Hope this helps. I cut my teeth in the shipyard doing ship repair. An inherent problem aboard ocean going vessels(re: electrical systems) is water-go figure.
Anyhow, learning how to use a megger was of the utmost importance. I used a Biddle 500 volt crank type megger. Aboard ship the electrical dist system is ungrounded as a ground on one phase does not interrupt the system from functioning. Last thing you want is to be dead in the water. So, as soon as a ground appeared on the monitoring sys. an electrical person was dispatched to find the culprit and clear it if possible as a ground on another phase would cause system failure.
Many of the older ships were DC (240/120 volt) and had ungrounded systems also. Most shorebird electricians have trouble understanding this concept (understandably) but if shipboard lighting/receptacle circuits had an intentionally grounded conductor most of the circuits would be compromised, i.e. not working, due to water in fixtures, etc...
The point of this is that a megger in the right hands is a very useful tool and has multiple uses such as phase to phase insulation testing, phase to ground insulation testing, and continuity of conductors from point A to B. On the ohms scale it can be used to measure resistance between conductors and of component parts, such as coils but is not as accurate as a VOM ,IMO.
