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Low-Voltage Fuses: A Technical Overview of CAN CSA C22.2 No. 248.7-00 (2005) Standard for Class H, K, and R Fuses
Understanding the Requirements and Compliance for Class H, K, and R Low-Voltage Fuses under the Canadian Electrical Code
Scope and Overview
The CAN CSA C22.2 No. 248.7-00 (2005) standard is a harmonized member of the CSA C22.2 series under the Canadian Electrical Code, Part II. It specifies the essential construction, performance, and testing requirements for non-renewable cartridge fuses rated up to 600 A and 600 V AC or DC. This standard specifically covers Class H, K, and R low-voltage fuses, which are widely used in industrial, commercial, and residential overcurrent protection applications.
Originally published in 2000 and reaffirmed in 2005, this standard aligns with the corresponding UL 248-7 standard to facilitate cross-border acceptance and ensure consistent safety levels across North America. The document replaces earlier individual standards for these fuse classes and integrates them into a common framework.
Tip: When selecting a replacement fuse, always verify the class designation and voltage rating to ensure the device matches the original specifications and provides adequate overcurrent protection.
Technical Requirements
Voltage and Current Ratings
Fuses under this standard are rated for AC or DC operation at voltages of 125 V, 250 V, or 600 V. The continuous current rating may range from fractional amperes up to 600 A, depending on the manufacturer’s design and intended application.
Interrupting Capacity
One of the most critical distinctions among Class H, K, and R fuses is their interrupting capacity (IC). The standard defines minimum IC values for each class, as shown in the table below:
| Fuse Class | Voltage Rating (V AC) | Current Range (A) | Interrupting Capacity (kA AC @ 600 V) | Typical Application |
|---|---|---|---|---|
| H | 250 / 600 | 0 – 600 | 10 kA | General-purpose, low-fault-current circuits |
| K1 | 250 / 600 | 0 – 600 | 200 kA | High-fault-current industrial panels |
| K5 | 250 / 600 | 0 – 600 | 100 kA | Commercial distribution equipment |
| K9 | 250 / 600 | 0 – 600 | 50 kA | Branch circuit protection |
| R | 250 / 600 | 0 – 600 | 200 kA | Service entrance, high-fault applications |
Note: The interrupting capacity may be lower at DC voltage ratings; consult the manufacturer’s documentation for specific values.
Dimensions and Construction
The standard prescribes precise dimensional requirements for fuse bodies and mounting configurations to ensure physical interchangeability within each class. Class R fuses incorporate a rejection feature (a slot or notch) that prevents the insertion of lower interrupting capacity Class H or K fuses into Class R fuse holders. This rejection mechanism is mandatory under the standard to maintain system protection coordination.
Endurance and Overload Tests
Fuses must successfully pass a series of tests including temperature rise, overload operation, and endurance cycling. For example, after prolonged loading at 110% of rated current, the fuse must not exhibit excessive heating that could impair the surrounding equipment.
Warning: Do not use a Class H fuse in a circuit where the available fault current exceeds 10 kA. Doing so may cause the fuse to rupture violently, creating an arc flash hazard.
Implementation Highlights
Application in Electrical Installations
Class H fuses are typically found in older equipment and low-fault-current systems, while Class K and R fuses are preferred in modern installations where higher available fault currents exist. The standard helps engineers and installers choose the correct fuse class based on the system’s maximum short-circuit capability.
Marking and Identification
Each fuse must be permanently marked with:
- Manufacturer’s name or trademark
- Type designation and catalog number
- Voltage and current ratings
- Interrupting capacity (for Class K and R, the subclass must be indicated, e.g., K1, K5, K9)
- CSA certification mark
Replacement and Compatibility
Only fuses of the same class and identical voltage/current ratings may be used for replacement. Class R fuses can be used as replacements for Class H or K fuses if the holder accepts the rejection feature, but the inverse is not permitted because of the rejection mechanism.
Best practice: When upgrading a system to a higher interrupting capacity, consider switching to Class R fuses. Their 200 kA rating at 600 V AC provides robust protection for modern high-fault-current environments.
Compliance and Certification Notes
Certification Process
Manufacturers seeking compliance with CAN CSA C22.2 No. 248.7-00 (2005) must submit samples to an accredited certification body (e.g., CSA Group, UL, or Intertek) for testing. The testing program covers:
- Dielectric voltage withstand
- Temperature rise and overload
- Short-circuit interruption
- Endurance cycling
- Mechanical strength and dimension verification
Differences from UL 248-7
While the standard is harmonized with UL 248-7, minor differences may exist in marking requirements, test acceptance criteria, or recognition of specific fuse types. CSA certification acknowledges full equivalency when testing is conducted to either standard, provided all deviations are documented.
Reaffirmation and Status
The 2005 reaffirmation reaffirmed the 2000 edition without technical changes. Users should consult the latest edition (currently 2021, as of 2025) for up-to-date requirements, although many installations continue to reference the 2000/2005 edition for legacy systems.
Danger: Never attempt to bypass the rejection feature of a Class R fuse holder. Inserting a Class H or K fuse into a Class R holder eliminates the fault current protection and exposes personnel and equipment to catastrophic failure during a short circuit.
Frequently Asked Questions
Q: What is the main difference between Class H and Class R fuses?
A: Class H fuses have an interrupting capacity of only 10 kA, while Class R fuses offer 200 kA. Class R fuses also have a rejection feature that physically prevents insertion into non‑Class R holders, ensuring the correct level of protection.
A: Class H fuses have an interrupting capacity of only 10 kA, while Class R fuses offer 200 kA. Class R fuses also have a rejection feature that physically prevents insertion into non‑Class R holders, ensuring the correct level of protection.
Q: Can I replace a Class H fuse with a Class K fuse in the same holder?
A: Yes, provided the voltage and current ratings match and the Class K fuse is physically interchangeable (Class K does not have rejection features that would block insertion). However, the system’s available fault current should not exceed the Class H rating unless the Class K fuse holder is designed for the higher IC.
A: Yes, provided the voltage and current ratings match and the Class K fuse is physically interchangeable (Class K does not have rejection features that would block insertion). However, the system’s available fault current should not exceed the Class H rating unless the Class K fuse holder is designed for the higher IC.
Q: Is the CAN CSA C22.2 No. 248.7-00 (2005) still current?
A: It was reaffirmed in 2005 but has been superseded by later editions (e.g., 2006, 2010, 2021). For new installations, the latest edition is recommended. For existing equipment, the 2005 edition remains a valid reference for maintenance and replacement.
A: It was reaffirmed in 2005 but has been superseded by later editions (e.g., 2006, 2010, 2021). For new installations, the latest edition is recommended. For existing equipment, the 2005 edition remains a valid reference for maintenance and replacement.
Q: Where can I obtain a copy of the standard?
A: Official copies are available from CSA Group’s online store or through authorized document resellers.
A: Official copies are available from CSA Group’s online store or through authorized document resellers.
Article updated 2026. This content is for informational purposes and does not replace the official standard text. Always consult the latest edition of CAN CSA C22.2 No. 248.7 for complete requirements.