Electrical System Use and Maintenance
Part II: High Voltage (AC) Systems
by David H. Pascoe, Marine surveyor
High Voltage (AC) Systems
No one who is not trained in marine high voltage systems should ever perform wiring on a boat. The potential for creating deadly system faults that are injurious to both people and the vessel itself is too risky. That includes something as apparently simple as wiring a new service outlet, for if you don't understand the principles of the system, you are likely to unknowingly create a problem. Just because you know how to do wiring around the house, doesn't mean you know what you're doing on a boat.
Grounds and Grounding
One of the least understood aspects of a boats electrical system, and the most troublesome, is the proper method of grounding. That we often get questions of whether AC or DC electrical equipment should be grounded to the boat's bonding system is illustrative of this point. AC and DC grounding systems are two separate systems, for distinctly different reasons. If you don't understand these systems, you run the distinct risk of creating a disaster. Actually, there are four separate ground systems: DC ground, AC ground, AC grounding (or bond), and the vessel's bonding system. You can add to this lightning and HF radio grounds as well. Do you know the principles of each? Are you sufficiently confused to discourage you from doing your own wiring? I hope so. For unless you understand each thoroughly, you're headed for trouble.
The AC ground and grounding systems are "free floating," meaning that they do not ground on the vessel, but only to shore. The ground, or neutral, is a current carrying conductor, and is the source of many troubles because people do not regard it as such. The grounding, bond or green wire is the "safety" intended to channel current safely to ground in the event of a short circuit. Both of these circuits are capable of conducting current and can be the source of electrolysis when there are system faults with the dock or marina wiring. This is very easy to test for.
There is only one point where the DC side is grounded, and that is at the battery. It, too, is a "free floating" system in which nothing is ever grounded to any metallic part of the vessel, most especially not the bonding system. Just like a car sitting on rubber tires, completely insulated from earth potential, the battery itself provides the negative potential.
The bonding system, also green wire, has nothing to do with electrical systems. Underwater metals are simply wired together to equalize differences in potential of different kinds of metal. Nothing should ever be grounded to the bonding system. Unfortunately, some people don't understand this and use it to ground electrical equipment, occasionally with disastrous results.
Bonding simply means wiring all the boats underwater metals together. This is done because of the galvanism caused by the different metals. By wiring them together, the differing potentials are equalized. Bonding does not solve problems of galvanism or electrolysis, but it does spread the the flow of current around over more metal, so that 1/4 volt or so won't cause any damage. Whereas if that 1/4 volt were going to one small seacock, it would probably eat it up in a hurry. In other words, bonding lessens the effect of small amounts of current. On the other hand, it also spreads it around to all underwater metals so that higher currents end up damaging everything.
Bonding systems use wire and ordinary crimped ring terminals. After a while these get wet and corroded. Electricity doesn't flow very well through corroded metal, so your bonding system after a while stops working. To maintain it, simply cut off the old terminals and install new ones. Do you have wires attached to sea cocks with hose clamps? Forget it. This is putting stainless and copper together, which are galvanically incompatible and it won't work.
Your Bottom Paint
What does bottom paint have to do with electrical systems? Nowadays, with copper based paints, a lot. If, the next time your boat is hauled and you see large ugly burn patterns around all your underwater metals, you got a stray current problem. Copper-based bottom paints react severely to stray current, and serves as a great indicator. Sort of litmus paper for electrical problems.
Of course, the common wisdom is that the stray current "is from the marina." Or it's always the other guy's boat that is causing your problem. Don't bet on it. Most stray current problems are sourced on the boat in which they appear. Otherwise, everybody in the marina would have the same problem.
Electrolysis is a word that is badly abused by boaters who don't really know what it means, so let me correct this right now. First, understand that all boats have an electrical potential. That's because of all the different metals on the boat which, themselves have differing electrical potentials. This is exactly the same principle that makes a dry cell battery generate electricity. This electrical potential is called galvanism and is the reason why we put zincs on boats.
Electrolysis is stray current escaping from the system and is most damaging. It is an abnormal condition. When this happens, it will eat up the zincs in no time, usually leaving that metal looking bright and shiny. Therefore: Shiny zincs = electrolysis. Dull eroded zincs = galvanism.
I spent two years putting a meter on every boat that was hauled for survey. The average boat generates about 1/4 volt DC current and going as high as 1/3 volt without causing damage. But when it gets up to 1/2 volt, you got a problem. Zincs will erode rapidly and underwater metals begin to be affected.
Shore Power Cords
The single largest cause of problems with shore power systems results from failure to maintain the connectors on both the cord and the boat connectors. These devices are exposed to water and over time suffer from corrosion and general wear. High resistance caused by corroded, bent or worn connectors results in high resistance which causes overheating, which further amplifies the power drop. This not only creates conditions for a potential fire, but causes electrical equipment to work harder, resulting in reduced life span of equipment. IT PAYS TO MAINTAIN SHORE POWER CONNECTIONS.
You can perform a very simple check just by placing your hand on the shore cord near the connection to determine if it is heating up. Obviously, this should be done while you have a lot of equipment turned on. If it's anything but slightly warm, not more than 110 degrees, suspect a problem. Shore power connectors should be dismantled at least once per year, cleaned and repaired as necessary. Most of these connectors have replaceable parts. If you drop your shorepower connector in the water, you must take it apart, clean and dry it. Otherwise, expect it to burn up.
We recommend that you buy only the highest quality power cords, as these will last longer and have the advantage of replaceable connector parts. Cheap connectors usually can't be taken apart. We also advise against ever using the three-pronger household type adapters as this type of connector is highly unreliable and prone to causing system faults and fires. Only the twist-lock type connector is suitable.
One more thing: If you are not turning off the dock breaker before disconnecting the power cord, start doing it now. Not only do you risk getting electrocuted, but disconnecting an energized connector damages the contacts. Also consider what happens if you drop the energized cord in the drink!
This is an issue only with 125 VAC systems since 250 VAC systems will not function with wrong polarity. Since you have three terminals on a shore connection, wrong polarity can mean that any of these wires are in the wrong position. Not only should you pay attention to the polarity indicator on your boat, we recommend that you keep a plug in polarity indicator aboard and use it every time you hook up to shore power at a different location.
Reverse polarity is not only an electrocution hazard, but can also damage electrical equipment. It is most often found with the three prong spade connectors (household type), but occasionally twist lock connectors as well, particularly in marinas with dilapidated equipment. Never trust the power supply at strange docks, but always check the polarity. When hooking up to strange docks, always check your volt meters to make sure you have adequate voltage. Low voltage is very damaging to electrical equipment. Turn on the stove or water heater and watch what happens to the meter.
Main Circuit Protection
Many people think that the circuit breakers on the dock protect their boat. They do not; they only protect the dock wiring. Your main circuit breaker protects your boat's systems. But what about that section of wiring and connectors between your main panel and the dock breaker? Well, the fact is that it is unprotected. which is why so many fires occur. Check out all the top end boats and you will find that they have circuit protection located directly at the shore connectors. Which is why we recommend that you should too. Having slow blow cartridge fuses installed directly at the connectors can go a long way toward preventing fires and burned up shore cords, particularly if you are a traveler and frequently rely on uncertain power supplies. Circuit breakers should NEVER be installed on the exterior of the boat. Only gasketed, water proof cartridge holders should be used.
Circuit breakers wear out, and when they do they work less well, or not at all. If you are using circuit breakers as ON/OFF switches, you are helping them wear out that much faster. It also damages breakers when you shut off equipment via the breaker. This causes arcing at the contact points which damages the points. When connecting and disconnecting shore power, you should always turn OFF equipment at the appropriate switch on the equipment. Then shut the main breaker off. Do not ever simply throw the main breaker off to shut down equipment that is operating. The circuit breaker arcs and damages it.
Also be aware that any equipment run by a motor, such as air conditioning and refrigeration equipment, start up with an initially much higher amperage than the normal running amperage. An air conditioner that runs at 14 amps may have a start up amperage of 20 amps, so that if you just go and turn all the equipment on at once, it overloads the system. Then the circuit breaker gets hot and won't stay engaged until it cools down. Ergo, start up heavy equipment one item at a time, allowing it time to cycle into its normal operating voltage before turning something else on. For example, don't turn the AC, refrigerator and icemaker all on at once and not expect the breaker to pop.
Chronic Breaker Popping
It's human nature that when a problem appears, we wish it to go away. Breakers that pop frequently are signaling that there is a problem, which could either be the breaker, or something in the circuit. Yet most people will keep on attempting to make the breaker engage. This can be dangerous because you may cause the contact points of the breaker to fuse together from arcing, in which case it will never trip again. The above photo shows what can happen when you do this. DO NOT ATTEMPT TO ENGAGE A BREAKER THAT IS OVERHEATED BY FORCING IT. You must allow it to cool down.
If you are experiencing chronic problems with circuit breakers popping, first check how much current draw is involved. A single 30 amp circuit is not much when you're running things like air conditioners, water heaters and battery chargers. One very simple way to check whether you're dealing with an overload problem is to add up the amperage draw of each piece of equipment. List both the start up and run amperages. You will usually find the amperage given right on the equipment label. By making a list of the total power demand, you'll get a good idea of what you can and cannot operate simultaneously, particularly when starting the equipment. If you have an ammeter on your panel, check it against the amperage tally you made. Ideally, you should try to hold power consumption at 80% or less than the line rating.
Check the breaker by allowing it one hour (or whatever it takes) to cool down. Turn the equipment off and, after it is cool, reengage the breaker. Now turn the equipment back on. Place your finger on the front of the breaker and note its temperature. If it does not heat back up again, then the problem was probably a start-up overload. If the temperature rises again, there is a fault in the circuit or the breaker. (Note: when the breaker contact points become eroded, the breaker itself can overheat).
Ground fault current interrupter service outlets are required to be installed in wet locations such as the galley or head. In reality, there's little chance of being electrocuted inside a boat because you are not grounded within the boat. A greater risk is from service outlets being located in places that get wet, such as below leaking windows, hatches or close to doors. Three pronged plugs are prone to shorting across the terminals when wet, so having all your service outlets changed to GFCI's is a good idea. Use only the highest quality devices from a reliable manufacturer like GE, and not the el cheapos from the local discount store. They aren't worth having. Service outlets located anywhere on the exterior of the vessel are an invitation to trouble for reasons that should be obvious.
Tips on Electrical System
|I||Introduction with 2 photos|
|II||High Voltage (AC) Systems with 0 photos|
|III||DC Systems with 6 photos|
|IV||Adding New Equipment with one photo|
Posted November 15, 1998 (First posted July 14, 1998 at www.yachtsurvey.com. Revised and added two pictures November 02, 1998. Page design changed for this site.