Brymen BM867s: Fuse contradiction - Page 1
Forum section: electronics / testgear

author: mcc
author_rank: Contributor
timestamp: November 12, 2016, 07:17:15 pm
content: Hi,( I am no native speaker -- sorry for any confusion my wods may spend...it is not intendedthis is my first posting to this forum.I want say a BIG THANK YOU to Dave for all the very informative videosand for makeing the way he make the videos! Great stuff! Thumbs up!I have a Brymen BM867s (no "T" before "BM"). On the front between the COM jackand the mA/uA it says: "MAX 0.6A, HBC fused".The manual says "0.44A HBC FUSE" for that.0.6 A makes sense to me ... due to the 50000 counts.0.44 A make lesser sense regarding this.Normally I would think: 0.6A to a 0.44A fuse would blow it up.What is wrong (and why it is so nevertheless):The printing on the meter orthe printing on in the manual?Thanks a lot for any help in advance!Cheersmcc

author: IanB
author_rank: Super Contributor
timestamp: November 12, 2016, 07:26:38 pm
content: I think nothing is wrong. It takes a very large over current through a fuse to break it. Passing 0.6 A through a 0.44 A fuse for short periods will be fine.

author: mcc
author_rank: Contributor
timestamp: November 12, 2016, 07:35:49 pm
content: Quote from: IanB on November 12, 2016, 07:26:38 pmHi Ian,thanks a lot for the information!How long is "a short period of time"?Cheersmcc

author: Hydrawerk
author_rank: Super Contributor
timestamp: November 12, 2016, 07:36:42 pm
content: Find a datasheet of a HRC fuse.

author: wraper
author_rank: Unknown
timestamp: November 12, 2016, 07:43:27 pm
content: At room temperature, fuses usually need almost two times of the rated current to blow at all. At lower currents they will hold infinitely.

author: mcc
author_rank: Contributor
timestamp: November 12, 2016, 07:50:11 pm
content: Quote from: Hydrawerk on November 12, 2016, 07:36:42 pmDid this before. 0.6A is not specified...Cheersmcc

author: Hydrawerk
author_rank: Super Contributor
timestamp: November 12, 2016, 07:58:03 pm
content: See thisSIBA 5021006.0,44.http://www.download.siba.de/pdf/artikel/5021006.pdf

author: mcc
author_rank: Contributor
timestamp: November 12, 2016, 08:01:29 pm
content: Quote from: wraper on November 12, 2016, 07:43:27 pm..that is: At room temperature a fuse does not blow at all when 0.6A is applied?Isn't that in contradiction to for they are build for: Protecting against overload... ?Only my two cents...your multimeter may vary ...Cheersmcc

author: Hydrawerk
author_rank: Super Contributor
timestamp: November 12, 2016, 08:04:07 pm
content: This graph says it clearly.http://www.littelfuse.com/~/media/electrical/datasheets/fuses/industrial-and-ul-fuses/littelfuse_fuse_flu_datasheet.pdf

author: mcc
author_rank: Contributor
timestamp: November 12, 2016, 08:06:35 pm
content: Quote from: Hydrawerk on November 12, 2016, 07:58:03 pmOk, that is more specific...had a BUSSMAN datasheet before.Nonetheless for me it is a contradiction, that a 0.44A fuse will blow afterthree yearswhen 0.6A are applied.Electronics are faster than this, I thnk....Cheersmcc

author: mcc
author_rank: Contributor
timestamp: November 12, 2016, 08:10:06 pm
content: Quote from: Hydrawerk on November 12, 2016, 08:04:07 pmOk...seems something like 'more than 1000secs'...Logarthmic scaling make estimations beyond paper sheet edgesa little diffcult...Cheersmcc

author: SeanB
author_rank: Super Contributor
timestamp: November 13, 2016, 07:35:33 am
content: 0.44A means the fuse will survive this forever in the operating temperature range. Thus if you drop below the maximum temperature by a large amount ( a lot of fuses are specced at 70C, as many of them are used in enclosures that will run at this internally from generated heat from curreent flow) then they can carry a higher current before the heating inside causes it to melt.As well there is variation in thickness of the wire or strip used, and this, along with slight changes in the alloy composition either in the batch or from batch to batch, gives this range. Remember the function of a fuse is to protect the cable, and to prevent fires from overcurrent, in most if not all cases they are not going to protect electronics. The fuse has to fail before the cable, so if the cable will handle the current for x minuted before melting the insulation, the fuse just has to blow before that time.