When we analyze a 2D cross-section of a thermal bridge, we need to ensure that the model is large enough to capture the full extent of the heat flow through the junction. The "effective length" refers to the dimensions of the flanking elements (the parts of the wall, floor, or roof that extend out from the junction) that must be included in the model.
As a general rule, BR 497 states that the flanking elements in the model should extend away from the junction by a distance of at least 1 metre or three times the thickness of the element, whichever is greater. This ensures that the model is large enough to accurately calculate the heat loss. There are exceptions for ground floor elements where 4 meters in width is required and the ground has to be also modelled.
AutoPSI helps you manage this by automatically creating models with the correct dimensions when you use the Projects or Quick Model features. When using the Free Model feature, you will need to ensure your model adheres to this principle.