dmm-b-44/100 fuse hack - Page 1
Forum section: electronics / testgear

author: kirkpennock
author_rank: Newbie
timestamp: April 07, 2013, 03:38:31 pm
content: I blew a fuse in my new extec MM560A DMM. It has the huge HRC fuse. Why can't I solder the old style glass fuse right on to the body of the dead fuse? This fuse is very over built for .5 amps. Whats the worst that could happen? Shattered glass inside the meter?My old meters all have glass fuses. HRC is supposed to be able to stop any arc after blowing versus the glass is a air gap and might not?Is this a bad thing to do?Thanks

author: Excavatoree
author_rank: Frequent Contributor
timestamp: April 07, 2013, 04:18:06 pm
content: If you fully understand that you'll be limited to low energy circuits, and you're certain that you or no one else will ever try to use that meter on anything mains powered, then you can do it. Many have advocated placing a label over the category rating printed on the meter face, to remind you.Personally, I'd sooner spend the money and replace it with another HRC fuse, but we all have to make some tough decisions. That's your decision whether it's worth the 5-15 US dollar or equivalent to do so.

author: Unknown
author_rank: Unknown
timestamp: April 07, 2013, 04:24:33 pm
content: Glass fuses are usually only rated for 250 Vac, no or very low DC voltages and can interrupt a current up to ten times the nominal current (eg. 5 A for a 0.5 A fuse). The DMM-B 44/100 fuse is rated for 1000 Vac and 1000 Vdc, and can interrupt a current up to 10,000 A.I think Gossen or Fluke might have some videos that show the worst that could happen if it is connected to a high-energy circuit. Worst case is that the glass fuse arcs over (HRC fuses are filled with sand to extinguish arcs). Current will then be able to flow uninterrupted, and whatever the weakest component is will act as a fuse. For example the test leads that you are holding with your hands might melt. The energy that is released might also send parts flying.

author: nanofrog
author_rank: Super Contributor
timestamp: April 07, 2013, 11:39:04 pm
content: Do yourself a favor, and replace the HRC fuse with one of the same ratings.The cost of the fuses aren't that horrible, and are much cheaper than say medical bills that would result from an injury, or worse.

author: Wytnucls
author_rank: Super Contributor
timestamp: April 08, 2013, 07:29:09 am
content: A typical 500 mA fast-acting F/FF 3AG (6.35x32mm) or M205 (5.0x20mm) glass fuse, rated for 250VAC, has a breaking capacity of 10x the rated current or 35A whichever is greater (10KA at 125VAC).It should interrupt the circuit without arcing or breaking, up to 35A at 250VAC. (UL and CSA testing)At higher currents and voltages, some arcing inside the fuse may occur, shattering the glass, with residual arcing possible across the broken fuse if high voltages are present. (The same thing can happen with HRC fuses, if their ratings are exceeded)If you're only working with low voltage circuits, it would be silly to blow that expensive 500mA HRC fuse regularly. Keep that meter for the day you have to work on something connected to the mains and buy one with cheap glass fuses for your low energy work.http://php2.twinner.com.tw/customer/jennfeng/Product/MFA(AA10081803)100901.pdf

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 08, 2013, 07:36:28 am
content: heybuild you'rself some CC or OCP circuit , nice project and you won't have to compromise on having stupide glass fuse instead of proper HRC oneCheers

author: M. András
author_rank: Super Contributor
timestamp: April 08, 2013, 06:43:47 pm
content: or simply measure it on the 10a fused range instead the 400ma much lower chance of blowing the fuse

author: Unknown
author_rank: Unknown
timestamp: April 08, 2013, 10:05:28 pm
content: Quote from: Wytnucls on April 08, 2013, 07:29:09 amBut what's the breaking capacity at 250 VDC (primary side of an SMPS), or even at 48 VDC (UPS battery pack)?Unless I already have a good estimate of the current, I will always start on the 10 A range. Another advantage is that the burden voltage is lower.You can always put a box with a fuse holder with a glass fuse in series for low energy circuits. Thos way the HRC fuse can still interrupt the current if things get exciting. The value should obviously be lower and it should be at least as fast as the fuse in the meter.If you decide to go against the advice of most people in this thread, please take a sharpie, cross out any CAT or high voltage markings and indicate that it's for SELV only. Both for your safety (you might pick it up later to measure a high energy circuit) and for anyone else that might pick it up. Unless electrocution is your way of discouraging people to use your tools.

author: Wytnucls
author_rank: Super Contributor
timestamp: April 10, 2013, 09:53:38 am
content: But what's the breaking capacity at 250 VDC (primary side of an SMPS), or even at 48 VDC (UPS battery pack)?Strictly speaking, these glass fuses are not designed to interrupt DC circuits and shouldn't be used in multimeters. They have no DC voltage interrupt certification.It doesn't mean that they can't interrupt some DC voltages safely, but there is no way of knowing without conducting specific tests on the circuit under consideration.At best, with fast acting fuses (F/FF), a reduction of 25% of rated voltage is to be expected, depending on circuit inductance time constant.To err on the safe side, I wouldn't use them in multimeters to test circuits with DC voltages higher than 30VDC.'There is a limit as to how much arc energy a fuse can absorb. The DC voltage rating of a fuse always has an associated time constant because both terms are needed to define how much arc energy the fuse can absorb. For fuses, the DC voltage rating is inversely proportional to the time constant. In other words, as the time constant of the circuit increases, the voltage capability of the fuse decreases.'