single-trip overcurrent protection - saskpower 3 book 2/9. single...installation of the various...

March 31, 2005

S T U D E N T M A N U A L

Single-Trip Overcurrent Protection

Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved

2 S T U D E N T T R A I N I N G M A N U A L

Prerequisites: • Single-Phase Transformer Load Checks module

Objectives: From memory, you will be able to describe the function, types and installation of the various fuses and thermal breakers used for overcurrent protection.

Rationale: An electrical system is susceptible to many types of overcurrent problems. Proper installation of overcurrent protection devices will keep damage to the system and customer’s equipment to a minimum. It also helps the operator to troubleshoot.

Learning Objectives• Describe the types, function and installation of fuses.• Describe the types, function and installation of thermal breakers.

Learning Methods• Self-learning + On-the-job• Self-learning + On-the-job

EVALUATION METHODS

• Written test• Written test

STUDENT RESOURCES

• Construction Standards Manual

Learning Steps1. Read the Learning Guide.2. Follow the steps outlined in the Learning Guide.3. Clarify any questions or concerns you may have.4. Complete the Practice and Feedback.

S I N G L E - T R I P O V E R C U R R E N T P R O T E C T I O N 3


5. Complete the Evaluation.



Lesson 1: FusesLearning Objective:Describe the types, function and installation of fuses.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test

Introduction

Fuses are relatively inexpensive protective devices, incorporated intothe electrical system to protect against overcurrent.

If it weren’t for fuses, apparatus in the system and customer connectedequipment may be damaged by overcurrent caused by overloads or shortcircuit conditions.

Fuses are intentionally weakened components made of metals such ascopper, lead, silver, tin or tin-lead alloy, which are designed to melt at aspecific current if maintained for a specific period of time. Whencurrent exceeding the fuse amperage rating flows through the fuse, theresistance of the metal causes heat to be produced which then causes themetal to melt. By doing so, it opens the circuit before damage to theelectrical system or connected equipment can result.

To prevent the burning metal from causing damage or starting a fire,fuses are usually enclosed in a holder to contain the debris and snuff thearc. Vaporized metal and a puff of smoke are usually present with theblowing of the fuse.

There are four main types of fuses used in an electrical distributionsystem:

• Plug• Cartridge• Underground• Link



Plug Fuse

The plug fuse is enclosed in a cup-type housing which screws into aninsulated socket base. The plug fuse is used in low voltage applicationssuch as streetlights, which have a maximum voltage rating of 150 voltsto ground.



Cartridge Fuse

Cartridge fuses are available in two main types - non-renewable andrenewable.

• non-renewable - the fuse element of this cartridge is not replaceableafter the fuse blows

• renewable - the fuse element of this cartridge may be replaced afterthe fuse blows

With a cartridge-type fuse, the fuse element is enclosed in an insulatedtube, and may be used on circuits up to 600V

Underground Distribution Fuse

The underground distribution fuse is an overload sensing fuse whichprotects the transformer coil. Live-front and dead-front transformersutilize three basic types of fuses.



Live-front Transformers



In live-front applications, the load break NX silver sand fuse is used toprotect the transformer. It is filled with silica sand which helps quenchthe arc and contain it within the fuse cartridge during an overcurrentsituation.



Dead-Front Transformers



Dead-front transformers utilize a bay-o-net fuse as their source ofprotection. It protects the transformer from overcurrent, and iscoordinated with an internal isolation link.

Bay-o-net fuses operate on overcurrent and high oil temperature.

CAUTION!Bay-o-net fuses must not be used to pick up load.



Isolation Link

The isolation link is an internally mounted fuse found in undergroundtransformers, and must be factory replaced in the event of a failure. Itprotects the transformer’s primary coils from overcurrent, and since itdoes not produce gases upon clearing a fault, it can be submerged in oil.

Link Fuse

The link fuse is used for high voltage applications in conjunction withcutouts on such voltages as 2.4kV, 4.1kV, 14.4kV, 25kV, or 72kV.



Figure 1. Components of a link fuse: consists of a button, upper terminal, fusible element, lower terminal, leader and sheath



Most link fuses are of coiled construction, enabling the fuse element toabsorb vibration and thermal shock caused from current fluctuations.

Most link fuses work on the expulsion principle. The fuse element iscontained in a sheath which is lined with a de-ionizing fiber.

When the fuse element melts, it heats the fiber which, in turn, emits agas. This de-ionizing gas acts as an arc snuffer. The gas is expelled outthe end of the cutout or fuse tube. This expulsion of gases helps blowout the arc, which is where we set the phrase “blowing a fuse.”

These fuses are available in a variety of ratings which allow for variousapplications.

---Note---The most common link fuses are the “Fit-All” and “Trip-o-Link,” which have the following ratings:

• “T” fuses are used for all coordinated rural protection andtransformer overload protection on urban as well as ruraldistribution.

• “X” fuses are used for transformer overload protection inoilfield and irrigation applications.



Figure 2. Fit-All fuse: The button is the upper terminal, and the leader is the lower terminal. It is used in the open cutout.



Figure 3. Trip-o-Link fuse: Pull rings are used on fuse links designed for open link cutouts. These rings are the upper and lower terminals. The sheath is used to protect the fuse during handling and aid

in the interruption of faults.

Fuses are rated for their continuous current-carrying capabilities. Thisis designated by a number (eg. 12 amp). Fuses are also rated by thespeed in which they melt under fault conditions (eg. type “T” 12 amp).

This speed is broken down into two parts:

• minimum melting time• maximum clearing time

Minimum melting time is the time it takes for the fuse element to meltand initiate arcing after a fault has occurred. Maximum clearing time isthe combination of the melting time and the arcing time added together.

Melting and clearing times become important when coordinating fuseswith other protective equipment such as OCRs and RVEs. For example,if a fuse has a slower clearing time than an OCR’s lockout time, theOCR will lockout before the fuse blows.

Fuse coordination is calculated by the engineering department. If the



system is going to work properly, a Powerline Technician must replaceblown fuses with the required size. If too small of a fuse is used, the fusewill blow before the OCR has time to operate. If too large of a fuse isused, the OCR will trip out, making the fault difficult to track down.



Lesson 2: Thermal BreakersLearning Objective:Describe the types, function and installation of thermal

breakers.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test

Introduction

Thermal breakers are installed to protect the transformer from faults oroverload situations on the customer’s secondary. The breaker does notserve as protection to the secondary other than to isolate it from currentflow under abnormal conditions.

A thermal breaker is a device which operates on the principle of heat. Itcontains two bi-metal strips in series with the line leads. When currentflows through the bi-metal strips, the heat produced causes them tobend. If the current rating of the breaker is exceeded, the strips bend tothe point of tripping a locking device which causes the breaker contactsto open. Once tripped, the breaker must be reset manually.

These breakers are also trip free which means the contacts will openunder overload even though the operating handle is held in a closedposition.

There are two main types of thermal breakers used in the electricaldistribution system:

• current totalizing• thermal magnetic

Both type of breakers serve the same basic purpose, although theyoperate a little differently from each other.



Current Totalizing Breaker (CT Breaker)



Current totalizing breakers are unique in that they trip at differentoverload values when they have a balanced load, as compared to animbalanced load based on current flow. Since current is used tocalculate the single-phase load, the breaker will handle the greatest loadin a balanced situation. These breakers are available in three commonsizes - 25/41A, 50/75A, 75/100A and 100/150A.

In a totally balanced condition, a 50/75A breaker will allow 50A to flowthrough each leg.

The load of a totally balanced circuit can be calculated as follows,considering a 240V circuit:



When a totally unbalanced condition exists, the same breaker will allowcurrent flow of 75A on one leg and 0A on the other. Any increase inload on either leg will cause the breaker to trip.

The load of a totally unbalanced circuit is calculated as follows,considering a 240V circuit:



Thermal Magnetic Breaker



Thermal magnetic breakers are somewhat different from CT breakers inthat they aren’t subject to different tripping values in a balanced orimbalanced situation. A thermal magnetic 100 amp breaker will handle100 amps total on each leg in a balanced or imbalanced situation. Ifeither leg exceeds 100 amps, the breaker will trip.

There are three different thermal breaker sizes: 50, 100 and 200 amp.

CAUTION!Because of a vent hole on the top of the thermalbreaker, the leads going from the breaker to the metersocket shall be the coated wire supplied in theconversion kit.



Installation of Thermal Breakers

Thermal breakers are installed in series inside farm power centres. Theyare connected on the load side of the meter before the secondary serviceor splitter.



Summary

To summarize this module, you have learned:

• The function, types and installation of the various types of fuses.• The function, types and installation of thermal breakers.

Practice Feedback

Review the lesson, ask any questions and complete the self-test.

Evaluation

When you are ready, complete the final test. You are expected toachieve 100%.



Review Questions

1. A fuse is used to:(a) Protect the electrical system from lightning.(b) Protect the electrical system and connected

equipment from overcurrent.(c) Protect the electrical system and connected

equipment from overvoltage.(d) Produce an overcurrent in the electrical system.

T / F 2. A fuse is an intentionally weakened component in an electrical system.

T / F 3. The main purpose for fusing a transformer is to protect it from overvoltage.

T / F 4. A plug fuse is one type of underground rural distribution fuse.

5. A fit-all fuse is one type of:(a) Underground distribution fuse.(b) Link fuse.(c) Plug fuse.(d) Cartridge fuse.

T / F 6. Cutouts incorporate cartridge fuses to operate.

T / F 7. Plug fuses are designed for use with voltages up to 150 volts to ground.

8. Melting of the fuse is caused by:(a) A fault on the line side of the fuse.(b) Overcurrent through the fuse.(c) Resistance of the cutout.(d) Overvoltage in the electrical system.

T / F 9. Minimum melting time is the time it takes a fuse to melt after arcing has started.

T / F 10. Maximum clearing time is the minimum melting time plus the arcing time added together.



T / F 11. Cartridge fuses are used on circuits up to 600 volts, and are available in renewable and non-renewable types.

T / F 12. The link fuse incorporates a cutout to operate.

13. Minimum melting time is:(a) The time it takes a fuse to melt during normal

current conditions.(b) The total time it takes the arc cot be snuffed out.(c) The time it takes a fuse to melt and initiate arcing

after a fault occurs.(d) The time it takes a fuse to start melting in a fault

condition.

14. Maximum clearing time is:(a) The total time it takes the arc to be snuffed out.(b) The time it takes a fuse to melt and initiate arcing

after a fault occurs.(c) The time it takes to clear the electrical system

faults.(d) The time between when a fuse starts to melt and

when it has completed arcing.

15. In an overcurrent situation, breakers are used to protect the:(a) Secondary.(b) Fuse.(c) Cutout.(d) Transformer.

16. The breaker is a thermal device, which means it operates on the principle of:(a) Voltage.(b) Heat.(c) Transformation.(d) Fusing.

T / F 17. Only the current totalizing breaker has a trip-free feature.



T / F 18. A 200A thermal magnetic breaker will handle a maximum of 200A on one leg in an unbalanced situation.

T / F 19. A 50/75A current totalizing breaker will handle a maximum of 50A on one leg in an unbalanced situation.

T / F 20. A 100/150A current totalizing breaker will handle a maximum of 150A on each leg in a balanced situation.

T / F 21. “Trip-free” means a thermal breaker’s contacts will open under overload even though the operating handle is held in a closed position.

T / F 22. The design of a current totalizing breaker allows it to handle a greater load in a balanced condition as compared to an imbalanced condition.

23. A current totalizing breaker is rated at 100/150A. In a balanced condition:(a) The breaker will handle 150 amps on each leg

before tripping off.(b) The breaker will handle 50 amps on both legs

before tripping off.(c) The breaker will handle 100 amps on each leg

before tripping off.(d) None of these

24. A 50/75A current totalizing breaker has 75 amps on one leg. The maximum amperage that can be on the other leg before the breaker will trip off is:(a) 0 amps.(b) 75 amps.(c) 50 amps.(d) 150 amps.

25. A CT breaker rated at 100/150A is part of a balanced 240 volt circuit. The maximum load (kVA) on the breaker would be:(a) 12kVA.(b) 24kVA.(c) 36kVA.



(d) 48 amps.

T / F 26. A thermal magnetic breaker rated at 100A will handle a maximum of 100 amps in a balanced or imbalanced condition.



Review Question Solutions

1. Protect the electrical system and connected equipmentfrom overcurrent.

2. T

3. F

4. F

5. Link fuse.

6. F

7. T

8. Overcurrent through the fuse.

9. F

10. T

11. T

12. T

13. The time it takes a fuse to melt and initiate arcing after afault occurs.

14. The time between when a fuse starts to melt and when ithas completed arcing.

15. Transformer.

16. Heat.

17. F

18. T

19. F

20. F



21. T

22. T

23. The breaker will handle 100 amps on each leg before trip-ping off.

24. 0 amps.

25. 24kVA.

26. T

single-trip overcurrent protection - saskpower 3 book 2/9. single...installation of the various...

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