The present invention relates to a circuit arrangement having a transformation apparatus, which has a primary side (P) and a secondary side (S), wherein the primary side and the secondary side (S) are DC-isolated from one another, wherein the primary side (P) has a terminal for a supply voltage (UE) and is connected to a first reference potential, wherein the secondary side (S) comprises a first and a second output line, which, in the normal case, are not coupled to a reference potential and which form an output terminal, at which a voltage for a load (LA) can be provided, wherein it furthermore comprises: an apparatus for tapping off a measured voltage potential (CP) on the secondary side (S); and an apparatus (10) for determining whether the measured voltage potential is in a permissible range. The invention moreover relates to an operating method for a lamp using a circuit arrangement.