Should ac and dc grounds be connected on a boat

Should ac and dc grounds be connected on a boat

Five of them are described here

1. Incorrect cable sizing

There are several problems in using the ABYC 105°C table to determine the size of cables in a ship's electrical system.

Using cables with insulation less than 105°C.

Connecting wires together or burying them in insulation.

However, in general, cable size is not an issue because.

The lifetime of most large DC loads is short.

Most DC cables are sized based on voltage drop and are therefore larger than the minimum values recommended in the ABYC tables.

Cables are sold with different insulation temperatures. The maximum temperature typically used for grounding cables is 90 °C and many of the most common cable materials are rated for 75 °C, 80 °C and 90 °C. The ABYC 105 °C table can only be used for cables rated for 105 °C.

Heat is generated in the cable due to current resistance. The temperature of the cable depends on the heat generated by losses in the cable and the efficiency of the installation to dissipate this heat. This heat escapes to the air or to a heat sink, such as a solid copper bus bar. Attempts to carefully "wrap" the cables by tying or tucking them between the hull and the sheet metal make the situation worse. The better we secure the cable with grommets and run it between the sheath and the shell, the harder it will be for heat to escape. Efforts to make the installation cleaner can be potentially dangerous if the cable size is not increased to compensate.

The biggest problem is the AC circuit feeding the socket, which can easily become overloaded.

Even with a cable rated at 105°C at maximum current, the cable may be too hot to touch without burning. At these high temperatures, corrosion of the terminals will accelerate and the life of the system will be shortened.

2. always use a full-strength fuse.

When it comes to matching circuit protection to the cables to be protected, two facts contribute to the complexity of this task.

Fuses blow and circuit breakers trip at amperages much higher than those indicated on the equipment.

Cables and circuit protectors can become very hot after a few minutes of charging to 100% capacity.

SEA, Maxi, ATO and AGC fuses and most circuit breakers trip at 130% of their rating. ANL fuses operate between 140% and 266% of their rating. When fuses conduct 100% of their rated current, they generate excessive heat. They also generate too much heat when the cable conducts 100% of its rated current. Combined, the heat generated by fuses and cables carrying high currents can melt the insulation of cables and fuse assemblies. This heat generation can become severe when the load is operated for long periods of time. Large diameter cables take a long time to heat up, so short duration operations, such as bow thrusters, windmills and manufacturers' pumps, rarely last long enough to cause this type of heat generation. For example, a 2/0 cable takes 25 minutes to reach its maximum temperature. On the other hand.

For loads and equipment that run continuously for 10 to 30 minutes, the circuits and line protection must be sized so that the current does not exceed 80% of the rated capacity.

3) Do not place the locking ring on a grounded power cord.

The locking ring on the grounded power cord provides a secure connection between the plug on the power cord and the socket on the housing. If this connection is not secure, the plug can move back and forth in the socket, affecting the electrical connection and causing dangerous overheating.

Grounded plugs may be subjected to electrical and mechanical stresses. A locking ring prevents the plug from slipping out and holds the two components together so that the connection does not move during normal movement. As the vessel moves, the connection between the plug and socket is constantly being driven, which can loosen the connection, increase corrosion and weaken the spring contact stress.

Power cords and plugs are the most overloaded areas of a ship, as they supply power to the entire AC system, including the plug circuits. Every new appliance added is a new load, with appliances such as electric grills, hair dryers, air conditioners and heaters all drawing about 12 amps. It's easy to plug in more than the system's rated capacity until the overload reaches 130% or more and the circuit breaker trips. In this case, a poor connection between the plug and the outlet can be a source of fire.

4. There is no "green wire" or bottom connection between the DC negative terminal and the AC safety ground.

If there is no good connection between the negative DC terminal and the AC safety ground, AC current may enter the DC ground system. In this case, AC current could enter the water around the boat and injure or kill nearby bathers.

The green wire is the safety ground wire, which connects the negative DC ground block to the AC safety ground rod. The purpose of this wire is to provide a low impedance ground path for any AC current reaching the DC ground system. There have been instances where AC current has entered the water around the boat through the motor shaft, resulting in the death of bathers near the boat.

This green connection has a negative side. This safety ground can also provide a pathway for electrochemical currents if the boat is not properly protected by an electrical insulator. However, most maritime organizations and professionals now install this cable as standard. Safety requires that the shore cable be wired directly or through the galvanic isolator or a properly installed marine isolation transformer. Some people ignore grounding wires, mistakenly believing that they provide galvanic protection, but forgetting that they put the safety of people on board, on the dock and in the water at risk. Drowning by electrocution is now considered a previously undocumented cause of death. The Coast Guard is funding research to isolate and investigate this risk.

Green wires can be tested and show continuity, but do not carry enough current to trip the circuit breaker in the event of a fault. There are ways to check the quality of the connection.

An ohmmeter test may show that the green wire has a very low installed resistance, but the wire may not be able to carry 30 amps or more, requiring a trip on failure. The minimum resistance value of an ohmmeter does not necessarily indicate whether a connection is compromised, for example, a connection made with only one conductor. There are special ground resistance testers that can obtain high amperage readings, but they are rare. A thorough visual inspection of the ground connection is useful, but even a careful inspector may have difficulty finding all the connections and tracing the conductor.

One way to check the quality of the green wire is to connect a reflector or other heavy 12-volt load, with the positive terminal connected to the boat's battery and the negative terminal connected to the cable's safety ground pin. On a properly wired boat, the safety ground pin should return to the negative terminal of the battery when connected to the AC panel. If the light glows brightly and evenly, there is probably a good ground connection. This is a good check if the boat has an unknown maintenance history, has been refurbished, or is being repaired after damage.

5 Check the integrity of the ground connection with an ordinary plug tester.

Ordinary plug testers are very sensitive and will show a good ground connection even if the only connection is through the propeller shaft or hull to water. A better way to check the integrity of the ground connection is to connect the ground wire to the boat and place the ground plug near the control panel. With the AC power to the boat disconnected, use an ohmmeter to check that the ground pin is properly connected to the boat's safety ground system. Check the "U" ground connection at each outlet by pulling the end of the ground wire and meter to each outlet to check.