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Technical Overview of CSA C22.2 No. 248.12-11 (2017) for Class R Fuses
Comprehensive Guide to the Canadian Standard for Low-Voltage Class R Overcurrent Protection
Scope and Purpose
CSA C22.2 No. 248.12-11 (2017) is a Canadian national standard that specifies the construction, performance, and testing requirements for low-voltage Class R fuses used in electrical overcurrent protection. As part of the CSA C22.2 No. 248 series, this standard is harmonized with UL 248-12 and applies to branch-circuit fuses with a rejection feature that ensures only fuses with high interrupting ratings can be installed in compatible holders. The primary scope covers fuses rated up to 600 V ac (50/60 Hz) and 300 V dc, with current ratings from 1 to 600 A, and intended for use in accordance with the Canadian Electrical Code, Part I.
Class R fuses are categorized into two subtypes—RK1 and RK5—which differ in their current-limiting performance and let-through energy (I2t) characteristics. The standard defines the rejection dimensions that physically prevent lower interrupting-capacity fuses (e.g., Class H) from being inserted into Class R fuse holders, thereby maintaining the integrity of the overcurrent protection scheme. This standard is essential for electrical engineers, system designers, and compliance professionals who must specify reliable overcurrent protection in industrial, commercial, and large residential installations.
Technical Requirements and Ratings
CSA C22.2 No. 248.12-11 (2017) establishes mandatory design and performance criteria that all Class R fuses must meet. The following subsections summarize the key technical parameters.
Voltage and Current Ratings
The standard covers fuses with ac voltage ratings of 250 V and 600 V, and dc voltage ratings of 125 V, 250 V, and 300 V. Current ratings range from 1 A to 600 A, with specific ampere ratings defined in preferred increments. The continuous current-carrying capacity must not cause a temperature rise exceeding the values specified in the standard under 100 % rated current conditions.
Interrupting Rating
Class R fuses are distinguished by their high interrupting capacity, which is a minimum of 200 kA RMS symmetrical for ac ratings and 20 kA for dc ratings when tested under defined conditions (power factor ≤ 0.15 for ac, time constant ≤ 10 ms for dc). The rejection feature ensures that only Class R fuses can be installed in holders designed for this class, preventing substitution with lower interrupting fuses.
Time-Current Characteristics
The standard specifies the maximum clearing times for various overload and short-circuit currents. Fuses must open within defined time bands at specific multiples of the rated current (e.g., 135 %, 200 %, 500 %, and 1000 %). These characteristics ensure coordination with other overcurrent devices and comply with the Canadian Electrical Code requirements for conductor protection.
Rejection Feature
A key technical requirement is the physical rejection feature that prevents insertion of Class H or other lower-interrupting fuses into Class R holders. The standard details the dimensions of the rejection slots (or notches) on the fuse ferrule or blade, as well as the matching protrusions in the fuse holder. This mechanical deterrent is critical for maintaining the intended interrupting rating of the protection scheme.
| Parameter | Class RK1 | Class RK5 |
|---|---|---|
| Voltage Rating (ac) | 250 V / 600 V | 250 V / 600 V |
| Voltage Rating (dc) | 125 V / 250 V / 300 V | 125 V / 250 V / 300 V |
| Current Range | 1 A – 600 A | 1 A – 600 A |
| Interrupting Rating (ac) | 200 kA RMS | 200 kA RMS |
| Interrupting Rating (dc) | 20 kA | 20 kA |
| I2t (let-through) @ 600 V | Low | Moderate |
| Peak Let-Through Current | More limiting | Less limiting |
Tip: When specifying Class RK1 vs. RK5 fuses, consider the let-through energy and peak current required for equipment protection. RK1 fuses are generally preferred for sensitive electronic loads, while RK5 may be suitable for motor circuits with higher inrush.
Implementation and Design Highlights
Applying Class R fuses in compliance with CSA C22.2 No. 248.12-11 (2017) requires consideration of both the fuse and the holder design. The rejection feature is not only a safety mechanism but also a design constraint—fuse holders must include the corresponding rejection protrusions to accept Class R fuses. These holders are often referred to as “Class R fuse holders” and are marked accordingly. The standard also addresses the mechanical strength and dielectric withstand requirements for the fuse assembly, ensuring reliable operation under normal service conditions.
From a system design perspective, Class R fuses are used in applications where high available fault current exists (e.g., near utility transformers or in industrial switchgear). Their high interrupting rating (200 kA) makes them suitable for most commercial and industrial installations without the need for current-limiting reactors or dedicated breakers. Coordination studies should account for the fuse’s time-current curve and the let-through characteristics to provide selective isolation of faults.
Caution: Never bypass the rejection feature by modifying the fuse or the holder. Doing so can allow a lower-interrupting fuse to be installed, resulting in a catastrophic failure during a fault event. Always use Class R holders with Class R fuses.
Additionally, the standard mandates that fuses be marked with the manufacturer’s name or trademark, the catalog number, rating (voltage, current, interrupting), and the words “CLASS RK1” or “CLASS RK5” as applicable. The rejection dimensions must be measured and verified to meet the gauge tolerances outlined in the standard, ensuring interchangeability among manufacturers.
Harmonization Note: CSA C22.2 No. 248.12-11 (2017) is harmonized with UL 248-12. Fuses certified to this standard are typically accepted in both Canadian and U.S. installations, simplifying approval for North American projects.
Compliance and Testing Notes
Certification to CSA C22.2 No. 248.12-11 (2017) requires that all fuses undergo a series of type tests in an accredited laboratory. The testing regimen includes:
- Temperature rise test: The fuse must carry 100 % of its rated current without exceeding allowed temperature rise limits at the terminals and body.
- Overload test: The fuse must open within specified time limits at 135 % and 200 % of rated current while not opening prematurely at lower overloads.
- Interrupting test: The fuse must safely interrupt the full interrupting rating (200 kA for ac) at maximum rated voltage, with no rupture, flame emission, or electrical continuity.
- Rejection test: Gauges are used to confirm that a Class H fuse cannot be inserted into a Class R holder, and that a Class R fuse cannot be inserted into a non-Class R holder.
- Conditional short-circuit test: To verify the fuse can withstand short-circuit currents without degradation of its overload characteristics.
Manufacturers must also demonstrate consistency through production-line testing (e.g., resistance measurement and dielectric strength) and maintain a quality control program as part of the certification. Periodic follow-up inspections by the certifying body (e.g., CSA Group) ensure ongoing compliance.
WARNING: Never replace a Class R fuse with a Class H or other non-rejection fuse. Under a high-fault condition, the lower interrupting-rated fuse may fail to clear the current, resulting in an arc flash, fire, or explosion. Always check the fuse marking and holder reference before installation.
For designers and specifiers, compliance notes also highlight that Class R fuses must be used only in circuits where the available fault current does not exceed the interrupting rating. When fuses are applied in systems with continuous currents near the fuse rating, ambient temperature derating factors may need to be applied. The Canadian Electrical Code (CE Code) references CSA C22.2 No. 248.12 for the installation of Class R fuses, making compliance mandatory in all Canadian jurisdictions that adopt the CE Code.
Finally, it is important to note the 2017 edition supersedes earlier versions, incorporating refined test conditions and updated rejection gauge dimensions to improve safety and inter-manufacturer compatibility. Users should always refer to the latest edition of the standard for current requirements.
Q: What is the difference between Class RK1 and RK5 fuses?
A: Class RK1 fuses have a lower I2t (let-through energy) and lower peak let-through current than RK5 fuses. This makes RK1 more effective at limiting fault current and is often specified for protecting sensitive semiconductor equipment. RK5 fuses, while still having the same high interrupting rating (200 kA), have a slightly higher let-through and are typically used in general motor and branch-circuit protection where a moderate level of current limitation is sufficient.
A: Class RK1 fuses have a lower I2t (let-through energy) and lower peak let-through current than RK5 fuses. This makes RK1 more effective at limiting fault current and is often specified for protecting sensitive semiconductor equipment. RK5 fuses, while still having the same high interrupting rating (200 kA), have a slightly higher let-through and are typically used in general motor and branch-circuit protection where a moderate level of current limitation is sufficient.
Q: Are Class R fuses mandatory in certain installations?
A: Yes, the Canadian Electrical Code requires Class R fuses (or equivalent high-interrupting current-limiting fuses) when the available fault current exceeds 10 kA RMS, which is typical in most commercial and industrial settings. The rejection feature ensures that a lower-interrupting fuse cannot be installed, maintaining the required level of protection.
A: Yes, the Canadian Electrical Code requires Class R fuses (or equivalent high-interrupting current-limiting fuses) when the available fault current exceeds 10 kA RMS, which is typical in most commercial and industrial settings. The rejection feature ensures that a lower-interrupting fuse cannot be installed, maintaining the required level of protection.
Q: Can a Class R fuse be used in a Class H holder?
A: No. The rejection feature of a Class R fuse is sized so that it will not fit into a Class H holder. Conversely, a Class H fuse cannot fit into a Class R holder. This physical incompatibility is intentional to prevent misapplication.
A: No. The rejection feature of a Class R fuse is sized so that it will not fit into a Class H holder. Conversely, a Class H fuse cannot fit into a Class R holder. This physical incompatibility is intentional to prevent misapplication.
Q: How is the interrupting rating of a Class R fuse tested?
A: The fuse is tested at its maximum rated voltage and at the full interrupting rating current (e.g., 200 kA for ac) under controlled circuit conditions (specific power factor and time constant). Three successive operations must be performed, and the fuse must clear the fault without rupture, arc flashover, or emitting flames or molten metal. After the test, there must be no electrical continuity through the fuse element.
A: The fuse is tested at its maximum rated voltage and at the full interrupting rating current (e.g., 200 kA for ac) under controlled circuit conditions (specific power factor and time constant). Three successive operations must be performed, and the fuse must clear the fault without rupture, arc flashover, or emitting flames or molten metal. After the test, there must be no electrical continuity through the fuse element.
— Published 2026. This article is for general technical information only and does not replace the official text of CSA C22.2 No. 248.12-11 (2017). Always consult the current standard and the Canadian Electrical Code for authoritative requirements.