This is a cutaway of a typical normally closed valve.
The main features are highlighted here.
The control head is made of black plastic.
It contains the piston and the large return spring that will return the valve to the closed position when there is no pressure applied.
The spindle connects the control head to the valve plate.
It's protected by some durable packing material and compressed with a spring.

В данном контексте It's protected - относится к spindle или valve plate предыдущего предложения?

Спасибо!