10A Fuse on Multimeter - Page 1
Forum section: electronics / beginners

author: skillz21
author_rank: Frequent Contributor
timestamp: January 08, 2019, 08:49:56 pm
content: I have a multimeter, and for its current measurement, it says it has a 10A fuse. On the back of the multimeter, it says that it contains 2 fuses, one rated for 600v 10A and another rated for 200mA 600v. For this question, I'm going to focus on the 10A fuse. I'm kinda confused about this. Wouldn't you need 6000 watts to blow that fuse? Because technically, isn't it the watts that determine how much heat something is going to create? Or am I very wrong? So wouldn't I, theoretically, be able to measure more than 10 amps? (this doesn't mean that I'm going to, the wires get pretty warm around 8 amps.

author: skillz21
author_rank: Frequent Contributor
timestamp: January 08, 2019, 09:15:55 pm
content: Quote from: tvl on January 08, 2019, 09:02:38 pmI understand that, yes. Could you please explain to me why the fuse would be blown at 600v 10A? I don't understand, it says it's rated for 600v 10A(that equals 6000w)...? (yeah I understand that 6000 watts is a crazy high number, that what I'm trying to make sense of here.

author: cowana
author_rank: Frequent Contributor
timestamp: January 08, 2019, 09:18:56 pm
content: The fuse is rated for 10A current. Above that (ignoring the time it actually takes), it will blow. Doesn't matter at all about the voltage.Note that because of the thermal mass of the fuse, it won't instantly blow at 10.001A - the time taken will depend on the current. It can likely survive 15A for several seconds.When it does blow, it will create an open-circuit. It can then cope with up to 600V across it, before it will begin to arc and conduct again (not properly breaking the circuit). Below this 600V limit, it should be fine. This is why fuses have a voltage rating - it just doesn't apply until it blows.

author: barry14
author_rank: Regular Contributor
timestamp: January 08, 2019, 09:21:35 pm
content: You are misinterpreting the fuse specification. A fuse rated for 600 volts and 10 amperes will blow when the current through it reaches about 10 amperes irregardless of the voltage of the source feeding it (fuses are not precision devices). Once the fuse blows, it is able to withstand up to 600 volts across it without shorting or arcing. The fuse will never see 600 volts across it and 10 amperes through it (at least not for long).

author: rsjsouza
author_rank: Super Contributor
timestamp: January 08, 2019, 09:21:48 pm
content: Quote from: skillz21 on January 08, 2019, 08:49:56 pmThe meter (and consequently the fuse) is placed in series with the load. In this condition the voltage across the meter and the fuse is very minimal as the fuse has a very small resistance. For example, if the fuse has 0.005Ω of resistance and the current flowing through it is 10A, the voltage across the meter would be: V = R×I => V = 0.005×10 => V = 0.05V. Consequently, the power dissipated on the fuse would be P= V×I => P = 0.5WIn case of an overcurrent event the fuse opens and stays opened, thus the entire supply voltage is present across the meter (and the fuse) terminals. The voltage rating of the fuse expresses how much voltage it can safely sustain across its terminals if interrupted.The 6000W you calculated would be only present on the load under test, if the supply is 600V @ 10A.

author: skillz21
author_rank: Frequent Contributor
timestamp: January 08, 2019, 09:24:39 pm
content: Ok, it makes so much more sense nowthe 600v is for after it blows, got it! One question, how does it actually know when its 10A? Doesn't the heat produced depend on the wattage?

author: wraper
author_rank: Unknown
timestamp: January 08, 2019, 09:26:59 pm
content: Voltage rating has nothing to do with when fuse will blow or how much power it will dissipate. All that matters is current rating. Voltage rating on the other hand shows what max voltage it is guaranteed to interrupt. If actual voltage is higher than fuse rating, it may start to arc instead of interrupting current flow.

author: cowana
author_rank: Frequent Contributor
timestamp: January 08, 2019, 09:27:18 pm
content: The fuse will have a certain resistance, say for example 0.05 ohms.When there is 10A passing through it, this will create a specific voltage drop (0.05 * 10 = 0.5V in this case). That equates to 5W dissipation in the element, which melts it.The fuse doesn't care that it was in line of a 12V rail (or whatever it is) - the power dissipation in the element just depends on the voltage dropacross the fuse.

author: Adam60
author_rank: Regular Contributor
timestamp: January 08, 2019, 11:23:13 pm
content: Your meter is probably rated CAT III and part of that rating system is explained vefy well above. Google ABCs of multimeter safety by Fluke and you can read about the different categories and their specifications. I believe there are currently 4 of them

author: Nerull
author_rank: Frequent Contributor
timestamp: January 09, 2019, 02:56:40 am
content: Quote from: skillz21 on January 08, 2019, 09:24:39 pmOhms law. The fuse has a fixed resistance (well, its probably not actually fixed, but we'll pretend it is). Since resistance is fixed, 10A will always result in the same power. P=I^2 * R.If a fuse requires 1W to blow and has a resistance of 0.1 ohms, than we can calculate that the fuse will blow at I = sqrt(P/R) = sqrt(10) = 3.1A. Thus we would rate that as a 3.1A fuse.The voltage throughout the rest of the device is completely irrelevant to the voltage across the fuse. At 10A a fuse will always drop a given voltage and dissipate a given power depending only on its resistance, no matter what voltages exist elsewhere.