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I am currently going through the conundrum of where to place a fuse in my DC battery-powered circuit to protect the circuit components and the DC battery. I've been googling for a definitive answer, but I came across diverging opinions... Which one is correct and why?

  1. Connect the fuse to the negative terminal of the battery since it's where the actual flow of electrons originate which is opposite to the conventional flow of current from the positive terminal.
  2. Connect the fuse to the positive terminal.
  3. Connect two fuses, one at the positive and one at the negative battery terminals.

Also, during my research, I came across a post that advised to connect a fuse at the positive terminal since it would protect both circuit and the battery, but if the fuse is connected to the negative battery terminal, then it only protects the battery. Is this true? It doesn't make sense to me.

So, I can't figure out which one is correct and why? I made a simple block diagram to illustrate my question.

Fuse position

So, is it position A or B? Or both? And why?

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    \$\begingroup\$ If the frame is connected to the negative terminal of the battery (like in cars), you need to put the fuse in postion A. If there is a short circuit to frame, the current may pass by the fuse in position B. \$\endgroup\$ – Huisman Jun 27 '19 at 20:49
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    \$\begingroup\$ What bad event are you trying to protect from? What part of the circuit might fail? That should tell you where to put the fuse. \$\endgroup\$ – Elliot Alderson Jun 27 '19 at 20:51
  • \$\begingroup\$ if you are really concerned about the health of the battery, then use two fuses, one on each battery terminal \$\endgroup\$ – jsotola Jun 27 '19 at 21:13
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    \$\begingroup\$ "the negative terminal of the battery since it's where the actual flow of electrons originate" ha, actually it is the positive terminal from which the actual flow of holes originate. \$\endgroup\$ – Glen Yates Jun 28 '19 at 17:56
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[Should I] connect the fuse to the negative terminal of the battery since it's where the actual flow of electrons originate which is opposite to the conventional flow of current from the positive terminal?

Forget about electron flow. It only causes confusion such as in your thinking. Electrical current flows in a circuit in the same way that a bicycle chain flows around. Any mobile charge carriers that leave one terminal of the power source must return on the other. A break anywhere will stop current flow.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Four possible scenarios.

  • Figures 1a and 1b are equivalent. If F1 or F2 blows then current flow will cease. As the power supply has no ground / earth / chassis connection there is no danger of a single fault causing an alternate return path.
  • Figure 1c is the way most vehicles are wired with a negative connection to the chassis. The fuses are placed in the positive lines from the battery and close to the battery. If a fault occurs on the line between the fuse and the load the fuse blows and current flow stops. A ground fault on the return line is unlikely to cause any problems as there is no significant voltage on it.
  • Figure 1d shows a poor arrangement with the fuse in the return wire. It should be clear that a ground fault on the positive wire would be unprotected, F4 would not blow but BAT4 would pass high currents.

Again, for circuit analysis it is normal to use the conventional current flow from positive to negative. Don't worry about electrons, just think of it as charge moving from + to -.

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Since the negative terminal of the battery is normally considered "Ground" or "Zero Volts", a fuse in the negative lead would leave the rest of the circuit "hot" - usually Not a Good Thing.

Recommended practice is to place the fuse near the positive terminal of the battery, so the whole circuit will be dead if the fuse blows.

(Of course, if the positive terminal is considered Ground, place the fuse near the negative terminal.)

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    \$\begingroup\$ What if the battery was reversed on accident, do you think the other fuse would provide protection? \$\endgroup\$ – lucasgcb Jun 28 '19 at 10:48
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    \$\begingroup\$ @lucasgcb: the fuses won't protect against polarity reversals. \$\endgroup\$ – Peter Bennett Jun 28 '19 at 18:57
  • \$\begingroup\$ @lucasgcb When the polarity is revered, the fuse may or may not be blown depending on your fault current, but very likely, the components are already destroyed even if the fuse does eventually blow. A slightly better way to protect your circuit from short circuit is connecting a power diode in reverse. When the polarity is reversed, the diode would be forward biased and conduct, shorting power and ground and blow the fuse, but you need to make sure the power rating of the diode is good enough and use a fast-acting fuse, so the fuse would blow before the diode is destroyed. \$\endgroup\$ – 比尔盖子 Jun 29 '19 at 20:51
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Think about your failure modes

You have to ask yourself, "What happens if one of the wires or some of the electronics shorts out to something else?"

Suppose I have a + and - pair of wires going out to a left turn signal light. The chassis is bonded to the - side of the battery terminal. What happens if the - wire frays and contacts chassis? nothing What happens if the + wire frays and contacts chassis? Spitzensparkzen. Fuse the + wire.

I'm on a Subway Car. Third rail is propulsion +DC volts, rails are propulsion -DC, which means chassis is -DC. My control circuits are 36V isolated, so that if one car's wheels go up on rusty rail, it doesn't try to return 400A of propulsion current to the next car through the tiny 10 AWG control circuits. What happens if control+ shorts to chassis? BADNESS. What happens if control- shorts to chassis? BADNESS. Fuse both.

Suppose I'm using LED turn signals, and to save wires, I hook both turn signals to the same pair of wires. Brown wire + means left signal. Blue wire + means right signal. Fuse both, or fuse the source of my reverser circuit.

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    \$\begingroup\$ I can't speak for Subway Cars. But I have installed everything from radio transceivers(Ham FM & SSB, Navy FM), taximeters(Over 500 and counting), car stereos(~5 only for family, I"m not a masochist) and computers(~10, of various types). It was pounded into me both when I was in the Navy(6YR ET) and later as Ham, you only fuse the hot side UNLESS your installing a radio with a antenna that if a grounding strap came loose, would cause your coax to be the ground for connection. Then you fuse both. Meaning only the tranceiver PS get both fuses. \$\endgroup\$ – GB - AE7OO Jun 29 '19 at 8:56
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I would add that cars are an exception here: in most low-voltage applicatons the battery is not directly connected to the conductive parts such as casing or heat sinks, and therefore it doesn't matter where you put the fuse.

Don't get the impression that connecting the battery the way it is done in cars is a good idea: unless you have to deal with high currents or saving on wires by using the metal case as a conductor is essential, keep your electrical appliance isolated from the case. It will help to prevent electrochemical corrosion should your device get exposed to moisture or water, and a single short to mass will be a non-issue. A blown fuse is better than a fire, but not having to replace the fuse is even better.

Also, if your battery has to be electrically connected to the metal frame, don't just put the fuse on the positive terminal: put it on the terminal opposite to the one connected to the frame.

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It's not a perfect analogy but you can think of the positive terminal like a suction inlet, rather than the negative terminal like a blasting hose. Fuse is like a normally open valve. If the valve closes (fuse blows) and its near the suction inlet, the rest of the tube has no negative pressure, so a hole anywhere along the tube won't result in a hissing leak. If it's near the end of the tube (negative terminal), a hole at any point will. Place your fuse so that the least amount of "tube" possible has negative pressure.

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  • \$\begingroup\$ This analogy is more complex than the concept it is trying to explain. \$\endgroup\$ – Transistor Oct 27 '20 at 21:13
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On a vehicle there will be a fuse on the negative and positive leads for anything not using the chassis as the return/ground/negative lead. Well there are some uses of a positive ground but that's rare outside of telephone gear. Generally the negative post on the battery is grounded to the chassis of a vehicle.

The reason for this was touched on in other answers, the need for two fuses is to cover some known failure modes. In automotive applications the answer is that with a large load, like the starter motor, a failed ground on the large load can find a return path through the vehicle chassis, through the device chassis, down the small ground wire, to the battery. Without a fuse on that negative lead a 500 amp starter could be grounded with a 12 AWG wire to the battery. That wire would then become the fuse and burn up.

Others mentioned the additional hazard of the antenna becoming part of the circuit on any kind of radio. Having fuses on both positive and negative will protect the radio and wires on most failure modes. There's going to be some rare failure modes not covered by the two fuses. For those hopefully a properly installed fuse for that circuit path protects the battery, wires, and people around them.

You are going to get seemingly conflicting information if people are discussing different environments. As a ham radio operator I will wire a DC supply differently at home than in a car. If we look at AC circuitry in a home as an example there is only a circuit breaker on one terminal, the supply or "hot". In fact there is a redundant return path, ground and neutral, to make doubly sure that path is not broken. There's a different philosophy on safety, and that calls for a fuse only on the supply path.

To decide what is safe one needs to consider failure modes. I guess that it is possible for a failure mode to conflict on where a fuse needs to be, or if a fuse is appropriate means to prevent damage or injury. To address the failure of a ground fault, as an example, there needed to be something other than a fuse to detect that and act on it. Such a device is called a ground fault circuit interrupter, or GFCI. That's getting out of scope though.

The point is to consider the environment, the failure modes that creates, and what to do to protect against them. I gave examples on where one fuse is enough, and where it can take two.

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We must remember that current flow is actually the flow of electrons from negative to positive. Therefore the negative terminal supplies the energy to the circuit. (Energy being supplied from the positive terminal was how things were thought to work before electron flow was discovered). I suggest therefore that overcurrent protection should be as close to the negative source as possible.

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    \$\begingroup\$ Read the other answers and you'll see why you are wrong. The direction of current flow has next to nothing to do with where the fuse goes. Also, take a look at your car electrical system some day. The fuses are all in the positive side on negative chassis vehicles. The reason is explained in the answers above. \$\endgroup\$ – JRE Jan 6 at 12:08
  • \$\begingroup\$ Welcome to EE.SE, John. You can edit your answer to correct it or delete it if you don't want to attract downvotes. Do read the other answers to improve your understanding. \$\endgroup\$ – Transistor Jan 6 at 13:43

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