AC Grounding & Common Ground - Why you NEED to connect them... | Ask The Expert with NIGEL CALDER
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- čas přidán 14. 12. 2022
- Nigel explains why we need RCDs/ELCIs, galvanic isolators and a connection of the AC grounding wire to our boat's common ground point. And how European and American standards converged in this matter over the last decades. Check out our modules on AC systems in our Advanced Marine Electrics program for in-depth info on the topic: boathowto.com/advanced-marine...
I have signed up for a BoatHowTo electrical course. This video accurately portrays the quality of what you will find in the course itself. One of the great benefits of the course is after you have signed up and paid for the course you can go back through and review the material to refresh it in your mind (I certainly need that). I was totally satisfied with my experience and received great value for the educational money spent.
Thanks a lot for these kind words, it means a lot to us to get this kind of positive feedback from our students! :-)
Excellent historical perspective on how we got where we are today. Thanks.
Sometimes technical questions require technical answers.
Great content! My 5 cents: Isolation transformer also avoids stray current corrosion, while Galvanic Isolators won't. So they are similar but not the same. Congrats!
Take the BoatHowTo courses, even if you're a novice boater, or even just interested in marine electrical. They're a real eye opener.
Thanks Glenn! We love this kind of endorsement by our students! :-)
I'm planning on taking it very soon.
Excellent summary of a complex topic. What about AC inverters? Is ground leakage affecting swimmers a concern with them? They're not protected by the shore ELCI. But, if all our AC outlets have GFCIs then "whole boat protection" is just about the few other AC circuits right? For us that's just the hot water heater.
The electric shock drowning (ESD) issue is associated with shorepower, not onboard sources of power. This is because fault current seeks a path back to the source of power. If an inverter has a shorepower connection on its input side (an inverter/charger), when it is in pass-through mode the boat will be on shorepower and as such needs to be protected by an ELCI/RCD at the shorepower inlet. When the inverter is in invert mode, the path for fault current is within the boat and as such ESD is not an issue. There may, of course, still be a shock hazard within the boat.
- Nigel
Great video! Thank you for sharing! Quick question please: where does onboard AC ground connect to ships ground? It’s on the ship side of the source selector, right? I guess I get confused because the AC ground and AC neutral are connected at the source only: ie on shore or within inverter or within generator. But aboard the boat, on ship side of source selector, the AC ground is always connected to DC negative and engine/underwater metals, ect. Right?
A source selector switch switches the hot and neutral wires simultaneously and makes sure that no two power sources can be connected at the same time. The grounding wire is just "passed through" and does not get switched. So it does not matter, where (i.e. before or after the switch) the connection to the ship's common ground point is made, as long as it's there! We discuss this in detail in our module on AC system with onboard power sources of our Advanced Marine Electrics program: boathowto.com/courses/onboard-power-ac/
- Jan
I think this is so complicated, despite it not being rocket science (as is de facto evidenced by the confusion found everywhere in multiple sources), you ought to drop the focus on history ( although interesting to be fair) and be very specific about what the gold standard is now, today. I am an old GRP catamaran, with isolated saildrives, have a GI ( with test button), mains tester ( to use before boat shore plug-in takes place), and RCD and MCBs. I do not earth AC to the boats common ground...why?....Because in your world, lets say you are plugged in on shore power, which let's say has an unknown variable fault/short/ leak appliance/boatside, and you have your AC earth connected to the boat's common ground ( DC ground too)....you work on your DC circuits ( very frequently I might add compared to AC), while someone then uses said AC kit, and, touching the DC ground point/ side now means you the have the increased risk to get AC electrocuted? Am I being dim here as I don't think the risk of having a potential 'live' common with AC is sensible - let the tested shore side earthing do its job. The last thing you want is (an unkown) hot Dc side. I'd love the message in Europe first to be the mandatory use of GI's if you want to connect to AC shore power. Second, to make mandatory the testing of a shore socket before you plug into it. In 2000 miles one season, I recon 50% of the marina sockets I tested were incorrectly wired! Europe's biggest ( expensive) problem is galvanic corrosion, so after life and death basics, this is the big issue, surely - so why aren't GIs mandated as a minimum, big time? Apologies if I've got the wrong end of all this....your course is a good idea.
I have a 32 ft cabin cruiser with a portable generator. I s there any saftey precautions i need to take? Check? New to boat not sure if i even have any of those items. It`s a 1987 wellcraft.
So if you retrofit an ELCI, is it OK to delete the GFCI's on the plugs? Is there any problem with having GFCI's downstream of an ELCI?
The ELCI, or an RCD, is adequate whole boat ground fault protection. It is always OK to add additional protection with lower trip thresholds at specific locations in the boat (e.g., galleys and head compartments) but not necessary, but if these are already installed there is also no need to pull them out. In other words, there can be multiple layers of ground fault protection with diminishing trip settings. In Europe there will also typically be an RCD upstream of the boat on the dock, and the latest standards in the USA also require multiple layers on the dock.
- Nigel
Guys, thanks for great content. I have a Charles model 93. Which ELCI should I have on the grounded secondary? 3-pole Blue Sea 3104 or 2-pole Blue Sea 3093 ?
Old school says don’t switch the neutral. I too am nervous about that. What are the real concerns? Thanks
Any ELCI has to disconnect both hot and neutral in case of a fault.Both the 3104 and 3093 model do this. The difference is that the 3104 model is made for 120/240V split phase systems which you can sometimes find in the US. If you don't have such a system, the 3093 is the right choice. - Jan
@@BoatHowTo Thanks for that Jan. Much appreciated. To be clear, you mean Single Split -phase, (L1 and L2 180 out of phase and a center-tapped neutral), correct? That’s what my secondary is doing, so the 3104 seems right. Initially, I wasn’t sure the secondary would like having all three poles opened with a trip. Though my main panel has three-pole breakers for Shore and Genset
Thanks again.
I hope I am using the Acronym correctly RCD. Have two RCD's Between a relay that uses Disruption to RCD one to initiate fault to secondary.
Can you explain what the purpose of this setup is? An RCD trips if there is an imbalance in current between the two hot wires, indicating current leaking to ground somewhere. I see no practical application where you might want to connect two RCDs via a relay... - Jan
But what if one of the RCD is faulty and doesn't trip when it should? Shouldn't the second RCD pick up the slack?
@@anthonymillevolte3470 With respect to two RCDs in series, yes, if one fails the other will still be operative. On U.S. boats with an ELCI there are typically additional GFCIs in heads and galleys which are effectively in series with the ELCI but with a lower trip threshold (5 mA versus 30 mA) so we have exactly this situation. - Nigel
Can't the AC ground wire be "smart switched" to the boat ground? Set some minimum AC voltage threshold before the boat's AC ground and the boat's DC grounds are connected (3-4V). Wouldn't this offer some safety redundancy to an ELCI and also prevent the galvanic corrosion that can occur with a permanent DC-AC ground connection? If tripped it would simultaneously alert users to three things: 1) There is a ground fault, 2) the ELCI didn't do its job, and 3) it would be a particularly bad time to go swimming in the marina.
The problem with any kind of switching, smart or otherwise, is it can fail in the open position. In theory, the diodes in a galvanic isolator also can, but one of the requirements of the ABYC galvanic isolator standard is that they must fail safe - i.e., shorted - at which point you no longer have galvanic isolation, but you have the AC safety ground. - Nigel
I love the videos there's a lot of very useful information that I didn't know:
But with this video I believe there are some significant wrongly stated risks around m7.56 sec associated with not joining a vessels AC and DC ground
(1) There are 2 RCD's, 1 in the boat and 1 in the supply pedestal
BOTH would have to fail OPEN to present any danger - this is VERY unlikely, also a user should test when they connect, at least the vessel's RCD.
(2) With no AC-DC ground connection; If by some remote chance the ONLY faulty conductor in a cable is the GROUND conductor (so NOT, Neutral OR NOT obviously Live ) you would still need a fault to the case AND faulty RCD's to be at any risk. not impossible but extremely unlikely
Almost certainly a fault to the case will trip over current protection, I doubt these days the 'live metal case' scenario is very common, significantly more likely is a fault is to Neutral, and in any case the required INTERNAL fuse kills the power, caps, diodes, burnt PCB tracks, all kill the power even should one of the 2 RCDS not trip.
(3) If there must be a AC-DC ground I would suggest the best place for a galvanic isolation is between the AC ground and the DC ground point (always the engine?) eliminating anything in the path of the protective AC ground to the supply pedestal while still protecting the boat from galvanic current.
Note that in the U.S. there is typically not a shoreside RCD or ground fault protection of any kind on the dock, although this is now required in new marina construction. Also, the idea of placing a galvanic isolator between the grounding conductor and common negative bus was at one time recommended by a leading manufacturer in the U.S. It resulted in accidental ground paths that bypassed the isolator, resulting in galvanic corrosion. The galvanic protection standards are very clear that this device needs to be next to the shorepower inlet!
- Nigel
I'm no expert but trying to understand it a bit better,
Looking at this statement: "It resulted in accidental ground paths that bypassed the isolator, resulting in galvanic corrosion"
- I guess this refers to Bonding? while this might be common in America it seems far less so in Euro-land? also surprised everyone has not moved to composites.
Also I'm confessing I'm very skeptical about the whole ground current loop corrosion thing:
To be a problem there would have to be
(1) 200 mV or greater potential difference in the seawater between two metal fittings - tricky as the sea is such an excellent conductor
* Seawater Ohms : 4,000-5,000 microsiemens per centimeter = 1/5000 Ohms = 0.0002 Ohms per CM - as there's a lot of cubic cms resistance must be very low?
* Potential difference: To enable a 200mV potential difference between 2 thru-hulls say 2 meters apart, using the above seawater resistance there would have to be a huge current
* Current: Anything out of order would trip protection
A whole series of experiments to empirically resolve this must have been done already? Googling didn't produce anything clever, still looking
(2) A common ground or no current will flow - Bonding, AC to DC
As an aside: I long ago gave up trying to explain the difference between Galvanic and Electrolysis to boat neighbors;
Dang... no more having my Golden Retriever fetch balls in the water at the marina!
Wonder if Natalie Wood knew this stuff.
Personally, I am not sure that A/C grounding is as important today, as it was back in the old days. In the old days, appliances were made with steel or aluminum casings, and back then A/C grounding was very important. Today, most devices are made with plastics instead of conductive metals and are double insulated rated. Modern outlet electrical boxes are made of plastic instead of steel, making them safer than they use to be. Most of my electrical devices have power cords that do not even have a grounding prong on them. As I write this, I am running an electric space heater that is drawing around 12-amps continuous, and its power-cord does not have a grounding prong. If A/C grounding was important as it used to be, why are most electrical things manufactured without a grounding prong?
We should never underestimate the importance of AC grounding! However, we are talking about two somewhat different things here. The one is the installed AC system, which will also likely be powering installed AC equipment, much of which is not double insulated and which is still mounted in metal cases and on metal frames (for example, air conditioning condensing units, battery chargers, inverters), and the other is some corded equipment which plugs into power outlets and which may not have a grounding pin on its plug. As noted, nowadays (but not in the past) pretty much any corded equipment which does not have a grounding pin on its plug is double insulated. There is no mechanism to connect this equipment to the AC grounding system and if it is double insulated it is, in any case, not necessary. But there is still plenty of plug-in equipment which is not double insulated and which has a grounding pin (for example, every microwave I have ever seen, induction stoves, kettles). With respect to this equipment, and the rest of the boat’s AC system, it can be a matter of life and death to have everything properly grounded. - Nigel
So two things from this. One, if someone is swimming in a harbor then that person is an idiot for a multitude of reasons, but if the idiot is swimming near your boat and is suddenly spasming disconnect shore power asap. Second, if you have enough solar to not need shore power than everyone is safer.
divers cleaning the hull are idiots? Clown comment.
American did it wrong three times. First, a fuse does protect equipment from fire, not protect humans - they are dead before it trips. Second, adding the ground to the water and then introducing galvanic corrosion and shock for swimmers, they adding isolators. Third, choose a five mA far too low RCD current. What a mess. The European got it right with 30 mA. It is disappointing that they agreed to keep the hazardous wire to the water connection. Buying a cheap RCD tester to validate its safety function is enough to solve this idiotic compromise.
Saltwater is more electrically conductive than freshwater, so shouldn't that make it more dangerous to swim in saltwater than freshwater? So, wouldn't freshwater give you a tingle, but saltwater will kill you?
You are right that saltwater is more conductive than freshwater. But this is precisely, why it is MORE dangerous in freshwater, due to a different voltage gradient. Simply put, in this case, your body has less resistance in relation to the surrounding water. So more of the current will flow through your body instead of being led to ground through the water. -Jan