If you work with building automation systems, industrial controls, or electrical testing, you’ve likely run into the term “floating output.” A floating output is a circuit that’s not directly tied to ground or another reference point.
While floating outputs offer flexibility and isolation, they can also be tricky to measure correctly. In this blog, we’ll explain what floating outputs are, why they’re challenging to measure, and the best practices you can follow to get accurate and safe readings.
All Functional Devices' alternating current (AC) power supplies, stepdown transformers, and isolation transformers have floating outputs, isolated from the input and without internal connection. Our engineers design with safety and flexibility in mind. Feel free to contact us for live help when measuring floating output voltages.
Understanding Floating Outputs
A floating output is an electrical output with no direct connection to earth ground or the system’s common. Instead, it “floats” relative to its surroundings, allowing it to assume whatever potential differences develop in the circuit. Common examples include isolated power supplies, transformer secondaries, and certain sensor outputs used in building automation hardware.
Floating outputs have several advantages including:
- Help prevent ground loops
- Reduce noise
- Improve safety by isolating sections of a system.
Because they aren’t tied to a fixed reference point, however, their voltage measurements can be misleading if taken incorrectly. Understanding how these outputs behave is the first step toward measuring them properly.
Challenges of Measuring Floating Outputs
Measuring a floating output isn’t as straightforward as attaching your multimeter leads and reading the display. Floating circuits can produce readings that vary dramatically depending on the measuring instrument’s impedance, how the test leads are connected, and the surrounding environment. There are two major challenges when measuring floating outputs: point ambiguity and safety.
First, point ambiguity: Because the output isn’t tied to ground, your meter may pick up stray voltages or “phantom” readings due to capacitive coupling. This can result in voltage measurements that appear to fluctuate or seem impossibly high or low.
Second, safety: Connecting a floating circuit incorrectly may inadvertently ground it, damaging equipment or putting the user at risk. Using improper tools or methods can also create paths for current that the system wasn’t designed to handle.
Best Practices for Measuring Floating Outputs
The good news is that measuring floating outputs accurately is possible once you understand the right approach. Follow these best practices:
- Use the Right Meter. A high-quality digital multimeter (DMM) with high input impedance (10 MΩ or higher) is crucial. Lower-impedance meters can load the circuit and distort your readings.
- Establish a Reference Point. Determine what you want to measure relative to—often, this means temporarily connecting one side of the floating output to a system common or pseudo-ground through a high-value resistor. This provides a reference point without forcing a direct short to ground. Ground cannot be used as the reference for that measurement; doing so in this situation will lead to erroneous readings.
- Use Differential Probes for Oscilloscopes. If you’re working with oscilloscopes, use a differential probe or an isolated channel to measure across the floating terminals. Connecting a standard grounded probe can inadvertently ground one side of the circuit, producing incorrect readings or damaging equipment. To accurately measure the voltage, the meter probes should be connected directly to the output terminals.
- Follow a Step-by-Step Approach.
- Identify the floating circuit’s terminals.
- Verify isolation using a continuity tester before applying measurement tools.
- Connect your measuring device carefully, maintaining consistent polarity.
- Observe readings over time to ensure stability.
- Adhere to Code. Adhering to local and national codes is crucial to ensure safety, regulatory compliance, and the long-term reliability of any electrical installation or building automation system.
By following these practices, you’ll get more reliable measurements and reduce the risk of introducing errors or hazards into your system.
Common Mistakes to Avoid
Even experienced technicians can run into problems when measuring floating outputs. Here are some of the most frequent pitfalls:
- Accidental Grounding. Connecting your meter incorrectly or using a grounded probe can unintentionally tie one side of the floating circuit to ground.
- Using Low-Quality Meters. Cheap or old meters with low impedance can create “ghost” voltages or load the circuit heavily.
- Ignoring Manufacturer Specs. Failing to review the product’s datasheet can lead to measuring beyond the device’s rated voltage or current.
Taking the time to double-check your setup and equipment helps you avoid these mistakes.
Functional Devices’ Components with Floating Outputs
Functional Devices always aims to craft products that are easy to install and use. Let’s unpack some of the specifics when it comes to our components and measuring floating outputs.
Our AC power supply and transformer outputs provide galvanic isolation for connected equipment. Separating the input and output circuits prevents ground loops from forming and helps reduce noise on the line. Ground loops can cause potentially damaging currents to flow in a system.
While our devices do come from our factory with the outputs isolated, it is possible to connect the primary and secondary when it is required or desired. The diagram below shows how the primary and secondary can be connected.
When connecting the primary to a source with a neutral, that neutral can be connected to the "Com" output of the transformer. It is important to make this connection properly, especially when dealing with phase-dependent applications. If the incorrect output is connected to the primary neutral, the output will be out-of-phase with the input. This can adversely affect operation. On Functional Devices’ power supplies and transformers, the primary "Neutral" or "Com" will be in phase with the secondary "Com" terminal or wire.
Float with Help from Functional Devices
Floating outputs are common in modern electrical systems but can be tricky to measure. Understanding how they work and using proper testing methods improves accuracy, safety, and efficiency.
Functional Devices offers RIB® relays, current sensors, and power supplies with built-in isolation to prevent ground loops, designed to work seamlessly with building automation systems. Have questions about floating outputs? Contact our technical support team to find the right solution.
About Functional Devices, Inc.
Functional Devices, Inc., located in the United States of America, has been designing and manufacturing quality electronic devices since 1969. Our mission is to enhance lives in buildings and beyond. We do so by designing and manufacturing reliable, high-quality products for the building automation industry. Our suite of product offerings include RIB relays, current sensors, power controls, power supplies, transformers, lighting controls, and more.
We test 100% of our products, which leads to less than 1 out of every 16,000 products experiencing a failure in the field.
Simply put, we provide users of our various products confidence and peace of mind in every box.