I received a query recently from a person who read a recent article in Boating Magazine entitled “Stop the Shock”. At the suggestion of Boating’s editor Kevin Falvey I thought I might take a few moments to answer the readers fundamental question, why don’t we need to worry about in water shock hazard when unplugged from the dock and anchored out somewhere with an AC generator running. Its a good question and one that has been asked before. I don’t recall ever answering it here. So, here goes.
We really need to look back in history just a bit to understand the reasoning behind requiring the ELCI device in the shore power system in the first place. Basically there is some really bad history with dock wiring in general. The essential is a break in continuity of the green grounding conductor in the dock wiring, a fairly common issue. When the dock wiring gets compromised, and particularly the grounding (green wire) conductor electrical faults on board boats plugged into the shore power are going to look for alternative pathways to ground. The way that ABYC compliant boats need to be wired today requires a connection between the boat’s AC and DC grounding systems. Doing this provides an emergency alternate path to earth ground that will help to keep people on board the boat shock free. Unfortunately, this involves dumping the fault current out through the underwater bits of metal connected to the boat’s bonding system. So, with the addition of an ELCI device, which I have explained here on EBT a few times (do a search, those posts will come up) this 30 mA rated ground fault sensing device will trip and shut off the power to the boat. This keeps the water the boat is floating in safe and free from potentially lethal level of leakage current.
So, the question is why don’t we need to have an ELCI in the system to protect against the generator circuit or if the boat has an isolation transformer installed? The premise here is simple. Assuming both a generator or the boat wiring on the secondary side of an installed isolation transformer is done in accordance with ABYC standards, a fault will trip a circuit breaker because of the low resistance in the on board grounding system. Statistically, the committee that creates these standards felt that shore power supplied systems had a lot of bad history whereas generators and transformers had a very good history and any faults would take the low resistance on board path back to the source of power on board and trip the breaker.
Personally, I still wouldn’t allow swimmers in the water if the boat were mine and I had either a dc to ac inverter running or and AC generator running. We have to remember that fault current is not necessarily going to be enough to trip a breaker, but it could be more than enough to cause an ESD event. Situations like this come up within the ABYC standards fairly frequently and people need to understand that the committees really do have to consider what is practical, and make qualified and educated risk assessments as a part of their decision making process. In this case the decision was that shore power system have a bad history and there is a strong probability of a fault. In the case of the on board systems, there simply is no such history to support the need for ELCI protection for the on board AC power sources. Hope that answer the question.