FAQ on creepage and clearance: Part 1

Regulatory creepage and clearance requirements are increasingly affecting mainstream designs; understanding and meeting them is a complicated, multifaceted process.

Electrical, mechanical, and packaging designers who work with lower power rails of just a few tens of volts usually have few or no concerns about product safety with respect to stray voltage and user shock. Now, however, many of these lower-voltage practitioners are increasingly involved with higher-voltage designs, which incur many safety-related constraints, requirements, and mandates.

Why the change? There are many reasons. Among them, there’s the proliferation of higher-voltage battery-related systems such as electric vehicles, renewable energy sources, energy storage systems, and power devices. Also, the increased use of smart power devices, including sophisticated AC power supplies and efficient mass-market products using mid-range motors, means that even smaller products may have many tens of volts on their “power” side even if the processor side is in the single volts.

An obvious question is this: “At what voltage(s) do these safety considerations begin to emerge?” The answer is simple: “It depends.” In general, the threshold of concern is around 30 VAC or 60 VDC, but the specific value depends on the application and the governing regulatory mandates and industry standards.

The 60 VDC threshold is why many systems restrict their operation to 48 VDC or lower, which is adequately below 60 VDC with a margin of safety. Further, while the AC and DC values are important, the higher DC value is often of more relevance since DC rails are much more common within systems and on circuit boards, even if the primary power is AC; there are exceptions, of course.

As designs go above those thresholds, new concerns and considerations enter into the configuration analysis. Among these are requirements for minimum creepage and clearance dimensions. This FAQ will examine what creepage and clearance are, along with the many perspectives associated with these two safety-related mandates.

Q: What are creepage and clearance?
A: Creepage and clearance are related to the spacing of conductors in a system and are often but not solely associated with conductor spacing on a circuit board:

Creepage: This is the distance between two conductors on the surface of the board or along the surface of the insulating material. It is the shortest separation along the surface of an insulating material between two conducting parts.

Clearance: This is the “line-of-sight” direct distance between two conductors through the air. It is the shortest separation in the air between two conducting parts.

Q: I’m not sure what this actually means or what the difference is between the two; can you clarify?
A: This is clearly a case where a picture (or pictures) is better than words. Here are some examples of creepage and clearance positioning in Figure 1:

Figure 1. These three diagrams show the physical manifestation of creepage and clearance from different perspectives (Image: Cadence Design Systems, Inc., Coil Technology Corp., Sierra Circuits).

Q: Why do we care about creepage and clearance?
A: The creepage and clearance spacing rules must be observed to prevent unintentional arcing or flashover between two conductors – it’s that basic and simple. This could damage components, induce a larger failure, and put the user at risk.

Q: Are the creepage and clearance dimensions the same?
A: They often are, especially for “simple” components on PC boards, as the distance between two component pads or other exposed areas of metal on an unobstructed portion of the board is the same using either metric. In many cases, however, the two dimensions are different, depending on how the components are placed or if there are any unusual contours of the board. They also differ for connectors, which have a more complicated three-dimensional structure.

Q: Why does much of the discussion on creepage and clearance center on printed circuit boards rather than, for example, connectors?
A: The reason is that the layout flexibility of PC boards is both an advantage and a disadvantage with respect to creepage and clearance. When you use a connector, the physical dimensions are inherently fixed, and the connector vendor can clearly and unambiguously specify the creepage and clearance standards and values that the connect meets.

In contrast, the tracks on a circuit board can be changed in location and spacing with respect to other tracks and can be moved and sited next to different tracks as part of a layout iteration. In other words, a layout that meets requirements may be modified for any of many reasons, and the creepage and clearance values may also suddenly change.

Q: Are creepage and clearance similar to the galvanic (ohmic) isolation sometimes needed in circuits and systems?
A: Yes and no. Isolation is used to prevent stray currents (whether leakage or driven by a higher voltage) from passing from one part of a circuit to another, to keep higher voltages away from lower-voltage sub-circuits, or to keep different grounds separate. To make this happen, there can be no metallic contact between the two isolated parts. In addition, isolation may even be needed in a low-voltage scenario.

Creepage and clearance also keep metal parts separated, but they can be from totally different, unrelated parts of a circuit that just happen to be in close proximity. Here, the concern is flashover, arcing, or sparking. The requirement only applies as voltage increases beyond certain defined thresholds.

The next part of this article goes into more detail on creepage and clearance specifics.