Background Information
X-type capacitors suppress differential mode conducted emissions in applications such as switch-mode power supplies, DC-DC converters, variable-speed motor drives, and other similar devices. The X designation means they are applied line-to-neutral, as shown in the simple diagram below.
The Issue
Manufacturers of X capacitors have their capacitors tested and certified by a Nationally Recognized Testing Laboratory (NRTL), and a certificate is provided. X-type capacitors are considered a safety critical component of the end-product, and as a compliance engineering professional, it is crucial that you know which subclass to use. Not selecting the correct sub-class for the end-products’ application will result in a non-compliance that will cause re-design and delay of production release of the product. Switching is not a simple process if you select an X2-rated capacitor and need an X1 instead. First, you must find a suitable X1-rated part and try to make it fit into the existing design. X1-rated capacitors are physically bigger than X2-rated parts.
X-type Capacitor Subclasses
The standard most often used to certify X (and Y) capacitor types is IEC 60384-14. This standard specifies three different subclasses of X-type capacitors depending on their peak impulse voltage rating.
X3 Subclass
The subclass X3 has a peak impulse voltage rating of less than or equal to 1.2 kV. This is not too high of an impulse rating, so only use X3-rated capacitors in benign environments (those protected from overvoltage transients). Since it is not to be used for robust design, the X3 subclass is not discussed further in this post.
X2 Subclass
The subclass X2 has a higher peak impulse voltage rating than the X3 type. The X2 subclass has a peak impulse rating of less than or equal to 2.5 kV.
X1 Subclass
Finally, the subclass X1 is even more robust than the X2 capacitor type. It has a peak impulse rating of greater than 2.5 kV and less than or equal to 4.0 kV.
Which Subclass Type Do I Use? X1 or X2?
To determine which subclass type to use (X1 or X2), you must know the end-product’s overvoltage category of the intended environment. This is something that is usually determined early in the product development cycle.
IEC 60664-1 (latest edition is dated 2020) describes the overvoltage categories. There are four overvoltage categories, designated as I, II, III, and IV. Devices installed in category IV locations are subjected to the most severe transients (surges), whereas category I locations are the least severe, protected environments. This post focuses on categories II and III. Examples of category II locations are household appliances plugged into the standard home wall outlet. Category III locations are closer to the voltage supply and subjected to harsher transient conditions. Examples are switches in the fixed installation and equipment for industrial use with permanent connection to the fixed installation.
Annex B (informative) of IEC 60664-1 describes nominal voltages of mains supply for different modes of overvoltage control. See Table B.1 – Inherent control or equivalent protective control. This table is something you need to look at.
For example, select 300V as the voltage line-to-neutral derived from nominal voltages AC or DC. The rated impulse withstand voltage for the equipment is provided for each overvoltage category. For category II, it is 2.5 kV; for category III, it is 4.0 kV.
Given this information, if your end-product is installed in an overvoltage category II location, then an X2-rated capacitor is acceptable. However, if it is installed in an overvoltage category III location (a location that requires a rated impulse withstand voltage of 4.0 kV), then an X1-rated capacitor is required.
What If I Select an X2 but Require an X1?
All is not lost if you inadvertently select an X2-rated capacitor but require an X1. You can install a clamping device (such as a metal oxide varistor) across the line that limits the transient overvoltage seen by the rest of the circuit to less than or equal to 2.5 kV and work with your NRTL to accept it. This workaround has some risks as it depends on the NRTL’s policy for dealing with this situation, which could change anytime.
