In short. A main-tie-main board has two incoming supplies, each with a main breaker, joined by a tie breaker between two bus sections. Normally the tie is open and each source feeds its own section. If one source fails, the tie closes and the remaining source feeds both sections, so each source and transformer must be rated for the whole load.
The arrangement
A double-ended substation has two transformers, each feeding one end of a bus with a tie in the middle. The three breakers (Main 1, Main 2, Tie) follow an interlock so only two of the three can be closed at once in normal operation. This stops two sources being paralysed unintentionally.
Normal and failure states
| State | Main 1 | Tie | Main 2 | Result |
|---|---|---|---|---|
| Normal | Closed | Open | Closed | Each source feeds its section |
| Source 1 lost | Open | Closed | Closed | Source 2 feeds both sections |
| Source 2 lost | Closed | Closed | Open | Source 1 feeds both sections |
| Maintenance on Source 1 | Open | Closed | Closed | Planned, one section at a time if needed |
Transfer logic
- Detect loss or out-of-tolerance voltage on a main after a short delay.
- Open the failed main, confirm it is open, then close the tie.
- Reject the transfer if there is a fault on the section, indicated by a protection trip, so the tie does not close onto a fault.
- On restoration, retransfer manually or automatically with synchronism checks if closed transition is used.
See main-tie-main transfer control.
Capacity and loading checks
- Each transformer, main breaker, bus section and cable must carry the load of both sections, or the loads must be reduced by shedding.
- Check voltage regulation and cable heating in the single-source state.
- Check generator or standby arrangement for each source.
See transformer loading.
Fault-level checks
With both sources closed through the tie, the fault level can be the sum of both. If sources are paralysed for a closed-transition transfer, short-term fault current is higher. Confirm every breaker and busbar rating covers the worst case. See short-circuit study screening.
Protection and interlocks
- Use hardwired interlocking between the three breakers, backed by key interlock or PLC logic.
- Block the tie from closing after an overcurrent trip on the section.
- Coordinate overcurrent and earth fault settings for both configurations.
Worked illustration
Two 1,600 kVA transformers feed a main-tie-main board with 1,800 kVA of load, 900 kVA per section. In normal operation each transformer runs at about 56 per cent loading. If one transformer fails, the other carries 1,800 kVA, which is 112 per cent of its rating. The design must either allow a short-term overload the transformer can withstand, shed 200 kVA of non-essential load, or choose larger transformers. This is the most common oversight in double-ended design: each side must be able to carry the entire critical load, not just half.
Testing
- Trip each main by simulated source failure and confirm the tie closes and loads recover.
- Test the lockout after a simulated fault.
- Record voltage, current and time during the transfer.
- Test the return to normal.
Double-ended distribution
Two incoming sources each feed one half of a board. A normally open tie connects the halves. On loss of one source, the tie closes and the other source feeds both halves.
| State | Incomer 1 | Tie | Incomer 2 |
|---|---|---|---|
| Normal | Closed | Open | Closed |
| Loss of source 1 | Open | Closed | Closed |
| Loss of source 2 | Closed | Closed | Open |
| Maintenance of a section | As needed | As needed | As needed |
Design checks
- Each incomer and transformer must be rated for both halves (total load), unless load shedding is accepted.
- Interlock so that all three breakers cannot be closed together unless paralleling is intended.
- Block the tie closing if the loss was from a bus fault.
- Check fault level with the tie closed and both sources connected (if momentary paralleling is allowed).
Loading (illustrative)
Two 1,600 kVA transformers each feeding 1,000 kVA normally. On loss of one, the remaining transformer carries 2,000 kVA, which exceeds its rating, so load shedding or forced cooling is needed, or each should be sized so that normal load stays near 50 to 60 percent.
Generators in the scheme
Generators may connect to each half or to a common section. Define the preference order for sources and the logic for transfer.
See main-tie-main control and bus architecture.
Common mistakes
- Rating each side for half the load.
- No fault lockout on the tie.
- Different settings on the two mains causing uneven transfers.
- Failing to test the return to normal.
What to send us
Send the single-line diagram, transformer ratings and loading. Send your application details. See also bus architecture.
Questions engineers ask
Why must the tie be interlocked?
To prevent paralleling two sources unintentionally, which can cause fault levels above ratings and circulating currents.
Can the tie be closed in normal operation?
It can, but fault level rises and protection becomes more complex. Many sites keep it open.
How much overload can a transformer take?
It depends on the design, cooling and ambient temperature, and the duration. Confirm with the manufacturer’s loading guide.
What happens if the tie closes onto a fault?
The fault transfers to the other source and may take it down too. That is why the lockout after a protection trip matters.
This guide gives general principles. Ratings, settings and interlocks require engineering design and testing.