An X capacitor is a type of electrical component used in circuits that serves as a regulating mechanism for the circuit, and it has an electrical charge storage capacity. These capacitors are often referred to as filter capacitors because they are cross-circuit electrical components that maintain a steady flow of current. If an X capacitor is overloaded and fails, the circuit may experience a surge in current that could result in an electrical fire. This is different from how a Y capacitor functions in a circuit, where it is built in a serial line along one electrical channel, whereas an X capacitor is built into the circuit in parallel fashion. If either an X or Y capacitor fail, the circuit is usually not broken and continues to function.
Since an X capacitor is a filtering mechanism for circuits, its role is referred to by electrical certifying agencies such as the Underwriter's Laboratories (UL) in the US as an “across-the-line noise” component. This reflects how the X capacitor is wired into the circuit, where it effectively shorts the flow of a portion of current between positive and negative channels to act as a noise-regulating device. In this respect, it has some similarities to inline parallel fuses, though part of its regulatory function also involves a storage cell or capacitance function for current while the circuit is on. When the X capacitor fails, its regulatory function ceases, which does not mean that the circuit has become open where it can cause an electric shock, but has merely become more susceptible to normal mode levels of circuit noise.
While an X capacitor is used in the role that most capacitors are used for in circuits as a secondary cell or condenser of electrical charge, a Y capacitor is built into parallel in the circuit to dispose of excess electrical charge. Because of this difference, a Y capacitor is also connected to a ground source and it channels common mode noise into the ground as the circuit functions. The UL reference for this functionality is “line-by-pass noise,” and, when a Y capacitor fails, there is a real risk of electrical shock if a part of the human body comes into contact with it.
The X capacitor has many common uses in circuitry. Among them include its role as a regulator of direct current (DC) devices to prevent overloads from spikes, as well as serving as a filter for radio frequency noise or interference (RFI), which can be introduced into circuits from external sources by the property of inductance. The X capacitor also aids in smoothing out current flow by its ability to build up and store an electrical charge, which serves as a minor backup source for reservoir capacitors and ensures that the circuit into which it is built has a steady power supply.