Circularity switch
LBOA toggle cell that breaks the interest loop on demand, letting a model recalculate cleanly when an error has propagated around it.
Also written: circuit breaker, circularity breaker, circ switch
It is a single input, usually a one or a zero, that the interest formula reads. Set to one, interest runs on the average balance and the loop is live. Set to zero, interest is charged on the opening balance or on nothing at all, the dependency disappears, and the model calculates in one pass like any other workbook.
The reason it exists is failure recovery. With iterative calculation on, an error inside the loop becomes the stored starting value for the next pass, so every input to the calculation is now an error and fixing the original cause does not clear it. Opening the loop gives the model something clean to calculate from.
The working sequence is flip it off, confirm the model calculates and the balance check reads zero, find and fix the real problem, then flip it back on. Build the switch before you need it, because adding a toggle takes a minute and rebuilding a model that has filled with unclearable errors does not.
The cost is that someone has to remember which state it is in. A model handed over with the switch off is quietly running the opening balance convention, which is defensible when stated and misleading when not, so the switch belongs in the assumptions block where a reader will see it.
Worked example
A model errors after a divisor goes to zero inside the interest calculation, and undoing the change leaves the errors in place.
Setting the switch to zero charges interest on the opening balance, the workbook recalculates, and the underlying error becomes visible in a single cell. Fix it, confirm the balance check is zero, set the switch back to one, and the average balance calculation converges again.