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 running through enclosed spaces, so the work may well pass initial tests.

“But we have no way of knowing if you’ve got a poor installation hidden behind a panel or up in a roof space – it could be just a ticking time bomb,” Mr Wilton said.

“Even if you contact Energy Safety, they would say ‘as long as they’re working on voltages below 50V, they don’t require a licence’ … but for a major state government infrastructure project, which is going to be a children’s hospital of all things … it doesn’t sit well.’

Mr Wilton said the lead contractor and builder John Holland had subcontracted the installation to a company that then used a labour hire firm to staff the work.

It was believed up to 40 per cent were backpackers or working tourist visa holders with little exposure to Australian standards, Mr Wilton said, and all of the workers had to act as a sole trader and acquire an ABN so the subcontractor didn’t have responsibility for superannuation and the like.

“There’s an absolute sham arrangement going down there,” he said.

“It’s not the way that an honest, reputable building contractor would have done things in the past.”

Mr Wilton said it was yet another example of “doing the job as cheap as possible” at the hospital, where there have already been problems with asbestos in ceiling panels, lead in the water, faulty water pipes and fire door frames suspected of not meeting Australian Standards.

“I understand the government has some financial woes at the moment,” he said.

“But if you’re going to build a children’s hospital, which is going to be one of your legacies as part of the ‘Bigger Picture’ campaign that they’ve been running, you think you’d spend a bit more time and attention making sure the fire system is going to be robust enough to hold up a hospital full of kids.”

Mr Wilton oversaw electrical work on the new, much-maligned Fiona Stanley Hospital and said Multiplex had better practices.

John Holland has been contacted for comment.