CAN CSA C22.2 No. 248.9-00 (2005): Low-Voltage Fuses – Part 9: Class K Fuses – Technical Overview and Compliance Guidelines

CAN CSA C22.2 No. 248.9-00 (2005) is the Canadian national standard for low-voltage Class K fuses, harmonized with UL 248-9. It applies to both renewable and non-renewable fuses rated 600 V or less for use in branch circuits and service entrance equipment in accordance with the Canadian Electrical Code (CEC). This article provides a detailed technical review of the standard’s scope, key requirements, and guidelines for compliance.

1. Scope and Purpose

The standard covers enclosed low-voltage fuses classified as Class K, which are intended to provide overcurrent protection in circuits where a high available fault current may be present. Class K fuses are further subdivided by construction type:

• K-1 (Renewable) – Replaceable fuse link assembly, typically used for circuits with continuous currents above 60 A (61–6000 A).
• K-5 and K-9 (Non‑renewable) – One-time use fuses with fixed current ratings (0–600 A). K‑5 and K‑9 differ primarily in their interrupting rating and allowable overload performance.

The standard establishes dimensional interchangeability, electrical performance characteristics, and testing criteria to ensure safe and reliable operation.

2. Technical Requirements

2.1 Voltage and Current Ratings

Class K fuses are available at nominal AC voltages of 125 V, 250 V, 300 V, and 600 V. The standard defines standard current ratings (in amperes) that increase with 10 A increments up to 200 A, then 300 A, 400 A, 600 A (non‑renewable), and up to 6000 A for renewable types.

2.2 Breaking (Interrupting) Capacity

The standard requires that Class K fuses successfully interrupt any current from 100 % of the rated continuous current up to their assigned interrupting rating without rupture, excessive ejection of material, or permanent damage to the fuse mounting. Four interrupting rating categories are defined: 10 kA, 50 kA, 100 kA, and 200 kA symmetrical RMS. The test is performed at a power factor consistent with typical fault conditions.

2.3 Overload and Short-Circuit Tests

All Class K fuses must pass a series of overload and short-circuit tests:

• Overload test: The fuse is subjected to 135 % of its rated current for a specified duration (typically 1 h for current ratings ≤ 1000 A, or 2 h for higher ratings). It must operate (open) during this test without causing external damage.
• Short-circuit test: The fuse is tested at its assigned interrupting rating with prescribed make‑and‑break sequences. The fuse must clear the fault and limit the let‑through energy (I²t) to safe levels.

2.4 Temperature Rise and Power Dissipation

The standard specifies maximum permissible temperature rises above ambient when carrying 100 % rated current. Typical limits are 100 °C for the fuse barrel and 75 °C for the ferrules or terminals. These values ensure compatibility with standard connector materials and equipment. Power dissipation limits are also provided for coordination with fuse holder and panel specifications.

2.5 Time-Current Characteristics

Class K fuses must operate within defined time-current curves. Key test points for Class K-5 and K-9 fuses are at 200 %, 300 %, and 500 % of rated current. The standard gives maximum allowable clearing times at each multiple. For example, at 200 % overload, the fuse must clear within 2 min for ratings ≤ 100 A. These characteristics are essential for coordination with other protective devices.

2.6 Dimensions and Interchangeability

To ensure physical interchangeability, the standard specifies critical dimensions for each fuse type, including:

• Overall length (e.g., 2.5 in or 4.5 in for non‑renewable types).
• Ferrule diameter (for cartridge fuses).
• Blade dimensions and slot/groove details (for knife‑blade types).
• Ventilation area for renewable links.

These dimensions are harmonized with the North American fuse family, allowing Class K fuses from different manufacturers to be used in standard fuse holders or disconnect switches.

3. Construction and Marking

Construction: The fuse barrel must be of non‑combustible material (often melamine, glass‑filled polyester, or ceramic). The fuse link is typically made of silver, copper, or a tin‑alloy element with controlled melting sections, surrounded by silica sand for arc quenching. Renewable fuses have a removable cap and replaceable link assembly.

Marking: Each fuse must be permanently marked with:

• Manufacturer’s name or trademark
• Catalog number
• Rated voltage
• Rated current
• Interrupting rating (e.g., 10 kA RMS Sym.)
• Fuse class (e.g., Class K-5)
• Year of manufacture or date code

4. Implementation and Compliance Notes

For manufacturers, compliance with CAN CSA C22.2 No. 248.9-00 (2005) typically requires product certification by a recognized testing organization such as CSA Group or UL. The certification process includes design review, routine production-line tests, and periodic follow‑up audits. Key compliance points include:

• Material approvals: All materials (housing, filler, link) must be acceptable under the standard.
• Test data documentation: Evidence of passing all overload, short‑circuit, temperature‑rise, and dimension checks must be retained.
• Production line tests: Routine tests such as high‑potential (Hi‑Pot) and resistance verification are required.
• Country-specific requirements: Although harmonized with UL 248-9, Canadian codes may impose additional marking language (English/French) or polarity warnings.

For engineers and facility managers, this standard affects fuse selection and inventory management. While Class K fuses are gradually being replaced by Class R or Class T fuses in many new installations due to rejection features, they remain widely used in existing panels and as replacements. It is crucial to understand the interrupting rating and time‑current characteristics to ensure proper coordination with upstream and downstream devices.

5. Frequently Asked Questions

Copyright 2026 – Technical reference article based on CAN CSA C22.2 No. 248.9-00 (2005). Always refer to the latest published edition for regulatory compliance.