Mains filters

Here you will find mains filter capacitors intended for use in power supplies, and delta filters which consist of several components including mains filter capacitors. Commonly referred to simply as mains filters, this type of capacitors is also referred to as X or Y capacitors – X and Y being the two safety classes – or just safety capacitors. The ratings and different safety classes are there for a reason, and should not be ignored – read more below.

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.

Safety Classification – X and Y
Mains filter capacitors are separated into two safety classifications: X and Y. Safety-ratings are there 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.

X capacitors
X capacitors are connected across a device’s supply voltage, either phase to phase (L - L) or phase to neutral (L - N). Its task is to filter out any interference appearing on the mains, and stop them from entering or leaving the device. This is achieved by short-circuiting high frequencies, while letting lower frequencies pass through unhindered. X-capacitors normally have capacitances in the range of 0.1 µF to 10 µF and are generally made of polypropylene for high-frequency applications.

Some older X-capacitors were made with paper, not least by the Swedish manufacturer RIFA. These capacitors have become somewhat notorious for failing, often in dramatic ways with soot, burning and the release of nasty-smelling smoke. This is not uncommon when a device has been out of use for a long time – sewing machines and retro computers are two classic examples. If restoring or simply taking such a device out of storage, it might prove prudent to replace the mains capacitor (normally just an X-capacitor) as a precaution before turning it on.

Y capacitors

Y capacitors are instead connected from the phase (L) and/or neutral (N) to a device’s earth (PE). As the Y capacitor is connected to earth – a device’s chassis – it poses a major safety risk, in that an appliance casing could potentially become live. Therefore, Y capacitors are insulated with very high insulation resistances, and can also handle high-voltages pulses. In addition, they are manufactured so that should they fail despite all this, they always do so as an open circuit (“fail open”), meaning that no current can pass through the broken component into a device’s enclosure.

A broken X capacitor may be replaced with a Y-rated capacitor, but never the other way around. Although this is permissible, Y capacitors usually have much lower capacitances than X capacitors, so it may be difficult to find a suitable replacement. Typical capacitances for Y capacitors are in the range between 33 pF and 4700 pF. Capacitors marked with both X and Y also exist – they can be used in both applications.

Subclasses – 1 and 2

Both X and Y capacitors come in two different subclasses, numbered 1 and 2. The number is given after the letter: X1, X2, Y1, Y2.

X2/Y2 are mainly used in domestic environments, typically up to 230 VAC. X1/Y1 are the safest types, and are used in industrial environments, typically up to 400 VAC.

A rule of thumb to remember how mains filter capacitors should be connected is that X capacitors cross between the phases, like an X, while Y capacitors go down to ground from the phases, like the shape of the letter Y.