What is "that hole" in the plug of the AC/DC Power Adapter?

Explaining the reason for the holes in the prongs of AC/DC Power Adapter. A power supply manufacturer explains the mechanism that prevents accidental unplugging, safety standards, and the differences between plug structures in Japan and overseas.

If you take a close look at the plug (AC blades) of an AC/DC Power Adapter, you will notice a small round hole near the tip. On the other hand, some overseas Products and chargers do not have this hole.

"What is the purpose of this hole?"
"Are Products without holes unsafe?"

These questions are frequently raised not only by general users, but also by professionals involved in Products design, procurement, and quality control.

In this article, from the perspective of UNIFIVE, a specialized power supply manufacturer, we clearly explain the role of the "hole in the blade" of an AC/DC Power Adapter and the safety standards behind it.

1. Why Is There a Hole in the Blades of an AC/DC Power Adapter? Three Main Reasons

The approximately 3 mm diameter hole found in AC plug blades has more significance than it appears.

Prevents Accidental Disconnection (Improved Retention)

The primary reason is to prevent the plug from unintentionally coming out during use.

Many outlets in Japan are designed with small internal protrusions on the blade contacts. When the plug is fully inserted, these protrusions engage with the holes in the blades, providing light retention.

*The presence of protrusions depends on the outlet specifications.

This makes it less likely for the plug to come loose when the cable is pulled or subjected to vibration.

Stable Contact and Secure Insertion

The engagement between the hole and protrusion allows users to feel that the plug has been firmly inserted all the way in.

Insufficient insertion increases the risk of poor contact, heat generation, and tracking phenomena (ignition caused by dust buildup).

Plugs with holes can be considered part of a mechanical safety design intended to reduce these risks.

Compliance with Japanese Safety Standards (Technical Requirements)

For AC plugs intended for the Japanese market, this hole is not merely a design feature.

Under the technical requirements (ministerial ordinance) of the Electrical Appliance and Material Safety Act, a hole of approximately 3 mm in diameter must be provided about 11.7 mm from the base of the blade.

This requirement is one of the standards established to ensure the safety of electrical appliances distributed in the Japanese market.

2. Is the Hole Mandatory? Differences Between Japanese and Overseas Standards

The approach to plug holes varies by country and region.

Under North America's NEMA standards, holes in plug blades are optional rather than mandatory. Products are considered compliant as long as they meet the relevant standards, regardless of the presence of holes.

In contrast, in Japan, this structure is standardized from a safety perspective, making it an important requirement for Products intended for the Japanese market.

3. Are Products Without Holes "Defective" or "Low-Cost Versions"?

In conclusion, the absence of a hole does not automatically mean a product is defective or dangerous.

• In overseas markets, plugs without holes are widely distributed as legitimate products
• Differences reflect design philosophy and standards, not necessarily quality superiority or inferiority

However, caution is required in Japan.

If electrical appliances sold for the Japanese market use plug shapes that do not comply with technical requirements, the business operator may be held legally responsible.

Therefore, rather than calling it a "defective product," it is more accurate to describe it as a "product that does not comply with Japan's safety standards (PSE-related technical requirements)."

4. FAQ: Common Questions About Power Plugs

Q. Is it safe to use an overseas adapter without holes?

If it is a Products designed for overseas specifications, significant safety issues are uncommon. However, since it may be more likely to come loose in Japanese outlets, using PSE-compliant products is recommended in Japan.

Q. Why are the left and right slots of an outlet different lengths?

In typical Japanese outlets, the left and right slots may differ in length. This structure distinguishes between the grounded side and the live side, enhancing safety.

Q. What should I do if I receive a plug without holes?

If the Products was purchased from a business operator in Japan, it may require confirmation with the seller. If you are concerned, we recommend contacting the seller to verify specifications or discuss a return.

5. Conclusion: Understanding Safety Standards Is Key to Proper Power Supply Selection

The small hole in the plug of an AC/DC Power Adapter is a design element for preventing detachment and ensuring stable contact based on Japan's strict safety standards.

UNIFIVE provides a wide range of AC/DC Power Adapter that comply not only with PSE requirements for Japan, but also with the standards and plug shapes of North America, Europe, China, Korea, and other destinations.

Strengths of UNIFIVE

• Lineup compatible with various international safety standards and plug shapes
• Extensive adoption track record with companies in Japan and overseas
• Expertise of a specialized power supply manufacturer supporting everything from design and certification to mass production

If you are unsure which standards or plug shapes are appropriate for Products intended for a specific country, please consult UNIFIVE.
We support your power supply selection needs with proven technology and experience.