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New EEVblog BM786 Multimeter

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Wytnucls:
Regardless, one needs to be careful when exceeding the fuse rated current for any length of time, especially in warm ambient condition.

I don't think Bussmann Cooper makes a 600mA fuse for Fluke multimeters, but SIBA does make a FF 630mA 1000V 32/6.3mm fuse, which would be a better match than the SIBA 400mA fuse installed at the Brymen factory.

Fungus:

--- Quote from: EEVblog on November 22, 2020, 10:54:05 pm ---Very common.
The Fluke 70 and 80 series are both 6000 count and use a 440mA fuse.

--- End quote ---

A 440mA fuse won't blow at 441mA. Fuses will often stand double their current, they just get very warm

Underrating the fuse in this way will make it much more likely to blow at the expected current (eg. 7-800mA in a 6000 count meter).

The best way was the way it used to be done, eg. the 600mA range would have a cheap 600mA glass fuse in series with an expensive 2A HRC fuse. The idea was to blow the cheap fuse in small slipups (we've all been there) but to still have HRC protection in industrial accidents.

I wonder why they stopped doing that, was it because of burden voltage?

Kleinstein:
There is a problem with a cheap fuse in series with the proper HRC fuse. With a DC source it is possible and not that unlikely that the cheap fuse can blow but not quench the arc, even with a current not large enough blow the higher rated HRC one. The extra heat will become a problem where the HRC fuse will not help with very much. The smaller fuse may still be OK, but should still be DC rated and thus not a really cheap one. This is kind of the price you have to pay for a CAT 3 rating.

A 440 mA fuse will likely not blow at 441 mA, but legally there is a chance it could. It usually takes more like twice the current to reliably blow the fuse, but some may bow earlier.

I see no real problem if the fuse would even well exceed the nominal range, like a 440 mA fuse in a 200 mA range, as long as the protection is up to it. Especially for the lower range a slightly larger fuse rating would be preferable and would also allow lower burden and less heat under normal conditions. However it still needs the protection (diodes) to be good for the higher current. This may need re-certification / testing and is thus not a change to be made easy.

Wytnucls:

--- Quote from: EEVblog on November 22, 2020, 10:54:05 pm ---
--- Quote from: bitseeker on November 22, 2020, 10:19:18 pm ---I guess they assume you wouldn't be measuring at full scale for extended periods of time. Nevertheless, an interesting mismatch of values.

--- End quote ---

Very common.
The Fluke 70 and 80 series are both 6000 count and use a 440mA fuse.

--- End quote ---
The Fluke 87 V has a a range of 400mA only, with a specified limit of 400mA continuous and overload 600mA for 18 hours.
The Fluke 179 has a range of 400mA only, with a specified limit of 600 mA overload for 2 minutes maximum, 10 minutes rest.

The Brymen 786 should have a limited mA range of 400mA, not 600mA.

dcac:
Will a blown 400mA fuse in BM786 be covered by the warranty - if customer claims it was only measuring 590mA when it happened?

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