Checking "amps" on a fuse can mean a few different things: finding the fuse's designed amperage rating, checking if the fuse is blown (which prevents current flow), or measuring the actual current flowing through the fuse in a circuit. Each requires a different approach.
Finding the Fuse's Amp Rating (Visual Inspection)
The most straightforward way to check the "amps" a fuse is designed for is to simply read the label on the fuse itself. The amperage rating is a crucial specification indicating the maximum current the fuse can safely carry before it blows and interrupts the circuit.
- Look for Markings: Fuse ratings are typically printed, stamped, or color-coded directly onto the body or end cap of the fuse.
- Common Units: The rating will usually be followed by 'A' for amperes or 'mA' for milliamperes (1000 mA = 1 A).
- Examples: You might see markings like "10A", "500mA", "F2A" (Fast-acting 2 Amps), or similar. Automotive fuses often have color codes corresponding to specific ampere ratings, though the rating is usually also embossed on the plastic.
Always replace a blown fuse with a new one of the exact same amperage rating and type (e.g., fast-acting, slow-blow).
Checking if a Fuse is Blown (Continuity Test)
While not measuring "amps" directly, checking if a fuse is blown is a common way to "check" its functionality related to current flow (a blown fuse stops flow). This is typically done using a multimeter set to measure continuity or resistance.
- Ensure Safety: Turn off the power to the circuit containing the fuse.
- Access the Fuse: Carefully remove the fuse from its holder.
- Set Multimeter: Set your multimeter to the continuity setting (often indicated by a symbol that looks like sound waves or a diode symbol) or the resistance setting (Ω).
- Test the Fuse: Touch the multimeter probes to the two metal contact points (terminals) of the fuse.
- Interpret the Reading:
- Good Fuse: The multimeter should indicate continuity (usually with a beep) and/or show a very low resistance reading (close to 0 ohms).
- Blown Fuse: The multimeter will show no continuity (no beep) and/or display a very high resistance or "OL" (Over Limit/Open Loop), indicating a broken internal element.
Here's a quick summary:
| Test Result | Fuse Condition | Multimeter Reading (Continuity) | Multimeter Reading (Resistance) |
|---|---|---|---|
| Beep or Solid Tone | Good | Continuity Indicated | Very Low (near 0 Ω) |
| No Beep | Blown | No Continuity | Very High or OL (Open Loop) |
Measuring Current Flow Through a Fuse (Using a Multimeter)
To measure the actual current (in amps) currently flowing through a fuse in a live circuit, you need to use a multimeter set to measure current. This method determines the amperage load the circuit is drawing, not the fuse's rating itself or if it's blown (unless the current exceeds the rating and the fuse blows during the test).
Measuring current requires placing the multimeter in series with the circuit, meaning the current must flow through the meter. This is different from testing voltage or resistance, which is done in parallel or across a component.
According to one method for testing, to measure current flowing through a fuse, you need to set the multimeter to measure current and select the appropriate range, such as 10A or 200mA. This involves safely interrupting the circuit where the fuse is located and placing the multimeter in line so the current flows through the meter. Following this method, you then need to connect the red probe to the positive connection point in the circuit where the fuse belongs and the black probe to the negative connection point, effectively substituting the fuse with the multimeter.
Important Safety Precautions:
- Always turn off power before connecting or disconnecting a multimeter when measuring current.
- Set the multimeter to the highest current range first (e.g., 10A or 20A) to avoid blowing the meter's internal fuse if the current draw is higher than expected. You can switch to a lower range if needed after confirming the approximate current.
- Ensure the probes are in the correct meter jacks for current measurement (usually labeled "A" or "mA"). Using the wrong jacks or setting while measuring current can damage the meter or the circuit.
- Measuring current can be hazardous. If you are not comfortable with this process, consult a qualified professional.
