The Importance of Fuses in Handheld Insulation Multimeters
Fuses should always be checked before making an insulation test on Fluke 1577 and 1587/FC insulation testers.
What can go wrong?
Unlike typical multimeters (where the fuse is only part of the circuit during an amperage measurement), the fuses in the Fluke handheld insulation multimeters are also part of the circuit made during an insulation test.
If the fuse is blown or missing, the meter will not give accurate results. A wire under test that would otherwise fail can appear to have very high resistance. Without keeping a sharp eye open, it is easy for this wire to seemingly pass the test and continue to be used when it should not.
We recommend checking to see if you have a blown fuse before you begin testing as part of your standard procedure.
Expected behavior with and without a fuse.
When the circuit is open (for example: with the test leads unconnected to anything or unplugged entirely) the meter will push the full test voltage, and the resistance measurement will reach it's maximum possible value.
In the below example, the meter pushes 1049 V DC and maxes out its resistance measurement at 2.2 GΩ.
If the meter detects a short (either in the wiring or if the test leads are shorted together on purpose), the meter stops its test and does not push the full voltage.
In the below example, the meter pushes only 1 V DC and shows 0.0 MΩ, indicating that the test is showing a direct short.
However, if the fuse is blown or missing, the meter cannot tell the difference between a short circuit and an open circuit.
In the below example, the fuse is removed, and the test leads are shorted together. However, instead of the meter noticing the short, it behaves as if the circuit is open: it pushes the full voltage and reaches 2.2 GΩ.
While a wire reaching the maximum resistance value could be suspicious, it is not obvious that the test is giving incorrect results unless the user knows to look for it. It is always safest to double check if a fuse is in working condition before beginning any set of insulation tests and should also be checked if the resistance reaches the setting's maximum value (see the Expected Maximum Resistance On an Open Circuit table below).
How to Test for a Blown/Missing Fuse.
Take one test lead and insert it into the V/Ω terminal. Set the meter to the Ω (Resistance) dial position and ensure the meter is in Auto Range.
Take the probe tip and insert it into the mA terminal. If the display reading is OL, the fuse is bad and should be replaced.
Expected Maximum Resistance On an Open Circuit
Replacement Fuse
Fluke P/N 943121 or P/N 203411 - 440 mA, 1000V, Fast Release