Resources / Where to Start
Where to start: planning for transfer switches
Seven questions settle most transfer switch decisions. Work through them in order and you will know which technology, which rating and which features your site needs before you talk to any vendor, us included.
The seven questions
- What rating?Rack-mount static switches are typically 30, 40 or 50 A, and relay types 12 to 30 A. Insist on a 100% continuous rating, allow room for growth, and size the feeder breaker one step up so the full rating is available.
- What is your power factor?Relay ratings are often quoted for resistive loads and can fall by half at 0.85. Check the rating at the power factor of your load.
- How critical is the load?Compare the cost of one outage with the cost of the switch. Where an outage runs to hundreds of thousands, a static switch is the only sensible choice. Where servers are redundant and budgets are tight, a relay switch can be the right one. Many sites use both.
- What does MTBF mean across the fleet?A single MTBF figure hides the arithmetic. Multiply by the number of units you plan to install.
- Is there a bypass?If not, every alarm ends in a load shutdown. Static rack switches usually include one. Most relay types do not.
- Have you verified the claims?For a large purchase, test. Scope the output during a transfer and a retransfer and look for contact bounce. Ask for out-of-phase transfer time, fault rating, how the preferred source is changed, and whether relay coils stay energized.
- Where is the fault protection?Fuses should be reachable from outside and replaceable in a minute or two. If the unit relies on an upstream breaker, get the approved breaker list.
MTBF across a fleet
Twenty switches run 175,200 hours between them every year. At a published MTBF of 100,000 hours, that is 1.75 failures a year. Try your own numbers.
Next steps
Ask us for a comparison chart with the TwinSource column already filled in. Complete the other columns from your own tests.
Questions engineers ask
Should I buy the largest rating that fits?
Often, yes. Within a family the enclosure and electronics are usually shared, so the higher rating adds protection for little extra cost and runs cooler at your load.
What should I look for on a transfer switch datasheet?
Four things that make switches comparable: the switching element (SCRs or relays), the transfer time with the sources out of phase as well as in phase, the short-circuit (fault) rating, and whether the current rating is continuous. If a datasheet leaves one out, ask the manufacturer before you compare.
Is a relay switch ever the better choice?
Yes, for less critical loads under 24 A where servers are redundant. The TwinSource DCC-I is built for exactly that.
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.