To Float or Not to Float? Unveiling the Dilemma in Electrical Testing
In electrical and electronic testing, precision and reliability reign supreme. As engineers and technicians develop increasingly complex circuitry and connectivity designs, they encounter a critical decision point: fixed or floating earth? This blog article explores the nuances, advantages and applications of both, helping you decide whether to use fixed or floating earth systems.
This article was written by Daryl Manley, Principal Systems Engineer, MK Test Systems.
Understanding fixed earth systems
Most circuits boast a local ground tethered to electrical earth, forming the backbone of electrical connectivity, stability and safety.
A fixed earth measurement system anchors internal measurement sources and electronics to electrical earth, allow measurements to be performed on a unit under test (UUT) provided it is not itself connected to electrical earth. For instance, testing a cable harness on an insulated workbench solely connected to the test equipment exemplifies this setup.
The example diagram below shows a simple measurement system to confirm electrical connection of a wire (UUT) using a 5Vdc power supply referenced to earth and voltmeter. The voltmeter will confirm UUT connection by measuring 5Vdc.
Yet when the UUT is connected to earth as shown below, both the power supply and voltmeter face an effective “short” via earth, disrupting the intended test. The voltmeter will measure close to 0Vdc, and the expected test is no longer possible if the UUT is connected to earth.
Introducing a floating earth system
The solution lies in “floating” the power supply, detaching it from earth reference. This allows the UUT to be grounded without affecting the test’s integrity, enabling the voltmeter to accurately measure 5Vdc.
It should be noted however in this scenario the power supply 5Vdc output is “tied” and at the same potential as Earth. Therefore the supply 0Vdc terminal will be at -5Vdc with respect to earth.
Addressing concerns with floating earth systems
It’s crucial to note that in floating earth measurement systems, it is not desirable for the potential difference between local earth and earth to become too great, or it risks overcoming the isolation. In other words, the potential difference between earth and the local ground stresses the isolation between power supply output and earth, particularly in mains-powered systems.
This therefore requires additional protection devices to prevent isolation breaches. Protection devices are fitted between local earth and earth to prevent this – typically 100Vdc. This precaution prevents the potential difference between local earth and earth from becoming excessive, thus averting the risk of isolation failure.
High Voltage Testing Considerations
During high-voltage testing in floating earth systems, protection devices play a critical role in limiting potential differences between local earth and earth. While low-voltage tests (<50Vdc) can be safely conducted, high-voltage tests, like insulation or hipot tests, pose risks if the HV output is connected to an earthed UUT.
Therefore, it’s imperative to perform HV tests only from non-earth connections to earthed ones, ensuring the integrity of the testing process.
Choosing between a fixed or floating earth system
As shown in this article, the decision between using fixed or floating earth systems hinges on the ability to isolate the UUT from earth, balancing practicality and safety concerns.
It is only possible to use a fixed earth test system if the UUT can be completely electrically isolated from earth. If it is not possible to isolate the UUT from earth whether for practical or safety reasons – such as testing the wiring within a train which is earthed via the rails – a “floating earth” system must be used.
MK Test’s team are able to advise on the best system for your testing scenario and requirements. For a no-obligation chat about how we can help, please contact us directly or via your nearest MK location.
Further Reading
For other useful training and informative articles, please browse our blog. Interestingly, this IET article asserts the term ‘floating’ is technically undefined, so is a meaningless expression….a somewhat philosophical answer to this topic!
This article was written by Daryl Manley, Principal Systems Engineer, MK Test Systems.