Bad fuzes

[rdagger]

I bought an assortment of 5x20 fuses on eBay.  I hooked up an 8A fuse to a 120VAC circuit and plugged in a 13A hair dryer.  I ran it on high for about 2 minutes and the fuse didn’t blow.  My meter read 12.9A.

They look like AC fuses.  The fuse is stamped F8AL 250V.  I know AC and DC fuses have different ratings.  Could they be DC instead? 

I guess I should have known better because the auction description described them as High Quality Fuze which seems applicable.

[schmitt trigger]

Fuses have so-called I-squared-T curves, and depending on the ratings, some can withstand an overload for quite some time.

Google: I2t

[Fsck]

a fast blow 8A fuse will take way longer than 2 min to blow at 13A usually.
A fuse is designed to interupt serious overcurrent, not just a tiny overcurrent.

Look at the time-current chart for an 8A fuse in this datasheet as an example:
https://www.mouser.ca/datasheet/2/643/ds-CP-5sf-5sfp-series-1313110.pdf

[wraper]

Fuses normally can survive like 1.3 - 2 times of the rated current for like a minute and a lot longer once you lower the current a bit. That multiplier usually is on the lower side for low current ratings and on high side for higher current ratings. Although I would not buy fuses from ebay, using safety devices from dodgy source is not smart. DC/AC does not matter for current they'll act. It matters for how high voltage and current they can break safely.

[rdagger]

Very helpful!
Thanks for taking the time to explain.