Monday, June 10, 2024

Measuring Resistance of Fuses with a Milliohm Meter and Kelvin Clips -1- ( Car fuses )

In the world of electronics, ensuring the integrity of fuses is crucial for the safety and reliability of circuits. Fuses act as protective devices, preventing excessive current from damaging components. Over time, however, fuses can degrade, leading to increased resistance, which may compromise their effectiveness. Today, I'll share my experience measuring the resistance of some 12 Volt car fuses using an affordable milli-ohm meter equipped with Kelvin clips. This method provides accurate results, essential for maintaining optimal circuit performance.

Why Measure Fuse Resistance?

Measuring the resistance of fuses serves several purposes:

1. Ensuring Reliability: 

Over time, fuses can develop higher resistance due to aging or exposure to harsh conditions. Regular checks ensure they are still effective.

2. Troubleshooting: 

Identifying a fuse with abnormal resistance can help diagnose and prevent circuit issues.

3. Preventive Maintenance:

Regular measurements allow for the replacement of fuses before they fail, avoiding potential downtime or damage.

Equipment Needed

For this task, I used the following equipment:

- Milliohm Meter: 

A device capable of measuring low resistance values accurately. I used a YMC01 2R handheld Milliohm Meter. For details see my previous blogpost. According to the manual this meter uses a testing current of about 100 mA (i did not check this),

- Kelvin Clips:

 Four-wire probes that eliminate the influence of lead and contact resistance, providing precise measurements. These where included with my meter.

- Fuses:

 For the first test i used some some new 12 Volt Car fuses.

Also i tested a fuse in a fuse holder.
Below a table of the color codes of the car fuses.

The Measurement Process

Here’s a step-by-step guide to measuring fuse resistance with a milliohm meter and Kelvin clips:

1. Calibration

Before starting, it’s essential to calibrate the milliohm meter. Follow these steps:

- Turn on the meter and allow it to stabilize.

- Use the provided shorting clip to connect the Kelvin clips together.

- Zero the meter according to the manufacturer’s instructions. This step ensures that the meter reads zero when there’s no resistance. My cheap meter does not have a zeroing function. H

2. Connecting Kelvin Clips

Attach the Kelvin clips to the fuse. Ensure a solid, clean connection:

- Place the clips at opposite ends of the fuse to measure the resistance accurately.

- Make sure there is no dirt or oxidation on the fuse terminals that could affect the reading.

3. Taking the Measurement

With the Kelvin clips properly connected, take the reading:

- Power on the meter.

- Read the displayed resistance value. The milliohm meter should provide a precise measurement free from lead resistance thanks to the Kelvin clips.

Voltage Drop Across the Fuse

For your milliohm meter with a 0-1 ohm range and a measurement current of 100 mA, you can calculate the voltage drop across the fuse using Ohm's Law (U = I × R).

Example Calculation:

1. Voltage drop at 100 mA : 

   - 100 mA = 0.1 Ampere

   - Measured Resistance: 0.004 ohms

   - Voltage Drop: U = 0.1 A × 0.004 ohms = 0.0004 volts (0.4 mV)

2. Voltage drop at current rate:

   - Fuse rating (e.g. 15 Ampere )

   - Measured Resistance: 0.004 ohms

   - Voltage Drop: U = 15 A × 0.004 ohms = 0.060 volts (30 mV)

2. Relative Voltage drop at current rate:

A voltage drop of 0.060 volts is at 12 Volt the relative voltage drop of 100% * 0.060/12 = 0.50 % 

Change in Resistance Due to Heating

When a fuse is subjected to higher currents, it can get hot, causing its resistance to change. This phenomenon is due to the temperature coefficient of resistance, which describes how the resistance of a material changes with temperature. For most metals, resistance increases as temperature increases.

Key Points:

1. Initial Resistance:

 The resistance of a fuse at room temperature is relatively low.

2. Heating Effect: 

As current flows through the fuse, it heats up. The amount of heat generated is proportional to the square of the current (P = I²R).

3. Increased Resistance: 

As the temperature of the fuse rises, so does its resistance. This change can be significant, especially for currents near the fuse’s rated limit.


If a fuse rated for 1A is subjected to a current close to its limit, it will heat up. The resistance can increase noticeably, potentially doubling or tripling depending on the fuse material and the current applied.

My Measurement Results

For this first test a set of 12 Volt Car fuses (see pictures above) and a 10A fuse with a fuse holder was used.

Table with Results:

Fuse Rated Current Measured ResistanceCalculated
Voltage Drop (100 mA) 
Voltage Drop at rated current
Calculated Voltage drop at rated current in % relative to 12 Volt
10A (without fuse holder)10   A 0.0080.00080.080.67 %
10A fuse in fuse holder10   A 0.0070.00070.070.58 %
Brown 7.5 A 0.0090.00090.06750.56 %
Red 10   A 0.0060.00060.060.50 %
Blue15   A 0.0040.00040.060.50 %
Yellow20   A 0.0030.00030.060.50 %
Clear 25   A 0.0020.00020.050.42 %
Green 30   A 0.0020.00020.060.50 %

Interpreting the Results

- Expected Values:

 Typically, a good fuse should have a very low resistance, often just a few milliohms.

- Anomalies: 

If a fuse shows significantly higher resistance than expected, it might be compromised and should be replaced.


These resistances are measured with the Milliohm meter. I expect all with around the same current of 100 mA. A higher current will heat up the fuse wire and change the resistance and to get real good conclusions other measurements will be needed. 

The measured resistances of these car fuses where very low (below 0.01 milliohm).

I only had two red 10A fuses (and for the other fuses only one) I measured a resistance of 0.0008 and 0.0006 for these fuses (and 0.0007 ohm for the first red fuse using the fuse holder),

The fuse holder did not add a significant resistance.

The lower rated fuses seem to have a slight higher resistance.

Measuring fuse resistance with a milliohm meter and Kelvin clips is a straightforward yet powerful method to ensure the health of your circuit protection devices. Regular checks can help you catch potential issues early, ensuring your electronics remain safe and functional. By investing in this affordable yet accurate setup, you can maintain the integrity of your circuits and prevent unexpected failures.

Stay tuned for more insights and results from my ongoing experiments and measurements. Happy measuring!

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