So most of the ship is a good conductor, I assume seawater is fair conductor, and ships have tons of high voltage motors, pumps, etc and sensitive gear all over the place, especially on big warships.

So how how do they design ships so that everyone's not always getting electrocuted, and everything's not always getting fried ?

 

I don't see the problem:
If you are standing on ground you don't touch 440vac and live.
If you are standing in/on a metal boat you ……. (same story)
Insulation on wires and keep your hands in your pockets.

 

What does seawater outside the ship have to do with what occurs inside the ship?

 

Waterproof fittings, good insulation and mainly try to keep water(specially sea water) on the outside. Think of how submarines cope, surrounded by sea water>

 

In the Marines we had many classes about these electrical issues... I recall them saying it took hundreds of years to figure out how to do things safely. It's not just the ship either, there are static electricity issues all over the place, just sending an anchor down can cause incredible voltages to develop... think about what happens with the steam pistons... anything that's insulated that is moving relative to something else... ships can be very dangerous. St Elmo's fire is caused by same. This is a very important topic and great observation.

 

Only a problem when you dock..

I work on dredgers.. Three cranes that dredge the sea floor.. More than once I've seem them kick into life of there own.. Funny to watch.. We had to set timer relays to dump the static build up...

 

When I've watched documentries on ship building, I don't see massive layers of insultation being placed everywhere, yet wires are run everywhere. What happens if a wire in a bulkhead' has it's insulation worn through? You could have a current going across the ship through metal wall/floors/etc.

IDK the resistance across multiple rooms of a ship, but what on a ship is not welded and bolted together, and hence connected to just about everything else ?

 

Source and return are contained within the ship, where the power is generated. Submarines can operate on VERY high voltages (not at liberty to say). IF water penetrates the hull, and it does from time to time due to malfunctioning vent pipes and valves and such, you can stand waste deep in energized water and that energy is flowing to the ship's metal hull. You'll feel it but there's not a whole lot of potential disaster unless YOU become the conductor between power and ground. (or power and hull)

 

When I've watched documentries on ship building,

What happens if a wire in a bulkhead' has it's insulation worn through?

Don't know what documentaries your watching, but any I've seen has most if not all power going through conduit to eliminate that problem, just like in an industrial setting.

 

They used a lot of Mineral Insulated Cable in ships. MIC is basically grounded copper tubing with a mineral filler that contains the conductor wires. It is more or less fireproof.

 

Speaking for US Navy war ships and only US war ships things work like this. Something I found of interest was the standard 120 VAC 60 Hz power. We had standard US type outlets but reading from either prong to ground resulted in 60 VAC. Page 320-1 of the link:
"320-1.2.1.1 System Grounding. All ac distribution systems on U.S. Navy ships are ungrounded. However, certain avionics shops, aircraft servicing systems, and landing craft air cushion (LCAC) servicing outlets are supplied from three-phase, four-wire systems that are supplied by transformers with a grounded, wye-connected secS9086-KY-STM-010/CH-320R2 320-1 ondary and single-phase, three wire grounded outlets with two pole ground fault current interruption (GFCI) protection are provided where required by the user equipment (see paragraph 320-1.2.1.3). Only one ground connection point is allowed for each grounded power system. Ungrounded electrical systems have no intentional connections between the electrical conductors and the ground (ship’s hull). These systems were chosen for their higher reliability because grounding of a single conductor will not typically produce enough fault current to interrupt power. Grounds must be removed as soon as possible to maintain this advantage, however, because a second ground on another phase will cause a power loss and could endanger personnel'.

The few cruise ships I have been on were similar as to single phase 120 VAC ground and wiring. Additionally any and all personal electrical equipment is subject to routine safety inspections and following an inspection Pass it is tagged accordingly. Most of my work focused around US Aircraft Carriers and Submarines so those are what I had the most exposure to.

Ron