Selling or even handling power supply systems in the world requires the meeting of relevant safety standards. These standards may vary depending on the power supply type in questions. For instance, an outdoor power supply unit may require extra provisions to account for weather conditions. An outdoor power supply may need to be waterproof even in the event of rain or excess humidity.
These standards are defined and enforced by national or international agencies and the compliance to these regulations is tested by government approved labs.
The primary goal of these standards is protection against phenomena like fire or injury. Power supply units have to comply with these regulations considering some of the very sensitive applications they have like in the military. Sensitive military operations like missile power supply will need equipment that is reliable round the clock. You can imagine a fighter pilot right in the thick of it and his missile power supply unit fails.
Products that meet these requirements may be identified by a safety mark from the relevant standards organization or simply a mark indicating compliance with local legislation.
Classes of Equipment
Safety standards identify different classes of equipment depending on how their power supplies (especially mains units) isolate secondary circuits and accessible parts from dangerous AC mains voltage.
Class 1
Equipment in class 1 achieve electric shock protection through basic insulation and protective grounding. This in particular requires all conductive parts that could assume dangerous voltage in case of insulation failure be connected to a protective earth conductor.
Class 2
Class 2 equipment provides protection using double or reinforced insulation and as such, no grounding is required.
Class 3
Equipment operates from a safety extra low voltage supply unit. What this means is it inherently protects against electric shock because it is impossible for hazardous voltages to be generated within the equipment.
Insulation and Isolation
There are 5 types of insulation to shield live components with hazardous voltages from other voltages and circuits.
Operational Insulation
Functional insulation is only necessary for the correct functioning of the equipment and does not provide any protection against electric shock. As applied in military DC DC converters, the output is isolated so there is no protection against electric shock.
In this setting, transformer primary and secondary windings are typically wound over one another relying on the thickness of the wire lacquer for insulation.
One advantage of this method of insulation is it is low cost and the output is a very compact sized transformer or military DC DC converter. Although the transformers may be small, they can withstand up to 4KV of DC isolation voltage testing – with limited duration though and it is a one-off test.
A typical application of functional isolation is non-safety-critical systems. The isolation is primarily used to break round loops or to provide a change in functional ground reference voltage. In applications like these, an isolation fault would not cause serious damage to the equipment or even stop the application from working. The isolation provides added security against interference but is not at all safety related.
Basic Insulation
Insulation applied to live parts to provide protection against electric shock. In basic isolation, secure insulation is required between the primary and secondary windings rated at the system voltage. Depending on lacquer insulation on winding wire like in functional insulation wont work because pin-holes can be present.
In this kind of insulation, the input and output windings are not wound directly over one another. Instead, they are separated by a minimum distance with a physical barrier. The physical barrier is often agency-approved insulating film.
This method is mostly applied in large transformers where size is not an issue so there is enough room to add layers of tape between the windings.
Supplementary Insulation
Independent insulation is applied in addition to the basic insulation as a backup to the basic insulation in case it fails.
Double Insulation
Double insulation comprises both basic and supplementary insulation.
Reinforced Insulation
Single insulation systems applied to live parts provide a degree of protection that is equivalent to double insulation.
Input and output windings are separated by a larger distance or at least two physical barriers each equivalent to basic insulation ratings.
The multiple layers of insulation make the transformer bulkier. A way around this is using triple insulated transformer wires which are already classified as offering reinforced insulation.