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SAE J156 Fusible Links: Engineering Specifications and Testing for Vehicle Electrical Systems
Fusible links are essential for protecting low voltage surface vehicle electrical systems from extreme current overloads. SAE J156 defines the supplemental requirements for primary cable used as fusible links, ensuring they reliably open and minimize wiring damage. This article covers the core conductor and insulation requirements, the short circuit flame test procedure, and critical design insights for proper application.
Core Conductor and Insulation Requirements
To serve as a fusible link, the cable must first meet the base requirements of SAE J1127, SAE J1128, or SAE J1678. The conductor must comply with those standards—whether bare copper, coated copper, or other approved materials. Insulation performance depends on cable size:
| Cable Size | Required Insulation Type |
|---|---|
| 8 gauge and smaller | SAE J1128 Type TWP |
| 6 gauge and larger | SAE J1127 Type STT |
If the fusible link size is not listed in those standards, the next larger size shall be used for testing and specification.
🛠️ Design Insight: The fusible link should be at least two gauge sizes smaller than the protected circuit cable. For example, a size 10 fusible link is recommended to protect a size 6 cable. This ensures the link sacrifices itself before the main wiring sustains severe damage.
Short Circuit Flame Test Procedure
The short circuit flame test validates that the fusible link opens quickly and safely under overload conditions. The test setup and criteria are as follows:
- Sample length: 150 mm, with 10 mm of insulation stripped from each end.
- Connected to a 4.3 m length of cable simulating the protected circuit—ideally at least two sizes larger than the fusible link.
- Battery source: a fully charged battery per SAE J537, with minimum 500 cold cranking amps at –18 °C.
- A relay is used to complete the circuit.
Key performance criteria:
| Parameter | Requirement |
|---|---|
| Opening time | ≤ 10 seconds after circuit activation |
| Flame before opening | None permitted |
| Flame after opening | ≤ 5 seconds, then self-extinguish |
| Subsequent current flow | Not permitted after circuit opens |
If the link does not open within 10 seconds, the test must be repeated with an additional battery in parallel. The number of batteries used must be recorded.
⚠️ Common Mistake: Using a fusible link that does not meet the required insulation type or failing to verify the sizing relationship between the link and the protected circuit can lead to test failure or hazardous wiring damage.
Frequently Asked Questions
What is the primary purpose of a fusible link?
A fusible link is a section of low tension cable designed to open the circuit under extreme current overload, protecting downstream wiring and components from damage.
Can any low voltage cable be used as a fusible link?
No. The cable must meet the conductor and insulation requirements of SAE J1127, SAE J1128, or SAE J1678, with specific insulation types (TWP for small sizes, STT for larger). It must also pass the short circuit flame test defined in SAE J156.
How is the fusible link sized relative to the protected circuit?
The standard recommends that the fusible link be at least two sizes smaller than the protected cable. This ensures the link melts and opens before the main circuit becomes damaged.
What happens if the fusible link fails to open within 10 seconds during the test?
The test procedure requires adding an additional battery in parallel to increase fault current, and the test is repeated. The number of batteries needed must be documented.
For complete details, refer to the latest SAE J156 standard.
