Final answer:
When using a shielded system for fire alarm or instrumentation cables, the drain wire should be connected to the cable shield and properly terminated. This ensures effective shielding against electromagnetic interference and proper grounding.
Step-by-step explanation:
If a fire alarm or instrumentation cable is designed with a shielded system, the drain wire should be connected to the cable shield. The shield provides protection against electromagnetic interference (EMI) and helps to prevent damage to the cable's conductors.
The drain wire acts as a ground for the shield, allowing any EMI to be safely conducted and discharged. It should be properly terminated at both ends of the cable to ensure effective shielding and proper grounding.