CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM Line H: Install Electrical Equipment H-7 LEARNING GUIDE H-7 INSTALL EMERGENCY POWER SYSTEMS

Transcription

1 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM Level 4 Line H: Install Electrical Equipment H-7 LEARNING GUIDE H-7 INSTALL EMERGENCY POWER SYSTEMS

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3 Foreword The Industry Training Authority (ITA) is pleased to release this major update of learning resources to support the delivery of the BC Electrician Apprenticeship Program. It was made possible by the dedicated efforts of the Electrical Articulation Committee of BC (EAC). The EAC is a working group of electrical instructors from institutions across the province and is one of the key stakeholder groups that supports and strengthens industry training in BC. It was the driving force behind the update of the Electrician Apprenticeship Program Learning Guides, supplying the specialized expertise required to incorporate technological, procedural and industry-driven changes. The EAC plays an important role in the province s post-secondary public institutions. As discipline specialists the committee s members share information and engage in discussions of curriculum matters, particularly those affecting student mobility. ITA would also like to acknowledge the Construction Industry Training Organization (CITO) which provides direction for improving industry training in the construction sector. CITO is responsible for organizing industry and instructor representatives within BC to consult and provide changes related to the BC Construction Electrician Training Program. We are grateful to EAC for their contributions to the ongoing development of BC Construction Electrician Training Program Learning Guides (materials whose ownership and copyright are maintained by the Province of British Columbia through ITA). Industry Training Authority January 2011 Disclaimer The materials in these Learning Guides are for use by students and instructional staff and have been compiled from sources believed to be reliable and to represent best current opinions on these subjects. These manuals are intended to serve as a starting point for good practices and may not specify all minimum legal standards. No warranty, guarantee or representation is made by the British Columbia Electrical Articulation Committee, the British Columbia Industry Training Authority or the Queen s Printer of British Columbia as to the accuracy or sufficiency of the information contained in these publications. These manuals are intended to provide basic guidelines for electrical trade practices. Do not assume, therefore, that all necessary warnings and safety precautionary measures are contained in this module and that other or additional measures may not be required.

4 Acknowledgements and Copyright Copyright 2011, 2014 Industry Training Authority All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or digital, without written permission from Industry Training Authority (ITA). Reproducing passages from this publication by photographic, electrostatic, mechanical, or digital means without permission is an infringement of copyright law. The issuing/publishing body is: Crown Publications, Queen s Printer, Ministry of Citizens Services The Industry Training Authority of British Columbia would like to acknowledge the Electrical Articulation Committee and Open School BC, the Ministry of Education, as well as the following individuals and organizations for their contributions in updating the Electrician Apprenticeship Program Learning Guides: Electrical Articulation Committee (EAC) Curriculum Subcommittee Peter Poeschek (Thompson Rivers University) Ken Holland (Camosun College) Alain Lavoie (College of New Caledonia) Don Gillingham (North Island University) Jim Gamble (Okanagan College) John Todrick (University of the Fraser Valley) Ted Simmons (British Columbia Institute of Technology) Members of the Curriculum Subcommittee have assumed roles as writers, reviewers, and subject matter experts throughout the development and revision of materials for the Electrician Apprenticeship Program. Open School BC Open School BC provided project management and design expertise in updating the Electrician Apprenticeship Program print materials: Adrian Hill, Project Manager Eleanor Liddy, Director/Supervisor Beverly Carstensen, Dennis Evans, Laurie Lozoway, Production Technician (print layout, graphics) Christine Ramkeesoon, Graphics Media Coordinator Keith Learmonth, Editor Margaret Kernaghan, Graphic Artist Max Licht, Graphic Artist Publishing Services, Queen s Printer Sherry Brown, Director of QP Publishing Services Intellectual Property Program Ilona Ugro, Copyright Officer, Ministry of Citizens Services, Province of British Columbia To order copies of any of the Electrician Apprenticeship Program Learning Guide, please contact us: Crown Publications, Queen s Printer PO Box 9452 Stn Prov Govt 563 Superior Street 2nd Flr Victoria, BC V8W 9V7 Phone: Toll Free: Fax: crownpub@gov.bc.ca Website: Version 1 Corrected, September 2015 Revised, April 2014 New, October 2012

5 LEVEL 4, LEARNING GUIDE H-7: INSTALL EMERGENCY POWER SYSTEMS Learning Objectives Learning Task 1: Describe the application of cells and batteries Self-Test Learning Task 2: Describe emergency lighting equipment Self-Test Learning Task 3: Describe standby generators Self-Test Learning Task 4: Describe uninterruptible power supplies Self-Test Answer Key CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 5

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7 Learning Objectives H-7 Learning Objectives The learner will be able to identify types of emergency power systems. The learner will be able to determine emergency power system requirements. The learner will be able to describe procedures to test emergency power systems. Activities Read and study the topics of Learning Guide H-7: Install Emergency Power Systems. Complete Self-Tests 1 through 4. Check your answers with the Answer Key provided at the end of this Learning Guide. Resources All resources are provided in this Learning Guide. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 7

8 BC Trades Modules We want your feedback! Please go the BC Trades Modules website to enter comments about specific section(s) that require correction or modification. All submissions will be reviewed and considered for inclusion in the next revision. SAFETY ADVISORY Be advised that references to the Workers Compensation Board of British Columbia safety regulations contained within these materials do not/may not reflect the most recent Occupational Health and Safety Regulation. The current Standards and Regulation in BC can be obtained at the following website: Please note that it is always the responsibility of any person using these materials to inform him/herself about the Occupational Health and Safety Regulation pertaining to his/her area of work. Industry Training Authority January CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

9 Learning Task 1: Describe the application of cells and batteries The cell (Figure 1a) is the basic unit that converts chemical energy to electrical energy. A battery is usually an arrangement of two or more cells (Figure 1b) that are series- or parallel-connected to supply the appropriate voltage or current to the load. Figure 1 Series/parallel connection of V cells In its most basic form, a cell consists of two conductive electrodes (or plates) of different material and an electrolyte solution (Figure 2). The electrolyte reacts chemically with the electrodes and acts as a medium for transferring electrons, creating a potential difference between the two electrodes. The value of this voltage depends upon the materials used for the electrodes and electrolyte. Electrochemical cells are rated by the voltage and ampere-hour capacity they can deliver under specified conditions of temperature and discharge. [Ampere-hours (Ah) = amperes hours, a measure of charge used to size the battery for a required operation time when the current drain is known.] Cells are divided into two fundamental types: Primary cells Secondary cells In the primary cell, the chemical action during discharge results in a progressive destruction of one (or both) of the electrodes. When fully discharged, the cell is chemically irreversible, and either the cell or the active ingredients must be replaced. The most common type of primary cell is referred to as a dry cell. Figure 2 Simple electrochemical cell CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 9

10 Learning Task 1 H-7 The secondary, or storage, cell is chemically reversible. It can normally be recharged by passing current through it in a direction opposite to its discharge path. Large-capacity rechargeable batteries are sometimes referred to as wet cells. Dry cells Dry cells are not really dry, but contain an electrolyte that is combined with various other ingredients to form a paste or gel. The ingredients are sealed in liquid-tight containers so that they can be operated in any position. Although generally considered as primary cells, certain kinds of dry cells can be recharged. The most common types of dry cells used are: Carbon zinc (Leclanché) Zinc chloride Alkaline Mercury Silver oxide Lithium Table 1 shows some comparative features of these primary batteries. Table 1: Battery characteristics Battery Family Carbon zinc Zinc chloride Alkaline Silver oxide Mercury Lithium Type Primary Primary Primary Primary Primary Primary Volts per cell 1.5 V 1.5 V 1.5 V 1.6 V 1.35 or 1.4 V 3.0 V Positive electrode Negative electrode Electrolyte Limits Temperature effects Carbon/ manganese dioxide Carbon/ manganese dioxide Manganese dioxide Silver oxide and manganese dioxide Mercuric oxide Manganese dioxide Zinc Zinc Zinc Zinc Zinc Lithium Ammonium and zinc chloride Lower efficiency at higher current drains Poor lowtemperature operation Zinc chloride Hightemperature performance same as carbon zinc OK at lower temperatures Potassium hydroxide Expensive for lowcurrent uses Good at lower temperatures Potassium or sodium hydroxide Expensive Good at lower temperatures Potassium or sodium hydroxide Poor performance at low temperatures Good at high temperatures; poor at low temperatures Lithium perchlorate 3 V uses only Wide operating temperature range 10 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

11 Learning Task 1 H-7 Battery Family Features Carbon zinc Readily available; low cost Zinc chloride Greater service capacity than carbon zinc Alkaline Silver oxide Mercury Lithium Very efficient at high loads Flat discharge; good shelf life Very flat discharge curve Higher voltage; excellent shelf life Carbon-zinc (Leclanché) cells The carbon-zinc (or Leclanché) cell is most commonly used because of its low cost, ready availability and reliable performance. Although carbon-zinc cells are available in several different shapes, the cylindrical cell construction is the most common. These cells come in a variety of physical sizes ranging from the small AAA code to D code (and larger) sizes. Figure 3 shows some of the constructional features of a typical carbon-zinc cell. This cell uses a zinc can enclosure as the negative electrode and a manganese-dioxide mixture surrounding a carbon rod as the positive electrode. The electrolyte is a solution of ammonium chloride and zinc chloride with powdered carbon added to improve the conductivity of the mix and to retain moisture. As the cell is used, the zinc is consumed and the reaction products begin to retard the chemical action of the cell, lowering its voltage. Figure 3 Cutaway of a general-purpose carbon-zinc cell The nominal or open-circuit voltage of the cell is 1.5 V. The working or closed-circuit voltage of the cell drops gradually as it is discharged. The cutoff voltage, or the voltage considered as the end of useful discharge, is usually about 0.9 V per 1.5 V cell. Figure 4 shows typical voltage CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 11

12 Learning Task 1 H-7 characteristics of a fresh, size D, carbon-zinc cell that is discharged at a rate of two hours per day at 20 C. Figure 4 Typical voltage discharge characteristics of a carbon-zinc cell Figure 5a shows a 1.5 V Leclanché flat cell. In this construction, the cells can be stacked so that the bottom zinc plate of one cell contacts the carbon of the next cell to form a duplex electrode. Figure 5b shows six cells stacked to form a 9 V transistor battery. Metal straps are used to attach the ends of the assembly to the battery terminals, and the entire stack is usually encapsulated in wax or plastic. (a) Flat cell construction (b) Flat cell battery assembly Figure 5 Leclanché flat cell Carbon-zinc cells are normally designed to operate at an optimal temperature close to 20 C. Higher temperatures during discharge will increase the energy output, but will dramatically shorten the life of the cell. Lower temperatures tend to decrease the chemical activity in the cell and reduce the output; nevertheless, lower temperatures (ideally 5 C to 10 C) are preferred for shelf storage. When removed from lower temperature storage, cells should be allowed to reach room temperature before being put into service. 12 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

13 Learning Task 1 H-7 Carbon-zinc cells are typically classified as general-purpose cells and are used as a comparison for other types of primary cells. They are best suited for intermittent operation at light-tomedium drain service in applications such as toys, flashlights or portable radios. Zinc-chloride cells Zinc-chloride cells (Figure 6) have the same constructional features and the same open-circuit voltage as carbon-zinc cells. Their essential difference is the omission of ammonium chloride from their electrolyte solution. This results in the cell having a much lower internal resistance, and permits higher rates of discharge for longer periods of time than the carbon-zinc cell. They cost slightly more than their carbon-zinc counterpart, but can deliver up to 50% more energy. Some manufacturers designate the zinc-chloride cell as their heavy-duty cell. - Figure 6 Cutaway of a zinc-chloride cell Alkaline cells The alkaline-manganese dioxide cell is more commonly referred to as the alkaline battery. It has a uniquely different electrolyte chemistry and construction that provide up to 10 times the energy capacity of standard carbon-zinc cells (Figure 7). The larger area of zinc and the higher conductivity of the electrolyte provide the greater capacity for this cell. Although alkaline cells discharge much more slowly than standard carbonzinc cells under similar load conditions, their performance excels on high discharge rates and more continuous service. They also have a long shelf life, but they are more expensive than carbon-zinc and zinc-chloride cells. Alkaline cells are best suited for high-intensity lamps, camera flash units, cassette recorders, clocks and other motor-driven devices. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 13

14 Learning Task 1 H-7 Figure 7 Cutaway of an alkaline cell Silver-oxide cells The silver-oxide cell (Figure 8) provides a high-energy, miniature power source that has found wide acceptance in electronic equipment such as hearing aids, watches, calculators and cameras. This primary cell is most commonly seen in the button configuration with capacities ranging from 35 to 200 mah. Figure 8 Cutaway of a silver-oxide button cell Figure 8 shows the typical construction of a silver-oxide button cell. The cell has a highly alkaline electrolyte. Potassium hydroxide is used as the electrolyte in hearing aid batteries to provide maximum power density at hearing aid current drains; sodium hydroxide is used in watch batteries to provide long-term reliability. 14 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

15 Learning Task 1 H-7 The open-circuit voltage of the silver-oxide cell is about 1.6 V, and it exhibits an almost flat discharge curve throughout its service life. The silver-oxide cell has good low-temperature performance (delivering about 70% of its rated capacity at 0 C and 35% at 20 C) and good shelf-storage life. However, because of the silver content, these cells are rather costly. Mercury cells Mercury (also called mercuric-oxide) cells (Figure 9) provide an almost constant output voltage throughout their working life. They are typically manufactured in button or small cylindrical shapes. Applications include hearing aids, watches and miniature electronic equipment. Figure 9 Cutaway of a mercury button cell The output voltage is about 1.35 V per cell, which can be increased to about 1.4 V per cell when manganese dioxide is added to the mercuric oxide. The alkaline electrolyte can be either potassium hydroxide or sodium hydroxide. Since these cells may be dimensionally interchangeable but functionally different, special care must be taken in selecting the proper cell for the application. Mercury cells do not perform well at low temperatures and are best operated at temperatures of about 20 C. Do not dismantle mercury cells, since mercury is poisonous and the cell electrolyte is corrosive. For ecological reasons, spent cells should be returned to the manufacturer. Lithium cells There are several classifications of lithium primary cells. One of the most widely used is the lithium-manganese dioxide cell. It is available in either a cylindrical form or flat coin (button) shape, as shown in Figure 10. The manganese-dioxide mix reacts with a thin layer of lithium that is electroplated onto a stainless steel container. The chemical reaction is supported through an electrolyte of lithium perchlorate suspended in propylene carbonate. This means that the electrolyte is virtually dry, and as a result the cell can be operated over a wide temperature range ( 20 C to +55 C). The unique chemistry also allows a shelf life exceeding five years. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 15

16 Learning Task 1 H-7 Figure 10 Cutaway of a lithium button cell The open-circuit voltage of a lithium cell is 3 V and remains virtually constant throughout the operating life of the cell. In early days, lithium cells were used mainly for watches and calculators. Now they are also used for audiovisual equipment, strobe lights, measuring instruments and memory backup. Secondary cells A secondary battery is an assembly of electrochemical cells that can be recharged electrically by passing current through them in the opposite direction to the discharge path. Larger capacity rechargeable batteries are commonly referred to as storage batteries (or wet cells). Storage batteries have a liquid electrolyte and vents to release gases that are produced during charge and discharge. For example, lead-acid batteries produce hydrogen gas and require proper ventilation, if located in confined areas. The design and construction of batteries vary according to the particular duty they must perform. The main categories of secondary batteries are: SLI or automotive batteries (for engine starting, lighting and ignition) Motive power batteries (for electric vehicles) Standby batteries (for emergency power to essential equipment and alarms) Portable batteries (for cordless tools and equipment) Three types of long-life storage batteries that have been in common use for years are: Lead-acid Nickel-iron Nickel-cadmium 16 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

17 Learning Task 1 H-7 Lead-acid battery The lead-acid battery is one of the most widely used in secondary battery systems. The conventional lead-acid (also called Planté) battery has positive-plate electrodes of lead peroxide and negative-plate electrodes of sponge lead insulated from each other and supported within a lead alloy grid structure. The electrolyte is a solution of sulphuric acid and water. Figure 11 illustrates some of the constructional features of a typical power station service battery. PVC Figure 11 General arrangement of a heavy-duty lead-acid (Planté) battery During battery discharge, both the positive and negative plates are chemically converted to lead sulphate. Using DC, the charging process restores the plates to their original chemical makeup. Lead-acid batteries should never be left in a discharged condition for any great length CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 17

18 Learning Task 1 H-7 of time because the lead sulphate may harden, preventing the battery from accepting a full charge. If the battery is to be placed in storage for a long period of time, it is recommended that the battery be: Fully charged before storage Periodically recharged (at least every six months) Stored in a cool, dry place (recommended between 0 C to 10 C) Most lead-acid batteries use electrolyte with a relative density (formerly called specific gravity) ranging from 1210 to Because the relative density of the electrolyte changes with the state of charge, the condition of the battery may be checked with a hydrometer. The hydrometer float with its graduations will rise with an increase in relative density, or charge. As Figure 12 shows, some hydrometers have three defined areas that are coloured (instead of graduated) to indicate state of charge. (a) Float markings (b) State of charge Figure 12 Hydrometer readings For stationary batteries, a full charge is usually indicated with a reading between 1210 to 1225; fully discharged is between 1180 and For portable batteries, a full charge is usually indicated by a reading between 1280 and 1300; and fully discharged is between 1100 and CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

19 Learning Task 1 H-7 If the level of the electrolyte is low, water should be added and the battery should be charged before a hydrometer reading is taken. The lowering of the electrolyte level in normal usage is caused by the evaporation of water. Use only distilled water to restore the electrolyte to the correct level. If electrolyte is lost because of spilling, it should be replaced with a sulphuric acid solution of the correct relative density. Individual cells are connected in series to provide different voltage levels for batteries; for example, three cells are needed for a 6 V battery, six cells for a 12 V battery, and so on. A nominal 12 V battery has an average discharge voltage of about 2.0 V per cell, as shown by the graph in Figure 13. The charging voltage should be approximately 2.3 V per cell, and a fully charged battery should indicate approximately 2.2 V per cell. It should be noted that the open-circuit voltage is not an accurate means of determining the state of charge for a battery. The internal resistance of the cells increases during discharge, resulting in a decrease of the battery s terminal voltage. This internal resistance decreases during the charging process. Figure 13 Typical voltage characteristics The vented type of battery requires a fair amount of maintenance. In recent years, different types of sealed lead-acid batteries have been produced to provide maintenance-free service. These have been designed so that they do not require the addition of water throughout their normal service life, they give off only small amounts of gas, and they can be located in areas without special venting requirements. Newer styles of standby and portable batteries use either a gelled or absorbed electrolyte that allows mounting of the cells in either vertical or horizontal positions without the risk of acid spillage. Refer to Figure 14. Lead-acid batteries can give many hundreds of charge/discharge cycles without deterioration, and can have a life expectancy up to 15 years. They are available in sizes up to 2.5 Ah in small, sealed cells, and up to Ah in large stationary batteries. (The Ah capacity is usually based on a constant 20-hour discharge rate for portable batteries, and on a constant 8-hour discharge rate for stationary batteries.) CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 19

20 Learning Task 1 H-7 - Figure 14 Portable, sealed lead-acid battery Nickel-iron battery The nickel-iron (also called Edison) battery is a durable, rugged, long-life battery that is almost indestructible. It can withstand electrical abuse such as overcharging, remain discharged for long periods, and even be short-circuited. The battery is best suited where high cycle life at repeated deep discharges is required (such as traction vehicle applications). Its limitations are low power density, poor low-temperature performance, poor charge retention, and hydrogen gas evolution. In recent years, the nickel-iron battery has been losing its market to the nickel-cadmium battery in many applications. However, a new generation of nickel-iron batteries is under development for mobile industrial equipment. The conventional nickel-iron battery has negative-plate electrodes of metallic iron oxide and positive-plate electrodes of nickel oxide. The electrolyte is an alkaline solution of potassium hydroxide with a small amount of lithium hydroxide added. The relative density of the electrolyte does not change during charging and discharging, so it cannot be used to determine the state of the battery. It has a relative density of about 1200 at 15 C. The state of charge must be determined with a DC voltmeter during charge or discharge. A fully charged battery should indicate about 1.37 V per cell. The nickel-iron battery has an average voltage of about 1.2 V per cell during discharge, and a cell should be recharged when it reaches 1 V. The maximum charging voltage should not exceed 1.8 V per cell. Refer to Figure CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

21 Learning Task 1 H-7 Figure 15 Typical voltage characteristics of a nickel-iron battery during constant-rate discharge/charge Nickel-iron batteries should be operated in well-ventilated environments to prevent the accumulation of hydrogen. If the battery is to be stored for more than a month, it should first be discharged, then short-circuited and left in that condition for the storage period. The filling caps should be kept closed during storage. Nickel-iron batteries have been available in sizes typically ranging from 5 Ah to 1200 Ah. Figure 16 shows a drawing of a single cell and how the cells are rack-mounted to form a multi-cell battery. (a) Single cell (b) Multi-cell battery Figure 16 Rack-mounted nickel-iron cells Nickel-cadmium (Ni-Cd) batteries The nickel-cadmium (Ni-Cd) cell is similar in many respects to the nickel-iron cell but is cheaper to produce (Figure 17). The pocket-plate nickel-cadmium cell uses nickel hydroxide as the positive electrode material and a cadmium-oxide mixture for the negative plate electrode. The electrolyte is an alkaline solution of potassium hydroxide in distilled water. The electrolyte has a relative density of about 1200 at 15 C and does not change during charge or discharge, so it is not useful in determining the cell s state of charge. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 21

22 Learning Task 1 H-7 Positive plate (nickel-plated steel containing nickel hydroxide) Figure 17 Vented pocket-plate, nickel-cadmium battery A more recent development of Ni-Cd cells has resulted in a sintered-plate construction (see Figure 18) that provides a thinner form than the pocket-plate cells. The sintered-plate battery has most of the favourable characteristics of the pocket type, although it is generally more expensive. 22 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

23 Learning Task 1 H-7 Figure 18 Features of a vented sintered-plate nickel-cadmium cell The Ni-Cd battery has a nominal (average discharge) voltage of 1.2 V per cell and will hold its charge for a long time if stored. A fully charged cell would indicate about 1.35 V. The state of charge for the battery must be determined using a DC voltmeter during charge or discharge. Although overcharging will normally not damage the cell, it may result in evolution of hydrogen gas and decomposition of water. In applications requiring negligible gassing and low maintenance, the battery is often float charged, where it operates at about 80% of the charged condition while using a constant voltage charger set at about 1.4 V per cell. Newer developments have led to sealed Ni-Cd batteries (Figure 19) requiring no maintenance other than recharging. These batteries have a long service life (over 500 cycles of charge/ discharge or five years of standby power) and can operate over a wide temperature range (from 40 C to +50 C). Ni-Cd battery styles are available in a variety of sizes typically ranging from 5 Ah to over 1200 Ah capacity. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 23

24 Learning Task 1 H-7 Cover (positive terminal) Resealable vent mechanism Insulating seal ring Positive tab collector Cadmium negative plate Separator (absorbent for alkaline electrolyte) Nickel oxide positive plate Negative tab Nickel-plated collector steel case (negative terminal) Figure 19 Sealed cylindrical Ni-Cd cell Rechargeable Ni-Cd cells are also available in the same popular cylindrical sizes as primary zinccarbon cells. These cells have a nominal voltage of 1.2 V and can lose their output if left idle for several days. Ni-Cd cells can also form a memory. Memory effect Memory effect is an effect observed in certain specific applications of sintered plate nickel cadmium rechargeable batteries, which causes them to recharge to less than full rated charge. Batteries appear to gradually lose their maximum capacity if they are repeatedly recharged after being discharged down to a partial level rather than completely discharged. The battery appears to remember the lesser charge level. The term memory effect is often mistakenly applied to almost any situation in which a battery appears to hold less charge than was expected. These cases are more likely due to battery age and use, leading to irreversible changes in the cells due to internal short-circuits, loss of electrolyte, or reversal of cells. Memory effect will not be a problem with a battery if any one of the following conditions is met in the charge and discharge cycle: Batteries reach full overcharge during charging cycle. Discharge is not exactly the same each usage cycle. Discharge is to less than 1.0 volt per cell. 24 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

25 Learning Task 1 H-7 Other problems perceived as memory effect Phenomena which are not true memory effect may also occur in other battery types than sintered-plate nickel-cadmium cells. Voltage depression A common process often ascribed to memory effect is voltage depression. In this case the peak voltage of the battery drops more quickly than normal as it is used, even though the total energy remains almost the same. This is a common problem with high-load devices such as digital cameras. Voltage depression is caused by repeated over-charging of a battery, which causes the formation of small crystals of electrolyte on the plates. These can clog the plates, increasing resistance and lowering the voltage of some individual cells in the battery. This causes the battery as a whole to seem to discharge rapidly as those individual cells discharge quickly and the voltage of the battery as a whole suddenly falls. This effect is very common, as consumer trickle chargers typically overcharge. High temperatures High temperatures reduce the charge accepted by the cells, which reduces the voltage level the cells are charged to. This may be mistaken for the memory effect. Deep discharge Some rechargeable batteries can be damaged by repeated deep discharge. Batteries are composed of multiple similar, but not identical, cells. Each cell has its own charge capacity. As the battery as a whole is being deeply discharged, the cell with the smallest capacity may reach zero charge and will reverse charge as the other cells continue to force current through it. The resulting loss of capacity is often ascribed to the memory effect. Age and normal usage Rechargeable batteries have a finite lifespan and will slowly lose the ability to store charge as they age, due to secondary chemical reactions within the battery. These reactions occur whether the battery is used or not. Some cells may fail sooner than others, but the effect is to reduce the output voltage of the battery Other causes Other problems that may be perceived as memory effect include: Operation below 0 C High discharge rates (above 25 C) in a battery not specifically designed for such use Inadequate charging time Defective charger CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 25

26 Learning Task 1 H-7 Battery maintenance and testing Since batteries have no moving parts, they generally require little care. However, there are some basic maintenance procedures that are essential to assure their dependability and long life. The frequency of maintenance depends on the actual type of battery installation. For example, with station batteries, monthly maintenance may consist of: Measuring the relative density of pilot cells Recording hydrometer readings (temperature-corrected) Checking electrolyte levels and adding water as necessary Measuring cell voltages with battery on float charge Cleaning accumulated dust from cell covers In large battery installations, pilot cells are chosen to provide a representative test indication for the battery installation. Typically, five pilot cells are used for tests, and are periodically advanced within the bank. For sealed-type batteries, cleaning and charging are normally all that is required for maintenance. For batteries in standby service, the charging voltage should be checked periodically and, if necessary, the current adjusted in accordance with the manufacturer s recommendations. Safety precautions Personnel involved in testing and maintenance of battery installations must be properly trained. To reduce the possibility of injury to persons or failure of the system, battery rooms usually have restricted access. Danger! Sulphuric acid burns The electrolyte solution in batteries is highly corrosive and must be handled with care. When servicing wet cells, always wear rubber gloves, a protective rubber apron and sealed eye goggles. The electrolyte in a lead-acid cell contains sulphuric acid, whereas an alkaline cell contains potassium hydroxide. If electrolyte splashes on skin or in eyes, flush immediately under running water and get medical help immediately. If full-strength acid has to be diluted, always add acid to distilled water. If water is added to strong acid, rapid heating and bubbling will occur, causing uncontrolled splashing of the acid. Potassium hydroxide is generally mixed from a powder by adding the prescribed amount of powder to distilled water and stirring with a glass rod. Do not handle the powder with bare hands. It is as corrosive as the liquid electrolyte. 26 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

27 Learning Task 1 H-7 Danger! Explosive gases There must be no smoking around battery installations. Both acid and alkaline electrolytes release hydrogen gas during the charge/discharge cycles of the battery. If a means of ignition is present during this gassing action, an explosion can occur. Avoid sparks, and keep flames away from the vicinity of storage batteries. Danger! Electric shock and burns Multi-cell systems can attain dangerous voltages. Insulate tool handles to protect against both electric shock and shorting of battery terminals. Remove watches, rings and other jewellery items so that they cannot contact battery terminals or busbars, resulting in injury. Never leave metal tools on top of cells since sparks due to shorting may result in an explosion of hydrogen gas. Disconnect AC and DC circuits before working on batteries or charging equipment. To safely move cells, use proper lifting straps. If electrolyte is spilled, it should be neutralized by using: Baking soda and water for acid batteries Vinegar and water for alkaline batteries The most important testing and maintenance tools for wet-cell battery installations include: A hydrometer An electrolyte thermometer A voltmeter A water container A bristle cleaning brush Most important: Personal protective gear. Safety goggles (or face shield), rubber gloves and a rubber apron should be worn to protect against contact with electrolyte solutions. A maintenance log should be kept to record relative density, temperature and cell voltages. This will help to detect potential problems before they result in battery failure. Battery manufacturers recommend the frequency and type of tests to be performed, and can provide appropriate log sheets for record-keeping. When measuring the relative density of the electrolyte, use a hydrometer with the appropriate range, since the measurements may be different for portable versus stationary batteries. A hydrometer used for lead-acid batteries should never be used for alkaline batteries, as any acid residue could destroy the alkaline cells. Refer to Figure 20. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 27

28 Learning Task 1 H-7 C 1280 Discharged Half-charged Charged 25ºC (a) Hydrometer (b) Indicator float on hydrometer (c) Battery thermometer Figure 20 Hydrometer and battery thermometer An electrolyte thermometer is used to measure deviations from recommended cell temperatures (25 C or 77 F) and apply any necessary corrections to hydrometer readings. For each degree above 25 C, 0.7 point is added to the relative density reading taken. For each degree below 25 C, 0.7 point is subtracted from the relative density reading. For example, if a relative density of 1200 is measured at 10 C, then the corrected reading for 25 C would be 1200 (15 0.7) = It is recommended that alcohol- rather than mercury-type thermometers be used for lead-acid batteries. The thermometer and hydrometer should be thoroughly flushed with clean water after each use, to prevent the buildup of residue that could obscure readings on the scale. For voltage measurements, a DC voltmeter with an accuracy of 1% is recommended. The cell voltage measured should be to two decimal points (e.g., 2.17 V), so it is generally best to use a digital meter. As battery cells are charged and discharged, some loss of water occurs from the electrolyte. To replenish the electrolyte, distilled water should be added, using non-metallic (hard rubber, plastic or glass) containers, so that it will not contaminate the battery electrolyte. The minimum/maximum electrolyte levels are generally marked on the side of each cell jar, and should not be exceeded. 28 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

29 Learning Task 1 H-7 A small torque wrench may be needed to periodically tighten any intercell connector links that may have loosened from temperature changes or vibration. Follow the manufacturer s torque specifications, and do not over-tighten! A non-metallic bristle brush should be used to periodically dust off the top of cell covers. All vent plugs should be properly in place before cleaning. Battery charging To recharge a storage battery after discharge, a direct current must pass through the cells in a direction opposite to that of discharge. Proper recharging is important to obtain the optimal life from any secondary battery. Battery manufacturers specify the ideal method and rate for recharging batteries. Although the charging methods can vary for the type and capacity of batteries, batteries can generally be charged at any rate that does not produce excessive gassing, overcharging or high temperatures. Generally, batteries can absorb a high current during the early part of the charging process but are limited to a lower current as the battery becomes charged. The type of battery, the service conditions, the time available for charging and the variation in the number of battery cells when charged in multiple are all factors in determining the type of charging method used. Following are several terms dealing with the types of charge. Initial Initial charge is the charging process conducted after the battery is initially filled with the electrolyte solution. The resulting electrolysis is used to properly form the plates. Normal Normal charge is a routine charge given to restore a battery to its fully charged condition. Stationary batteries are usually recharged in 8 hours based on a normal discharged condition. Trickle Trickle charge is a constant charge given to a standby battery (no external load connected) to maintain it in a fully charged condition. Float Float charge is a charging system in which the battery and its load are connected in parallel with the rectified power source. Emergency Emergency (or quick) charge is a method sometimes used for automotive (SLI) batteries to recharge the battery in a minimum amount of time. This charging rate is much higher than that normally used for charging. There are two fundamental methods used to charge batteries: Constant-voltage Constant-current CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 29

30 Learning Task 1 H-7 Constant-voltage method In the constant-voltage method, the charging voltage is held at a constant level while the charging current is reduced until the battery comes up to a fully charged condition. Figure 21 illustrates this method for a lead-acid battery. Figure 21 Constant-voltage charging method Constant-current method In the constant-current method, the charging current is held relatively constant until the battery comes up to a fully charged condition. This method is often used for sealed Ni-Cd batteries. Figure 22 Constant-current charging method Basic charging circuits The most basic battery-charger circuits utilize either half-wave or full-wave, single-phase rectifier circuits. The most common input for these circuits is 120 V, 60 Hz, which is stepped down using transformers, as shown in Figure 23. A resistor is used for current-limiting, and no filtering or smoothing is necessary. 30 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

31 Learning Task 1 H-7 (a) Half-wave rectifier circuit (b) Full-wave rectifier circuit using centre-tap transformer (c) Full-wave bridge rectifier circuit Figure 23 Single-phase rectifier circuits for battery charging Although the circuits in Figure 23 are basic and economical, they must be carefully monitored and manually controlled. An improvement in circuit design can provide a regulated charger (Figure 24). In this circuit, once the battery is fully charged, the charging process will cycle to maintain a trickle charge. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 31

32 Learning Task 1 H-7 Figure 24 Regulated battery-charger circuit Figure 25 shows a battery charger designed for charging both standby and on-line batteries. Battery and load conditions are continually sensed to provide the proper charger output rate as determined by the battery system. As battery voltage reaches the preset charger voltage, the charging current tapers to the charge-preserving rate. 32 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

33 Learning Task 1 H-7 Figure 25 Circuit features of stationary battery charger Courtesy of GNB Battery Technologies Battery installations Section 26 of the Canadian Electrical Code provides rules that apply to storage batteries in larger industrial installations. In installations such as power stations, secondary batteries are located in a battery room (Figure 26). These batteries are usually mounted on stands (racks) that are constructed of corrosion-resistant material and bolted to the floor. The batteries can be either mounted on the racks as single-tier, or stacked to provide multi-tier (tower) arrangements (Figure 26a). The racks can be arranged in a single row against a wall, or in a multi-row layout, where they can be accessed from both sides (Figure 26b). CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 33

34 Learning Task 1 H-7 (a) Rack elevation (B-B) (b) Floor plan Figure 26 Typical layout of battery room The batteries are usually housed in rooms (vaults) that are well ventilated and dry. Wall, ceiling and floor finishes are generally treated with acid-resisting paint. Such a room may also house all maintenance equipment, protective clothing and services. It is normal practice to provide corrosion-resistant luminaires in battery rooms. Refer to Rules to of the CEC, which deal specifically with battery rooms and the installation of batteries on stands. Connections between batteries are usually made to suitably sized copper bus bars that are marked at intervals with red (+) and black (-) paint or rubber tape for polarity identification. The battery switchgear is usually located immediately outside the battery room. Cable connections to the respective chargers or distribution boards are all made within the switchgear enclosure. Refer to Rule of the CEC for the approved wiring methods between batteries and their associated equipment. Other types of wiring (e.g., lights, receptacles, etc.) in a battery room shall be done in accordance with the requirements for a dry location as specified in the CEC. Now do Self-Test 1 and check your answers. 34 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

35 Learning Task 1 H-7 Self-Test 1 1. What is the difference between a cell and a battery? 2. What is the difference between a primary cell and a secondary cell? 3. What does the term dry cell imply? 4. What is another name for the carbon-zinc dry cell? 5. Which primary cell with similar features is considered as the heavy-duty counterpart to the carbon-zinc cell? 6. In order to form a 9 V transistor battery, how must the individual carbon-zinc cells be connected? 7. What two advantages does the alkaline cell have over the standard carbon-zinc cell? 8. In what type of case shape are silver-oxide and mercury cells commonly constructed? CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 35

36 Learning Task 1 H-7 9. Which type of primary cell has the highest open-circuit voltage? 10. Which type of primary cell is generally considered as the most expensive for its energy capacity? 11. What is a storage battery? 12. What is meant by the term wet cell? 13. List the four main categories of secondary batteries. 14. The relative density of the electrolyte in a battery is measured with an instrument called a. 15. For lead-acid type batteries, state the relative density expected for a fully charged: a. portable battery b. stationary battery 36 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

37 Learning Task 1 H What happens to the internal resistance of a lead-acid battery during normal discharge? 17. What is the terminal voltage expected for a fully charged, six-cell, lead-acid battery? 18. State how many cells are in a conventional 12 V: a. lead-acid battery b. nickel-iron battery c. nickel-cadmium battery 19. Compared to acid (in lead-acid batteries), what type of electrolyte is used in nickel-cadmium batteries? 20. What is the relative density expected for a fully charged nickel-cadmium stationary battery? 21. The relative density of a nickel-cadmium battery cannot be used as a reliable measurement of its state of charge. Why not? 22. How can the memory effect be overcome in a nickel-cadmium cell? CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 37

38 Learning Task 1 H List three tasks involved in the monthly maintenance of lead-acid station batteries. 24. List three items of personal protective equipment required when servicing wet-cell batteries. 25. Why is it important that there be no smoking or open flames near stationary battery installations? 26. State the type of electrolyte solution used for: a. lead-acid cells b. alkaline cells 27. Why is it good practice to insulate tool handles for use on battery installations? 28. Would the indicator float in a hydrometer sink deeper or float higher in a fully charged leadacid battery? 29. If the relative density of an electrolyte read 1280 at 30 C, would you expect the temperaturecorrected relative density at 25 C to be higher or lower in value? 38 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

39 Learning Task 1 H Why should metal containers not be used to store distilled water for batteries? 31. Why should a non-metallic bristle brush be used to clean the tops of battery cells? 32. Voltmeter readings of cell voltages should normally be recorded to how many decimal places? 33. What is the usual charging time required to fully restore a stationary battery from its discharged condition? 34. What is the name given to the charging system in which the battery and its load are connected in parallel with the rectified power source? 35. What type of charge is given to a standby battery in order to maintain it in a fully charged condition? 36. What are the two fundamental methods used for charging batteries? 37. In accordance with the CEC, what is the minimum spacing between battery cells? 38. The mounting surface of a battery rack is usually required to have a covering of insulating material with a dielectric strength of at least V. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 39

40 Learning Task 1 H Sufficient ventilation should be provided in battery rooms to prevent the hydrogen gas from building up to a level of % by volume in the room air at any time. 40. Lead-acid batteries should not be located in areas where the temperature is likely to fall below C or rise above C. 41. Is it permitted to use MI cable for wiring between batteries? Go to the Answer Key at the end of the Learning Guide to check your answers. 40 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4