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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 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 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 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 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 at 10:48
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    \$\begingroup\$ @lucasgcb: the fuses won't protect against polarity reversals. \$\endgroup\$ – Peter Bennett Jun 28 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 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 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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