When a half-period or two is missing.

The Danish power grid (the mains supply) delivers a voltage that is mostly completely stable and without disturbances. Most private homes rarely need to reset the alarm clock and refrigerator because of power outages. When CE marking electronic appliances, all appliances are tested with a small selection of voltage dips and power outages, as a kind of minimum clarification of the function, if such a disturbance should one day occur.

Steep pulse edges in latest standard

EMC testing for CE marking entails several tests with voltage variation and voltage outages. The test standard is EN/IEC 61000-4-11, and the latest version has just been released in 2019. The standard clarifies that the drop from full voltage to 0 V must be performed with a very steep flank. Especially the fall time for voltage drop-outs must be between 1 μs and 5 μs. Peak current from the test generator should not be less than 500 A. These are strict requirements for the testing equipment and has, for even modern testing equipment, meant that the generator has to contain a special semiconductor switch to be able to deliver a large and fast current pulse.

The standard contains an entire paragraph arguing that the steep edges do occur if an appliance suddenly short-circuits the mains voltage, so it quickly drops to 0 V. When this occurs, the impedance of the network is very low, which equals a large power availability. Figure 1 shows an example of a steep voltage drop (the drop time appears to be 1 ms or shorter), whereas when the voltage returns to normal, this occurs more smoothly.

The input filter is drained of energy

When electronics are tested for network outages, the power supply loses part of its ability to maintain a constant voltage. Modern switched-mode power supplies can easily keep up as the voltage drops, but at some lower voltage, it is no longer possible to still provide the correct output voltage when the input voltage is missing. This may lead to a reset or stop of the electronics. If the voltage has been zero for a while, the reservoir capacitors in the power supply can be completely drained of energy, and then these must be recharged before the power supply operates normally again.

When the grid recovers

Most circuits detect when the mains voltage disappears and can save vital data or assume a safe state. However, the combination of the changed grid voltage and the duration of the drop-out should be tested, so that the response pattern of the voltage monitoring circuit is known. Therefore, the developers should test far more combinations of voltage and duration than the minimum required by the product standards. 

Product standards usually indicate Performance criterion C for voltage outages. This means that it is acceptable for the electronics to be restarted by an operator after an outage. However, it is, of course, more advantageous for a user if the electronics can restore itself without the need for external intervention.

Figure 1: Three-phase voltage outage to approximately 17% of the nominal value measured on the Danish power grid. The outage lasts approximately 75 ms or approximately 3.5 periods of the mains frequency (50 Hz). The voltage subsequently returns to normal.

This article has been published in SPM Magasinet, August 2019.