Products > Test Equipment
New EEVblog BM786 Multimeter
Frank987:
Now I have read the SIBA homepage. I agree, the left curve is the minimum time not the typical time for the fuse to break and curve is only valid for 1A 1.6A and 2A fuses. Better calculation is with the "SchmelzintegralI" I²ts Value, which is 0.07 A²s for the 400 mA fuse. Now with 0.6 Amp the time to break is 0.07 / 0.6² = 0.194 s. I can not read the Multimeter in this short time as it does only 5 measurements / s. So before 1 measurement is done the fuse is broken...
HKJ:
--- Quote from: Frank987 on February 14, 2021, 04:37:33 pm ---Better calculation is with the "SchmelzintegralI" I²ts Value, which is 0.07 A²s for the 400 mA fuse. Now with 0.6 Amp the time to break is 0.07 / 0.6² = 0.194 s. I can not read the Multimeter in this short time as it does only 5 measurements / s. So before 1 measurement is done the fuse is broken...
--- End quote ---
It sounds like the integral specified the minium time, i.e. the fuse will at least last 0.2second, but may never blow at all.
Wytnucls:
t is the time for the fuse to completely open with overcurrent.
600 mA equals 150% of In and the FF fuse must not open for an hour, by IEC regulations 60127.
UL 248 standards are different:
4 hours at 110% and 60 minutes max at 135%.
A fuse made to UL/CSA/ANCE 248-14 and operated at its rated current will
eventually open. For this reason UL fuses are customarily operated
at no more than 75 percent of their rated current.
Currents between 110 and 135 percent of fuse ratings present a
severe challenge to the designer because they can subject parts to
high heat for extended periods of time, and because fuse behavior
at these currents can be difficult to predict. Littelfuse
EEVblog:
--- Quote from: Frank987 on February 14, 2021, 10:39:01 am ---There is a problem with the 400 mA Fuse: The BM786 manual says a SIBA Fuse 7017240;0.4A/1000V is used for the 600 mA range. In the SIBA datasheet https://download.siba.de/pdf/artikel/SIBA-GSS-7017240.pdf you find that the 400 mA fuse blows typical after 0.1 s at 1,5 * 400 mA = 600 mA. The I²t value of the fuse is 0.07 A²s, giving about 0.2 s at 600 mA. So something must be wrong as it s not possible to do measurements in less than 0.2 s. Either the manual is wrong or the 600 mA current rating of the meter or the meter is supplied with the wrong fuse, should be SIBA 7017240.0,63 with 630 mA.
--- End quote ---
The BM235 uses the same 400mA fuse for its 600mA range and it's never been an issue.
I asked Brymen years ago if a 500mA fuse could be used on the BM235 and their response was
--- Quote ---Not recommended.
For measuring 600mA in short period, there is no problem to use that fuse. But for measuring 600mA continuously, it will heat and the internal ambient temperature around that fuse may reach to the one out of plastic case temperature spec.
--- End quote ---
Wytnucls:
It seems to me that Brymen is obfuscating here.
A SIBA 630mA fuse will only heat up by a few degrees when subjected to 600mA, whereas the 400mA fuse, at 150% of its current rating, could possibly melt plastics, before it opens one hour later.
The Bussmann 440mA fuse temperature only rises by 6 degrees above ambient (datasheet), when operating at its max rated current.
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