The basics of AC-line isolation for safety, Part 2: The solution
Isolation from the AC line is essential for user safety in many situations and can be implemented using a special isolation transformer as well as other means.
Part 1 of this FAQ looked at the multiple-fault and failure scenario that can put users at risk from AC-line connected products, including appliances, instruments, or medical devices. This part looks at ways to solve this problem and ensure safety.
Q: So, how is the problem avoided?
A: There are several ways. For electronic equipment, the most common way is to use an isolation transformer, which allows the AC voltage to reach the secondary side and its circuit (the load) while preventing the flow of current through the user and back to the neutral line (Figure 1).
Q: What does the isolation transformer do?
A: The isolation transformer does not have a wire from neutral to Earth, so the current will not flow through the user even in the fault situation, Figure 2. Note that isolation transformer may have a 1:1 turns ratio, so its input and output have the same voltage, or it may be a step-down transformer with a lower secondary-side voltage, which often simplifies conversion, rectification, and regulation of the circuit’s power rails. Regardless, the principle is the same. (Aside: in my research into the different ways of explaining how an isolation transformer provides safety, most of the explanations – even from credible technical sites – said something like “the isolation transformer provides safety because it isolates the user” which is a circular and meaningless explanation!)
Beyond the AC line
Q: What about shock risk from battery-operated devices with no AC-line connection?
A: These devices do not pose a shock hazard, even with high-voltage batteries. If the case gets connected to one of the battery terminals and thus to the user, there is still no current path from the user back to the other battery terminal (always keep in mind that current has to flow in a complete path). Of course, if a user grabs one battery terminal with one hand and the other terminal with the other hand, current will be driven across and through the chest, which is potentially a dangerous scenario.
Q: There are line-operated power tools which do not have safety grounds yet don’t need isolation transformers – how is this possible?
A: Until a few decades ago, construction and medical power tools such as drills had metal cases. If there was an internal fault which made the case become “live” the current path could be through the user (or patient or medical-care professional). The metal case was connected to the ground terminal of the unit’s AC cord to prevent this situation. However, this was always a risky solution since, as noted, in many real-world scenarios, the cord’s ground wire was not really connected to Earth ground due to faulty cord, outlet, or use of a “cheat” three-wire-to-two-wire adapter for non-grounded outlets.
Q: How was the problem solved? Was an isolation transformer used?
A: Some construction sites did use such a transformer as part of an external, standalone power distribution unit (PDU), but it’s another thing to keep it properly maintained and not abused. The solution which is now widely used is a “double insulated” design (Figure 3) for tools, medical devices, and even consumer products such as table lamp. The tool’s internal electrical circuits are insulated as usual, of course, and the case itself is also non-conductive with no exposed conductive parts.
In this way, even if there is an internal fault and short circuit to the case – or a drill bit hits a live AC wire in a wall – the user is still protected from current flow. Double-insulated tools fully meet the National Electrical Code (NEC) standards and are preferred because they don’t rely on an often-absent grounding connection in a three-wire plug and socket. In fact, double-insulated tools and instruments only have a two-wire plug (Hot and Neutral connections) and no ground wire at all.
Q: How is isolation achieved in situations where the voltage is in the thousands of volts?
A: An isolation transformer could be used for AC situations but would be impractical or unwieldy, especially as the insulation would have to be very thick to provide a sufficiently high breakdown voltage. There are alternatives which go back to basics, however. As a dramatic demonstration of the importance of understanding where the current can and cannot flow, isolation from ground, and minimizing risk, note that there are powerline-repair specialists who work on live lines at hundreds of kilovolts (kV) both AC and DC, but are wearing only thin, non-insulated work gloves. They do this while perched on a helicopter (Figure 4) or from a fiberglass bucket, which is super-insulated from ground (it even uses optical fibers for signaling from the bucket to the control motors rather than wire-based circuits). The wire’s current does not go through them, and so they feel no shock even as they handle the live wire.
Similarly, birds who land on a single power line are safe since the current does not pass through them to ground or to a line at a different voltage (which would mean a potential difference). But if the bird touches two lines simultaneously (most power lines are three-phase systems, of course), there will be a potential difference resulting in current flow and likely lethal shock. In areas where large-winged birds roost, the power lines on the support insulators are spaced further apart.
Q: Will any transformer without a return path be suitable for isolation?
A: Yes, and no. For basic isolation, they will work. But for medical applications, they are not necessarily adequate since medical products and their probes may be in direct contact with a person’s exterior or even interior tissue. Medical products’ standards place strict and extremely low limits on leakage current flowing from the primary side to the secondary side due to various imperfections and despite the formal insulation. Isolation in medical devices is part of the Means of Patient Protection (MoPP) strategy.
Further, there is leakage current, which does flow due to the capacitive coupling between primary and secondary windings. Due to this unintended current flow, isolation transformers for medical products must meet additional stringent tests, and their design requires limiting the interwinding capacitance to picofarads. These transformers use various advanced materials and manufacturing techniques to ensure that leakage current is below the allowed limits.
Conclusion
Line isolation is an important tool for providing user safety in AC-line powered products of all types. Depending on the application, it can be achieved using a special isolation transformer, double insulation, or physical separation. It is different from the non-safety isolation needed to achieve proper or superior performance unrelated to safety and must be addressed as such.
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External References
- Autodesk, “How does ground work in electronics?”
- Analog Devices, “Isolation in Digital Power Supply—Why and How”
- Associated Research, “Use of an Isolation Transformer While Performing Leakage Current or Functional Run Tests”
- EtechoG, “Applications, Advantages, Disadvantages of Isolation Transformer”
- Lumen Learning, “Electrical Safety: Systems and Devices”
- Voltech Instruments, Inc., “InterWinding Capacitance”
- OpenStax CNX (Rice University), “Electrical Safety: Systems and Devices”
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