Safety Capacitor Classes

In our prior blog, What Are Safety Capacitors, we provided an overview safety capacitor functionality and how these passive components play a crucial role filtering EMI in a circuit. Now, we will dive deeper into the functions of safety capacitors and how to select the capacitor that will meet all your needs by exploring the different classes and subclasses for these components. 

Let’s start with the two classes of capacitors – Class-X and Class-Y:

Class X Capacitors: Class-X capacitors, also known as "across-the-line capacitors,” are used between the wires carrying the incoming AC current. These capacitors offer line-to-line protection, so if there is a failure, a short may occur but there is no risk of shock. This is because an X capacitor failure usually results in a short circuit, triggering a fuse or circuit breaker to open.

Class Y Capacitors: Class-Y capacitors, also known as "line-to-ground capacitors" or “line bypass capacitors,” offer line-to-ground protection, which generally means that if a failure with the ground occurs, there is a risk for shock. Class-Y safety capacitors must meet rigorous specifications, and are designed to fail open, minimizing the chance of electric shock.

X/Y Capacitors: Some safety capacitors handily combine classes, for example, X1/Y2 (details below). This simply means that the capacitor can be used as an X1 capacitor in an across-the-line application or as a Y2 capacitor in the line-to-ground portion of the circuit.  

Figure 1. A representation of where Class X and Class Y safety capacitors are placed and the associated risks.

Safety Capacitor Subclasses

As mentioned, both X and Y capacitors are further broken into the following subclasses according to their pulse voltage withstand capability and rated voltage (more details in Table 1):

• X Capacitor Subclasses:
  • X1: For higher surge levels (≥2.5 kV and ≤4 kV)
  • X2: Most common in consumer electronics (≤2.5 kV)
  • X3: For ≤1.2 kV, less common
    
• X Capacitor Subclasses:
  • Y1: For higher insulation and impulse voltage (>500 VAC, up to 8 kV)
  • Y2: Common for 250 VAC operation
  • Y3/Y4: Lower ratings, used in less critical applications
    

*2x Y2 or Y4 rated may bridge double or reinforced insulation when used in series

Table 1. A high-level overview of the key specifications for a range of different safety-certified MLCC classes.

By understanding the differences between the different capacitor classes and subclasses, you can optimize product development while ensuring user safety, product compliance, and reliable circuit performance.

In the next post in our series on safety capacitors, we will explore how ceramic and film capacitors compare for use in safety capacitor applications.

Learn more about Knowles Precision Devices extensive line of safety capacitors.