electricity fundamentals, 13 electrical distribution · ac power source, the line voltage is ¾ u...

© Festo Didactic 89688-00 269

When you have completed this exercise, you will be introduced to the notions of power network and distribution network. You will also be introduced to three-phase circuits, and to wye and delta configurations. You will be familiar with the operation of fuses, circuit breakers, disconnect switches, and electrical panels. You will be introduced to the Disconnect Switches module and Circuit Breaker module of the training system.

The Discussion of this exercise covers the following points:

Introduction to the power network and distribution network

Three-phase circuitsPhase sequence. Wye and delta configurations. Distinction between line and phase voltages, and line and phase currents.

Circuit protectionFuses. Circuit breakers. Magnetic circuit breakers. Thermal circuit breakers. Ground fault circuit interrupter GFCI breaker. Circuit breaker symbols.

Electrical panels

Disconnect switch

Power circuit versus control circuitPower circuit. Control circuit.

Training system modulesCircuit Breaker module. Disconnect Switch module.

Introduction to the power network and distribution network

A power network is an interconnected network that delivers electricity to consumers. It consists of three basic networks: the generation network, the transmission network, and the distribution network, as shown in Figure 213.

Electrical Distribution

Exercise 13

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

Exercise 13 – Electrical Distribution Discussion

270 © Festo Didactic 89688-00

Figure 213. Typical electrical power generation, transmission, and distribution networks.

The generation network comprises all power generating stations, such as fossil-fuel plants (mainly coal plants), hydropower plants, wind faRMS, and photovoltaic power stations. This part of the network produces the electrical power supplied to customers. Power stations are often located near their power source (e.g., coal and water). For this reason, and also for security purposes, power stations are often located far from consumers (large population centers).

Figure 214. View of the Grand Coulee Dam on the Columbia River in the state of Washington, USA. Hydropower plants are usually very large power stations that supply power to thousands of consumers.

The transmission network comprises all equipment necessary to transfer power from the generation network to the distribution network. The transmission

Power generating stations

Step-up transformer

AC transmission lines

Step-down transformer

Electrical power consumers

Generation network

Transmission network

Distribution network



network thus mainly consists of transmission lines whose length varies depending on the remoteness of the power stations. At the input of the transmission network, step-up transformers increase the voltage of the incoming electrical power. This minimizes the power losses occurring across the transmission lines. Because of this, transmission lines often carry electrical power with a very high voltage.

The distribution network comprises all the equipment necessary to distribute power to the consumers, be they residential or industrial. The distribution network thus consists of relatively short distribution lines. At the input of the distribution network, step-down transformers decrease the voltage of the incoming electrical power to a voltage more suitable for residential or industrial consumption.

Figure 215. Transmission substation used for decreasing the high voltage present in the transmission lines before the power is distributed to consumers.

Three-phase circuits

Phase sequence

A three-phase circuit is powered by three voltage sine waves having the same frequency and magnitude and which are displaced from each other by 120°. The phase shift between each voltage waveform of a three-phase ac power source is

therefore 120° (360° 3 phases). Figure 216 shows an example of a simplified three-phase generator (alternator) producing three-phase ac power. A rotating magnetic field produced by a rotating magnet turns inside three identical coils of wire (windings) physically placed at a 120° angle from each other, thus producing three separate ac voltages (one per winding). Since the generator’s rotating magnet turns at a fixed speed, the frequency of the ac power that is produced is constant, and the three separate voltages attain the maximal voltage value one after the other at phase intervals of 120°.



Figure 216. A simplified three-phase generator.

The phase sequence of the voltage wavefoRMS of a three-phase ac power source indicates the order in which they follow each other and attain the maximal voltage value. Figure 217 shows an example of the voltage wavefoRMS produced in a three-phase ac power source. The voltage wavefoRMS follow the

phase sequence , , , which, when written in shorthand form, is the sequence A-B-C. This phase sequence is obtained when the magnet in the three-phase generator of Figure 216 rotates clockwise.

The phase sequence of a three-phase ac power source is important because it determines the direction of rotation of any three-phase motor connected to the power source. If the phases are connected out of sequence, the motor will turn in the opposite direction, and the consequences could be serious.

Figure 217. Voltage wavefoRMS produced in a three-phase ac power source.

Phase 3 Phase 2

Phase 1

N

S

Vo

lta

ge

Time



Wye and delta configurations

The windings of a three-phase ac power source (e.g., the generator in Figure 216) can be connected in either a wye configuration, or a delta configuration. The configuration names are derived from the appearance of the circuit drawings representing the configurations, i.e., the letter Y for the wye configuration and the Greek letter delta ( ) for the delta configuration. The connections for each configuration are shown in Figure 218. Each type of configuration has definite electrical characteristics.

As Figure 218a shows, in a wye-connected circuit, one end of each of the three windings (or phases) of the three-phase ac power source is connected to a common point called the neutral. No current flows in the neutral because the currents flowing in the three windings (i.e., the phase currents) cancel each other out when the system is balanced. Wye connected systems typically consist of three or four wires (these wires connect to points A, B, C, and N in Figure 218a), depending on whether or not the neutral line is present.

Figure 218b shows that, in a delta-connected circuit, the three windings of the three-phase ac power source are connected one to another, forming a triangle. The three line wires are connected to the three junction points of the circuit (points A, B, and C in Figure 218b). There is no point to which a neutral wire can be connected in a three-phase delta-connected circuit. Thus, delta-connected systems are typically three-wire systems.

Figure 218. Types of three-phase system configurations.

Distinction between line and phase voltages, and line and phase currents

The voltage produced by a single winding of a three-phase circuit is called the

line-to-neutral voltage, or simply the phase voltage, . In a wye-connected three-phase ac power source, the phase voltage is measured between the neutral line and any one of points A, B, and C, as shown in Figure 218a. This results in the following three distinct phase voltages: , , and .

The voltage between any two windings of a three-phase circuit is called the line-to-line voltage, or simply the line voltage, . In a wye-connected three-phase

ac power source, the line voltage is (approximately 1.73) times greater than

the phase voltage (i.e., ). In a delta-connected three-phase ac

(a) Three-phase wye configuration (b) Three-phase delta configuration

,

N

A

B

C

A

BC



power source, the voltage between any two windings is the same as the voltage

across the third winding of the source (i.e., ), as shown in Figure 218b. In both cases, this results in the following three distinct line

voltages: , , and .

The three line wires (wires connected to points A, B, and C) and the neutral wire of a three-phase power system are usually available for connection to the load, which can be connected in either a wye configuration or a delta configuration. The two types of circuit connections are illustrated in Figure 219. Circuit analysis demonstrates that the voltage (line voltage) between any two line wires, or lines,

in a wye-connected load is times greater than the voltage (phase voltage)

across each load resistor. Furthermore, the line current flowing in each line

of the power source is equal to the phase current flowing in each load resistor. On the other hand, in a delta-connected load, the voltage (phase voltage) across each load resistor is equal to the line voltage of the source. Also,

the line current is times greater than the current (phase current) in each load

resistor. The phase current in a delta-connected load is therefore times smaller than the line current.

Figure 219. Types of load connections.

The relationships between the line and phase voltages and the line and phase currents simplify the analysis of balanced three-phase circuits. A shorthand way of writing these relationships is given below.

In wye-connected circuits:

and

In delta-connected circuits:

and

The following figure shows

the electrical symbol repre-

senting a three-phase ac

power source. Notice that

lines A, B, and C are some-

times labeled lines 1, 2,

and 3, respectively.

(a) Wye-connected load (b) Delta-connected load

Line 1

Line 2

Line 3

Neutral

Line 1

Line 2

Line 3

,



Circuit protection

Since electricity can be dangerous and destroy electrical equipment, all electrical circuits must have protection. Fuses and circuit breakers are common devices used to protect electrical circuits against overloads, ground faults, and short circuits by interrupting the flow of electricity.

Fuses

A fuse basically consists of a low-resistance resistor that can be added in series to a circuit in order to prevent overcurrents and damage to the equipment. Most fuses are made of a metal wire (filament) that melts when the current flowing through it is too high. When this happens, the fuse is said to be blown and current can no longer flow in the circuit. Once a fuse is blown, it must be replaced in order to make current flow in the circuit again.

Figure 220 shows a typical low-capacity fuse. Notice the wire connecting one end of the fuse to the other. This wire melts when the current flowing through it reaches too high a value.

Figure 220. Typical cylindrical fuse (glass type).

Fuses come in many types and shapes, each with its particular current rating. The current rating indicates the maximal current value that the fuse can tolerate before it eventually blows. When adding a fuse to a circuit, it is important that the current rating of the fuse matches the maximal current rating of the equipment it protects. For example, to prevent overcurrents in a light bulb with a rating of 1 A, it is necessary to use a fuse with a current rating slightly above 1 A. Therefore, if a malfunction occurs and a current value higher than normal flows in the light bulb, the fuse will immediately blow and prevent damage to the light bulb. The electrical diagram symbol for a fuse is shown in Table 25.

Table 25. Fuse symbol.

Component Symbol

Fuse

Most fuses have markings on the body or end caps that indicate their ratings. Common characteristics of fuses are the rated current, rated voltage (the rated voltage of a fuse must be equal or higher than the operating voltage of the component to protect), fuse speed (slow blow, fast acting, and very fast acting), breaking capacity, and maximum power dissipation.

Metal wire (filament)



Figure 221. Power circuits with fuse protection.

Circuit breakers

Circuit breakers are another type of component commonly used in electrical circuits. Circuit breakers are basically automatically actuated electrical switches that allow a circuit to be opened in order to prevent undesirable operating conditions. Therefore, just like fuses, circuit breakers are added in series to a circuit in order to prevent overcurrents and damage to the equipment.

The main difference between fuses and circuit breakers is that, while fuses must be replaced when blown, circuit breakers simply need to be reset when tripped (after removing the source of the tripping). Most circuit breakers are reset by pushing a button or toggling a switch. Therefore, the main advantage of circuit breakers over fuses is that they have a much longer service life, since they can be used multiple times instead of only once.



Figure 222. Examples of circuit breakers.

There are many types of circuit breakers. The two most common types, magnetic circuit breakers and thermal circuit breakers, are presented in the following two subsections. Note that it is also common for a circuit breaker to combine the operating mechanisms for both magnetic and thermal circuit breakers, resulting in a thermal-magnetic circuit breaker.

Magnetic circuit breakers

Magnetic circuit breakers use the principles of solenoids to prevent overcurrents. When current flows through a magnetic circuit breaker, it passes through a solenoid. This solenoid produces a magnetic field that interacts with the circuit breaker contact. When the current flowing in the circuit breaker reaches a current value equal to its rating, the magnetic field created by the solenoid is strong enough to open the contacts of the circuit breaker, thereby opening the circuit and preventing current flow. Figure 223 shows the cross-sectional view of a magnetic circuit breaker when the contact is closed, while Figure 224 shows the same view when the contact is open.

Just like fuses, magnetic circuit breakers have a current rating. This rating indicates the maximal current value which the circuit breaker can tolerate before tripping. For example, to prevent overcurrents in a kitchen oven with a rating of 15 A, it is necessary to use a circuit breaker with a current rating slightly above 15 A. Therefore, if a malfunction occurs and a current value higher than normal flows in the oven, the circuit breaker will immediately trip and prevent damage to the oven. Once the source of the malfunction is removed, the circuit breaker can be reset.



Figure 223. Cross-sectional view of a circuit breaker when the contact is closed (photo courtesy of Kae).

Figure 224. Cross-sectional view of a circuit breaker when the contact is open (photo courtesy of Kae).

Thermal circuit breakers

This type of circuit breaker is designed to detect temperature increases. When the temperature in the circuit breaker becomes too high, its contact opens, thereby opening the circuit and preventing current flow. This reaction is usually achieved in thermal circuit breakers by a mechanism combining two metals whose heating expansion rates are different. As the thermal circuit breaker heats, one metal expands more than the other, causing the contact to open.

Closed contact

Open contact

Magnetic protection(solenoid)

Thermal protection



Thermal circuit breakers are mainly used to detect overcurrents. This is because, as mentioned before, whenever current flows through a material with a resistance value, heat is produced. The higher the current is, the greater is the heat. Therefore, if a thermal circuit breaker has a current rating of 3 A, it will allow 3 A to flow in it without producing too much heat. However, if the current flowing through the thermal circuit breaker increases above 3 A, it will gradually produce heat and eventually trip. The more the current flowing in the circuit breaker exceeds its current rating, the faster it will trip.

Ground fault circuit interrupter GFCI breaker

A ground fault circuit interrupter GFCI is a type of breaker designed to interrupt a circuit if the current flow in the “hot” wire is not equivalent, within a few thousandths of an ampere, to the current flow in the neutral wire. If a device is working properly, all electricity that the device uses will flow from the hot wire to the neutral wire. Any imbalance between the current flowing in these wires indicates a current loss. Ground fault circuit interrupters mount in the service panel and are required for receptacles in the kitchen, bath, unfinished basements, garages, and all those which are outdoors.

Circuit breaker symbols

The different electrical diagram symbols for circuit breakers are shown in Table 26.

Table 26. Circuit breaker symbols.

Component Symbol

Circuit breaker

Magnetic tripping

Thermal tripping

When representing a circuit breaker in an electrical diagram, it is necessary to use the general symbol for a circuit breaker (first row of Table 26), followed by the mechanism(s) by which the circuit breaker trips. For example, a magnetic circuit breaker would be represented by the symbol in the first row of Table 26 followed by the symbol in the second row. Note that it is possible to use more than one tripping mechanism. For example, a thermal-magnetic circuit breaker would be represented by the symbol in the first row of Table 26, followed by the two other symbols.

Electrical panels

Electrical panels, also known as distribution boards, are widely found in distribution networks. They consist of a metal panel or enclosure that subdivides the incoming electrical power in individual circuits, thus allowing control over each circuit. For example, electrical panels found in houses commonly subdivide the incoming electrical power into a separate circuit for each room or area of the house.



Most electrical panels have a single input that can be disconnected using a main circuit breaker. The main components found in electrical panels are circuit breakers and fuses for each individual circuit. These add a level of protection to the electrical panel by preventing overcurrent conditions. The circuit breakers also allow individual circuits to be opened for maintenance or reparation.

Electrical panels are available in many sizes, shapes, and types. The most common types are residential electrical panels, which are usually relatively small. Larger electrical panels are also often found near residential areas, where they distribute power to a multitude of individual residences, such as the one in Figure 225.

Figure 225. Large street electrical panel used to control the power supplied to nearby residences (photo courtesy of Geoprofi).

Disconnect switch

A disconnect switch, or isolator switch, is used to ensure that an electrical circuit is completely de-energized for service or maintenance. Such switches are often found in electrical distribution and industrial applications, where machinery must have its power source removed for adjustment or repair. The disconnect switch is not intended for normal control of the circuit, but only for safety isolation.

Disconnect switches consist of an enclosure containing a device capable of maintaining an electrical circuit open. The device is usually a switch operated by a handle or a rotary button, a breaker, or fuses that can be removed from the circuit by pulling out the fuse block. Disconnect switches have provisions for a padlock so that inadvertent operation is not possible.



Power circuit versus control circuit

Many electrical circuits are represented by power circuits and control circuits. Figure 226 shows the ladder diagram of a heating circuit controlled using a low voltage thermostat. The name ladder diagram is based on the observation that the diagram resembles a ladder, with two vertical rails and a series of horizontal rungs between them. The operation of the circuit is as follows:

The circuit is powered through terminals L and N.

A double-pole disconnect switch allows the heating circuit to be isolated from the power source.

The circuit is fuse protected.

On a low temperature, the low-voltage thermostat in rung 2 closes. This causes the coil CON of the contactor to energize, the NO contact CON-1 in rung 1 to close, and the heating element to start heating.

When the temperature attains the value set on the thermostat, the thermostat opens. This causes the coil CON of the contactor to de-energize, the NO contact CON-1 in rung 1 to open, and the heating element to stop heating.

Figure 226. Ladder diagram showing the power and control circuits of a heating element controlled by a low voltage thermostat.

Power circuit

Control circuit

CON

CON-1

Thermostat

Line (L) Line (N)

H1 H2

X1 X2

Disconnectswitch

Heating element

By convention, the designation Hrefers to the higher-voltage

winding, and the designation Xrefers to the lower-voltage winding.

Rung 1

Rung 2

24 V

The dashed line indicates a mechanical link.



Power circuit

In the circuit of Figure 226, the section above the transformer is called a power circuit. A power circuit is the section of a circuit that provides power (energy) to electrical loads. Power circuits often carry high voltages and consist of incoming main power.

Control circuit

Also in the circuit of Figure 226, the section under the transformer is called a control circuit. A control circuit is the section of a circuit that includes a control unit and the devices required to manage the operation of the loads. In the circuit of Figure 226, the control circuit is connected to the power line through a transformer that reduces the voltage from line voltage to 24 V ac.

Training system modules

Circuit Breaker module

Figure 227. Circuit Breaker module (220-240 V version).

The Circuit Breaker module of your training system operates at the ac power line voltage. The circuit breaker has a current rating of 0.2 A and a voltage rating of 250 V, and is of the thermal-magnetic type. This means that the circuit breaker trips whenever the temperature in the circuit breaker is too high (thermal), or the current flowing through the circuit breaker is too high (magnetic). The circuit breaker can be reset by setting the reset button to the I (on) position.

The module is provided with a load resistor controlled by a push button. The load resistor is used to observe the behavior of the circuit breaker when an overload condition occurs.

Reset button

Load resistor

Push button tocontrol the use of

the load resistor

IEC symbol for a circuit breaker



Disconnect Switch module

Figure 228. Disconnect Switch module.

The Disconnect Switch module of your training system is provided with fuses having a current rating of 10 A and a voltage rating of 250 V. The fuses are of the RK5 class, which means that they can sustain overcurrent and overvoltage conditions for a certain time before blowing. Considering the rating of the fuses, this means that the fuses can indefinitely carry a current of up to 10 A and a voltage of up to 250 V. If these values are exceeded, however, the fuses will eventually blow after a certain time.

To isolate a circuit branch from the power source, turn off power source and remove the fuse block from the disconnect switch (or reverse the orientation of the fuse block). See Figure 229.

Figure 229. Inside view of the disconnect switch.

Fuse block

IEC symbol for a disconnect switch (fuse type)

Exercise 13 – Electrical Distribution Procedure Outline


The Procedure is divided into the following sections:

Setup

Operation of a fuse

Operation of a circuit breakerCircuit with an excessive load. Short-circuited circuit breaker.

Resistance, current, and voltage measurements in a disconnect switchCurrent measurement. Voltage measurement.

High voltages are present in this laboratory exercise. Do not make or modify any

banana jack connections with the power on unless otherwise specified.

Setup

In this section, you will install the training system modules in the workstation.

1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform this exercise.

Install the equipment required in the workstation.

Make sure that all fault switches are set to the O (off) position.

Make sure that the main power switch on the Power Source module is set to the O (off) position, then connect it to an ac power outlet.

Operation of a fuse

In this section, you will verify the status of a fuse by visually inspecting the fuse and by measuring its resistance. Then you will confirm that there is continuity between the terminals of the fuse-protected low voltage winding of the Control Transformer module.

2. On the Control Transformer module, remove fuse F3 by pushing and turning the fuse cap counterclockwise. Note that the rating of the fuse is indicated on the metallic cap of the fuse.

Observe the fuse. Is the filament broken? Is there discoloration to the transparent fuse casing?

Yes No

PROCEDURE OUTLINE

PROCEDURE

Exercise 13 – Electrical Distribution Procedure


3. Measure the resistance of the fuse. Record the value below.

Resistance of the fuse

4. From the observations you made in step 2 and the resistance you measured in the previous step, what can you conclude regarding the status of the fuse? Briefly explain.

5. Replace the fuse in the Control Transformer module.

Measure the resistance of the low voltage winding protected

by fuse F4 at the secondary of the transformer. Record the value below.

Resistance

6. What can you conclude from the resistance you measured in

the previous step regarding the continuity between the terminals of the low voltage winding? Briefly explain.

A high voltage is present in the next part of this laboratory exercise. Do not make

or modify banana jack connections with the power on.

Operation of a circuit breaker

In this section, you will connect a circuit containing a circuit breaker as well as the components required for testing the circuit breaker. You will make current flow in the circuit, measure it, and analyze the results. You will let the circuit operate until the circuit breaker trips, and determine whether it tripped thermally or magnetically. You will then connect a short-circuited circuit protected by a circuit breaker. You will make current flow in it and observe what happens. You will let the circuit operate until the circuit breaker trips, and determine whether it tripped thermally or magnetically.



Circuit with an excessive load

In this section, you will operate a circuit protected by a circuit breaker with a load (test resistor) that exceeds the rating of the circuit breaker, and observe the behavior of the circuit breaker.

7. Set up the circuit shown in Figure 230. Use the components on the Circuit Breaker module to implement the different elements in the circuit. Also, make sure that the circuit breaker is not tripped by setting the reset button to the I (on) position. The indicator light is used as a load to prevent the power

source from being short circuited when push button is released.

Figure 230. Circuit containing the circuit breaker as well as the components required for testing the circuit breaker.

8. Turn the power source on, then press and hold push button .

9. Measure the current flowing in the circuit. Record the value below.

Current A

10. What happens in the circuit when you turn the power source on and press

push button ? Does the circuit breaker prevent current from flowing in the circuit?

Test resistor

Push button


L

N



11. Compare the current you measured in step 9 to the current rating of the circuit breaker indicated on the front panel of the Circuit Breaker module. What can you conclude?

12. Wait for a few seconds while holding push button . What happens when you let the circuit operate in the present conditions? Briefly explain.

13. Was the trigger for the tripping of the circuit breaker thermal or magnetic. Briefly explain.

14. Release push button , then turn the power source off.

Reset the circuit breaker by setting the reset button to the I (on) position.

Short-circuited circuit breaker

In this section, you will short circuit a circuit protected by a circuit breaker and observe the behavior of the circuit breaker.

15. Modify the circuit in Figure 230 by adding a normally open push button ( ) in parallel with indicator light . The modified circuit is shown in Figure 231.

Pressing push button will short circuit the power source.

Use the NO push-button switch of the Push Buttons module to implement

push button .



Figure 231. Circuit containing the circuit breaker as well as the components required for testing the circuit breaker.

16. Turn the power source on, then press push button .

Release the push button. What happens when you turn the power source on

and press push button ? Does the circuit breaker prevent the flow of current in the circuit? Briefly explain.

17. Was the trigger for the tripping of the circuit breaker thermal or magnetic? Briefly explain.

18. Turn the power source off.

Reset the circuit breaker.

Disconnect your circuit.

Test resistor

Push button


L

N

Push button



Resistance, current, and voltage measurements in a disconnect switch

In this section, you will verify the status of the fuses in a disconnect switch, then you will perform current and voltage measurements in a disconnect switch. The objective of these measurements is to familiarize yourself with that type of measurement.

19. Before setting up the next circuit, check the status of the fuses in the disconnect switch. To do so, measure the resistance between the Line and Load terminals of each pole of the switch.

Resistance between the Line and Load terminals (left side)

Resistance between the Line and Load terminals (right side)

20. Do the resistances you measured in the previous step confirm that the fuses are not blown? Briefly explain.

21. Remove the fuse block from the disconnect switch by pulling the fuse block handle.

Record the current and voltage ratings indicated on the fuse body.

Current rating A

Voltage rating V

22. Return the fuse block in the disconnect switch.

Set up the circuit shown in Figure 232. This circuit is the same as in Figure 226 except that the heating element has been replaced by an indicator light. Use the contactor having a 24 V coil in the Contactors module to implement contactor CON.

Connect the Line terminals of the Disconnect Switch module to terminals L and N on the Power Source module.



Figure 232. Familiarization circuit with the use of disconnect switches.

23. Make sure that all fault switches are set to the O (off) position.

Set the thermostat to the lowest temperature (lever fully at the left position).

Turn the power source on.

Is the indicator light on or off when the thermostat is set the lowest temperature?

24. Set the thermostat at the highest temperature (lever fully at the right position).

Does the indicator light turn on when the thermostat is set the highest temperature?

25. Does the circuit operate as described in the discussion?

Yes No

CON

CON-1

Thermostat

Line (L) (connect to terminal L on the power source)

Line (N) (connect to terminal N on the power source)

H1 H2

X1 X2

Disconnectswitch

Indicator light

Control transformer



26. Turn the power source off.

27. Open the disconnect switch box as shown in Figure 233.

Figure 233. Open the disconnect switch.

28. Insert a small screwdriver into the opening of the nameplate as shown in Figure 234, and then remove the nameplate.

Figure 234. Remove the nameplate.

Current measurement

29. Before you work with live lines (power source turned on), familiarize yourself with the manipulations described in this step. Do not hesitate to ask your instructor for help if you are not sure of how to make the measurements.

Press the trigger of the clampmeter to open the jaw, then fully enclose the wire as shown in Figure 235. Repeat for all wires.

Insert a small flatscrewdriver into the

hole



Figure 235. Current measurement using a clampmeter.

30. Once you feel comfortable with the manipulations, turn the power source on. Being careful not to touch the terminals in the disconnect switch, measure the current flowing in each fuse when the indicator light is on.

Current flowing in the left fuse when the indicator light is on A

Current flowing in the right fuse when the indicator light is on A

Voltage measurement

31. Using a voltmeter, measure the voltage across the Line terminals in the disconnect switch as shown in Figure 236 (the line terminal of the right fuse corresponds to the neutral line of the power source).

Voltage across the Line terminals in the disconnect switch when the indicator light is on V

Figure 236. Voltage measurement.

Exercise 13 – Electrical Distribution Conclusion


32. Turn off the power source and the measuring instruments.

Disconnect your circuit.

Return the leads to their storage location.

In this section, you were introduced to the notions of power network and distribution network. You became familiar with the operation of fuses and circuit breakers, and learned how to troubleshoot them. You also became familiar with electrical panels. You were introduced to the Circuit Breaker and Disconnect Switch modules.

1. What are the three basic networks comprised in a power network?

2. Briefly define what a distribution network is.

3. Briefly explain what a fuse is and how it operates.

4. Briefly explain what a circuit breaker is and how it operates.

CONCLUSION

REVIEW QUESTIONS

Exercise 13 – Electrical Distribution Review Questions


5. Explain the conditions required for a magnetic circuit breaker to trip and those required for a thermal circuit breaker to trip.

electricity fundamentals, 13 electrical distribution · ac power source, the line voltage is ¾ u...

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