Understanding Fuses: How They Work, Why They Blow, and How to Test Them
Hey guys, welcome back to the shop! Today we’re diving into some good old-fashioned shop talk — specifically about fuses. We’ll cover what causes a fuse to blow, how to properly check them, and the difference between an overload and a blown fuse — which many people often confuse.
What Does a Fuse Panel Look Like?
Behind me is an old fuse panel used for demonstration purposes. You might still find these in older homes. They house a variety of fuses, and it’s important to understand how to read whether one is blown or simply popped.
A common issue we see is when people say, “I checked all my fuses and they don’t look blown.” Well, just because they don’t look blown doesn’t mean they’re still working. In fact, a fuse could have popped due to an overload — and you wouldn’t know by appearance alone.
What Happens Inside a Fuse?
Let’s look closely. Inside each fuse, there’s a small element. That element is designed to break if more current flows through it than it’s rated for — say, over 20 amps on a 20A fuse.
Now here’s the tricky part:
- If a fuse blows from an overload, the internal element simply pops.
- This does not blacken the front of the fuse.
- It can be almost impossible to tell by looking.
But if the fuse blows due to a dead short (a hard electrical fault), the front will typically turn black. That’s the visual clue everyone knows.
So the visual signs of a blown fuse only show up in short circuits — not overloads.
The Right Way to Check a Fuse
The only reliable way to test a fuse is with a multimeter — not just a voltage tester or circuit tracer.
What you need:
- A multimeter with ohms or continuity (beep) mode
- Two probe leads
We’ll link some good, affordable multimeters in the description below.
How to test:
- Set your multimeter to the continuity or ohms setting (look for the squiggly line).
- Touch the two leads together — you should hear a beep. This confirms the meter is working.
- Now place one lead on the copper side of the fuse and the other on the center tip (like testing a lightbulb).
If you hear a beep — the fuse is good.
If not — the fuse is blown.
This applies to standard screw-in fuses and cylinder-style fuses used in larger appliances like stoves or dryers.
Testing Cylinder Fuses
For these longer, cartridge-style fuses:
- Simply touch each end with your multimeter leads.
- A beep = good. No beep = blown.
Final Thoughts
We know fuse panels are becoming a thing of the past — most modern electrical systems use breakers now. But fuse panels are still out there, especially in older homes, and you’ll need to know how to test them.
So keep a good multimeter or fuse tester on hand. Otherwise, you might end up with a pile of fuses that look fine but are actually bad.
Thanks for watching!
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