What are X Y Capacitors?

Radio Frequency Interference (RFI) and Electromagnetic Interference (EMI), emanating from electronic devices, switching circuits, chopping circuit devices using SCR, IGBT, etc. are a major source of disturbance or nuisance for sensitive electronic circuits, radio and video equipment etc.  The interference may be from incoming supply, or it may come from external sources by radiations.

Incoming supply source can have high frequency interference from switching on or off circuits like switching devices connected to the same line or supply system. These are conducted interference. This can produce common mode and differential mode interference at AC input mains.

There could be disturbances appearing due to other sources outside the circuit, which affect the circuit by radiation. A change in current in a conductor creates a changing magnetic field around it, which in turn produces changing electric field. Frequency and amplitude of this magnetic field decides by the frequency and amplitude of electric field so produced.

X and Y safety capacitors are designed for use in AC line filtering in many electronic circuits. Their purpose is to provide safety from EMI (electromagnetic interference) and RFI (radio frequency interference) to the device or equipment. These are also called EMI suppression capacitors and AC line filter capacitors. They help minimize generation of EMI and negative effects of EMI and RFI.

Safety capacitors are also known as AC line filter capacitors, or EMI / RFI Suppression Capacitors. Class X and Y capacitors help minimize the generation, or the negative effects of received EMI / RFI interference. These capacitors are connected directly across input terminals (line and neutral), and are therefore subject to over-voltages and transient spikes / pulses. They must be designed to withstand these safely since they are directly across supply terminals. AC supply usually has three terminals viz., line, neutral and ground.

X capacitors

Function of X capacitor is to bypass all high frequency interference or signals from source and not allow them to enter the circuit. They are connected directly across supply input terminals (L and N), and have large values in farads. X capacitor is therefore also called ‘across the line capacitor’.

Failure of this capacitor will normally activate overcurrent protection (or short circuit protection) devices like fuse or circuit breaker. (In the absence of such protection, there is real danger of explosion / burning.) therefore failure of X capacitor does not present a shock hazard.

Y capacitor

This may also be called ‘line to ground capacitor’ or ‘line bypass capacitor’, since it is connected between line and ground, (L and G), and neutral and ground (N and G). Failure of this capacitor therefore directly leads to shock hazard due to ground connection (ground is different from neutral), even though the RFI protection is gone and equipment or circuit is deprived of RFI / EMI noise or interference protection.

X and Y capacitors are classified based on

1. Peak or rated voltage, and
2. Peak impulse voltage they can withstand

Table 1. Classes of X capacitors

Sub-class (IEC60384-14)	Peak Voltage Pulse While in service	Peak Impulse before endurance test
X1	>2.5 KV >4.0 KV	4 KV per C ≤ 1 μF
X2	≤2.5 KV	per C ≤ 1 μF 2.5 / √C  > 1 μF
X3	≤ 1.2 KV	None

Table 2. Classes of Y capacitors

Sub-class (IEC60384-14)	Rated Voltage	Peak Impulse before endurance test
Y1	≤ 500 V AC	8 KV
Y2	150 V AV ≤ V < 300 V AC	5 KV
Y3	150 V AV ≤ V < 250 V AC	None
Y4	≤ 150 V AC	2.5 KV

Applications of X and Y capacitors

Subclass X2 and Y2 are used most widely.  These are found on most common household appliance. X2 and Y2 are used in industrial applications. This is the general thumb rule, but one may select X2 and Y2 capacitors for household applications, though they may be costlier. Some capacitors are designed to pass both X and Y capacitor safety standards, and these are termed X1/Y1, X1Y2, X2/Y3 etc. A capacitor rated X1Y1 may be used either as X1 in line-to-neutral or as Y1 line-to-ground application.

A Y2 capacitor can be used instead of X2 capacitor, but reverse is not applicable, because X2 capacitor is not designed to withstand high voltage needed for Y2 capacitor, meaning it does not meet ground safety requirements. Y2 capacitors are more robust, can withstand high voltage peaks, and fail in open circuit mode. X2 capacitor may fail in short-circuit mode, which can endanger humans if connected to ground, and also can lead to fires. Failure of Y capacitor will cause high frequency interference to pass through, but will not cause shock or fire hazard. Value of X capacitor could be in μF range, governed by circuit considerations, while Y capacitors are small, in pF range (e.g, 4700 pF). They may be ceramic, or plastic film capacitor.