slow blow fuse specs - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: October 28, 2015, 08:25:16 am
content: I am looking at these fuses:http://www.farnell.com/datasheets/68897.pdfI am a little confused about the 14'400 second (2 hr) delay time, it's too long to be of any practical use but short enough to cause a problem.

author: tron9000
author_rank: Frequent Contributor
timestamp: October 28, 2015, 08:41:16 am
content: me too. Is it time to open circuit or time circuit is open for?

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2015, 08:55:23 am
content: This is very common, especially for larger fuses which may never blow with just a 35% overload, depending on the ambient temperature. In fact there needs to be a 200% overload before one can be certain the fuse will definitely blow.The resistance of the wiring should be low enough to ensure the short circuit current is high enough to blow the fuse, especially when you're dealing with mains because the voltage on any exposed part should not be allowed to exceed 50V and the fuse should cut the power within a specified length of time.

author: Simon
author_rank: Unknown
timestamp: October 28, 2015, 08:57:07 am
content: @tron9000 Well it's a fuse, when it's open it's open, what I am considering is the minimum current I want to flow without any chance of accidentally blowing but also I don't want to make it so bit that it does not blow when it needs to.

author: Simon
author_rank: Unknown
timestamp: October 28, 2015, 08:57:57 am
content: Quote from: Hero999 on October 28, 2015, 08:55:23 amWhich is my concern

author: tron9000
author_rank: Frequent Contributor
timestamp: October 28, 2015, 10:52:56 am
content: Sorry, from its packaging it looks poly-fuse-like, hence why I questioned time open for.

author: retrolefty
author_rank: Super Contributor
timestamp: October 28, 2015, 04:40:35 pm
content: Quote from: Simon on October 28, 2015, 08:57:07 amIf you meant whatmaximumcurrent you can flow without accidentally blowing the fuse, then as the datasheet states 70% of the devices rating is the recommended max continuous current.

author: SeanB
author_rank: Super Contributor
timestamp: October 28, 2015, 06:21:24 pm
content: It is a manufacturing and usage spread. The worst case is that the fuse is at the thick side of the allowed wire tolerance, and the length is at the minimum length, so the heat generated with that overload is lower. It also considers that the alloy blend has some variability in melting point from batch to batch, and you have as well one with the high end of melting point. It also considers usage, you have it in a walk in freezer or outside in the Arctic in mid winter in Yakusk, where it is -70C, so the heat generatec has to warm up a considerable amount of the fuse and the big massive brass Russian made fuse holders that are keeping the ends at -30C even with the overload. Then it will take up to that time for the fuse to blow.Other end, you have the thinnest wire, lowest melting alloy and it is sitting inside an oven at 130C, and it should not blow at the rated current.Either way you are not going to win. Select a lower blowing point and run it more dangerously, or use the slow blow and rate the wiring such that a short will blow it before the wiring melts or catches fire, which is what a fuse is for. Take running current, stall current and select to blow under the stall current, and use either the controller to limit start up power or keep start up time under the blowing curve.