Resources / Transfer Time and Source Quality Basics
Transfer time and source quality basics
A transfer is successful when the load never knows it happened. That depends on two things: how long the load can run without input, and how long the switch takes to notice a bad source and move to a good one.
What the load can tolerate
The ITIC curve, once known as the CBEMA curve, describes the input voltage that information technology equipment should ride through without malfunction. A complete dropout is acceptable for up to 20 ms. A sag to 70% of nominal is acceptable for half a second, and a sag to 80% for 10 seconds. Steady-state voltage should stay within 10% of nominal.
The curve was written for 120 V single-phase equipment, so treat it as a guide elsewhere. Control circuits can be less forgiving: an electrically held contactor typically drops out in one to two cycles when its voltage disappears. For those loads the speed of the transfer switch matters more, not less.
What makes up transfer time
Two intervals add together. Sensing is the time to decide that the preferred source has left its window. Switching is the time to stop conducting from one source and start from the other, break-before-make. TwinSource static switches complete both as fast as a quarter cycle per phase. Set the phase angle between the sources and the type of switch to see what reaches the load.
Source quality
The switch needs a definition of "good." You set a voltage window for each source, and the switch transfers when the preferred source falls outside it and the alternate is inside it. It will not move the load onto a source that is itself out of tolerance. When the preferred source has been back inside its window for a set time, the load returns. Front-panel indicators show whether each source is available and which one is carrying the load, and the same status is available on alarm contacts and RS-485.
Out-of-phase transfers
Datasheets often quote transfer time with the two sources in phase. In service, the transfer that matters happens when a UPS is failing, and a failing UPS cannot hold synchronization. Some relay switches tolerate only 15 to 20 degrees of difference, or add a long delay when the angle is large. Ask for both numbers. TwinSource switches transfer at any phase angle. If a transformer sits downstream, an out-of-phase transfer can drive it toward saturation, so involve our engineers early.
Questions engineers ask
Is faster always better?
Up to a point. Once the transfer is well inside the ride-through of the load, reliability of the switch, out-of-phase behavior and serviceability matter more than another millisecond.
Why do datasheets quote such different numbers?
Some quote switching time only and leave out sensing. Some quote in-phase only. Ask for total time from the start of the disturbance to restored output, at 0 and at 180 degrees.
Does the switch react to sags as well as outages?
Yes. It transfers whenever the preferred source leaves its voltage window, whether the cause is an outage or a sag.
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.