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I'm converting a van to an RV. I have a small DC distribution/fuse box bought from Amazon. On some of the fused paths I read almost 13V When I remove the fuse, I get about 11.5V or so. In some spots, I get no voltage, but on most, I get voltage (11.5V or so) even with no fuse. Does this trigger a "I know what you did wrong" for anybody, or is it just that the distribution box is somehow messed up?

To be clear, when I say I get voltage from the removed fuse, I mean the wires I have connected to that empty fuse, I can meter THOSE wires and get 11.5V. That is how I discovered the problem because the wires connected to the fuse were live even after removing the fuse. While testing the hot "nut" on the box for the other spaces, I get 11.5V on some, zero (less than 1V) on some and then almost 13V on those with a fuse. On the wires connected to the fuse in question, I get 11.5V when the fuse is pulled and like 12.95V when the fuse is inserted. This is a measure from the ends of the wire.

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    \$\begingroup\$ It's an "I know what you did wrong", I'm afraid. You've purchased from a vendor that sells product without datasheets. We have nothing to go on, no brand or part number, can't check that you've wired it properly, or what it's internal wiring is. \$\endgroup\$
    – Transistor
    Feb 13 at 22:57
  • \$\begingroup\$ Sorry, I just cannot see what you did at all. This could e.g. be an effect of open load detection circuits, a wrong connected load or missing GND paths. \$\endgroup\$
    – Jens
    Feb 13 at 23:36
  • \$\begingroup\$ I'm wondering if you're running afoul of a high-performance DC voltmeter? Suppose you substitute it for a crude sensor - a 12v incandescent lamp. Also, try choosing various return paths (ground?) to the battery. \$\endgroup\$
    – glen_geek
    Feb 13 at 23:46

2 Answers 2

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Let's be clear about how to measure those voltages:

schematic

simulate this circuit – Schematic created using CircuitLab

Ignore R3 for the moment.

What you read on your voltmeter will depend entirely on where you measure. There are four voltmeters above, and with or without a fuse, it's quite possible to read 12V (see VM2 and VM3).

For load 2, without any fuse in place, if you measure across where the fuse should go (between D and E), you will see 12V, since the load is still present, "pulling" the potential at E down towards ground.

In the left scenario, it is assumed that the fuse has no resistance, and it will connect node B directly to battery positive. With node B at a fixed +12V, now you will read zero volts across the fuse, and 12V across the load.

So, what you measure is meaningless without understanding what you are measuring. A meaningful measurement would really be across the load, between B and C, or between E and F above. Note that the C and F are the chassis, in a negative-ground vehicle, so even if you don't have direct access to the load's negative connection, you can consider the vehicle's chassis to be the same node. That's an assumption, of course, and if the manufacturer/tinkerer has done something different, then the rules change.

It might still be possible to read non-zero across the load when the fuse is absent. You might fall foul of that if, for instance, a fuse-less load is somehow connected to some other load that is properly fused. It's not likely, though, that the head-lamp circuit would be connected to the radio in some way, so common sense is needed to determine if that's an issue.

Also, if you're using a modern multimeter, with incredibly high input resistance, you might find yourself reading potential differences where you would expect none, due to dirt and grease (or your own fingers) bridging terminals. One way to mitigate this would be to place a 10kΩ resistor in parallel (across) the meter terminals, as I've done for VM3 above.

Of course, I haven't included switches for the loads, and I have assumed the vehicle's chassis (ground) is connected to battery negative. For older vehicles that may not be the case, but the same principles still apply. I wish it were possible to summarise expected readings in an all-encompassing, one-schematic-for-everything, but I'm afraid a general understanding of Kirchhoff's laws and Ohm's law is the only way you're ever going to really grok this.

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  • \$\begingroup\$ So to be clear, the reason I noticed the problem is because the wires connected to one of the fuse ports still had electricity running through them even after I pulled the fuse. So ignoring all other things, the wires connected to (let's say fuse spot 2) still had almost 12v when the fuse was pulled. Pulling the fuse did not kill the electricity to the wires connected. It did decrease the power from almost 13v to about 11.5. Replacing the fuse brings it back up. But I am metering the wires (at their end) from the box and they still draw power even after the fuse is pulled. Like 1.5v less? \$\endgroup\$ Feb 14 at 6:31
  • \$\begingroup\$ I am open to the idea that my ground is bad, but would that cause this problem? And would it make only some of the fuse spaces draw power and others not? I am confused that some of the empty spaces show 11.5v and others less than 1v (assuming that is zero but a little for the LED). If they all read the same, I would feel better, but as some read power and others do not (just the empty), I am confused. \$\endgroup\$ Feb 14 at 6:33
  • \$\begingroup\$ My thought was maybe I ran a hot wire to the neutral bus, but I feel like that would make ALL the spaces show power? And I double-checked them. Then I thought my ground was bad, but I also feel like that would also make them all show the same voltage? \$\endgroup\$ Feb 14 at 6:36
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    \$\begingroup\$ @TommySpangler You measure voltage between two places, and I have no idea which two you are talking about. I suggest you draw a crude schematic of what you are measuring. The tools are in the editor. \$\endgroup\$ Feb 14 at 6:42
  • \$\begingroup\$ I am just going with "I have no idea how electricity works". To make this simple, I connected a single 5ft wire (double wire) to fuse #2. With no fuse, I measured 11.88v on the wire (the red lead to the red wire, the black to the black). With the fuse, I read 12.6 (matching my batteries). However, when I connected a DC outlet to the wires and measured the wires again (the wires, not the outlet), I now read like 3v. If I unplug one wire from the dc outlet, I again read almost 12v on the wire. (with no fuse). A light plugged into the outlet lights with the fuse and goes out with no fuse. \$\endgroup\$ Feb 14 at 18:19
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If the fuse panel has LEDs to indicate blown fuses, the current through those LEDs will probably cause a meter to indicate a voltage on the output side of the fuse if no other load is connected to the fuse socket.

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  • \$\begingroup\$ Yes, I understand some voltage with the LED, I did some research before asking and I do see less than 1v on some of the empty spots. However, I get almost 12v on the other empty spots. I feel if I messed up the wiring I would get the 12v on ALL the spots, the fact that I get it on some and not others makes me wonder. Why 13v on the ones with a fuse and 11.5 on the others, but still less than 1 on some? I feel if I messed up the wiring I'd get 11 on ALL of them. But I can't imagine a scenario where the box was faulty that I would get this either. I was hoping I made a common mistake. \$\endgroup\$ Feb 14 at 4:59

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