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What is the purpose of "that hole" in the plug of the AC/DC Power Adapters — Desktop & Wall-Mount?
Explains the reason for the holes in the blades of the AC adapter. A power supply specialist describes the anti-removal mechanism, safety standards, and differences in plug structures between Japan and overseas.
If you take a close look at the plug (AC blade) of an AC adapter, you’ll notice a small circular hole near the tip. On the other hand, some overseas products and certain chargers may not have this hole.
"What is the purpose of this hole?"
"Are products without this hole unsafe?"
These kinds of questions come not only from general users but also from professionals involved in product design, procurement, and quality control.
In this article, we will clearly explain the role of the blade hole in AC adapters and the underlying safety standards, from the perspective of Unifive, a specialist power supply manufacturer.
1. Why Are There Holes in AC Adapter Blades? Three Main Reasons
The approximately 3mm diameter holes seen in AC plug blades are more significant than they appear.
Prevention of Unintentional Disconnection (Retention Strength)
The main reason is to prevent the plug from unintentionally slipping out during use.
Many electrical outlets in Japan have a structure that includes a bump inside the socket. When the plug is inserted all the way in, this bump fits into the hole and gently secures the plug.
*Whether bumps are present depends on the outlet design.
This mechanism helps prevent the plug from coming loose due to tugs on the cable or vibration.
Connection Stability and Proper Insertion
The combination of holes and protrusions provides a tangible feeling that confirms the plug is fully inserted.
If the plug is not fully inserted, it can lead to poor contact, heating, or even tracking (fire caused by dust build-up).
Plugs with holes are part of a mechanical safety design aimed at minimizing such risks.
Compliance with Japanese Safety Standards
For AC plugs used in Japan, these holes are not simply a design feature.
The technical standards of the Electrical Appliance and Material Safety Act require a hole approximately 3mm in diameter to be located 11.7mm from the base of each blade.
This requirement is one of the standards for ensuring the safety of electrical products distributed in the Japanese market.
2. Are Holes “Required”? Differences Between Japanese and Overseas Regulations
Regulations regarding plug holes vary by country and region.
| Item | Japan (PSE) | North America (NEMA) | Europe / China |
|---|---|---|---|
| Presence of Holes | Required by regulation | Optional | Generally none |
| Key Concept | Prevention of disconnection and ensuring safety | Design flexibility in manufacturing | Different plug structure altogether |
| Sales in Japan | – | Non-compliant products not allowed | Not permitted for sale |
Under North America's NEMA standard, plug holes are not mandatory. As long as the plug meets regulatory standards, it is legally acceptable with or without holes.
In contrast, in Japan, holes are standardized for safety, and are an important requirement for products intended for the domestic market.
3. Are Products Without Holes "Defective" or "Cost-Reduced"?
In conclusion, simply not having holes doesn’t mean a product is defective or dangerous.
- Hole-less plugs are widely distributed as standard products in overseas markets.
- Differences are due to design philosophy and standards, not necessarily product quality.
However, caution is needed within Japan.
If an electrical product for sale in Japan uses a non-compliant plug structure, the supplier may be held legally responsible.
Therefore, it’s more accurate to see it as “a product not compliant with Japan’s safety standards (PSE-related technical standards)” rather than as a “defective item.”
4. FAQ: Common Questions About Power Plugs
Q. Is it okay to use an imported adapter without holes?
If the product is designed for overseas specifications, it usually does not pose major safety issues. However, plugs may become loose more easily in Japanese outlets, so using PSE-compliant products is recommended in Japan.
Q. Why are the plug slots in Japanese outlets different lengths?
In typical Japanese outlets, one slot may be longer than the other. This design helps distinguish between the grounded (neutral) side and the voltage (hot) side for safety purposes.
Q. What should I do if I receive a plug without holes?
If a product purchased from a domestic business has a plug without holes, it may be a matter for the seller to confirm. If concerned, we recommend checking the specifications or consulting the seller about returns.
5. Summary: Understanding Safety Standards is Key to Choosing the Right Power Supply
The small holes in AC adapter plugs are safety design features based on Japan’s stringent regulations to prevent unintentional disconnection and ensure stable contact.
Unifive offers a wide range of AC adapters for the Japanese market that comply with PSE standards, as well as versions tailored to North America, Europe, China, Korea, and more.
Strengths of Unifive
- Comprehensive range compatible with each country's safety standards and plug types
- Extensive track record with domestic and global companies
- Expertise from design to certification and mass production, all under one roof
If you are unsure about which standard or plug type to choose for a product in a specific country, feel free to consult Unifive.
We support your power selection needs with proven expertise and reliable solutions.