If you are using the CXN/851N in digital 'pre-amp' mode in conjunction with the CXA-60/80, then the CXN/851N’s audio output may be too low for the CXA’s APD (auto power down) detection to recognise the audio signal, causing the amplifier to switch itself off after 30 minutes . You can disable the Auto Power Down function (APD) to avoid this happening. A guide to disabling APD is located here.
If you are using an integrated amplifier such as the CXA60/80, then instead of using digital pre-amp mode on the CXN/851N, you may use the control bus function to control the volume of the amplifier. More information regarding control bus is located here.