How do you know that fuse is protecting your circuit?

How do you know that this fuse will protect your rig?

How do you know that this fuse will protect your rig?

I blog for a leading manufacturer of circuit protection devices, so I keep my eye out for articles on fuses, ESD diodes and the like. Recently, I came across the article, “Fuse selection factors critical to circuit design.” Among the factors discussed is:

11. Application testing/verification prior to production.
Request samples for testing in the actual circuit to verify the selection. Before evaluating the samples, make sure the fuse is properly mounted with good electrical connections, using adequately sized wires or traces. The testing should include life tests under normal conditions and overload tests under fault conditions to ensure that the fuse will operate properly in the circuit.

Being a former test engineer, this got me to thinking about how one would actually do this. For example, how would you test that a fuse will actually protect a circuit board? Would you inject faults, i.e. deliberately short-circuit nodes? If so, which ones?

What measurements would you make to ensure that the fuse was working as you hope? Would you measure the time elapsed between the time you injected the fault until the time the fused actually blew? How about measuring the current profile over that period of time? That might be important and/or tell you something about the failure.

What kind of fault analysis should you perform after the fuse has blown? I suppose at the very least you’d want to replace the fuse and ensure that the circuit is functioning again. I would say that you should also run a full performance test to ensure that the fault didn’t adversely affect the board’s performance. Also, I’d think that you’d want to visually inspect the board to ensure that the fault current didn’t damage the board or traces at all.

I’m curious if any of you have had any experience with this kind of development testing. If so, please e-mail me or comment here.


  1. Dave, N8SBE says:

    Automotive electrical testing would require the above and much more, including so-called “four corners” testing for temperature, humidity, and even altitude (electronics that need to dissipate heat are derated for high altitude). Thermal shock would be included, as well, where a component is allowed to ‘soak’ at a given high or low temperature, then moved quickly into the opposite environment.

    Then, there’s the electrical ‘dips ‘n drops’ testing, where you find out if the power-on-reset circuit you gave to your newbie engineer to design actually holds up in real world environments, where the voltage may bounce around during engine cranks or ‘load dumps’, or may be subject to momentary interruptions as the ignition key is cycled, or from a flaky connector when you go bouncing over a railroad crossing.

    If I had a nickel for every poorly designed power-on-reset circuit (or cases where the designer just copied the sample given in the chip manufacturer’s application note), I’d be filthy rich by now.

  2. Sometimes the goal of the fuse is not to protect the circuit, but the house in which it is installed. The primary goal of some fuses is to protect the wiring / home by limiting the chances of fire. In these cases, the circuit under ‘protection’ might well be seriously damaged by the time the fuse blows. A well designed fault detection / crow-bar can limit the damage to the ‘well designed’ locations, but not always the case.

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