Mains filter capacitors: X and Y classifications

Mains filter capacitors are used to filter out EMI (electro-magnetic interference) and RFI (radio-frequency interference) from and to the power grid. This applies primarily to interference which is conductively coupled, i.e. along wires such as a device’s power cables.

Mains filter capacitors are commonly used in devices which can create large amounts of EMI, such as switched-mode power supplies. Another type of equipment that can cause interference any appliance with an AC motor – mains filters can thus often be found in kitchen blenders, sewing machines or power tools like drills and hacksaws.

The image below shows the connection within one common type of EMI/RFI mains filter with both X and Y capacitors and a choke (L). It also shows how X and Y capacitors are meant to be connected.

You can find Electrokit’s range of mains filter capacitors here.

Safety classification – X and Y

Mains filter capacitors are separated into two safety classifications: X and Y.

Safety classes have been created to minimize the risk of dangerous electric shocks or fire hazards. Hence, it is important that mains filter capacitors are used correctly, and also that damaged capacitors only be replaced with properly rated components of the correct type (X or Y).

As safety capacitors are commonly the very first component in a circuit, they are the first victim of harmful spikes from the power grid. Therefore, they must be able to withstand large surges and transients. This is one reason why only properly rated safety-grade capacitors should be used.

A good mnemonic for remembering how safety capacitors are used is that X capacitors cross between the phases, like an X, whilst Y capacitors run down to earth from the phases, in the shape of the letter Y.

Subclasses – 1 and 2

Both X and Y capacitors are available in two different subclasses, numbered 1 and 2. The number is indicated after the letter: X1, X2, Y1, Y2. X2/Y2 is mainly used in domestic settings, typically up to 230 VAC. X1/Y1 is the safest class and is used in industrial settings, typically up to 400 VAC.

The X-capacitor

X-capacitors are connected across a device’s phases (L to N). They typically have capacitances in the range of 0.1 µF to 10 µF and are manufactured from polypropylene for high-frequency applications.

Some older X-capacitors were manufactured using paper, not least by the Swedish manufacturer RIFA. These capacitors have become somewhat notorious for failing, often in dramatic fashion, resulting in soot formation, burnt circuit boards and foul-smelling smoke. This is particularly likely to happen when the device has been unused for many years – sewing machines and retro computers are two classic examples, where it may be a good idea to replace the capacitor as a precaution before switching it on.

Class	Peak pulse voltage	Typical application
X1	>2.5 kV ≤4.0 kV	3-phase 230–400 V, industrial
X2	≤2.5 kV	1-phase 230 V, domestic

The Y capacitor

Y-capacitors are connected from the phase (L) and/or neutral (N) to the earth (PE) of an appliance. As Y-capacitors connect directly to earth – an appliance’s chassis – they pose a significant (potential) safety risk. If such a capacitor failed and formed a short circuit, the appliance casing would become live. For this reason, Y-capacitors are insulated with very high insulation resistance and high resistance to voltage surges. They are also designed so that if they do fail, they do so in a way that creates an open circuit. This ensures that no current can pass through the faulty component to the appliance’s casing.

A faulty X-rated capacitor may be replaced with a Y-rated capacitor, but never the other way round. However, Y-rated capacitors normally have lower capacitance than X-rated capacitors. Typical capacitances for Y-rated capacitors range from 33 pF to 4700 pF. There are also capacitors marked with both X and Y – these can be used in both applications.

Class	Peak pulse voltage	Insulation	Rated voltage
Y1	8 kV	double	≤ 250 V
Y2	5 kV	single	≥ 150 V, ≤ 250 V