SAFE LABORATORY PROCEDURES

Northampton Community College

To develop a healthy respect for electricity, it is important to understand:

how electricity acts,

how electricity can be directed,

what hazards electricity presents,

and how to minimize hazards through safe laboratory procedures.

HOW SHOCK OCCURS

Shock occurs when the body becomes a part of the electric circuit. The current must enter the body at one point and leave at another.

HOW SHOCK OCCURS

Shock may occur in one of three ways. The person must

come 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” by being in contact with an energized wire, while the person is in contact with the circuit ground.

Hand to hand contact allows a current path through the heart and lungs

Left hand to left foot allows a current path through the heart and lung

Right hand to right foot allows a current path through the lung but bypasses the heart

A foot to foot current path allows current to pass through a critical part of the body.

This could cause the victim to fall and sustain even more damage from other current paths.

SEVERITY OF THE SHOCK

The severity of the shock received when a person

becomes a part of an electrical circuit is affected by three

primary factors: the rate of flow of current through the body, measured

in 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 Body

The currents are stated in milliamperes, which is another

way of saying in thousandths of an ampere.

(1 ampere = 1,000 milliamperes)

From the table you can see that a difference of only about

50 milliamperes exists between a current which can just

be perceived and one which can be immediately fatal.

On low-voltage circuits, if the person cannot let go of the

circuit and is not rescued from it, the ratio between a

current which can just be perceived and one which is

dangerous may be less than one to five.

This factor should be kept in mind with respect to live

parts of low-voltage circuits, as the difference in resistance

between dry skin and skin wet by either water or

perspiration will usually vary by considerably more than a

factor of five.

Further, in low-voltage shock, there is much greater

danger of having current in the range, which will cause

ventricular fibrillation (convulsive movement) of the heart, a

condition for which there is usually no field treatment.

On the other hand, high-voltage shock frequently causes

paralysis of breathing and applying artificial respiration

saves many victims of this.

The resistance of the body governs the amount of

current flowing through the body.

The skin offers about the only resistance presented by

the human body to the flow of current.

But the skin’s humidity varies over wide limits.

A person working in high temperatures may perspire

freely and when the skin, and possibly clothing, becomes

wet, the skin’s resistance to electric current drops

radically, quite easily to approximately 1,000 ohms.

If working on damp or wet surfaces, or there is a break in

the skin, it could drop even more, at times to a few

hundred ohms.

Remembering Ohm’s law states that the number of

amperes flowing in a circuit with a given voltage will be

inversely proportional to the resistance, it is apparent

that great variations of current are possible even with the

same voltage.

Assuming a 120-volt circuit, and under ideal conditions – a

person with a dry skin of 100,000 ohms resistance

standing on a wood floor with a resistance on the order of

100,000 ohms – the amperage passing through the skin

could be calculated as 120 ÷ 200,000 = 0.001 ampere (1

milliampere), which would not be particularly harmful.

If however, the resistance of the skin were reduced to

1,000 ohms because of perspiration, and if the person

were standing on a wet or damp ground, the current

passing through the body would be in the nature of

120 ÷ 1000 = 0.1 ampere (100 milliamperes) – more

than enough to kill.

The length of time the body is in the circuit may also be

important, particularly with respect to the severity of burns.

Burns break down the resistance of the skin, the more

extensive the burn, the greater the flow of current and the

more severe the shock.

The preceding information was based OSHA pamphlet 3075.

LABORATORY PROCEDURES

Jack Schreiber

Laboratory experimentation can be the most

interesting way of learning ever devised.

You perform experiments so that you can better

understand the material in your textbook.

Most industries use the same equipment you will be using

in your lab projects.

Consequently, you will be acquiring knowledge of

electricity and learning techniques at the same time – not

only for the grade you get in this course, but to help you

the rest of your life.

Laboratory experimentation can be interesting – or it can be

a drag.

The best way to make sure you find it interesting is to get

yourself prepared before you start the experiments.

The best way to make sure you get maximum benefit from

each lab session is to follow good laboratory practices.

The more seriously you approach laboratory

experimentation, the more actual enjoyment you will get out

of learning the fundamentals of electricity.

ADOPT AN INQUISITIVE ATTITUDE

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 more

important aim than just performing it successfully.

Your purpose should be to learn something by

performing the experiment safely and successfully.

Therefore, before you even go into the lab, you should have

studied your textbook thoroughly and absorbed as much as

you could out of it.

If you understand the material, experimentation reinforces

that understanding.

If there are points still not clear, lab work should clear them

up. If you need help with the lab project as the instructor .

Before you energize you project have the instructor inspect it.

ORGANIZE YOUR WORK AND YOUR TIME

You will accomplish more if you get in the habit of

organizing every lab period.

Always start by looking over the entire experiment.

As you gain more and more experience in laboratory

experimentation, you will be able to estimate pretty

accurately how much time it should take you to do each

experiment.

If there is a lot of data to be gathered, concentrate on taking

the necessary readings.

Be alert for any readings that appear not to be in keeping

with what you expect. The time to recheck those is before

the equipment is put away.

Unless required to do so, leave the computations and

analysis until last.

Usually, you are assigned to work with another student – as

a two-person team.

It is suggested that the two of you alternate as team leader.

The other student not only follows the instructions of the

leader, but also acts as a checker on the accuracy of every

phase 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 its

predictability.

We know if we make certain connections, certain things will

happen.

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 the

power is going.

•Always protect yourself and the circuit by using the fused

disconnect (1 amp fuse)

•Most lab sources are variable.

•That is, even though they are ON, when the control knob is

in its minimum position there is minimum power output.

•The knob, then, can be turned clockwise to obtain any

voltage between zero and the maximum for that source.

For example, a 0-140V AC power source can be set to

supply any voltage up to 140 volts. These terms will be

explained in detail.

Always have the knob in the minimum position before you

turn the power ON.

Always turn the power OFF immediately after you complete

an 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 supplies

This means that if the supply is called upon for more power

than it is capable of, it will disconnect the output to protect

itself and the equipment.

So, if you don’t have power, turn the main switch off and the

control knob back to zero, check the fuse and go over your

circuit 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 can

harm you under normal circumstances; there are

particular situations where caution is advised.

It’s important to know the location of the main disconnect in

case of emergency.

All students should be aware of elementary first aid and what

to do if an accident occurs, either to himself or herself or

another student.

The instructor will point out the main switch and disconnects

for 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 passes

through the heart. Electrical leads should be handled with one

hand only, while the other is safely out of the way.

(2) Under certain conditions, electricity can produce a painful

burn.

(3) A sudden, unexpected shock causes a fast reaction and the

reaction can result in injury, either to the person getting shocked,

or a bystander.

Be especially cautious when the circuit contains coils and

capacitors.

These can cause shocks after power has been turned off.

Remember the following for

SAFE 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 the

voltage 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 to

zero.

Always test the circuit for power before working on that

circuit.

Always retest the meter when checking that the power is

off.

Follow instructions.

DON’T ever clown around.

Make notes, but don’t complete the lab report until you

have completed the project and have put all the tools and

materials away.