Despite many web-resources, manufacturers’ datasheets and books about this subject, questions about isolation and insulation grade remain one of the most commonly asked tech support topics.The subject is complex, with many layers, standards and definitions, but it is not complicated. This guideline intends to outline the essentials. If there are still questions remaining, please feel free to contact RECOM technical or sales support!
Isolation prevents unwanted current from flowing between two or more parts of a system. This current can flow because of a potential difference between the supply rail grounds (for example, high voltage mains to a low voltage board-level power supply), because the grounds are not the same across the system (power ground and signal ground perhaps at different potentials) or because there is the possibility of an external voltage that can be injected into the system (noise interference, faults or misuse.) The unwanted current can be a danger to life (shock hazard), can cause equipment damage (faults) or it can simply be a nuisance (erratic data measurements), so there are different levels of prevention required. These levels are called the isolation grade.
The two parts of the system are galvanically isolated from each other. Ground loop currents and cross-interference from one supply rail to another are blocked and protection against certain fault conditions (for example, output short circuits) can be realized. In a power supply, functional isolation is commonly achieved using a transformer. The input and output windings can be wound directly over each other with just the wire coating providing electrical isolation. There are no minimum creepage or clearance separation distances required as the isolation provides no protection against electric shock.
Typical applications: avoiding ground loops, signal interference isolation (separating quiet and noisy power rails), board-level power supplies (isolating the load from the supply), ability to generate positive or negative voltages by choice of grounding point and multi-channel or bus-systems (a short circuit on one output does not pull the whole system down).
Basic Isolation
Functional isolation plus an additional isolation layer to protect against electric shock in conjunction with another measure of protection. In a DC/DC power supply, Basic isolation between input and output is commonly achieved using a transformer with the input and output windings separated with a layer of solid insulation. Basic isolation to ground is typically achieved by insulation or physical separation. There are minimum creepage or clearance separation distances required, depending on the voltage differences in the system, over-voltage category of the supply, pollution degree of the environment and for clearance, altitude
Typical applications: DC voltage systems which are considered to be potentially hazardous (above 60 VDC) or functional isolation applications requiring an additional single-fault protection for reliability. Basic Isolation between AC input and ungrounded outputs is not applicable for mains-powered applications because two means of protection are required by the safety standards. However, Basic isolation between AC input and a grounded housing is required.
Double Isolation
Provides functional isolation plus two independent isolation layers, each capable of protecting against electric shock (two means of protection). In a power supply, Double isolation is commonly achieved from input to output using a transformer with the input and output windings separated with two separate layers of solid insulation of a minimum thickness. There are also minimum creepage or clearance separation distances required, depending on the voltage differences in the system, over-voltage category of the supply, pollution degree of the environment and for clearance, altitude.
Typical applications: AC/DC mains powered systems in plastic housings with no earth wire and high reliability DC/DC isolation applications requiring double-fault protection.
Reinforced Isolation
Provides functional isolation plus a single insulation layer that is equivalent to two independent protections against electric shock. In a power supply, Reinforced isolation is commonly achieved using a transformer with the input and output windings separated by set creepage and clearance and a single thick insulator or with a single thinner layer of solid insulation and one winding using triple insulated wire. The advantage of the latter approach is a much more compact transformer compared to a double-insulated design. There are minimum creepage or clearance separation distances required, depending on the voltage differences in the system, over-voltage category of the supply, pollution degree of the environment and for clearance, altitude.
While functional isolation simply requires that the isolation barrier withstands the high-voltage test voltage placed across it, the other grades of isolation additionally require minimum separations across the isolation barrier. These have two basic definitions:
Creepage
The minimum distance across the isolation barrier measured along the connecting surface. For example, on a printed circuit board, the creepage distance would be the closest the tracks on the primary and secondary side approach each other, measured across the PCB.
Clearance
The minimum distance across the isolation barrier measured ‘as-the-crow-flies’ between two conductors. For example, on a printed circuit board, the clearance distance would be the closest two exposed metal components, one on the primary and one on secondary side, approach each other, measured through air.
The industry standards have tables that list the minimum creepage and clearance separation distances depending on several application-specific classifications such as the operating voltage, maximum surge voltage, the type of environment the power supply will be used in and the maximum altitude. All of these factors influence the ability for current to track along the surface of a part or to arc across between two points, so the minimum separations are the worst-case distances depending on all of these factors.
In addition, different standards define different limits depending on the type of application. For example, an AC/DC power supply might meet all of the safety requirements listed for use in a harsh industrial environment but still not meet the minimum creepage and clearances for use in a clinical environment. Even though the environment in a hospital is generally far more benign than in a factory, personnel are judged to be more vulnerable.
There is a constant battle between designing a power supply or a circuit board with the smallest possible dimensions and meeting the minimum creepage and clearance separations required by the standards. Certain techniques such a milling slots into the PCB to increase the creepage distances or wrapping tape around components to move the effective clearance between two conducting points can be useful, but the best method is to understand the application, use and anticipated environment thoroughly and design the layout accordingly.
Who decides on which isolation grade is required?
The isolation grade requirements for a given system are determined through industry standards, government regulations and safety directives, so the requirements are both application and safety standard specific.
Product safety laws require manufacturers to protect users from electric shock hazards under all normal operating conditions, so products must be safe during normal use, under single-fault conditions and during foreseeable misuse. The simplest way to show compliance with the laws and regulations is for a power supply manufacturer to certify the product to a recognized industry safety standard. The test house will use the intended application and the voltage ratings of the device to determine the required level of isolation.
We are here to help
Understanding the isolation grade required for a design depends on the reason why isolation is needed, the type of intended application and for which safety standard compliance is required. Although it might be tempting to always go for the best possible level of isolation, this is not always the best approach. Using a basic isolated part in a low-voltage battery powered application that is not intended for connection to a hazardous voltage is an example of overkill and simply adds cost without bringing any benefit. Using a reinforced isolation DC/DC converter in a gate-driver power supply application can actually be detrimental as the isolation capacitance in a Reinforced isolated DC/DC converter is higher than in a functional isolated DC/DC converter. On the other hand, if the isolation grade does not match the intended use for the power supply, the test house will reject the isolated part as being unsuitable, leading to high re-design and re-certification costs.
In such cases, it is worthwhile contacting RECOM technical support or our experienced sales engineers for advice. RECOM manufactures tens of thousands of different AC/DC and DC/DC converters and LED drivers and one of the reasons for such a wide product portfolio is that we have many different products that have been specifically designed to meet different safety and performance standards, whether that be for LED lighting, railway, industrial, household or medical applications.
The power supply unit, whether it be off-board or on-board, is usually the main component for ensuring compliance with the safety laws and regulations. Using pre-certified AC/DC or DC/DC converters does not remove the need for due diligence in designing a safe system, but it does make the compliance procedure simpler, faster and cheaper.
