Checking your Shore Power System

By Wayne Canning,AMS
www.4abetterboat.com

Most modern boats over about 25 feet are now equipped with some sort of shore power system. These systems add convenience and comfort to your boating experience but they can also bring hidden dangers. Your boats shore power is potentially the most dangerous equipment you have onboard. The real danger is in that you cannot always see, smell, or feel the danger. However there are some simple tests we can make to make sure our system is safe and stays that way. Many older boats were wired prior to today’s safety standards used by most boat builders. Add to that years of additional equipment installed and corroded connections and you have the potential for some real problems. As a marine surveyor and boat owner I have seem many systems that are accidents waiting to happen. Even some newer boats are not as safe as one would hope. Most electrical standards are voluntary on the part of the builder. Although most builders do their best to produce a safe product mistakes are made. I have seen some serious errors on brand new boats built by major manufactures. So never assume your system is safe and free from defects.

The good news is there are some simple tests you can do to determine the safety and integrity of your system. Although you do not need an electrical background to perform the needed tests some basic understanding of electricity will help. Before you begin I would recommend reading some of the great books out there on marine electrical systems to familiarize yourself with the basics before starting. Of course I know some of you are just going to jump right in so I will try to keep things simple. These tests will not expose all potential problems but should give you some basic piece of mind and will hopefully find the big problems. If you find anything not right I would recommend consulting an ABYC certified marine electrical technician for further help and advice.

Tools needed:
A Mutimeter or VOM (volt ohm meter.) I like the digital ones with a rugged case. I prefer a mid range meter, the cheap ones are just that and will not give you good service and the expensive ones have too many bells and whistles making them hard to use. Look for one with diode testing ability. You should have one onboard in your tool kit already but you if do not now is a good time to get one. A decent meter should cost less than $50.00
A 3 light outlet tester. These are cheap and can be gotten at any hardware store. Try to get one with a test button for GFIC outlets.
A AC clamp on AMP meter. If you do not have one of these I would try to borrow one from someone around the marina. They are not expensive but not something you will use on a day to day basis. Most of these have other functions that are useful as well so if you have to buy one it would be a handy tool to have onboard. Try to find one with a large diameter clamp ring so that it will fit around 2 30 amp shore cords at the same time.

A non contact IR thermometer. These have gotten cheaper the last few years and are now affordable for basic testing. These can be handy for checking your engine as well so once again a good tool for your onboard kit.
A male 30 AMP 125 volt to female 15 AMP adaptor. You can buy these pre-made or make one up yourself. If you make one pay extra care to the polarity of the wires if that is wrong all your tests you do with this will be wrong. This can also be a handy tool for working on your boat at the dock as you can use it to plug in power tools to a 30 AMP receptacle.
Finally basic tools like screw drivers to access the back of your electrical panel.

Safety:

Always use common sense and disconnect power sources while working in exposed panels. If your boat is equipped with an inverter, be sure the DC power is disconnected from that before proceeding. Inverters can be as dangerous as shore power and some will not show voltage until a load is applied and that load could be you! Just to be sure plug something in and turn it on to verify you no longer have power the meter alone my not be enough to turn it on. If the boat is on land make sure there is a ground wire connected to the prop shaft or other good ground. This should be done anytime you haul your boat and should be in place when ever shore power is connected while on land. I like to use a 15 AMP plug with just a long green ground wire with a clip on the end. Plug this into a close 120 volt outlet and clip the other end to your prop shaft.
Always check to verify any parts have been de-energized before touching or using tools. Double check with a volt meter or test light just to be on the safe side. Unplug the sore cord while working if you do not need the system energized. Never assume just turning off the breaker is enough. Leave a note at the power box if you will be working below and out of sight this will prevent anyone from coming along and plugging the cord back in thinking it has fallen out, this has happened to me. Check and recheck you might not get a second chance. And of course wear rubber soled shoes and do not stand in water while working on electrical equipment.

Testing the source:
The first thing to do is a quick visual inspection of your system starting with your shore cord. Unplug your cord or cords and inspect for chafing, cuts or cracks in the cover or other damage. Make sure the ends have molded covers or properly fitted weatherproof boots and that the lock rings are intact. Look at the prongs and contacts for signs of burning or heat damage; these are signs of other problems. Check your boats inlet fittings for heat damage and burning as well. Make sure the cord has not been spliced and that if the ends are not original that they are marine type and properly fitted with covers and lock rings, we will check for proper wiring shortly. If there are any defects they should be corrected first. If the cord is old or in bad shape I would just replace it. Also make sure the cord is of marine type. It should be yellow or white and market for marine use. Marine cords are designed for flexibility, resistance to oil and solvents and have heavier covers to protect from chafe and cutting. So even though a good quality extension cord may be cheaper and look like a good alternative it likely is not suited to the task. 30 AMP cords are not that expensive and in many cases the cost to repair properly is simply not worth it. 50 AMP cords can be more expensive and may well be worth repairing but only if the cord itself is in good condition and the problem is just a bad end. With the rising cost of wire you will just have to evaluate the parts and labor verses simply replacing. If the cord has had a replacement end fitted it is a good idea to check the polarity of the cord. You can do this with the cord unplugged and using your VOM. Check the continuity of each contact making sure they are the same on both ends. You can also use you 3 light tester if you have a 30 AMP to 15 Amp adaptor. With the adapter plugged in and the three light tester plugged in, plug the cord in and observe the indicator to verify the cord is correct. Before performing this test and several others we will get to, it would be a good idea to verify that your power source is good. Using your 3 light tester plugged into the adaptor, plug the adaptor into the power source you use on the dock power pedestal. If a fault shows try a different location. If you have faults and are sure your adaptor is good report the problem to your dock master and do not plug in your boat till the problem is corrected.
After you are sure your power cord or cords are good and your dock source is good, plug them into the boat and energize the system. The correct order for connecting your boat to a shore side power source is to turn off all breakers on the power pedestal and in your boat. Once the cord is properly connected and the lock rings are in place turn on the power at the dockside pedestal first. Check your polarity and power indicator lights on your panel. Also check the voltage on your meter if you have one. If everything is correct turn on your main breaker followed by any other breakers you wish to have on. Plug your volt meter into a 125 volt outlet with no other loads plugged into it and turn on the breaker to this outlet. Make sure this outlet is not on a string of outlets with loads applied. In other words if the outlet is on a breaker labeled port outlets make sure everything is unplugged from the other port outlets. Observe and record the voltage. Now turn on most of your normally used appliances including the water heater and air conditioning if you have them and recheck the voltage. You should not have a drop of more than 10% or around 12.5 volts. If it is more than that you may have a problem. With your non contact thermometer check the temperature of the shore inlet, the wiring running from the inlet to the back of the panel. (Remember to turn off the power before opening the panel.) If the temperature is higher then 10-20 degrees above ambient you may have excessive resistance and should find the source of the problem. Often on older boats the wire from the shore inlet to the panel is undersized for the loads. If the boat was built 20-30 years ago not much electrical equipment was used at that time so there was little need for a heavy wire. This is particularly true if air conditioning and or an inverter/charger have been installed. These items can draw a heavy load particularly during start up. Burnt cord ends and a large voltage drop are sure signs of either an overload for the system or excessive resistance in the system or both. You should have a main breaker within 10 feet of the shore power inlet. The closer the better. If the wire run to the main panel is greater than 10 feet with no breaker I would consider adding a breaker as close to the inlet as practical.

Checking polarity:
If everything looks good to this point it is time to look at other parts of the system. First check your main panel polarity lights and make sure they are not lit. If they are lit then go back and start at the source and check your way back to the boat. If everything is good up to your inlet then you have a problem with the wiring from the inlet to the main panel. If the reverse polarity lights are not lit you are good to continue. Armed with your 3 light tester, test all the 125 VAC outlets in the boat. Note any that show faults. Check the GFIC outlets with the test button on your tester. When you do this make sure you know where all the GFIC outlets are or listen carefully for a click when you do. Sometimes these can be hidden and hard to locate. If you do not have any GFIC outlets then replace all standard outlets in wet locations (head galley etc.) with GFIC outlets. Be sure to check hidden outlets like those behind TVs and micro wave ovens as well. It can be surprising where builders will hide these outlets. If any outlets show faults find the cause and repair as soon as possible. Most of the time the fault is in the wiring of the outlet, at the outlet, so check this first. If multiple outlets show faults it is possible the whole boat has a reversed situation or that circuit is wired incorrectly at the panel. Finally with the power off open the panel and do a visual inspection of the wiring. Find the ground (green wire) and neutral (white wires) buss bar. All black wires should go to the breakers all white wires to the neutral buss bar and all green wires to the ground buss bar. If you have a 240 VAC system you will also have red wires going to the breakers. You may also want to check appliances that have been hard wired with your multi meter. This is a little harder as you have to access the wires at the appliance. But in some cases it can be worth the effort. The reading should be 120v between the hot (black) and Neutral (white), 120v between the hot and ground (green) and less than 3v between the neutral and ground. If you get anything else you have reversed polarity and it should be corrected at the source. While you are at it, make sure the appliance neutral is not connected to the ground. This is common in many household appliances and should be checked. We talk more about neutral ground connections shortly.

Ground Connection:
A good ground is key to a safe system. Electricity is always seeking a path to ground so by providing this path you can avoid it using you as a path. So this is a good place to start. Try to determine if your boat is equipped with a Galvanic Isolator or not, this will help knowing how to test your ground. It is not always accurate to test you shore ground with an ohm meter. It can look fine yet have resistance under load. One more reason to be sure the ground on your vessel is in good condition. I have had the argument from several owners that as long as the ground is connected to the shore ground there is no need for the AC and DC ground to be connected on the boat and that connecting them just contributed to underwater corrosion. This argument does not take into account the fact the electricity will find a path to ground with the least resistance. If there is resistance in you shore cord or either end of the cord the electricity might find that you are the path of least resistance. You could be using a power tool and touch a DC ground and suddenly find you are a conductor a dangerous situation not worth the risk.
If you have a galvanic isolator installed on your ground you may want to test that as well. Many of these units have built in self test systems but not all. Early units did not have the built in monitors. A galvanic isolator is a device installed in your AC shore ground wire consisting of diodes used to block low voltage DC currents from the shore connection. I will not go into the full theory and reasoning for this unit right now but rather just focus on making sure it is functioning correctly. To test the proper function bring the shore end of your shore cord into the boat. Next with your digital multimeter set to the diode test function, put one lead on the ground lug of your cord and the other end onto a ground (engine, ground buss, receptacle ground etc.) The reading should rise to approximately 0.8 +- volts. Reverse the leads and repeat. If you get less or more there is likely a problem and you should have the isolator replaced or tested by a professional.

Ground Neutral Connection:
The ground and the neutral wires should only be connected at the source, meaning on the dock, at the generator, and/or at the inverter. If it is connected on your boat it can allow current to flow from your boat ground back to the source thru the water thus becoming a shock hazard and possibly causing galvanic corrosion. To check this unplug your cord from the power pedestal . With the boat main breaker on and if you have one, the shore generator selector switch set to shore, use your meter set to ohms on the neutral and ground pins of your shore cord. You should have a reading of more than 25,000 ohms. If you have a generator switch place it in generator position. Your reading should change to less than 1 ohm. If you have an inverter you will get a reading of 1 ohm or less on those circuits that are connected to the inverter. Inverters should have an automatic switch to connect the neutral ground when shore power is disconnected and the inverter is in inverter mode. If loads not connected to the inverter such as a battery charger show a neutral ground connection as well, you may be sharing the inverter neutral with the shore power neutral. These should be separated so that you have a neutral buss for shore loads and one for inverter loads and these are not to be connected.

The AC ground needs to be connected to the DC ground as well. This is important so that any fault will have a low resistance path to ground. Electricity is always looking for a path to ground and will find it thru any path it can find including you. By connecting the AC ground to the DC ground you are providing any fault current, a path to ground thru the engine and your bonding system. With your power disconnected at the shore cord use you meter to check between the AC ground and the DC ground. The best way to do this is at the main panel, look for the AC ground buss bar (greens wires) and the DC ground buss bar (black or yellow wires). Your meter should read less than 1 ohm between the 2. If not you do not have a proper connection. Most of the time you can just visually inspect for a connecting wire. This wire should be large enough to handle a full system fault, at least a #10 or #8 in most cases. If you have more than one ground buss bar check them all.

Ok we have checked and made sure the power coming into the boat is correct, we know our polarity is correct and that we do not have a neutral ground connection unless the generator or inverter are on and operating. If everything is good and any faults corrected we are ready to move onto checking for any AC ground faults. Most of the time you would assume an AC ground fault would result in a tripped breaker but this is often not the case. 80% of all faults are undetected because they do not trip the breaker. These faults are a shock hazard for both people on the boat and in the water near the boat. They are also a fire hazard (heat from resistance) and can add to underwater metal corrosion and excessive zinc usage. That last one may have gotten your attention as this is the single most common complaint I hear about and often the most noticeable.

Testing for an AC ground fault starts once again back at your shore cord and at the power pedestal. This time using the clamp on amp meter turn the breaker off at both the shore source and on your boat. Check the reading on the meter, you may see something but just note it as this is the background reading. Faults on other boats or the shore system will often try to find a path back to ground and this could be thru your boat. This is not necessarily a problem so do not worry yet. And do not be too quick to blame you neighbor in the next slip as this can be caused by the local electric grid as well. Now turn on the shore breaker and your boats breaker. And turn on as many loads in the boat as possible. The difference between this reading and the first background reading is the net fault current being caused by your boat. If the reading is zero or did not change then life is good and you are done with this test. If the reading increased this is showing the amount of current flowing thru the ground wire and out the boat into the water. If you have multiple cords try to clamp around both at the same time and check your reading. If your meter clamp will not fit around both measure each one separately and take the total as your leakage. How much would be too much at this point depends but considering a fire can be started with as little as 300MA. Anything over 250MA for salt water and 100MA for fresh water should be investigated. At this point the meter is only measure the fault to the water to find the total fault you should measure just the ground wire by itself. At this point I would recommend you get an ABYC certified electrician to look at your system, explain the tests made and have him check for the cause.

Hopefully at this point you have a better understanding of how to test your system for faults. The goal here is not to turn you into an electrician put to help you better understand your system and give you the knowledge to test the health of your system. I would also recommend reading some of the good books out there on marine electrical systems. This can be a fascinating subject and having a good understanding of the fundamentals will help you understand the problems. It is a good idea to check your system at least annually and after any major changes or installations.
Many faults can lie hidden, you may have trouble with excessive zinc usage or you may not notice any problems till it is too late. I have been in more than one boat and gotten shocked when I inadvertently touched an appliance and a ground at the same time. These days we cannot live easily without AC power but there are things we can do to keep it safe. Spending a couple of hours every spring to test your system will go a long way to protecting yourself and those on and around your boat. Always use common sense when working around electrical equipment. Always double check with a meter before touching any potentially energized parts. And finally when faults are found or suspected consult a qualified marine electrical technician. Tell them what tests you have done and if possible be present when they fix the problem so you can learn more. And finally enjoy your boats electrical system knowing you are safe.