Unlicensed electricians are installing the fire detection and control system at the $1.2 billion Perth Children's Hospital, potentially leaving a "ticking time bomb" of risks, a union says.

Under WA regulations, low-voltage components of the system can be installed by unlicensed electricians, but must be inspected, commissioned and connected to high voltage power supply by licensed electricians. Electrical Trades Union branch president Greg Wilton said it was impossible for the inspectors to fully check all of the cabling…