Fuse Time-Current Characteristic (Time-Current Curve)
The curve showing at what current a fuse blows and how quickly. Just above the rated value the blowing time is measured in minutes, while at several times the current it falls to milliseconds. In motor applications, fuse selection is made by comparing this curve with the starting current profile.
The condition for a correct selection is this: the point defined by the motor's starting current and its duration must stay outside the blowing region of the curve. At the same time the fuse must definitely blow at some point above the current the cable can carry.
Practical method:
- Measure the continuous current under load.
- Choose the fuse rating at roughly 125–150 percent of that current.
- Measure the starting current and its duration; typically 5–8 times the rated value and 0.5–3 seconds.
- Verify that this point does not fall into the blowing region of the curve of the fuse you chose.
Do not forget one point: the fuse protects the cable, not the driver. Semiconductors are damaged far faster than a fuse; what protects them is electronic overcurrent protection. So the cable cross-section must be able to carry the selected fuse rating, and the fuse rating must not be raised beyond what the cable allows.
Where this term is used: DC Motor Driver Selection Guide