Loaded Measurement
This is a measurement taken while the real operating current is flowing, and the majority of electrical faults only become visible under that condition. In an unloaded circuit the supply voltage reads normal, but when the load is connected a high-resistance connection drops the voltage noticeably.
The resistance of a connection only shows itself when current passes through it. The voltage range of a multimeter draws current in the microampere region; a value measured with that current says nothing about what happens at a real 20 A load.
- What changes: a system reading 12.6 V unloaded can drop to 10 V at the device terminals under load. The difference lies in the total resistance of the cable, fuse, connector and chassis connections.
- How it is done: run the load and take the measurement at the device's own supply terminals, not at the battery post. The difference between the two measurements gives the total loss on the line.
- Records: if possible use an instrument that holds the minimum value or logs; short collapses at the moment of starting are not caught by eye.
- Safety: the measurement is made on an energised system; take care that the probe tips do not touch a neighbouring terminal.
On crane, lift and driver installations, the description "it works unloaded, it stops under load" is almost always line-related. Measuring under load shortens diagnosis time.
The context in which this term is used: DC motor and driver fault diagnosis matrix