Potential Hazards and Safety of Capacitors
Time：2022.01.26
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1. Potential dangers and safety of capacitors
After the capacitor is charged and the power is turned off, the charge in the capacitor may still be stored for a long period of time. This charge is sufficient to produce an electric shock, or destroy connected equipment. A disposable camera flash module is charged from a 1.5V AA dry battery, which seems safe, but the capacitors in it may be charged to 300V, and the electric shock generated by the voltage of 300V can be very painful and even fatal.
Many capacitors have low Equivalent Series Resistance (ESR) and therefore draw large currents when shorted. Before servicing equipment with large capacitors, make sure that the capacitors are fully discharged. For safety reasons, all bulk capacitors need to be discharged before assembly. If the capacitor is placed on the substrate, a bleeder resistor can be connected in parallel next to the capacitor. In normal use, the leakage current of the bleeder resistor is small and will not affect other circuits. When powered off, the bleeder resistor provides a path for the capacitor to discharge. High-voltage large capacitors need to short-circuit their terminals during storage to ensure that the stored charges have been discharged, because if the capacitors are suddenly discharged when the capacitors are installed, the generated voltage may cause danger. The large, old-fashioned oil-immersed capacitors of electric welding machines contain poly-chlorinated biphenyls (poly-chlorinated biphenyls), so they need to be disposed of properly. Polychlorinated biphenyls are carcinogens, and trace amounts can affect the human body. Larger capacitors are more dangerous and require extra care. New electronic parts are free of PCBs.
2. Potential dangers of high voltage capacitors
Capacitors operating at high voltages and currents are more dangerous than usual.
High voltage capacitors have the potential to be catastrophically damaged when operated beyond their nominal voltage. Failure of insulating material can cause arcing in small cells filled with oil (usually the oil acts as a barrier to air) causing the insulating liquid to evaporate, causing capacitors to bulge, rupture or even explode, which can get flammable oil everywhere All, fire, damage to nearby equipment. Hard-packed cylindrical glass or plastic capacitors are more likely to burst than the usual cuboid-packed capacitors, which are less prone to cracking under high pressure.
Capacitors used in RF circuits and long-term high current environments can overheat, especially the reel in the center of the capacitor. Even if the external ambient temperature is low, the heat cannot be dissipated in time, and the accumulation inside may quickly lead to high internal heat and lead to capacitor damage.
In a capacitor bank working in a high-energy environment, if one of the capacitors fails and the current is suddenly cut off, the energy stored in the other capacitors will flow to the failed capacitor, which may cause a violent explosion.
High voltage vacuum capacitors emit certain X-rays even when used correctly. Proper sealing, fusing and preventive maintenance will help reduce these potential hazards.
