We have been using NI PXI DMMs for some time to measure small resistance values (typically under 30ohms). We of course use 4-wire measurements to eliminate lead resistance and we also ensure that the lead resistance is less than the specified 10% of the range.
When looking at the latest NI-DMM manual (http://zone.ni.com/reference/en-XX/help/370384V-01/dmm/4-wire/ ) I noticed that it indicates that if the Maximum 4-wire lead resistance is exceeded (10% of range) there is a potential of damage to the DMM (specifically the current source used to provide the 1mA test current in our scenario). While it is clear that the voltage drop over the test leads would cause a problem, it raises a few other questions.
If I was measuring a slightly larger resistance in 4-wire mode (say 120 ohms in the 100ohm range) could that damage an NI DMM? Our scenario is an automated test specifically looking to weed out bad product - I have no guarantees that bad product won't exceed the range.
Given the same current source is used for 2-wire measurements, does that mean that a large resistance greater than the specified range (say 120 ohms measured in the 100ohm range) could damage an NI DMM? If it does I would find this odd as the specification is listed as "Maximum 4-wire lead resistance" with no mention of 2-wire or otherwise.
The "Maximum 4-wire lead resistance" specification is not specific to NI. I see that other DMM manufacturers list the same specification but the only reference from them I have found for exceeding this is that it impacts the stated accuracies. Why are the NI DMMs different in this regard?
I have asked these questions through the appropriate channels but am interested to see if anyone in the community could help shed some light on this.