Future Electronics – XY Caps: Built to Fail…Safely

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Some examples of X capacitors in captivity


By: David DeLeonardo, Analog Specialist AE, Future Electronics

In this Tech View, we will take a close look at the common, but commonly misunderstood, components known as “XY” or “safety” capacitors. For each type, we will look at how they are used, constructed, rated, age and fail. Finally, at the end of this article, a chart is provided that compares the two types on the basis of each of these parameters.

X is for across. X caps are sometimes called safety caps, or AC line filter caps. They are a class of capacitors that are intended to be connected across the AC lines. Due to this connection, if they were to fail in a shorted condition, no shock hazard is created since they have no connection to the chassis, as we will see is the case with Y capacitors. Of course, if they were to fail in a shorted condition, there is the risk of fire. Therefore, they are intended to be connected downstream from an appropriately rated FUSE as seen in Figure 1.

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Figure 1

As can be seen in Figure 2 (which is a detail taken from Figure 1), if either of the X caps Cx1 or Cx2 fail, the resulting fault current would pass through the fuse which, in turn, would blow open and end the fault condition. At no time would a shock hazard be present, since no connection would be made between the AC lines and the chassis. However, even with a fuse in line with the fault current, there is the risk of a fire occurring from the failed capacitor. Therefore, X caps must be constructed so that they are self-extinguishing in the event that they do happen to fail short. To accomplish this, most X caps are enclosed in a plastic box structure or encapsulated in a plastic cylindrical case.

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Figure 2

Self-Healing Property of Film Capacitors
The vast majority of X caps are metalized film type capacitors. Thus, they consist of a thin plastic film with metallic coating on both sides. When a pinhole or other imperfection occurs in the film, it is vulnerable to arcing, even when the applied voltage is below the rating for the device. However, when the device does break down in the area of the flaw, the resulting electrical arc changes the device in the area of the flaw in such a manner so as to heal the flaw. However, while the device can not withstand voltages that caused arcing prior to the healing event, it should be noted that the capacitance of the device will be reduced due to the reduction in the effective area of the plates. Also, the leakage of the device will be increased due to the presence of carbon in the newly created void channel. This process is illustrated in the three-panel graphic below:

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Figure 3: The presence of a flaw, (void or impurity) in the plastic film creates a point of failure in the event of an over-voltage.

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Figure 3: High voltage pulse drives current flow, carbonizes plastic film, and “burns back” metal to form a “void channel.”

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Figure 3: The device now has lower capacitance due to reduced film area and higher leakage due to carbon in “void channel.”

Classes of Safety Caps
All safety caps are grouped into industry standardized classes. These classes are characterized by their peak service voltage and their peak test pulse voltage. The classes and their defining characteristics for both X and Y caps are detailed below:

Sub-GroupPeak Service Voltage (Volts AC)Peak Pulse Test Voltage
X12500 ≤ X1 ≤ 40004500V, C ≤ 1µF {(4k)/[(C)^(0.5)]}V, for C > 0.1µF
X22500 ≤ X22500V, C ≤ 1µF {(2.5k)/[(C)^(0.5)]}V, for C > 0.1µF
X3X3 ≤ 1200None
Y1Y1 ≤ 5008,000V
Y2150 ≤ Y2 ≤ 3005,000V
Y3Y3 ≤ 500None
Y4Y4 ≤ 5002,500V

The specific circuit and conditions relating to these pulse tests are defined in industry standards such as UL 1414 or 1283, while in Canada, the relevant standards are CSA C22.2 No.1 and No.8, and in Europe the standard is EN132400. The international standard is 60384-14. All products that claim to be a particular class of X cap must be certified as passing the tests of their corresponding class. Such products will be marked with one or more of the following symbols:

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Figure 4

The symbols shown in Figure 4 are used by UL, TUV and CSA to indicate that the component bearing this marks has be constructed so as to meet the relevant safety standards for the US, Germany and Canada, respectively. Most other countries that do not have product safety agencies of their own simply mandate that products imported into their territories must have the marks (and the attendant approvals implied thereby) of one or more of these agencies.

Y is for…the other kind of safety cap. Y caps are sometimes referred to as line by-pass or line isolation or line to ground caps. (The name Y may have originated from the graphical pattern created on schematics by the two Y caps connecting to a shared GND node.) They are specifically designed for use where a shock hazard would be present if they were to fail short. This is due to the fact that most products that have a metal chassis connect the chassis to GND. Thus, they are designed to fail open when overly stressed.

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Figure 5 : Some examples of Y capacitors in the wild

Figure 5 is a close-up of the input filter for an AC input power supply. The X capacitors are the large grey boxes. The much smaller Y caps are the blue disks, C10 & C11. Oftentimes, an additional Y cap, as opposed to the simpler two-cap design shown here and in Figure 1, is connected in series between GND and the shared connection point of the two Y caps that are connected to the AC lines. This lowers the AC current through the caps and makes a short to GND even more unlikely. It also helps balance the AC lines to avoid false triggers of a GFCI.

Another point that is illustrated in Figure 5 is the difference in mechanical stability between the X and the Y caps. While the Y caps require the addition of a glue (white silicon in this case) to keep them from vibrating during mechanical shock and vibration tests, no such provision is required for the X caps. This is due to the fact that the X cap box structure firmly supports it against the PCB and cancels any range of motion. However, the leaded through hole package of the Y caps allow relative easy range of motion in at least one axis. This must be prevented if the product is to survive shock and vibration test which mimic conditions commonly encountered in the products application environment.

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Figure 6

As can be seen in the detail in Figure 6, if either of the Y caps Cy1 or Cy2 were to fail in a shorted condition, then fault current would flow from the AC lines to GND. If the system chassis is connected to GND, then there would be AC line voltage present on the metal chassis. Thus, Y caps must be extremely robust and designed to fail open. Since the Y caps and the chassis share a common connection to GND, this places an upper limit to how much Y capacitance can be present. This is due to the fact that, if the connection to external GND is lost, then the path to GND through the Y caps would be completed by the body of a person touching the chassis. Thus, Y caps must be limited to much lower values than X caps. This is especially true of device intended to be used in medical applications.

Wrapping It All Up
In the table below, we present values for the defining characteristics of both X and Y capacitors. As can be seen, they are quite different in every parameter as befitting their very different application requirements. Of course, this is also reflected in their size and cost relative to each other. That is, X caps are nearly always film capacitors and thus are much larger and more expensive than a corresponding Y capacitor which is nearly always a ceramic device.

TypeClassPeak Service VoltageIn Circuit ConnectionFault RiskValue RangeMechanical StabilityThermal StabilityTime/Age StabilityOver-Voltage StabilityVolumetric Efficiency {Volume/(C*V)}Cost per C*V
XX12500 ≤ X1 ≤ 4000Across the AC mainsFire only; no shock1nF to 47µFVery HighVery HighVery HighCan self healHighHigh
XX22500 ≤ X2Across the AC mainsFire only; no shock1nF to 47µFVery HighVery HighVery HighCan self healHighHigh
XX3X3 ≤ 1200Across the AC mainsFire only; no shock1nF to 47µFVery HighVery HighVery HighCan self healHighHigh
YY1Y1 ≤ 500From each AC line to GNDFire and shock470pF to 1µFLowLowLowFails to openLowLow
YY2150 ≤ Y2 ≤ 300From each AC line to GNDFire and shock470pF to 1µFLowLowLowFails to openLowLow
YY3Y3 ≤ 500From each AC line to GNDFire and shock470pF to 1µFLowLowLowFails to openLowLow
YY4Y4 ≤ 500From each AC line to GNDFire and shock470pF to 1µFLowLowLowFails to openLowLow

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