When reading across a fuse the voltage will take the less resistant path and go through the fuse, not your meter which has resistance built in. When the fuse is blown, there is no voltage flowing though it. Connecting your meter, you create a new path for the voltage to take, this getting a reading.
Edit: the potential is still on the one side of the fuse. You are just creating a new bridge. Also that should be the only voltage you see unless there is a transformer or the like.
A fuse is a short. It is just a wire. In normal operation you read no voltage across it because you are just reading two sides of a wire. If the fuse is blown, it becomes an open, two separate wires. One side is connected to your power supply, the other is connected to ground through the rest of your device. Since it is two separate wires and one of those wires is powered you will have a voltage across the fuse.
You can think of voltage like the water pressure in a pipe, and a fuse like a valve.
When the fuse is intact it allows current to flow, so it’s like an open valve. And if you measure the water pressure right in front of an open valve, and right behind it, it’s basically the exact same pressure, so the difference in pressure, or the potential, will read zero. The same when you measure across an intakt fuse, the difference in electrical potential between the two ends, which is what your multimeter measures, is pretty much zero.
Now you close the valve. On the side with the water you still have that same high pressure, but on the other side there’s no longer any water pressure at all, so now you have a pressure difference.
Same with a blown fuse, on the PSU attached side the voltage is still the same, but on the disconnected side there is no longer any voltage, so there’s now a difference between the two sides.
An unblown fuse is supposed to be close to a short circuit,which brings the voltage either side of the fuse to close to the same level.
When the fuse blows no current goes through the fuse, but current can still go through the rest of the circuit, so the voltage is only really pulled down by whatever current is going through the meter. As most meters have a very high input resistance when measuring voltage, the rest of the circuit usually conducts enough that you see close to your power supply voltage across a blown fuse.
Think about the circuit, 12v -> fuse -> incandescent lamp -> gnd. Normally the lamp is what’s limiting current, so the voltage across the lamp is close to 12v. If the fuse blows, it’s the fuse that’s limiting current, but electricity can still flow through the lamp, so the fuse side of the lamp gets pulled down to gnd.
Measuring voltage is measuring the potential difference between two points. If you measure an intact fuse, there is zero potential difference between the two probes of your meter and therefore the difference (voltage) reads zero on your meter.
This is the same as measuring two points on the same live cable. There is no difference so no voltage is measured. However, if you measure between phase and neutral you will see a difference. In a 230VAC T-N-C-S system, the phase is at 230V potential whereas the neutral is bonded to ground at the incoming supply and therefore has 0V potential. The potential difference between phase and neutral is 230V and you therefore measure 230VAC on your meter.
This is similar to your blown fuse situation. Think of the incoming side of your fuse as the phase at 230V or whatever voltage your system is at. The load side of your blown fuse now has the same potential as your neutral which is 0V in most cases. So, if you measure across a blown fuse you get 230V.
If you measure between neutral and earth (ground) in a T-N-C-S system, you should also get 0V on your meter as these are connected together at the incoming connection as mentioned previously.
Let’s say you have an old appliance that is not connected to a GFCI (RCD) and it has a short circuit to earth in it. If you were to measure the frame of the appliance to earth, you will get something close to 230V, and that is where the danger comes in. There is now a potential difference between the frame of your appliance and earth, and if someone completes that circuit, with their two arms for example, their left arm will be at 230V and their right arm will be at 0V and they will get a shock across their chest (most dangerous kind of shock).
The people saying that electricity flows through your voltmeter are incorrect. A voltmeter is high-resistance/impedance to prevent electricity flowing through it. If electricity flowed through your meter when you connected your probes, you would have a big problem i.e. a short circuit.
They may be confusing a voltmeter with an ammeter which measures current. An ammeter has a low (close to zero) resistance/impedance because it NEEDS the electricity to flow directly through it so it can show how much current is flowing in the circuit.
Not exactly ELI5 but hopefully makes sense.
Source: Am trade qualifed Electrician
This is a massive misunderstanding about how multimeters read
Voltage is the DIFFERENCE between 2 points.
So fuses that are good will read 0 volts between the input and output of the fuse. This doesn’t mean it’s safe to touch.
Fuses that are bad will read voltage between in the input and output.
If you hypothetically were able to put a multimeter on a power line up above your house then put the other probe on that same line the reading would be 0.
It would only be if you put the probe on one of the other lines you would see the thousands of volts running through the line.
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