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Isolation transformers, and ELCI for DC stray current?

Apr 17, 2016
13
projected to own one as I said ;-) none yet
As I am slowly approching the time when it comes to buy my own boat, and because I care about the boats electric, I have two questions that are not dependent (but connected somehow).

Number 1: It is clear that the boat-AC-ground must not be connected to the boat-AC-neutral as long as boat-AC-ground is connected to shore-AC-ground. However when an isolation transformer is installed (in isolation mode) then boat-AC-ground must be connected to boat-AC-neutral (both the secondary side of the transformer).
Question: shouldn't I install an ELCI on the transformer secondary side just before the ground/neutral connection?

Number 2 (esp. for metal boats): In my understanding for the 12V (or 24V) DC system the same rule should be valid - current back (e.g on the negative side) must be identical to current forward (positive side), or there would be some DC leak which might induce stray current corrosion, even if it is never harmful to persons and wouldn't be even felt. Assuming that the DC lines are generally carried out in a two-wire system (and not using the hull) couldn't be stray current detected by measuring/comparing amperage of positive and negative lines?
Question: is this reasoning somehow correct, and if yes do ELCIs for 12 (24) V DC exist?
 
Dec 25, 2000
4,439
Hunter Passage 42 Shelter Bay, WA
Hi Ernest, no electrician here, but I do most all of my own electrical work, whether on the boat or at home. The questions you pose were addressed in some detail in the latest 48 degree north magazine. That article can be found here on page 24, "How To Stop Leaking Current". Well worth the read and should answer your questions.

 
Jan 11, 2014
5,758
Sabre 362 113 Fair Haven, NY
@ErnestV Saw this today, thought some of the information might be helpful to your questions. (scroll down to the ELCI piece)


Steve D'Antonio is a well respected and knowledgeable guy. His site is well worth the read.

Number 1: It is clear that the boat-AC-ground must not be connected to the boat-AC-neutral as long as boat-AC-ground is connected to shore-AC-ground. However when an isolation transformer is installed (in isolation mode) then boat-AC-ground must be connected to boat-AC-neutral (both the secondary side of the transformer).
Question: shouldn't I install an ELCI on the transformer secondary side just before the ground/neutral connection?
Yes. See the above referenced article from Steve D'Antonio. The issue is a short to some metal that is in the water, not necessarily directly back to the AC supply. The isolation transformer is there to block DC current being carried over the AC ground and to physically isolate the boat's AC system from shore power.

Number 2 (esp. for metal boats): In my understanding for the 12V (or 24V) DC system the same rule should be valid - current back (e.g on the negative side) must be identical to current forward (positive side), or there would be some DC leak which might induce stray current corrosion, even if it is never harmful to persons and wouldn't be even felt. Assuming that the DC lines are generally carried out in a two-wire system (and not using the hull) couldn't be stray current detected by measuring/comparing amperage of positive and negative lines?
Question: is this reasoning somehow correct, and if yes do ELCIs for 12 (24) V DC exist?
I suppose in theory you would be correct. However, boats are not wired in the same manner as houses. It is probably more typical for the positive and negative wires to be run separately. This can simplify wiring by using a negative bus bar located near a collection of devices and then run one negative line back to the main negative bus bar. The radar, chart plotter, wifi, AP, and router are all located near each other, instead of running 5 negative wires back to the panel, they are connected to one bus bar with one larger gauge wire running to the main negative bus bar near the battery bank. This also shortens the total run which reduces line loss.

The ELCI is not about corrosion, it is about keeping AC current out of the water. AC current leaking into the water is deadly for people, and not so deadly for boats. DC is the opposite, it causes metals to corrode quickly. Metal boats are a particular concern and need special care in wiring. They aren't cars, yet some get wired that way.
 
Apr 17, 2016
13
projected to own one as I said ;-) none yet
@Terry - thanks for the link, very informative!
@dlochner - thanks as well, I begin to see a bit clearer ;-) And your NauticalReads is a treasure ;-)
 
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