The Importance of Isolation (Working vs. Test Voltages)

When selecting a power supply for any application — whether medical, telecommunications, industrial, or otherwise — engineers must consider a wide range of factors. The first priority is to ensure that the supply can handle the required AC and/or DC input voltage range and deliver the necessary output power. Other critical considerations include efficiency, thermal management, size, regulatory compliance, electromagnetic compatibility (EMC), and environmental robustness.

One often-overlooked aspect is proper isolation. Choosing a power supply with suitable isolation characteristics is crucial for long-term system reliability, safety, and standards compliance. For mission-critical and safety-critical installations, such as industrial, medical, and automation applications, understanding the difference between test voltage and working voltage is essential for informed design decisions.

In power supplies, the isolation barrier is a deliberate electrical separation between the input (primary side) connected to the mains and the output (secondary side) supplying the load. This barrier prevents direct electrical current from flowing between the two sides, while still allowing safe and effective transfer of power and data. This isolation is typically achieved through transformers for power transfer, complemented by optocouplers or digital isolators for feedback of control signals.

The term “working voltage” (also known as “continuous voltage”) across an isolation barrier refers to the maximum voltage difference that can continuously exist between the two isolated domains during normal operation. This value — crucial for determining the insulation requirements and safety ratings of the power supply — depends on the system configuration, not merely the input voltage.

The term "dielectric withstand voltage" (also known as “test voltage”) refers to the maximum voltage that an insulating material (the "dielectric") can withstand without breaking down. In other words, it is the threshold at which the material maintains isolation without allowing current to pass through in a way that would compromise safety. The “dielectric withstand test” (also known as the “high-potential test” or “hi-pot test”) is based on this concept. In this test, a high voltage (usually well above the working voltage) is applied across the isolation barrier. The purpose is to confirm that the insulation remains intact, with no breakdown or excessive leakage, demonstrating that the product is safe and resilient even under transient or fault conditions.

During the dielectric withstand test, the high voltage is applied for a specified duration, often 1 second or 60 seconds, to verify there is no leakage or breakdown. One-second tests are commonly used in production to prevent component degradation, while 60-second tests are more rigorous and typically reserved for type approval and certification. Different standards define various multipliers of the working voltage, test durations, and even test waveforms to validate insulation integrity.

The safety and reliability of a power supply require attention to the supply’s electrical characteristics and physical design. Standards specify requirements for the following:

• Working voltage (continuous voltage across the barrier)
• Dielectric withstand voltage (short-duration high-voltage test)
• Insulation type (basic, supplementary, reinforced, or double)
• Clearance distance
• Creepage distance
• Pollution degree
• Overvoltage category (OVC)
• Altitude
• AC vs. DC inputs (in AC designs, peak voltages and transients are more severe, requiring more robust insulation and spacing)

The clearance distance is the shortest distance through the air between two conductive parts, where clearance prevents “flashover” (arcing through air). In contrast, the creepage distance is the shortest path along a surface between two conductive parts, intended to prevent “tracking” (current flowing along the surface of the insulating material). The required distances depend on several factors:

• Working voltage
• Overvoltage category
• Pollution degree
• Material group (tracking resistance of insulation)
• Altitude

The overvoltage category (OVC) represents the mains hierarchy where the power supply is intended to operate. Higher categories correspond to larger transient voltage peaks, which require increased clearances and higher test voltages to maintain safety. Below is a high-level overview of these categories:

• CAT I: Protected electronic circuits (battery-powered, limited-energy secondary circuits)
• CAT II: Local appliances (household, portable tools)
• CAT III: Building installations (distribution panels, fixed wiring)
• CAT IV: Origin of installation (service drop, primary distribution)

The pollution degree (PD) describes the amount of conductive contaminants that may accumulate on the power supply's insulation during normal operation. Higher levels of pollution require increased creepage distances to maintain safe isolation. Below is a high-level overview of the categories:

• PD 1 (0.1x): Clean, dry (e.g., sealed electronics, space)
• PD 2 (1.0x): Normally non-conductive, occasional condensation (typical indoor)
• PD 3 (1.5x): Conductive pollution or humidity (e.g., kitchens, factories)
• PD 4 (2.5x): Persistent conductive pollution (outdoors with heavy salt spray, chemical plants)

Each pollution degree has a corresponding multiplier that is applied to the base creepage distance per kV of working voltage. For instance, if the base creepage is 2mm/kV and the power supply is intended for a PD 4 environment, the requirement becomes 2mm × 2.5 = 5mm/kV of working voltage. In addition, the altitude effect must be considered. As altitude increases, air density decreases, which reduces dielectric strength and increases the risk of arcing. Standards such as IEC 60664-1 apply a derating factor to clearance for elevations above 2000m. For example, at 5000m, the required clearance might need to increase by 50% or more compared to sea level.

There’s an old engineering joke that says, “Standards are great … everyone should have one.” The challenge is that many organizations indeed have their own. In the case of power supply isolation, there are many standards, each with distinct requirements. Some of the more common standards for insulation and safety in power supplies are as follows:

• IEC 60950-1: IT and telecom equipment (now superseded by IEC 62368-1)
• IEC 62368-1: Audio/video, IT and telecom (modern replacement for IEC 60950-1)
• IEC 60601-1: Medical equipment (includes patient/user protection)
• IEC 61010-1: Laboratory and industrial (focuses on test and measurement gear)
• IEC 60204-1: Industrial machine systems (often paired with IEC 61010)
• IEC 62109-1/-2: Solar inverters and PV (for high-voltage solar gear)

The challenge is further complicated by the presence of various regulatory and certification bodies, each with its own set of requirements. Key organizations include UL (Underwriters Laboratories – United States), CE (Conformité Européenne – European Union), CCC (China Compulsory Certificate – China), and PSE (Product Safety Electrical Appliance & Material – Japan). For example, CE may call for a 1-second test at a specific voltage level, while UL might demand a 60-second test at another level. This results in a complex “patchwork of requirements” that manufacturers must navigate to achieve global compliance.

RECOM’s extensive portfolio of power supplies addresses the needs of a wide range of applications and markets: from tiny converters used in embedded and IoT systems to heavy-duty converters for industrial and automation applications. Notably, RECOM recently received the Red Dot Award for Product Design 2025 for its RACPRO1 DIN Rail Series of industrial and automation power supplies. RECOM also serves many other markets, including medical, telecommunications, networking, rail, and transportation.

Each RECOM power supply is specifically designed to meet the isolation and working vs. test voltage requirements of its intended application. Additionally, RECOM’s experts are available to guide you through the complexities of isolation regulations.

The RECOM AC/DC, DC/DC, and EMC Book of Knowledge offers an extensive collection of insights and expertise needed to implement effective AC/DC power supply designs. In addition, RECOM’s Energy Efficiency in AC/DC Power Supplies: Quick Guide to the Essentials blog provides a helpful introduction to ecodesign regulations. These regulations are a set of standards and guidelines established by governments or regulatory bodies to ensure that products are designed to minimize their environmental impact throughout their entire lifecycle. The primary focus is on improving energy efficiency and promoting sustainability by reducing waste, emissions, and resource consumption.