Best practices for cabling and test leads in electrical calibration

You might not think of a cable as an electrical instrument; however, leads and cabling are the foremost cause of measurement errors in the calibration lab. Here are three things to keep in mind so you can avoid unnecessary errors:

1. Cabling and leads must be suitable for the work you’re performing. Consider:
   1. Safety and category (CAT) ratings
   2. The accuracy required for your measurement
   3. The type of measurement being performed
   4. Whether the leads will be used in a controlled or non-controlled environment
2. Make the simplest connection possible with the least number of adapters.
    
3. Invest in the correct cables and connectors. You’ll save many hours of troubleshooting, and you’ll be more confident in your measurements.

Recommended cabling techniques for specific measurements

DC voltage measurements

• Use low thermal, low EMF cables for best performance.
• Low voltage dc measurements are affected more adversely than ac measurements by dissimilar metals that can produce thermal EMFs..
• DC measurements are the least error prone. With good cables and connections, there are not many other error sources to consider.

Low resistance measurements

• Cable/connector resistance should be low and constant.
• Use a copper wire or spade lug securely tightened to the binding post.
• Using four-terminal techniques minimizes the effects of cable and connector lead and contact resistance.

High resistance measurements

• It is common to use banana jack connections. In fact, most high resistance workload consists of DMMs with only banana jack connections.
• Shielding and insulation are key to error-free measurements. The cable must have high insulation resistance.
• Never use low-voltage wire coated with other materials for high-resistance applications.
• Keep the environment controlled and don’t touch the wire during the measurement.
• Give the measurement time to stabilize.
• Don’t forget about dielectric absorption, settling and effects, and voltage memory.

4-wire Ohms measurements

• 4-wire techniques are used to minimize the effects of cable and connector lead and contact resistance.
• Adding a set of wires to sense the voltage drop across the unknown resistance (in which no current flows) eliminates the effect of cable resistance.
• For highest accuracy, use a 4-wire measurement with lower value resistances when possible.
   

2-wire Ohms measurements

• The measurement leads are not perfect conductors and have some series resistance of their own. By driving the current through the measurement leads, we see not only the voltage drop across the load, but also the voltage drop for each lead. Thus, we end up measuring the combined resistance of the positive lead, the load?, and the negative lead.
• If we use four leads, the source current and the measurement can be separated. The meter terminals are called Source for the current supply and Sense for the voltage input.

Want to learn more?

Download this application note:
How cables and connectors impact measurement uncertainty »

Watch this on-demand web seminar:
Electrical Calibration - Best Practices for Cabling and Test Leads »