In this article, you can read about the basics of electrical systems and the concepts of DC and AC.

In order for the article not to be so extensive, I have broken out the parts that are about

1. Ohm’s law
2. How to use a multimeter
3. More about alternating current

Electrical polarity

In order for an electrical voltage and current to occur between two points, one point must have an excess of positively electrically charged particles with a higher electrical potential (electrical voltage) than the other point.

The point that has a surplus of positively charged particles is called the Plus pole and the one with a deficit of the Minus pole.

Electrical voltage

Between the poles, there is an electrical voltage that depends on the electrical potential of the positive charges. How an electric potential arises is beyond the scope of these articles, so you simply have to accept the fact that there is an electrical potential and that it determines the electrical voltage.

• Voltage (U) is measured in the Volt (V) unit

In a battery, such a polarity has been created chemically. The plus pole of a battery is also called the Cathode and the minus pole Anode. There are several different chemistries for batteries and the most common on boats are those based on lead-acid or lithium-iron-phosphate (LiFePO₄ ).

Electrical conductor

An electrical conductor is something that conducts an electrical current, in cables, the metal copper is most often used. There are other substances that also conduct electricity, such as water.

Electric resistance

All electrical conductors have electrical resistance, which slows down the electrical current. This reduces the voltage along the conductor. How much it decreases depends on the resistance and in the section on Ohm’s law you can read more about resistance.

• Resistance (R) is measured in the unit Ohm (Ω)

Electric surge

The voltage can cause an electrical surge in the air (a flash) that can ignite something. This happens when two poles with different polarities get so close to each other that the electrons jump through the air to the other pole. In the boat, this happens if you, for example, disconnect a cable from one of the battery terminals when a current passes through the cable. It happens regularly inside switches when you turn a lamp on or off. In dry air, approximately 100V per mm of air is required between the poles for the air to start conducting current and a flash to form. If the air is humid, as little as 10V per mm of air between the poles may be enough for the air to start conducting current.

Electric current

An electric current consists of positively or negatively charged particles moving through an electrical conductor connected to the positive and negative poles of a battery or generator.

• Electric current (I) is measured in the Ampere (A) unit

The electrical voltage between the poles means that the difference in positive charges is evened out by electrons that are negatively charged moving from minus to plus through the conductor. The higher the electrical voltage (potential energy) between the poles is, the greater power the current will be able to have.

This transport of electrons from minus to plus reduces the positive charges in the plus pole and one can still (theoretically) consider the current as a movement of positively charged particles in the opposite direction.

The current (I) is measured in amperes (A). How large the current becomes depends on the consumer’s electrical resistance which is called electrical resistance (R) and is measured in ohms (Ω). In the article on Ohm’s law, you will learn to calculate how big the current will be.

All cables have a resistance that depends on the length and thickness of the cable. This requires that cables must be dimensioned so that they can transport the current (amperes) a consumer requires without creating too great a voltage drop (volts).

A 12V USB charger consumes about 0.1 A while a 4 Hp strong bow thruster takes 500A at 12V DC and that is why cables for strong motors need to be much thicker.

Overheated cables

If the cable is too thin, it gets a higher resistance (ohms) and we lose voltage (volts) on the way to the consumer, who can then get too low voltage. The same thing if there is poor contact in a coupling, eg at the battery, then the resistance in that coupling increases. If the cable is much too thin, it heats up faster than it has time to cool down and in the long run, it then becomes so hot that the insulation around it begins to melt and in the worst case it starts to burn in something nearby that is more flammable than cable insulation. It can also melt the insulation on a nearby cable and cause a short circuit or other strange faults in the electrical system.

Remember that it is the current that causes heating in a cable.

Series connection and parallel connection

When connecting electrical components, they can be connected in series (one after the other) or in parallel (next to each other). see the picture below.

It is also possible to connect batteries in series or in parallel as shown in the picture below. Assume the capacity of one battery is 100Ah.

When connected in parallel, the voltage becomes the same and the capacity is doubled to 200 Ah

When connected in series, the voltage from each series-connected battery is added and the capacity is the same.

Analogy to electricity

To describe what electrical voltage and current are for something, one usually compares it with water that flows down a waterfall.

• Electrical voltage corresponds to the height of the waterfall.
• Electric current corresponds to the amount of water flowing over the edge every second.

If you fill up with water at the top, gravity makes the water flow over the edge. The higher the waterfall, the higher the gravitational energy (potential energy) the water has and resulting in a higher speed of the water when it comes down (it hits the water surface with greater force, effect).

Different forms of electricity

DC voltage 12V or 24V

DC means that the nominal voltage in the cable is constant with constant polarity. Onboard a boat, this means 12V or 24V.

The cables connected to the plus and minus poles always have the same polarity and the same nominal voltage (12V / 24V and 0V respectively).

Alternating current 230V / 400V 50Hz

In electrical AC systems with 230V (RMS) and 50Hz, the voltage in the cable changes polarity 50 times per second. RMS (root mean square) is a mathematical formula for calculating the average value of the voltage during an oscillation and this means that you can use the RMS voltage when calculating resistance, current, and power as if it were DC.

The picture below shows the voltage variation during two oscillations. Note that the RMS value is constant.

If you want to delve a little deeper into the different properties of AC, you can read more in the article about AC systems and what properties they have.

Electrical systems

In an electrical system where all parts have electrical (metallic) contact, there can only be one form of electricity and there must also be the same nominal voltage in the whole system.

Do we transfer this requirement to a boat where there may be equipment that needs DC 12V or 24V and maybe also AC 230V 50Hz or as in the US 120V 60Hz, so you understand that the electrical system on a boat actually consists of several separate electrical systems that are interconnected.

Energy balance

Another requirement that exists is that the system must be able to produce exactly the same amount of electricity consumed at any given moment, i.e. the power supplied must be equal to the power of the consumers. If not, the voltage level changes.

If the supplied power is too low, the voltage in the system drops until the consumed power is equal to the supplied power. This relationship is determined by the power formula, which says that the power consumed decreases when the voltage decreases, read more about the power formula in the article about Ohms law. When the voltage becomes too low, equipment may stop working.

If the supplied power is too high, the voltage in the system increases until the consumed power is the same as supplied. This relation is also determined by the power formula, which says that the power consumed increases when the voltage increases. If this happens, there is a risk that something will break.

An AC system responds in the same way to an imbalance regarding produced and consumed power in terms of voltage level.

This is why an electrical system must be regulated and keep the nominal voltage and frequency of the system within certain ranges.