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Considering that the regular treatment of a fuse is to place it on the positive side, "before" a load, is this just common practice, or is it backed by an actual reason?

Can a fuse be placed on the negative side, "after" a load? Considering that 1, there is a school of thought that current flows from the negative side to the positive side, and 2, current should be equal at all points in a series circuit (which is why a resistor can go on either side of an led and still regulate the current). Is there any specific reason that the fuse is placed on the positive side aside from just convention?

And for that matter, can a fuse be placed in the middle of a circuit?

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  • \$\begingroup\$ There's no such thing as 'after a load' really. The fuse is between the source and the load. The power amplifiers I service have fuses in both rails. \$\endgroup\$ – user207421 Oct 9 '15 at 23:05
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Depending on what the fuse is designed to protect, and what behavior is desired when the fuse blows, it could potentially be placed anywhere in series in the circuit that must be interrupted on fault conditions.

For a simple circuit like this, all 3 fuse locations shown are valid and will protect the LED, the CL25 driver and the battery in case something goes awry:

schematic

simulate this circuit – Schematic created using CircuitLab

For something slightly more complex like this, note that Fuse F2 protects the load without protecting the regulator, while Fuse F1 protects the regulator, while not protecting the load for load currents lower than F1's fusing limits:

schematic

simulate this circuit

In such situations, using multiple fuses for protecting individual sub-circuits is common.

Also note that as the circuit grows more complex, having a fuse on the ground return path becomes increasingly undesirable: A "typical" fuse necessarily introduces some resistance into the path, by the very fact that heating of this resistance due to current through it causes the fuse to blow. A changing current through the ground return therefore ensures a changing voltage across the fuse, and hence a varying ground voltage as seen by the following parts of the circuit.

This may be immaterial in low current designs, where the voltage generated across the fuse even at maximum within-spec load is insignificant compared to the circuit voltages. Thus, you will see a return path fuse on some automotive circuits.

In all other cases, this variable ground voltage behavior is undesirable, hence fuses on ground return would be avoided.


As suggested by rawbrawb, a footnote on why low-side fusing is avoided in higher voltage designs, i.e. where the supply voltage is either DC, or AC at mains voltage or sufficiently high voltage such as to be harmful or painful to accidental touch:

The ground return is also the "no voltage" or safety return path for a circuit, essentially zero Volts, safe to touch, and in circuits with a non-isolated power supply, often connected to the device chassis and eventually to building earth.

A natural perception in a non-operational device is that other than the supply line itself, the rest of the circuit should be safe to touch. When such a device is fused on the return path, the rest of the circuit will rise to the supply voltage, in other words will be "live" or electrically "hot" when the fuse blows, since there is now no return path. Touching such "hot" portions of the circuit (pretty much all of the circuit) would then make the human being the return path for the supply voltage.

Until humans get bioenhancements which incorporate internal fuses, this exposes users to potential risk of electrocution or injury during device diagnosis, from what should have been a "dead" circuit. Hence, in high voltage devices, having the fuse on the high side is pretty much mandatory. Yes, additional fuses for individual sub-circuits might be used as well, for the low voltage sections for instance.

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  • \$\begingroup\$ You implied it in your low voltage comment, but maybe you can expand upon what happens in low side fusing with high (dangerous) voltage levels? \$\endgroup\$ – placeholder Aug 4 '13 at 16:19
  • \$\begingroup\$ Hmm, what would be considered low current designs? < 0.1A? 1A? 5~10A like standard accessory car fusing? If fuses act like resistors, are we talking about < 1Ω values? Or higher? \$\endgroup\$ – Passerby Aug 4 '13 at 16:27
  • \$\begingroup\$ @Passerby That would depend on the circuit's supply voltage and current draw: For a 5 Volt circuit that can deal with a 0.1 Volt variation, and draws up to 1 Ampere, any fuse of resistance greater than 0.1 Ohms is a problem. For the same supply and tolerance, but only 100 mA current draw, a 1 Ohm fuse will work. Picking a random glass-fuse datasheet and selecting a 1 A fuse AGX-1, we see the cold resistance is 0.16250 Ohms, which will float up to ~0.16 Volts. \$\endgroup\$ – Anindo Ghosh Aug 4 '13 at 16:41
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A fuse "after" the load would change your ground reference, making it more difficult to make accurate measurements on the load. Most of the time the ground reference of a meter is simply clipped to the chassis. And although there will be a voltage on the load's high terminal, no current will flow.

schematic

simulate this circuit – Schematic created using CircuitLab

Also you'll have a fairly high chance that the fuse is shorted by the chassis housing, be it aware or accidentally, as nobody expects a fuse in that position. When measuring resistance while the circuit is powered off, the fuse looks like a short and the load appears as if directly connected while it isn't.

And notice that when the fuse blows, the load will still be connected to live power supply which is an unsafe situation.

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  • \$\begingroup\$ Could you please explain how the ground reference changes as the voltmeter? I am confused as both are connected to the gnd or in this case the negative side of the battery. \$\endgroup\$ – mindentropy Oct 13 '13 at 7:52
  • \$\begingroup\$ @mindentropy A voltage will buid up across the fuse when a current flows. So when you connect the voltmeter to ground and measure the hot side of the load, you actually measure the voltage across load and fuse. \$\endgroup\$ – jippie Oct 13 '13 at 12:17
  • \$\begingroup\$ ok got it. You want to measure the voltage drop only across the load. \$\endgroup\$ – mindentropy Oct 14 '13 at 7:14
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Placing the fuse before the load (in your language by 'before' you mean the highest potential) is purely a safety convention. In the event of a fault it isolates the load from the voltage source. The reason for the fuse to blow is always because of a fault, that fault may have safety implications, this way at least the system becomes fail safe.

If the fuse were placed 'after' the load (by after in your language this means towards the lowest potential), the source potential could still be present and dangerous at the load even though the device is not operating and may even have a critical fault.

Really you should say 'placed towards the largest potential' (which may be negative). As any circuit is a loop, there is no before or after.

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The point of a fuse is to disconnect overcurrents. To decide where to put fuses you need to think about where overcurrents can potentially flow under fault conditions, where best to disconnect them and what impact the opening of fuses will have on the voltages in your system.

If your circuit is as simple as a floating source driving a floating load it really doesn't matter which side you put it in or even if you put it in the middle between a pair of loads connected in series. It will disconnect the overcurrent either way.

In more complex circuits it's normal practice to designate one node in the circuit as "circuit ground" or "0V". Depending on the application this node may or may not be connected to "mains earth" and/or the general mass of the earth. Most loads in your circuit will have one end connected to "circuit ground".

Power rails may be positive, negative or even AC relative to circuit ground.

In such circuits normal practice would be to put the fuse away from circuit ground. That way as well as protecting against a fault where the load shorts it's two terminals it also protects against faults where the load's power terminal somehow gets shorted to circuit ground. It also means that when the fuse blows the load does not have a voltage on it relative to circuit ground which is usually considered a good thing (especially if circuit ground is referenced to mains ground).

Fusing the ground referenced end of a device can mean that if/when the fuse blows a connection that was normally at (approximately) ground potential rises to a hazardous voltage. Some electrical standards forbid fuses in neutral conductors for this reason. Even in a system that doesn't involve dangerous voltages such a rise in voltage may cause damage.

In a split rail DC system or a multi-phase AC system fusing the neutral conductor can lead to overvoltages as loads on different poles/phases end up in series.

Not all circuits fit that model though and in such cases you have to think about where overcurrents can flow and how to protect against them. In some cases it may be deemed nessacery to include more than one fuse (or better still a multi-pole common trip circuit breaker) to provide adequate protection. This is especially true when you start dealing with "open collector" style outputs that switch the "ground" end of the device.

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