Ground fault

[tomh]

If I have this right, you actually have a ground fault circuit interrupter, but have a shared common with another light circuit. Ground faults occur when current is leaking somewhere, in other words, electricity is escaping to the ground. By using a common that is shared with an unprotected circuit, the GFCI detects a fault and properly trips. You need to provide a dedicated ground and common on that circuit. Sharing neutrals is a dangerous shortcut on any circuit, and it will always cause a ground fault on a monitored circuit.