Power Transformer Safety Needs: Creepage, Clearance, and Insulation

Transformer safety needs depend upon the creepage distance, clearance distance, distance through solid insulation, and the dielectric withstand voltage needed for each design. These factors vary for each design and can be determined by the end application’s function, working voltage, and insulation type.

Creepage – The shortest path between two conductive parts measured along the surface of the insulation; the shortest path between the primary and secondary sides of the transformer that is measured along the surface of the insulation.

Clearance – The shortest path between two conductive parts measured through air; the shortest path between the primary and secondary sides of the transformer that is measured through air. Often times your clearance distance is less than your creepage distance and can be the more critical distance.

Dielectric – The peak voltage that the insulation under consideration is required to withstand. In simple terms, an electrical strength test used to verify the insulation strength between two conductors.

To determine the creepage, clearance, and dielectric, first look to the end application and its intentions. If the customer intends to use the design for LED lighting it will not have the same requirements as using the design for medical equipment. Then, you can establish what working voltage is necessary. The working voltage is the highest voltage the insulation or component can withstand when the equipment is normally operating. Lastly, the necessary insulation type must be decided. There are three main insulation types to consider - functional, basic, and double/reinforced.

Functional Insulation – Functional insulation is required only for proper functioning of the transformer. It is usually associated with DC/DC applications in which the end user is not exposed to hazardous voltages.

Basic Insulation – Basic insulation is not basic. It is typically requested when the end application has a means of providing additional isolation from the hazardous input voltage in case the basic insulation fails.

Double/Reinforced Insulation – This is the most common form used in offline applications. The end user is completely isolated from hazardous input voltage by reinforced insulation without the need for additional isolation.

Check back for further information about insulation types and how they affect transformer sizes.

Flickr Image