Setting Up a Calibration Lab: Best Practices for Cabling and Test Leads

Cable management has an important but often overlooked role in calibration lab setup. Cables and leads that aren’t managed or maintained correctly can introduce uncertainties that influence measurements and ultimately impact calibration accuracy.

In fact, improper lead and cable management is the leading cause of measurement errors in the laboratory. However, setting up a calibration lab using best practices for cable management helps eliminate these errors. In this article, we’ll cover how cables and test leads intersect with calibration, how to choose them, best practices for maintaining them, and more.

What’s the Role of Cabling and Test Leads in Calibration?

Test leads and cables are the electrical link between the calibrator and the unit under test (UUT). While you may not think of cables and leads as electrical instruments in a calibration lab, they can introduce variables like resistance, inductance, and capacitance, as well as dielectric absorption — all of which can impact measurements. Interference from electromagnetic fields (EMF) can also be a factor, if the cable isn’t properly shielded.

Choosing the correct leads and cables, selecting the right connectors, and even managing the cables themselves are key factors to precise calibration. An investment in the correct cables and connectors could save you hours you might spend on troubleshooting measurement errors.

How to Choose the Right Cables and Test Leads

In precision metrology, you want to make the simplest connections possible, using connectors, adaptors, or leads made of the same materials. When choosing the cables and test leads for your calibration lab setup, there are several factors to consider. Let’s review them below.

1. Choose the Connector Material

The most important aspect of choosing the connector is its metal type. You’ll want to ensure that the connector uses the same metal as your cable. That’s because using dissimilar metals can lead to problems such as thermal voltages, especially in low-level voltage DC measurements; this is also known as the Seebeck effect.

Whenever possible, choose pure copper connectors and cables to reduce uncertainties due to thermals. For best results, use low-thermal, tellurium-copper, or gold-plated connectors and avoid nickel plating.

2. Choose the Leads and Cables

While there are many types of leads and cables that metrologists use, the three most common types are patch cords, test probes, and coaxial cables. Let’s review each below so you can choose the best type of cable for your calibration lab setup.

Patch Cords

Patch cords are best suited for calibrator-to-digital-multimeter (DMM), calibrator-to-UUT connections, and transfer measurements. They typically have the same type of connector, often a banana plug, on each end. They’re easy to obtain, and you can easily stack many of them, which lets you make multiple connections if necessary.

Typically, inexpensive sets don’t have a shield, so they aren’t suitable for some precision measurements. Additionally, the metal flares inside banana plugs can loosen over time, leading to measurement errors.

Test Probes

Test probes allow for direct, precise measurements by placing the probe on a circuit board or other circuit. They also often come with many accessories, such as alligator clips, giving more connection options and making it easier to, for example, connect to hard-to-reach areas of a test device. 

However, not all test probes are created equal. Higher-quality probes often come with safety features that make them more reliable, like retractable shields and compatible clip accessories.

Coaxial Cables

Coaxial cables are easy to connect and manage, and they typically provide good shielding, which can reduce measurement errors. They come with many types of connector ends, such as N-TYPE straight plug, BNC, or SMA, and they’re a good option to use when the UUT has a coaxial connector. You can also use an adaptor so that you can use coaxial cables with banana jacks. 

Most coaxial cables use fine-wire braid as the outer conductor and have between 80% and 98% coverage. A few cables on the market use a foil shield instead of a braid to increase coverage to 100%, but these are more expensive and more susceptible to damage. 

Despite their high coverage and ease of use, coaxial cables may also have high capacitance, which can reduce signal quality. Some also have a specified bend radius, and adhering to this bend radius is crucial for ensuring you don’t damage the shielding.

3. Choose the Correct Shielding Level

Different types of cabling and cables can help avoid picking up unwanted signals in the circuit. While there is no absolute right or wrong shielding, different types of shielding are better in certain situations. Consider how much shielding you need for your application and whether the shielding is effective when considering both electrostatic and magnetic interference.

Best Practices for Cable and Lead Maintenance in Your Calibration Lab

Before you start any calibration work, you need to consider your cabling setup as part of the entire calibration lab setup. Here are key items to consider: 

• Safety: All leads and cables you use should have the correct voltage or current rating for the application.
• Category Ratings: Test leads have a category rating ranging from Category I to Category IV. Make sure to use leads with the proper rating.
• Precision: The difference between measuring with a 3.5-digit multimeter or an 8.5-digit multimeter is significant­, and the cabling requirements vary.
• Environmental Factors: What’s the environment where the leads will be used? Is it controlled? Are there outside influences such as magnetic or electrostatic fields that may impact measurement uncertainty?

Connector Care and Cleaning

Maintaining clean connectors that are in good working condition is key to taking accurate measurements, and it’s especially important when taking radio frequency (RF) or microwave measurements.

Keep the inside of the connectors clean and free from dirt and debris by using isopropyl alcohol, also known as isopropanol, to remove any dust or contamination. You can also use compressed air from an aerosol can, but avoid using factory airlines, because they may be contaminated with oil from the compressor.

Minimize Temperature Impact

While using dissimilar metals can cause thermoelectric EMF, different temperatures between leads and the UUT can also result in thermoelectric EMF. You can avoid this by waiting to take measurements until temperatures of every unit and cable match the ambient temperature of the surrounding area. You can also protect the measurement area from drafts by covering joints and terminals with thermal insulating material. Additionally, avoid handling cables close to the joints when changing connections.

Reduce Electromagnetic or Static Radiation

Radiation usually comes from something that may be present in the environment, outside of the measurement. It can include electrical noise that comes from power transmission lines or other equipment like motors, cooling fans, or air conditioning units. Communications equipment like Wi-Fi access points and even cell phones used in the lab can also produce high-level signals close to the instrumentation, leading to issues with even low-frequency DC measurements.

Shielding can mitigate these effects and eliminate radiation from the environment or the equipment. Most well-designed instruments have appropriate internal shielding, though shielding may not be present in lower-cost models. Using shielded leads can also help prevent radiation and improve measurement accuracy.

While shielding can help with measurement issues that stem from radiation or electromagnetic interference, awareness and careful handling of the environment and equipment are just as crucial.

• Make sure any mains wiring in the laboratory is routed through a grounded metal conduit.
• Shield primary power circuits with a power line ground, shield digital circuits with the digital supply common, and use local shielding such as a Faraday shield for sensitive analog circuits.
• Ensure fluorescent fixtures and LED lighting have grounded metal enclosures to reduce interference.
• Be cautious about keeping unnecessary equipment nearby during sensitive measurements.

A quick way to check for interference is to take some low-level AC measurements, such as millivolt readings. If you’re seeing unexpectedly high results, it’s possible you’re picking up interference. You can try turning off nearby equipment to see if the results improve. This may give you an indication of the source of the interference.

Magnetic Couplings and Loops

Large loops in wiring are more susceptible to picking up interference than tight loops. Keeping wires close together minimizes the voltage differential and reduces the chance of picking up interference. Coaxial cables are especially effective, because the current-carrying conductors run concentrically, canceling out electric and magnetic fields. 

A magnetic field must cut through a loop perpendicularly to induce current, so arranging leads at right angles to the magnetic field can minimize interference. Twisted pairs also help reduce loop area and are especially beneficial when working with high currents. Adding shielding around the leads, or moving them away from potential sources of interference, can also help reduce the magnetic field’s influence.

Instrument Placement or Stacking

Instrument placement, such as stacking, can also impact readings. For example, an instrument like a mains transformer can radiate magnetic fields with a frequency of 50 or 60 hertz, which can radiate through the instrument case and into another instrument. If you’re getting unusual readings, try rearranging your setup to see if there are any improvements.

Grounding

Ground loops from poor or mismatched grounding can cause measurement errors; such a setup could allow for some of the signal current to flow through the mains safety ground loop.

Here are a few tips to reduce the risk of ground loops:

• Plug the mains power of the calibrator and the UUT into the same receptacle.
• If possible, connect to another circuit any equipment that is putting large amounts of current into the safety ground plug.
• Keep system interconnections as short as possible with low-resistance cables to reduce resistive and reactive impedances. Coaxial cable is ideal, especially for RF signals.
• Never operate equipment where someone has removed the safety plug or bypassed it with a “cheater plug.” This removes protection from electric shock.

Instrument Shielding

Shielding your instrument with something like a Faraday shield ensures sensitive measurement circuits avoid problems from electrostatic coupling or electric field coupling. Using a material like mu-Metal can also shield from magnetic effects.

What Fluke Cable or Test Lead Products Are Best for Your Calibration Lab?

The best cable or test lead products will depend on the type of measurements you’re taking in the lab.

Best Cables for Measuring DC Voltage

Low-resistance applications: Use low-voltage, low-EMF cables for the best performance. Thermals that result from dissimilar metals more easily affect DC measurements. We recommend using a copper wire or spade lug securely tightened to the binding posts instead of using banana jacks.

High-resistance applications: For high-resistance applications, use a cable with very high insulation resistance and high-quality banana plug cables. Polytetrafluoroethylene coating is usually the best insulation option.

Best Cables for Measuring AC Voltage

Cable capacitance has a greater impact on AC measurements, so it’s important to choose a cable with the lowest possible capacitance. A high-quality, shielded, insulated, twisted-pair cable with a capacitance below 20 picofarad per foot works well for minimizing capacitance and preventing EMFs from affecting the measurement.

You should only use coaxial cables in moderate-accuracy situations, because coaxial cables have high capacitance.

Conclusion

Understanding the factors that test leads and connectors can introduce to the accuracy of your measurements and calibration — and mitigating those factors through cable selection and management — ensures more precise calibration and avoids many potential measurement problems.

Want to learn more about the importance of cables and test leads for calibration? Please check out these resources: