TSD 5x20mm IEC Time-Lag Glass Fuse for Controlled Inrush
Not every overcurrent condition is a true fault. Some circuits draw a brief surge of current during startup, then settle into normal operation. Motors, transformers, power supplies, and capacitor-input circuits are all common sources of temporary inrush current.
That is where a time-lag fuse becomes important.
The OptiFuse TSD / TSD-P is a 5x20mm glass tube fuse designed specifically for circuits that need IEC-style time-lag protection. It helps tolerate short, harmless inrush events while still opening when an overcurrent condition lasts too long.
What makes the TSD series especially important is not simply that it is a slow-blow fuse, but that it is designed around IEC 60127 standards. That makes it a strong choice for electronic equipment and industrial designs built to international miniature fuse specifications.
IEC vs. UL Time-Lag Fuse Testing
When choosing a 5x20mm tube fuse, physical size and amp rating are only part of the selection process.
Two fuses may look similar and carry the same current rating, yet still be evaluated to different standards. For comparison, the OptiFuse TSC is also a 5x20mm time-lag glass fuse, but it is tested to UL 248-14 and CSA C22.2 No. 248.14. ne of the major differences is how each fuse’s overload performance is verified.
The TSD is tested differently because it is built around IEC 60127 requirements. Instead of only looking at 135% and 200% overload points, as tested by UL 248-14 standards, the TSD was tested on IEC-style time-lag performance at 210%, 275%, 400%, and 1000% of rated current. For ratings from 125mA to 10A, the TSD must open within 2 minutes at 210%, between 600ms and 10 seconds at 275%, between 150ms and 3 seconds at 400%, and between 20ms and 300ms at 1000%.
Why the Standard Matters
This distinction matters because the standard defines how the fuse is evaluated, not just its physical size. A 5x20mm time-lag fuse tested to UL 248-14 is not automatically interchangeable with a 5x20mm time-lag fuse tested to IEC 60127. The correct selection depends on the equipment standard, agency file, bill of materials, and the approvals the final product must maintain.
The interrupt rating structure is also different. The TSD uses an IEC-style interrupt rating of 35A or 10In, whichever is larger. The TSC lists interrupt ratings by current range and voltage, including higher interrupt ratings at certain 125VAC and 250VAC conditions. This is another reason engineers should avoid selecting by physical size and amperage alone.
In simple terms, the TSD is the better fit when a design calls for an IEC 60127 5x20mm time-lag fuse. The TSC is the better fit when the design calls for a UL 248-14 / CSA C22.2 No. 248.14 5x20mm time-lag fuse.
Power Supply Input Protection
A common application for the TSD is AC input protection on small power supplies, such as control power supplies, adapters, and electronic equipment with capacitor-input circuits.
When power is first applied, the input capacitors inside the power supply draw a short surge of current as they charge. This is normal startup behavior, not necessarily a fault. A fast-acting fuse may open during this brief inrush event, even though the circuit is operating correctly.
The OptiFuse TSD helps solve this problem through its time-lag response.
Key Specs at a Glance
- Fuse size: 5 x 20mm
- Response type: Time-lag
- Voltage rating: 250VAC
- Current range: 100mA to 20A
- Interrupt rating: 35A or 10ln whichever is larger.
- Available styles: Cartridge and axial lead versions
- Standards: Tested to IEC 60127, with selected ratings carrying cURus recognition
The clear glass body allows for quick visual inspection, which can help during troubleshooting and service. For more information, read our How to Choose a Tube Fuse blog.
For more inquiries, contact OptiFuse at [email protected]. See our Fuse Selection Guide for complete context.
Sebastian Castañeda is a circuit protection specialist and technical writer with application-focused experience in technical support and custom protection design. He contributes practical, application-driven insights to the OptiFuse Blog.