200 to 500 ms; total interruption under 3 s without generator.
See utility and DG transfer and interlocking.
The master engineering question
When one source is lost, can the logic tell the difference between a lost source and a fault on the bus, and can the remaining source carry the combined load?
What to send us
Send the one-line diagram and transformer data. Send your application details.
Questions engineers ask
Why is the tie normally open?
To limit fault level and keep sections independent.
What if B cannot carry both sections?
Shed non-essential loads automatically.
Is retransfer automatic?
Can be, with delay and conditions, or manual after review.
Can two utility sources be out of phase?
Yes, so closed transition needs sync checks.
This guide gives general principles. Confirm with a project design.