Measuring Resistance of Fuses with a Milliohm Meter and Kelvin Clips -2- (250V Fuses measurement results )

Ensuring the integrity and reliability of 250V fuses is crucial for maintaining the safety and functionality of high-voltage electrical systems. In this post, I'll share my experience measuring the resistance of 250V fuses using an affordable milliohm meter equipped with Kelvin clips, highlighting the key considerations and benefits of this method.

Why Measure Fuse Resistance?

Regularly measuring the resistance of 250V fuses is essential for several reasons:

1. Ensuring Reliability:

Over time, fuses can develop higher resistance due to aging or exposure to harsh conditions.

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.

Tools Needed

For this task, I used:

- Milliohm Meter:

 Capable of measuring low resistance values accurately.

- Kelvin Clips: 

Four-wire probes that eliminate the influence of lead and contact resistance, providing precise measurements.

The Measurement Process

Here's a quick overview of the process:

1. Calibration:

Ensure the milliohm meter is calibrated using the shorting clip and zeroing function.

2. Connecting Kelvin Clips: 

Attach the clips to the fuse, ensuring a solid, clean connection at opposite ends.

3. Taking the Measurement: 

Power on the meter and read the displayed resistance value.

Voltage Drop Across the Fuse

For a milliohm meter with a measurement current of 100 mA, the voltage drop across the fuse can be calculated using Ohm's Law (V = I × R).

Example Calculation:

- For a measured resistance of 0.005 ohms: 

  - Voltage Drop: V = 100 mA × 0.005 ohms = 0.5 mV

- For a measured resistance of 0.002 ohms: 

  - Voltage Drop: V = 100 mA × 0.002 ohms = 0.2 mV

Change in Resistance Due to Heating

When a fuse is subjected to higher currents, it can heat up, causing its resistance to change:

- Initial Resistance:

The resistance at room temperature is relatively low.

- Heating Effect:

Higher current flow increases temperature, raising the resistance.

- Increased Resistance:

This change can be significant, especially for currents near the fuse’s rated limit.

Measured resistance

I having a long time a box with slow 250V glass fuses with values 50 mA, 160 mA, 200 mA, 315 mA, 500 mA, 800 mA, 1 A, 2 A, 2.5 A. It would be nice to use these fuses as a test-set. 

My box with fuses (open)

I first did think it was needed to measure each fuse several times. However during my first (not documented) test i discovered that remeasuring a fuse id almost gave no variation in the result. (max 1 or 2 in the last digit).  The variation between different fuses of the same rating was much bigger. Therefore i decided to measure all the fuses once and put the results in a table (1).

However there were some limitations: 

- According to the manual, the resistance is measured with a testing current of about 100 mA. ( i did not check this yet.) Therefore i decided to skip to 50 mA fuses as they would be blown an not measured.

- The fuses are relative old. I expect at least 25 years. The nice thing is that it is a set of in total more than 140 fuses of different values and the aging effect of the fuses is included.

- The box with fuses had at (least) two sources. A box with fuses that i did buy many years ago and a similar box with fuses that i received some year ago from someone else. I did merge the content of these boxes a year ago to save some space.

- I noticed some fuses had a resistance far above 1 ohm. (And i expect some of these fuses where bad. I measured with a multi-meter around 200 Ohm and some above 10 Mega Ohm and even some fuses that made no connection at all. I removed these fuses from the box with fuses, kept them apart and did not include the results. I expect the fuses that made no connection at all are blown fuses and i did throw them away.  

- All my 2.5 A fuses had corrosion and the measured resistances where above 1 Ohm. I don't know if this is normal or this was due to aging and/or the corrosion. The strange thing is that all the 2.5 A fuses that had visible corrosion and almost none of the other fuses.  I don't know the source of the corrosion. Perhaps the fuses are of a different material or there was humidity that mainly reached the 2.5 A fuses and not the other fuses. Therefore i excluded all of the 2.5 A fuses.

Results

Below a table with the measured resistance , the calculated voltage drop at 100mA and the voltage drop at the rated current.

Table 1: Measured resistances with calculated voltage drops

Result /  measurement Number	mA	Measured  resistance	Calculated Voltage drop at 100mA	Calculated Voltage drop at rated current
1	500	0.447	0.04470	2.235
2	500	0.477	0.04770	2.385
3	500	0.446	0.04460	2.230
4	500	0.434	0.04340	2.170
5	500	0.520	0.05200	2.600
6	500	0.461	0.04610	2.305
7	500	0.510	0.05100	2.550
8	500	0.536	0.05360	2.680
9	500	0.462	0.04620	2.310
10	500	0.432	0.04320	2.160
11	500	0.502	0.05020	2.510
12	500	0.482	0.04820	2.410
13	500	0.452	0.04520	2.260
14	500	0.465	0.04650	2.325
15	500	0.424	0.04240	2.120
16	500	0.456	0.04560	2.280
17	500	0.469	0.04690	2.345
18	500	0.461	0.04610	2.305
19	160	0.579	0.05790	0.926
20	160	0.570	0.05700	0.912
21	160	0.537	0.05370	0.859
22	160	0.640	0.06400	1.024
23	160	0.409	0.04090	0.654
24	160	0.658	0.06580	1.053
25	160	0.664	0.06640	1.062
26	160	0.531	0.05310	0.850
27	160	0.613	0.06130	0.981
28	160	0.568	0.05680	0.909
29	160	0.677	0.06770	1.083
30	160	0.581	0.05810	0.930
31	160	0.604	0.06040	0.966
32	160	0.619	0.06190	0.990
33	160	0.494	0.04940	0.790
34	160	0.615	0.06150	0.984
35	160	0.689	0.06890	1.102
36	160	0.557	0.05570	0.891
37	160	0.409	0.04090	0.654
38	160	2.900	0.29000	4.640
39	1000	0.088	0.00880	0.880
40	1000	0.089	0.00890	0.890
41	1000	0.092	0.00920	0.920
42	1000	0.087	0.00870	0.870
43	1000	0.089	0.00890	0.890
44	1000	0.089	0.00890	0.890
45	1000	0.087	0.00870	0.870
46	1000	0.089	0.00890	0.890
47	1000	0.087	0.00870	0.870
48	1000	0.094	0.00940	0.940
49	1000	0.091	0.00910	0.910
50	1000	0.096	0.00960	0.960
51	1000	0.089	0.00890	0.890
52	1000	0.087	0.00870	0.870
53	1000	0.089	0.00890	0.890
54	1000	0.092	0.00920	0.920
55	1000	0.091	0.00910	0.910
56	1000	0.090	0.00900	0.900
57	1000	0.090	0.00900	0.900
58	315	0.700	0.07000	2.205
59	315	0.562	0.05620	1.770
60	315	0.761	0.07610	2.397
61	315	0.706	0.07060	2.224
62	315	0.852	0.08520	2.684
63	315	0.805	0.08050	2.536
64	315	0.607	0.06070	1.912
65	315	0.723	0.07230	2.277
66	315	0.681	0.06810	2.145
67	315	0.612	0.06120	1.928
68	315	0.646	0.06460	2.035
69	315	0.884	0.08840	2.785
70	315	0.760	0.07600	2.394
71	315	0.719	0.07190	2.265
72	315	0.871	0.08710	2.744
73	315	0.766	0.07660	2.413
74	315	0.681	0.06810	2.145
75	315	0.620	0.06200	1.953
76	315	0.759	0.07590	2.391
77	315	0.602	0.06020	1.896
78	2000	0.039	0.00390	0.780
79	2000	0.039	0.00390	0.780
80	2000	0.039	0.00390	0.780
81	2000	0.042	0.00420	0.840
82	2000	0.038	0.00380	0.760
83	2000	0.037	0.00370	0.740
84	2000	0.035	0.00350	0.700
85	2000	0.703	0.07030	14.060
86	200	0.331	0.03310	0.662
87	200	0.485	0.04850	0.970
88	200	0.545	0.05450	1.090
89	200	0.405	0.04050	0.810
90	200	0.486	0.04860	0.972
91	200	0.584	0.05840	1.168
92	200	0.490	0.04900	0.980
93	200	0.573	0.05730	1.146
94	200	0.472	0.04720	0.944
95	200	0.442	0.04420	0.884
96	200	0.446	0.04460	0.892
97	200	0.508	0.05080	1.016
98	200	0.515	0.05150	1.030
99	200	0.429	0.04290	0.858
100	200	0.489	0.04890	0.978
101	200	0.395	0.03950	0.790
102	200	0.491	0.04910	0.982
103	250	0.268	0.02680	0.670
104	250	0.517	0.05170	1.293
105	250	0.382	0.03820	0.955
106	250	0.388	0.03880	0.970
107	250	0.385	0.03850	0.963
108	250	0.340	0.03400	0.850
109	250	0.348	0.03480	0.870
110	250	0.366	0.03660	0.915
111	250	0.249	0.02490	0.623
112	250	0.322	0.03220	0.805
113	250	0.338	0.03380	0.845
114	250	0.405	0.04050	1.013
115	250	0.263	0.02630	0.658
116	250	0.361	0.03610	0.903
117	250	0.350	0.03500	0.875
118	250	0.247	0.02470	0.618
119	250	0.349	0.03490	0.873
120	250	0.322	0.03220	0.805
121	250	0.342	0.03420	0.855
122	250	0.371	0.03710	0.928
123	250	0.292	0.02920	0.730
124	800	0.151	0.01510	1.208
125	800	0.136	0.01360	1.088
126	800	0.139	0.01390	1.112
127	800	0.127	0.01270	1.016
128	800	0.133	0.01330	1.064
129	800	0.131	0.01310	1.048
130	800	0.141	0.01410	1.128
131	800	0.128	0.01280	1.024
132	800	0.128	0.01280	1.024
133	800	0.133	0.01330	1.064
134	800	0.136	0.01360	1.088
135	800	0.143	0.01430	1.144
136	800	0.127	0.01270	1.016
137	800	0.128	0.01280	1.024
138	800	0.135	0.01350	1.080
139	800	0.136	0.01360	1.088
140	800	0.132	0.01320	1.056
141	800	0.147	0.01470	1.176
142	800	0.132	0.01320	1.056

Some of the fuses gave results that made me doubt if the fuse had indeed the current rating i expected. However i did check the rating it on the fuse! 

Table 2: Summary of measured resistances  (Some outliers are removed.)

Fuse rating (mA)	Average resistance	Min resistance	Max resistance
160	0.5797	0.409	0.689
200	0.4756	0.331	0.584
250	0.3431	0.247	0.517
315	0.7159	0.562	0.884
500	0.4687	0.424	0.536
800	0.1349	0.127	0.151
1000	0.0898	0.087	0.096
2000	0.0384	0.035	0.042

 

Fuses with a higher rating seem to have a lower resistance, however even in this box with fuses not always!  I decided to already publish the measured results with a quick summary. In an upcoming blog-post i will do a more in depth evaluation of the results.

My box with fuses (closed)

Conclusion

Measuring the resistance of 250V fuses with a milliohm meter and Kelvin clips is a straightforward yet powerful method to ensure the health of your high-voltage circuit protection devices. Regular checks can help you catch potential issues early, maintaining the integrity and safety of your electrical systems. It was also a good exercise to measure these fuses to check the health and variation between these fuses. This affordable and accurate setup is a valuable addition to any toolkit, providing peace of mind and reliable performance.

Stay tuned for more insights and results from my ongoing experiments and measurements and a more in-depth evaluation of these measured results. Happy measuring!