120v Inverter/Shore Power switch or relay?

smckean (Tosca) · Posted: Thu Feb 04, 2021 5:17 pm Post subject:

I believe we have some talk of apples and some talk of oranges going on here at the same time....

Grounding is an issue in 2 completely different situations. One is protecting against a surge due almost always to lightning; and the other is providing an Equipment Grounding Conductor (EGC.....this is the "green wire") to protect humans from a ground fault in a circuit. We've mixed up these 2 protective practices in this conversation.

B95054 is correct when he says: "....we have to understand why to ground in the first place. It is there to protect you and your appliances from surges.". But I presume B95054 is talking about a surge created by lightning. Only lightning surges need to be directed to the earth (Note electricity does not "seek the earth", but rather it seeks to return to its source by whatever and all paths it can find with the most amps flowing in the paths with proportionally lesser resistance.) In the case of lightning, going to the earth is going to its source; but in the case of home wiring, the source is the transformer down the street; or while on shore power, the source is the transformer supplying the power on the dock; or in the case of a inverter or generator, it seeks to go back to the inverter or generator from whence it came. As journey on explains, the use of the green wire (EGC) is totally different than the surge (lightning) case. The EGC is there in case there is a short to the metal case of the drill, toaster, or whatever appliance you are using. Since the ground (green wire) is connected to the metal case (or other conducting surface you might touch), if a short like this happens, the circuit breaker (over current device) trips since the green wire provides a ultra low resistance path back to the source (instead of your body perhaps being part of some other higher resistance path back to the source).

My project and discussion is about providing a EGC. I have no lightning protection on my boat, and I don't feel that I need any here in the PNW.

B95054 · Posted: Thu Feb 04, 2021 7:54 pm Post subject:

Surges are created by many more issues than lightning strikes. Actually over 75% of surges are created within the home. When you are using a high draw appliance and it shuts off abruptly a surge occurs, Surge Protective devices help but the ground rod is the path that the excess energy will ultimately follow.
On a floating ground you are correct....a breaker or fuse trips stopping the flow of electricity through the circuit.

thataway · Posted: Thu Feb 04, 2021 9:31 pm Post subject:

Several thoughts: If you get a lightning strike--the grounding on the inverter will most likely be one of the least of your problems on a C Dory.

Since 2014, updated code required marinas to install Ground Fault Protection (GFP). which would trip at 30mA or more. ABYC recommended boat manufacturers install a similar safety device at the entry point of the boat’s shore power, called an Equipment Leakage Circuit Interrupter (ELCI). Great for new boats--not so good for our older batons.

Since 2018 many marinas which re-wired, were forced to set the ground fault current at the pedestals to 5 to 6 mA. This became a problem for many vessels, since their grounding systems were based on the older standard of 30 mA. Also the main circuit ground fault was set at 30 mA vs the older standard of 1000mA. There has been a technical bulletin clarifying the rule for individual boat slips vs "floating housing". The design engineer can now use the old standards and design a system at that level or below.

Many of the Marinas along the ICW and in the Southern states where hurricanes have demolished docks, and new replacement pedestals were installed are ones at issue.

So the transient boater is still subject to possible tripping of the ground fault protection device at many upgraded marinas. Inverters are close to the top of the list of appliances which will trip the ground fault interrupter. Corrosion of fittings, older Galvanic Isolators or faulty Galvanic isolators, as well as defective power cords are also on the list of problematic items aboard.

smckean (Tosca) · Posted: Fri Feb 05, 2021 12:41 am Post subject:

B95054 · Posted: Fri Feb 05, 2021 9:40 am Post subject:

The current comes into the house on the hot lead and returns to the transformer via the neutral. In a voltage surge situation, let’s use a surge protector as an example...the excess energy gets collected in a storage device. In most cases this is a capacitor. The surge protector is tied to ground and will bleed off to the ground and not back to the transformer. I will stake my 30 plus years reputation as a consultant to electrical and electronics engineers on this. An electrical circuit requires balanced power to flow and any imbalance requires either equipment to rebalance (ie a surge protector) or it gets absorbed by your appliances. Over current in appliances can cause anything from failure to fire and/or safety risks. In the case of the floating ground the breaker serves to stop all flow of electricity when the imbalance occurs because you have nowhere else for it to bleed off to.
And btw - yes over 75% of surges are generated inside the home. Utilities have huge surge suppressors on the line that greatly reduce the effects of lightning strikes and also have the equivalent of breakers on the line to interrupt the circuit rather than allow the surge into your panel.That is not to say that lightning strikes cannot make it into the home (and they do) but in general they are greatly attenuated and not the number one cause of over current failures.
Take the example of a welder. It uses a high draw with sudden stops. Every time there is an abrupt stop there is a wave of current that needs to go somewhere. Hence the need for large surge suppressors in machine shops. On a smaller scale think of anything that pulls a high draw in your home. Turn it off abruptly and it will affect the waveform of all other running circuits. This is an over current situation generated in the home know as a surge. This is the number one failure of electronics.
I’ll tell you a little story...
A few years ago we had a situation at a Sobeys store where they were burning out their flouresecent ballasts at a rate of one to two per day. They were complaining that ballasts are built too cheaply and were laying the blame on the manufacturer. When I went on site you could see that there was a GM plant on their right and a machine shop on their left. All on the same grid. When I explained to them that they likely had a dirty power scenario they put up an argument that their power was clean and gave a very similar argument as above that all excess power would return to the utility. Not their fault, most people don’t have a clear grasp on electrical but somehow I had to convince them. So I suggested they install a surge suppressor with a hit counter and we will evaluate it after thirty days. If there were less than a couple of hits I would pick up the tab for it. If there were more than a few hits they pick up the tab. We did it and 30 days later it had registered 72 hits with no ballast failures. At that time I also noticed they have a wall of TV sets so I spoke with the electronics dept manager and asked if he had any tv failures. He explained that they are garbage and he loses on to two TVs a month. They just don’t manufacture them like they used to he said. So I offered him the same deal. He took it and when I went back to follow up after a year they still had not lost another tv since the installation of the surge protector.
So yes excess current is generated internally and does not return to the transformer. One way or another it needs to find its way to ground. If the ground is overloaded you need to find a way to slow down the amount of surge it needs to handle but one way or another it needs either a ground to bleed off to or an interruption in the circuit to stop all electricity from flowing.

smckean (Tosca) · Posted: Fri Feb 05, 2021 3:58 pm Post subject:

B95054 · Posted: Fri Feb 05, 2021 6:08 pm Post subject:

Absolutely correct and shame on me for attempting to oversimplify the flow of current into the house. There is still the issue of surges and how they are handled through the ground. But I will leave that subject alone. Suffice to say that leaving the ground unconnected as somebody posted above is a bad idea ( maybe that is just what I should have said in the first place and not over complicated things). I’ll leave it alone now too.