A question about how multimeter overload protection works. - Page 1
Forum section: electronics / beginners

author: Multinoob
author_rank: Newbie
timestamp: December 04, 2017, 08:11:47 am
content: I have researched this on google this for 3 days and absolutely cannot learn the answer!Some multimeters that are certified CAT III 600 VOLT rated have 2 inputs and some have 3. With 3 inputs, 2 are dedicated to measuring AMPS. These 2 are fused. With 2 inputs, one input combines the amp and voltage inputs.With 3 inputs, the voltage input does not have a fuse. With 2 inputs, the voltage input does have a fuse.A certified CAT III 600 VOLT meter can safely sustain transient surges up to 6000 volts. A fuse is not needed on the voltage input to do this. It uses ptc thermistors or whatever else for overload protection.From what I can gather, a fuse does not blow from voltage alone. It blows from heat which is created by voltage AND amps. And the voltage rating on a fuse only pertains to how much voltage an already blown fuse can handle before arcing. So it makes sense to me that a transient voltage surge would not blow a fuse when properly measuring voltage because current is not flowing through the meter. Is this correct?Does a meter with 2 inputs have overload protection besides fuses like a meter with 3 inputs does? Obviously a fuse is not needed on the voltage input. So when there is a fuse on the voltage input, does a voltage surge blow the fuse?I think this is a simple question really. I'm asking about the differences in how the two different types handle transient voltage surges. I am well aware that sticking the leads into a wall outlet while on the amp setting will blow a fuse. That has nothing to do with my question. Thank you MUCH for any help or insight you can offer.

author: IanB
author_rank: Super Contributor
timestamp: December 04, 2017, 08:35:43 am
content: Yes, a meter that combines voltage and current on the same input should have the same input protection as a meter that separates them.However, it is best to avoid such meters as it is too easy to turn the range switch while connected to a power source and blow the fuse.

author: Multinoob
author_rank: Newbie
timestamp: December 04, 2017, 08:38:59 am
content: Ok, that was easy! So a transient voltage surge would not blow the fuse? And the fuses are only there to protect against accidentally turning the range while connected to power?

author: Multinoob
author_rank: Newbie
timestamp: December 05, 2017, 12:18:38 am
content: Alright... thanks everyone!

author: retiredcaps
author_rank: Super Contributor
timestamp: December 05, 2017, 12:22:02 am
content: These videos, if you haven't seen them during your 3 day search, might help?BTW, the Fluke service manual that Dave references is also good at explaining input protection.

author: Russ
author_rank: Regular Contributor
timestamp: December 07, 2017, 02:55:40 am
content: The fuses (pack) for my Eevblog/Brymen 235 should last quite some time, assuming I don’t get stupid and careless?ThanksRuss

author: IanB
author_rank: Super Contributor
timestamp: December 07, 2017, 03:59:32 am
content: Quote from: Russ on December 07, 2017, 02:55:40 amI have never blown a multimeter fuse. For me they would last forever.

author: Russ
author_rank: Regular Contributor
timestamp: December 07, 2017, 04:13:51 am
content: Quote from: IanB on December 07, 2017, 03:59:32 amIanThanks. I’m a newbie to all of this, and those fuses come from Dave, which means that it takes a bit of time to get here. The 400mA fuses cannot be bought locally. 😬😢. I believe the larger one is somewhat common. But the smaller one are not.ThanksRuss

author: IanB
author_rank: Super Contributor
timestamp: December 07, 2017, 04:47:15 am
content: A few simple rules:1. Never leave the red lead plugged into an amps jack2. Never measure current if you can measure the voltage drop across a resistor instead3. If you must measure current, always use the 10 amp range first just in case there is something wrong with your setup4. Only switch to the mA or µA jack after you are sure nothing is wrong

author: Russ
author_rank: Regular Contributor
timestamp: December 07, 2017, 05:11:09 am
content: Quote from: IanB on December 07, 2017, 04:47:15 amAwesome.Thank you 🙏

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 07, 2017, 01:49:10 pm
content: The cat or voltage rating has more to do with pcb creepage design etc, it has little to do with fuse.. fuse is amperage rating and it has more to do with amp mode. The fact you havent blow a fuse is because you havent short a battery through a dmm ampere mode... and if you have seen a brand name fuse seller ads video showing a cheap china dmm blown to pieces due to hi volt application and they blame the cheap crap fuse, its bull sheet, the pcb or components were failing/shorting, not the fuse...fwiw..