Electrical Fast Transients

Electrical fast transients (EFT) are disturbing signals with little energy content. The high repetition frequency is typical for this type of signal. We can find this disturbance often when a switch changes (at high speed) from an isolating to a conducting position, e.g. mechanical driven relays-switches.

The pulse of the generator used for simulating this disturbance during testing is displayed at the bottom of this page. EFT’s due to switching-procedures can generate pulses with a frequency of 10 kHz up to 10 MHz and voltages up to 4 kV. The duration of one total burst is 15 ms and the repetition rate is 300 ms. So there are packages of bursts delivered to the equipment under test, to test the immunity. The tests are performed on the power supply port, and on I/O signal, data and control ports. There are different types of setups for the EUT. The test setup for laboratory-test is displayed below.

There are two types of coupling the EFT’s to the ports that have to be tested:
• Direct coupling; via a coupling unit as stated in the standards, the EFT’s are directly injected to the port under test. This is commonly used for the mains power ports;
• Capacitive coupling; Due to a large variety of cable and connectors, this method is a great help. The cables are lead through a capacitive coupling clamp with characteristics as stated in the standards. The clamp shall be closed as much as possible to provide maximum coupling capacitance between the cable and the clamp. The EFT’s are injected to the clamp and by means of capacitive coupling carried to the cable in the clamp.

When performing the tests it is required that the EUT and possible auxiliary equipment are exercising all normal modes of operation. The test results may be classified on the basis of the operating conditions and functional specifications of the EUT, according to the following performance criteria:

• (A) Normal performance within the specification limits;
• (B) Temporary degradation or loss of function or performance which is self-recoverable;
• (C) Temporary degradation or loss of function or performance which requires operator intervention or system reset;
• (D) Temporary degradation or loss of function or performance which is not recoverable, due to damage of equipment (components) or software, or loss of data.