If it’s discovered that the entire PA system isn’t working the relevant MCBs will be tripped and reinstated to try and rectify the problem.
I’m almost certain the rule book states a train without a working cab to cab or PA can’t begin a journey, it also says to try to lock the coach out of use if the PA is defective in that area. The problem is the guard can’t feasible know if their PA is working correctly. It’s like trying to check your own brake lights without the use of a reflection.
You've never held the brakes on for a bit then put your hand on the bulbs to see if they're warm?
In seriousness, of course it is difficult to check your own pa system but i have caught services that are the first working of the day where it hasn't worked at all in the entire train so unfortunately whilst i appreciate the rules, they aren't being followed everywhere. I do feel it's likely better to risk assess and run the service if it is deemed safe to minimise disruption and delay but they should do what they can to ensure passengers are informed of the situation, especially if changes to stopping pattern are made. Sadly, many guards don't keep passengers informed with a working system. It does bring me back to the generic concept that likely MOST of the problem is poor technique from staff so maybe additional training would go some way to solving this. Likely swinging off topic a little i guess.
It is interesting to see that http://www.realtimetrains.co.uk/train/W91030/2017/12/14
The service yesterday did call at those stations and still managed to make up 5 minutes of lost time between Fareham and Hilsea.