Overload prevention in any given design is serious business, which means that the choice of safety capacitor shouldn’t be taken lightly either. Areas to consider in the decision process include safety requirements, type of filtering, the pros and cons of different device types, the consequences of device failure, and much more. This article provides a quick reference to the most important of these factors when selecting a device.
1. The class of capacitor you need depends on whether you’re doing differential-mode or common-mode filtering
- Differential-mode interference is when pulses run along the wires (L-N) in opposite directions. For differential-mode filtering, you need Class X capacitors connected between the lines, effectively returning high-frequency interference back to its source.
- Common-mode interference is when pulses run in the same direction in both wires (L-N) in the same device. For common-mode filtering, you need Class Y capacitors connected between the wires and ground, bypassing the interference pulses from the wires to the ground.
Figure 1. The full-range, ac-filtering film capacitors shown here (MKP1847H) can withstand demanding temperature-humidity-bias (THB) testing—85°C, 85% RH for 1,000 hours at rated voltage—without altering their electrical characteristics. The capacitors feature segmented film, a rated capacitance from 1 to 35 µF, and ESR down to 3 mΩ. The devices provide ripple current capabilities up to 24.5 A and rated voltages of 250, 310, 350, and 480 V ac.
2. Safety requirements differ depending on the type of capacitor
- The noise level in all electronic/electric devices has to be kept below a certain level, which is defined in the standard for the particular device.
- Filtering is done by components such as capacitors or chokes, or a combination of these components.
- Since safety capacitors operate directly connected to the mains, they have to meet the requirements of the IEC 60384-14 safety standard.
- The safety requirements are much higher for Y capacitors, because a short/failure