Proving dead unearthed IT electrical systems

Hi all,

Would appreciate your input.

What is the correct procedure to adopt to confirm no voltage/prove dead on an unearthed IT electrical system, HV and LV systems.

I am unfamiliar with this system having only ever worked on resistive earthed systems. Were a single suitable HV test stick is proven, the system tested, stick re-proven, before any electrical work/application of a CME is carried out.

With no theoretical earth in an IT system am I correct to say you prove dead by using a live line 2 pole tester confirming no volts between phases?

Any advice for HV & LV systems is much appreciated.



Are you trying to check if 480v and 120v systems are live? 480v should be no different than anything else you have dealt with. To check for power you just put your meter phase to phase. Typical shipboard systems do not run a neutral on their 480 generation systems. There is nothing to bond to ground except for the generator frame. Ok, but here is where you get tricky. If you plug into your utility for your shore power you may be plugging into a 480 volt Y system which generates a neutral that is bonded to ground. In this case you can get 277v from phase to hull if you connected the ground in your shore power cable so you may want to check this as well.

Now, here is where you’re going to see a big difference between your shoreside and marine electrical, the 120v. Many older vessels run a 120v delta system. There is no neutral on a 120v delta system. Both sides are live. Your switches have to be two pole switches in order to isolate the equipment. This is one of my biggest peeves are companies that hire shoreside electricians who buy their switches at Home Depot (which sells single pole switches). A delta system with one leg open will read 0 volts on a leg to leg test but can read up to 1/2 system voltage even with no significant ground fault and can read full system voltage with a dead short ground fault.

And your non-contact voltage tester does not work on armored wire often found in marine construction.