Complex sentence: It is often necessary to install new electrical cables or mechanical systems piping through a hole in a fire rated barrier which has been properly fire stopped during initial construction, in which case the firestops are referred to as being "re-entered".
Split into simpler sentences: It is often necessary to install new electrical cables or mechanical systems piping through a hole in a fire rated barrier which has been properly fire stopped during initial construction. In this case, the firestops are referred to as being "re-entered".