Electrical-marine-wire and electrical-theory

Electrical-marine-wire and Circuit Basics

Understanding the basic theory of circuits is essential in understanding how various circuit elements will be used and how they will be configured. It is important to also understand how to make basic instrument tests.

After completing this lesson unit you will

1. Understand the theory of series circuits

2. Understand the theory of parallel circuits

3. Understand the theory of basic electrical circuits

4. Understand the use of circuit diagram notations and symbols

5. Understand how to use a test instrument

What is a Series Circuit?

A series circuit is one where all of the loads are connected end to end in a row.

R1 + R2 + R3

In a series circuit there is only ONE path for the electricity to flow. If this circuit was a string of light bulbs, and one blew out, the remaining bulbs would turn off.

What is a Parallel Circuit?

A parallel circuit consists of two or more paths for the electricity to flow in. This means that the loads or resistances are connected in parallel with ach other. The loads are usually represented by resistor symbols. If the loads are lamps in a typical cabin lighting system, when one lamp element blows the other will remain operational. This is because there is still a direct electrical path from the battery positive to negative terminals

To find the equivalent resistance (the total resistance offered to the flow of current) we invert the values and add them. Then we invert the result. For example take 2 ohms and 4 ohms in parallel. Inverted 1/2 +1/4 = 3/4 Invert this 4/3 = 1.33 ohms A quick check on your answer is that it should be smaller in value than the value of the smallest resistor.

If these resistors were connected across a 10 volt supply Ohms Law says about 7.5 amps would flow. The formula can be written as 1/Rtotal = 1/R1 + 1/R2 + 1/R3 etc etc.

If only two resistors are involved then use (R1 x R2) divided by (R1 + R2)

What is a Combination Circuit?

Many circuits consist of a combination of both series and parallel circuit resistances. When calculating circuit resistances and voltage drops they will need to be considered differently.

In practical circuits two lights in a circuit are connected in parallel and the cable resistance supply both is a series resistance

What is a Simple or Basic Electrical Circuit?

Most electrical circuits are basically the same. Every electrical circuit can be divided into functional blocks. This is a good practice when considering any circuit.

1. The Power source or supply. This may be a battery, alternator, solar panel etc

2. The current path, which is the conductor, wire or cable, and can be also a switchboard busbar

3. The Load which may be a lamp, a pump motor, or other connected device

4. The control device which is usually a switch to isolate power to the circuit or open the circuit and stop electron flow. This will make the circuit Open Circuit or Closed Circuit

5. An indicator, which in this case is an ammeter connected in series to show the amount of current flowing in the circuit. Many circuits do not have indicators and they are optional depending on the application, such as a charging circuit or main switchboard.

Another example of a simple DC circuit is that of a flashlight. The batteries in the flashlight provide the DC voltage source. The inside of the battery case usually acts as the negative conductor. The lamp bulb is the load and is a resistor that heats up to emit light.

The flashlight has an ON and OFF switch. This controls the flow of electricity. Because there must always be a complete path for current to flow, the switch stops the flow when it is in the OFF position. When the switch is ON, the circuit is complete and current flows, lighting the bulb.

When the batteries run down the bulb gets dimmer as the electromotive force from the battery reduces and less current flow. If the switch or other part of the circuit is loose or makes poor contact, such as at the battery terminals, the bulb will not work as the circuit opens. The usual “repair” method is to tap the flashlight several times as we have all experienced.

Circuit Diagrams and Symbols

Many people are intimated by circuit diagrams and prints. In general this is because circuit diagrams or schematics include all the systems in one diagram. The IEC has several standards defining symbols. The standard IEC60617 graphical symbols for diagrams which replaces IEC117 and contains several parts, each one dedicated to particular families of symbols. Part 1 provides some general information and includes an index to help find the symbol of any device. This list also contains the letter code of each device which is used in circuit diagrams. Part 7 contains the symbols for switches. The 3 main switches found in single-line diagrams are shown below. Although the symbol for a circuit-breaker is generally easy to remember, the disconnecting switch (disconnector) and load interrupter switch (switch-disconnector) are often confused.

The differences arise where symbols used in US drawings are different. These do not conform to IEC formats so people will have to bear in mind that schematic symbols will vary depending on the source of the equipment

When preparing your own diagrams look at using simple notations to avoid confusion and if preparing for clients make them as simple as possible. The general convention is a circuit is drawn from left to right, from power source or supply to the load. I also tend to use a vertical format with power supply at the bottom to the top load or circuits

How To Use a Multimeter

The majority of tests can be carried out using a multimeter. A multimeter as the name suggests is able to perform multi electrical measurements. There are two types of multimeters, analog and digital. An analog meter has a needle to show the readings. The digital meter (DMM) displays the test values numerically on a display. Manual ranging meters require selection of the measurement ranges, and auto-ranging automatically types select the measurement range.

Making Voltage Tests (AC & DC)

As we discussed the volt is the unit of electrical pressure, and is the force required to cause a current to flow against a resistance. It is the most useful of all measurements, either to detect that it is present or to precisely measure the voltage levels. I perform 95% of all my troubleshooting on complex oil rigs and commercial vessels with this function alone. The voltmeter is connected across the supply or equipment, which is negative probe to negative and positive-to-positive to measure the voltage potential between the two. Reversal of probes will simply show a negative reading. If the DMM is not auto-ranging, set the scale to the one that exceeds the expected or operating voltage of the circuit under test. To analyze results:

1. If the voltage is missing this indicates that the circuit supply is switched off, or the circuit is possibly broken, such as a connection or a wire (positive or negative), or a faulty switch or circuit breaker.

2. If the voltage is low this indicates that the supply voltage to circuit from the battery is low, or that additional resistance is in the circuit, such as faulty connection.

Making Continuity Tests

The continuity test requires the use of the Resistance setting. It is simply to test whether a circuit is closed or open. Many multimeters also incorporate a beeper to indicate a closed condition. Power must be switched off before testing. Set the scale to one of the Ohm ranges. Touch the probes together to verify operation, and then place the probes on each wire of the circuit under test. What you are looking for is a simple over-range reading if the circuit is open, and low or no resistance if it is closed. Prior to testing, touch the probes together to see that the meter reads zero.

Making Resistance Tests

If DMM is not auto-ranging set the range switch to the circuit under test, typically the 20ohm range is used. Turn off circuit power, and discharge any capacitors. When testing, do not touch probes with fingers as this may alter readings. Prior to testing, touch the probes together to see that the meter reads zero.

Making Current Tests

The ammeter function of a multimeter is rarely used or required although some use it for measuring so called leakage currents. The switchboard ammeter normally can be used for most measurements. The ammeter is always connected in series with a circuit, as it is a measurement of current passing through the cable. The circuit should be switched off before inserting the ammeter in circuit. Most DMM have maximum DC measurement ratings of 10 amps only.

Looking after electrical test meters

Look after your meter. Do not drop the meter or get it wet. There are a few basics that ensure reliability and safety:

1. Probes

Ensure that probes are in good condition. On many probes the tips sometimes rotate out, and a probe may came out and short across the terminals under test. Another problem is the solder connections of test leads break away due to twisting and movement.

2. Cables

Cables should be kept clean and insulation undamaged. Cables can age and crack. If a cable is damaged replace the cable. Do not attempt to test higher voltages, in particular AC voltages, if the cables are damaged. People have received severe shocks or been killed due to faulty leads.

3. Batteries

Replace the internal battery every 12 months, or at least carry a spare. Many meters will have a low battery warning function.