SAFE LABORATORY PROCEDURES Northampton Community College
To develop a healthy respect for electricity, it isimportant to understand: how electricity acts, how electricity can be directed, what hazards electricity presents, and how to minimize hazards through safe laboratoryprocedures.
HOW SHOCK OCCURSShock occurs when the body becomes a part ofthe electric circuit. The current must enter thebody at one point and leave at another.
HOW SHOCK OCCURSShock may occur in one of three ways. The person mustcome in contact with: both wires of the electric circuit, one wire of an energized circuit and the ground, an ungrounded metallic part that has become “hot” bybeing in contact with an energized wire, while the person isin contact with the circuit ground.
Hand to hand contactallows a current paththrough the heart and lungs
Left hand to left foot allowsa current path through theheart and lung
Right hand to right foot allowsa current path through thelung but bypasses the heart
A foot to foot current path allowscurrent to pass through a criticalpart of the body.This could cause the victim to falland sustain even more damagefrom other current paths.
SEVERITY OF THE SHOCK The severity of the shock received when a personbecomes a part of an electrical circuit is affected by threeprimary factors: the rate of flow of current through the body, measuredin amperes, the path of the current through the body, the length of time the body is in the circuit.
Other factors, which may affect the degree of shock, are: the frequency of the current, phase of the heart cycle when shock occurs, the physical and psychological condition of the person.
Effects of Electric Current in the Human BodyThe currents are stated in milliamperes, which is anotherway of saying in thousandths of an ampere.(1 ampere = 1,000 milliamperes)
From the table you can see that a difference of only about50 milliamperes exists between a current which can justbe perceived and one which can be immediately fatal.
On low-voltage circuits, if the person cannot let go of thecircuit and is not rescued from it, the ratio between acurrent which can just be perceived and one which isdangerous may be less than one to five.This factor should be kept in mind with respect to liveparts of low-voltage circuits, as the difference in resistancebetween dry skin and skin wet by either water orperspiration will usually vary by considerably more than afactor of five.
Further, in low-voltage shock, there is much greaterdanger of having current in the range, which will causeventricular fibrillation (convulsive movement) of the heart, acondition for which there is usually no field treatment.On the other hand, high-voltage shock frequently causesparalysis of breathing and applying artificial respirationsaves many victims of this.
The resistance of the body governs the amount ofcurrent flowing through the body.The skin offers about the only resistance presented bythe human body to the flow of current.But the skin’s humidity varies over wide limits.
A person working in high temperatures may perspirefreely and when the skin, and possibly clothing, becomeswet, the skin’s resistance to electric current dropsradically, quite easily to approximately 1,000 ohms.If working on damp or wet surfaces, or there is a break inthe skin, it could drop even more, at times to a fewhundred ohms.
Remembering Ohm’s law states that the number ofamperes flowing in a circuit with a given voltage will beinversely proportional to the resistance, it is apparentthat great variations of current are possible even with thesame voltage.
Assuming a 120-volt circuit, and under ideal conditions – aperson with a dry skin of 100,000 ohms resistancestanding on a wood floor with a resistance on the order of100,000 ohms – the amperage passing through the skincould be calculated as 120 ÷ 200,000 = 0.001 ampere (1milliampere), which would not be particularly harmful.
If however, the resistance of the skin were reduced to1,000 ohms because of perspiration, and if the personwere standing on a wet or damp ground, the currentpassing through the body would be in the nature of120 ÷ 1000 = 0.1 ampere (100 milliamperes) – morethan enough to kill.
The length of time the body is in the circuit may also beimportant, particularly with respect to the severity of burns.Burns break down the resistance of the skin, the moreextensive the burn, the greater the flow of current and themore severe the shock.
The preceding information was based OSHA pamphlet 3075.
Laboratory experimentation can be the mostinteresting way of learning ever devised.You perform experiments so that you can betterunderstand the material in your textbook.
Most industries use the same equipment you will be usingin your lab projects.Consequently, you will be acquiring knowledge ofelectricity and learning techniques at the same time – notonly for the grade you get in this course, but to help youthe rest of your life.
Laboratory experimentation can be interesting – or it can bea drag.The best way to make sure you find it interesting is to getyourself prepared before you start the experiments.The best way to make sure you get maximum benefit fromeach lab session is to follow good laboratory practices.The more seriously you approach laboratoryexperimentation, the more actual enjoyment you will get outof learning the fundamentals of electricity.
The first and most important thing to realize is that laboratory experimentation is a means to an end; the end being to learn.
When you begin an experiment you should have a moreimportant aim than just performing it successfully.Your purpose should be to learn something byperforming the experiment safely and successfully.
Therefore, before you even go into the lab, you should havestudied your textbook thoroughly and absorbed as much asyou could out of it.If you understand the material, experimentation reinforcesthat understanding.If there are points still not clear, lab work should clear themup. If you need help with the lab project as the instructor .Before you energize you project have the instructor inspect it.
You will accomplish more if you get in the habit oforganizing every lab period.Always start by looking over the entire experiment.
As you gain more and more experience in laboratoryexperimentation, you will be able to estimate prettyaccurately how much time it should take you to do eachexperiment.
If there is a lot of data to be gathered, concentrate on takingthe necessary readings.Be alert for any readings that appear not to be in keepingwith what you expect. The time to recheck those is beforethe equipment is put away.Unless required to do so, leave the computations andanalysis until last.
Usually, you are assigned to work with another student – asa two-person team.It is suggested that the two of you alternate as team leader.The other student not only follows the instructions of theleader, but also acts as a checker on the accuracy of everyphase of the experimentation.Your lab report is your work not a team effort.
APPROACH LABORATORY EXPERIMENTATION IN A BUSINESSLIKE MANNER OBSERVE SAFETY RULES.
Electricity is often referred to as a servant of humanity.
The reason we are able to turn electricity into a servant is itspredictability.We know if we make certain connections, certain things willhappen.
We know, too, that misuse of electricity can bring disaster.Electricity is inherently dangerous and must be treated with respect.
•Never turn on the source unless you are sure where thepower is going.•Always protect yourself and the circuit by using the fuseddisconnect (1 amp fuse)•Most lab sources are variable.•That is, even though they are ON, when the control knob isin its minimum position there is minimum power output.•The knob, then, can be turned clockwise to obtain anyvoltage between zero and the maximum for that source.
For example, a 0-140V AC power source can be set tosupply any voltage up to 140 volts. These terms will beexplained in detail.Always have the knob in the minimum position before youturn the power ON.Always turn the power OFF immediately after you completean experiment.
Except in the experiments where you are specifically told to do so, never connect or disconnect circuits with the power ON.
Fuses protect you and our power suppliesThis means that if the supply is called upon for more powerthan it is capable of, it will disconnect the output to protectitself and the equipment.So, if you don’t have power, turn the main switch off and thecontrol knob back to zero, check the fuse and go over yourcircuit to be sure all your connections are correct.Then replace the fuse with one supplied by the instructor.
Remember the power levels used in this program canharm you under normal circumstances; there areparticular situations where caution is advised.
It’s important to know the location of the main disconnect incase of emergency.All students should be aware of elementary first aid and whatto do if an accident occurs, either to himself or herself oranother student.The instructor will point out the main switch and disconnectsfor each table.
First, remember that electric shock is no joke – for three reasons:(1) A shock, even a small one, is more harmful if it passesthrough the heart. Electrical leads should be handled with onehand only, while the other is safely out of the way.(2) Under certain conditions, electricity can produce a painfulburn.(3) A sudden, unexpected shock causes a fast reaction and thereaction can result in injury, either to the person getting shocked,or a bystander.
Be especially cautious when the circuit contains coils andcapacitors.These can cause shocks after power has been turned off.
Remember the following forSAFE LABORATORY PROCEDURES
DON’T turn power on until the instructor checks the circuit. Always use the fused disconnect (1 amp fuse)Always set the power supply to zero before setting to thevoltage required for the project. Be ready to turn the power off fast. Make meter connections with one hand. Turn the power off after every use and set the supply tozero.
Always test the circuit for power before working on thatcircuit. Always retest the meter when checking that the power isoff. Follow instructions. DON’T ever clown around. Make notes, but don’t complete the lab report until youhave completed the project and have put all the tools andmaterials away.