Resources / STS vs ATS
Static transfer switch vs automatic transfer switch
Both move a load between two sources without anyone touching them. The difference is the switching element: semiconductors in a static switch, moving contacts in an ATS. That one choice drives transfer time, behavior between unsynchronized sources, service life and cost.
Three kinds of transfer switch
Side by side
| TwinSource STS | TwinSource MagLatch ATS | Conventional standby ATS | |
|---|---|---|---|
| Models | DCC-II, DCC-III, DCC-4 | DCC-I | Not a TwinSource product |
| Switching element | SCRs, no moving parts | Patented magnetically latched relays, no return spring | Contactors or breakers |
| Transfer time | As fast as ¼ cycle per phase | Under ½ cycle | 50 ms and up; 20–45 s with generator start |
| Out-of-phase sources | Any phase angle | In or out of phase | In phase only |
| Contacts | None (solid state) | Held by a permanent magnet, no holding current | Contacts arc and wear |
| Service under load | Bypass isolation or maintenance bypass | Replace the unit | Bypass-isolation models available |
| Ratings | 10 to 600 A, single and three phase | 12 to 24 A, single phase | 50 to 4000 A |
| Best for | Loads that must never drop | Less critical or redundant loads | Utility to generator transfer |
Will your load notice?
Set how long your load can run with no input power. IT power supplies built to the ITIC curve tolerate 20 ms. Contactors and relays in control circuits can drop out in one or two cycles, and some process equipment is more sensitive still.
When a relay ATS is the right answer
A well-designed site often uses both technologies. Relay switches suit budget-driven applications where servers are redundant and an occasional unit failure is tolerable. Static switches belong on the loads whose loss stops the business. If the target is a mean time between failures above a million hours, SCRs are the only practical route: relays have moving parts, limited overload capability and coils that age.
If you compare relay products, check the rating at your real power factor. Many relay ratings are for resistive loads and fall sharply at 0.85 power factor. Ask how the unit behaves when the sources are 120 or 180 degrees apart, because that is the condition it will meet when a UPS is failing.
Where to startDCC-I MagLatch ATSDCC-II static transfer switch
Questions engineers ask
Is transfer time the way to tell a static switch from a relay switch?
No. Good relay switches also finish inside the 20 ms that IT equipment tolerates. The differences that matter are out-of-phase behavior, overload capability, service life and whether the unit can be bypassed for maintenance.
Does an STS replace the generator ATS?
No. They do different jobs. The standby ATS brings the generator to the building. The STS gives an individual load two live sources and a transfer it cannot feel.
What makes MagLatch different from an ordinary relay?
A permanent magnet holds the contacts in either position, so no coil has to stay energized and there is no return spring to pull them back. A short pulse on the coil moves them. The design is covered by TwinSource patent US 7,535,129.
Put it to work
Narrow the range to your voltage, current and mounting, or send us your one-line diagram and talk to an engineer.