Customers have no control over incoming supply fault levels and utility suppliers have no obligation or policy to keep customers informed of changing fault levels. Fault levels may change due to switching arrangements or changing of cables and equipment for the purpose of the increasing capacity. For this reason it is unreliable to depend on utility suppliers for accurate information on fault levels. As the transformer is not the source of the fault current, it is safer to base calculations on transformer(s) let-through levels. Generator contributions can then be added if applicable.