Test leads are more than a means to connect your digital multimeter (DMM) to the unit under test—they’re an integral part of the complete measurement system.
Poorly made, worn, or under-rated test leads can cause inaccurate readings and may pose a serious shock or electrocution hazard if you touch live wires that the meter has read as being de-energised.
Using the correct test probe for the application is important. If your work is limited to circuits below 30 volts, performance and quality remain important but issues such as shock hazard concerns are reduced.
However, if you use the leads for measuring medium to high voltage, or high energy circuits such as distribution systems (240 volts to 600 volts), using leads that are in good condition with the proper markings and ratings is critical. Always take precautionary steps to verify the condition and rating of your leads before testing.