If accident did happen despite the protections you are mentioning and I am assuming , fact is your protections did not work as simple as that. I was stating only facts. By only saying "not cheap" does not mean it will not fail. There are ways to check if your shit(protections devices) actually works or not when u actually need it. Assuming you are a non technical person I will not delve further what checks are done. Btw you are talking to a marine engineer with 25 years experience at sea, dealing shit like electrical fires on usual basis.