Fast transients or bursts are generally a power line phenomenon, but it can also be a problem on signal lines due to inductive or capacitive coupling. It can occur when a power switch or a relay with an inductive load is operated: When the current is disconnected, a series of small sparks will put high-voltage spikes on the power line.
The figure above, Fast Transient Burst, shows the fast transient burst pulse train used for EMC testing. The next figure, Close-up of Burst, shows a close-up of a burst. Note that the pulse is only about 50ns wide, this is much smaller than the figure indicates. See IEC 1000-4-4 for details of the pulses and the test setup.
Test voltages on power supply lines are typically 1kV for protected environment and 2kV for industrial environment. Severe industrial environments may require up to 4kV transient testing.
Test voltages on I/O lines are half the values used for power supply lines.
On an I/O line, the pulse may seem similar to an ESD pulse, but there are some very important differences:
- The energy of a single transient pulse may be higher than an ESD pulse at the same voltage, depending on the coupling path into the system.
- ESD testing is performed once or only a few times, with several seconds cool-down time between each pulse. The fast transient pulse is repeated at 5kHz (2.5kHz @ 4kV) for 15ms: this is one burst. The burst is repeated every 300ms.