d. Addendum to Section 14

briefly alluded to in the preceding article, a buildup of oxygen in liquid nitrogen containers over

a period of time can become a problem if care is not taken to keep the cap on or to change the

entire volume occasionally. If the liquid takes on a blue tint, it is contaminated with oxygen and

should be replaced. The contaminated liquid should be treated as a dangerous, potentially

explosive material. Most users fill Dewars from larger ones, usually by pressurizing the larger one

with nitrogen from a cylinder, thereby forcing the liquid into the smaller one. In order to not waste

liquid nitrogen by evaporation in a warm container, neither of the two Dewars are usually allowed

to become totally empty, again leading to possible oxygen contamination. If these practices are

continued for a sufficiently long time, the oxygen content of the cryogenic liquid may become

dangerously high.

There are two relatively common ways to maintain a supply of liquid nitrogen at a facility,

one being to have a large reservoir of up to several thousand liters capacity from which individual

users fill their smaller Dewars. The boil-off from a large reservoir can be used to provide a supply

of ultra-clean “air” to laboratories to use to clean surfaces. Liquid nitrogen is also usually

available, if reasonably close to a distributor, in 160-liter pressurized containers delivered directly

to the laboratory. In either case, the quantities actually needed for most small laboratories can

be obtained frequently enough to avoid having an elaborate piping and control system from a

large central reservoir, with the associated problems of avoiding blockage of the system by ice

plugs. There are, of course, applications for which automatically controlled systems are necessary

that provide safety relief and warning devices.



REFERENCES

1.



Cryogenics, Marsh & McLendon Chicago, IL, 1962.



2.



Industrial Gas Data, Air Reduction Sales Co., Acton, MA.



3.

4.



Matheson Gas Data Book, 47th ed., The Matheson Co., Inc. East Rutherford, NJ, 1961.

Precautions and Safe Practices for Handling Liquid Hydrogen, Linde Company, New York, 1960.



5.



Precautions and Safe Practices for Handling Liquified Atmospheric Gases, Linde Company, New York,

1960.



6.



Braidech, M.M., Hazards/Safety Considerations in Cryogenic (Super Cold) Operations, Conference of

Special Risk Underwriters, New York, 1961.



7.

8.



Honre, Jackson, and Kurti, Experimental Cryophysics, Butterworths, London, 1963.

MacDonald, D.K.C., Near Zero, An Introduction to Low Temperature Physics, Anchor Books, Dou bleday

& Co., New York. 1961.



9.



Nears R.M., Handling Cryogenic Fluids, Linde Company, New York, 1960.



10.



Scott, R.B., Cryogenic Engineering, D. Van Nostrand , Princeton, NJ, 1959.



11.



Timmerhaus, K.D., Ed., Advances in Cryogenic Engineering, Vol. 7, Plenum Press, New York, 1961.



12.



Vance, R.W., and Duke, W.M., Eds., Applied Cryogenic Engineering, John Wiley & Sons,

York,1962.



13.



Zenner G.H., Safety engineering as applied to the handling of liquified atmospheric gases; in Advances

in Cryogenics Engineering, 6, Plenum Press, New York, 1960.



14.



Cryogenic Safety. A Summary Report of the Cryogenic Safety Conference, Air Products,

1959.



15.



Spencer E.W., Cryogenic safety, in CRC Handbook of Laboratory Safety 2nd ed., Steere, N.V., Ed., CRC

Press, Cleveland, OH, 1971.



16.



Prudent Practices in the Laboratory Handling and Disposing of Chemicals, National Academy Press,

Washington, D.C., 1995, pp 128-130.



©2000 CRC Press LLC



New



Allentown, PA,



15. Cold Traps *

Cold traps are used in instrumentation and elsewhere to prevent the introduction of vapors

or liquids into a measuring instrument from a system, or from a measuring instrument (such as

a Mcleod gauge) into the system. A cold trap provides a very low-temperature surface on which

such molecules can condense and improves pump-down (the achievable vacuum) by one or two

magnitudes.

However, cold traps improperly employed can impair accuracy, destroy instruments or

systems, and be a physical hazard. For example, many of the slush mixtures used in cold traps

are toxic or explosive hazards, and this is not indicated in the literature.

The authors (of this article) became aware of the deficiencies in tunnel instrumentation, where

it was necessary to measure pressures in the micron to 760 torr region (a torr is equal to a pressure

of 1 mm Hg). The instrumentation system used Stratham gauges for ambient pressure down to

100 to 150 torr or about 2 to 3 psia and NRC alphatron gauges for pressures to 5 x 10-2 torr. To

prevent calibration shifts and contamination of the NRC transducers by oil fumes from the

vacuum pump and possible wind tunnel contaminants, a cold trap was placed in the line.

The cold trap was filled with liquid nitrogen, and the valve to the tunnel line shut off. When

the valve was opened, cold gas shot out, shown by the condensation; the over-pressure

developed in the system destroyed the Stratham strain gage bridge, although it was not sufficient

to rupture the transducer diaphragm. As no satisfactory explanation was forthcoming, a glass

cold trap was procured and set up in a dummy system. The cause of the phenomenon soon

became apparent: air in the trap and system lines was liquified in the trap. When the valve was

opened, this liquid air was being blown into the warmer lines by atmospheric pressure. The

resultant volatilization of liquid into gas was practically an explosion.

Nevertheless, cold traps are often the only satisfactory means of removing contaminants,

although in ordinary experimental work the charcoal trap is occasionally acceptable. A charcoal

trap will remove oil and condensable vapors so that pressures to10-8 tor or better may be secured,

but it presents a serious restriction on pumping speed.

The errors introduced by the water vapor, when measuring low pressures, depend on the

vacuum gauge used. The presence of water vapor also affects the magnitude of vacuum that can

be achieved. The equilibrium point of a dry-ice-acetone slush is -78"C (-108.4"F), which, although

sufficient to trap mercury vapor effectively, does not remove water vapor; a temperature of at

least -100"C(-148"F) is required to eliminate water vapor or, alternatively exposure to anhydrous

phosphorous pentoxide (P205). This material is usually rejected for field use because of possible

biological, fire, and explosive hazards: in absorbing water it produces heat and reacts vigorously

with reducing materials.

Slush mixtures using liquid air and liquid oxygen were considered and dropped, either because

of the explosive hazard or toxicity of the vapors or because they were not cold enough. Table

4.14 lists many common thermal transfer and coolant fluids with their hazards and limitations.

a. Virtual Leaks

If the cold trap is chilled too soon after the evacuation of the system begins, gases trapped

will later evaporate when the pressure reaches a sufficiently low value. The evaporation of the

refrigerated and trapped gases is not rapid enough to be evacuated by the system, but is enough

to degrade the vacuum, producing symptoms very similar to those of a leak.To avoid these virtual

leaks, keep the trap warm until a vacuum of about 10-2 torr is obtained. The tip of the trap is then

cooled until the ultimate vacuum is reached, at which time the trap may be immersed in the coolant



*

This section is taken from the article “Cold Traps,” by Kaufman and Kaufman in the second edition of this

handbook.



©2000 CRC Press LLC



to full depth.

b. Safety Precautions

If liquid nitrogen is the coolant, liquid air can condense in the trap, inviting explosion. Liquid

air, comprising a combination primarily of oxygen and nitrogen, is warmer than liquid nitrogen.

Depending on the nitrogen content, air liquifies anywhere from -190"C (-310"F) (5"C warmer than

liquid nitrogen) to -183"C (-297.4"F) (liquid oxygen). If liquid nitrogen is used, the trap should

be charged only after the system is pumped down lest a considerable amount of liquid oxygen

condenses, creating a major hazard. Handle any liquid gas carefully; at its extremely low

temperature, it can produce an effect on the skin similar to a burn. Moreover, liquified gases

spilled on a surface tend to cover it completely and intimately, and therefore cool a large area.

The evaporation products of these liquids are also extremely cold and can produce burns.

Delicate tissues, such as those of the eyes, can be damaged by an exposure to these cold gases

which is too brief to affect the skin of the hands or face. Eyes should be protected with a face

shield or safety goggles (safety spectacles with or without side shields do not give adequate

protection). Gloves should be worn when handling anything that is or may have been in contact

with the liquid; asbestos gloves are recommended (this is no longer true because of the concern

about airborne asbestos fibers from products containing asbestos. Gloves made of an artificial

material such as Kevlar™ or Zetex™ are recommended as an alternative), but leather gloves may

be used. The gloves must fit loosely so that they can be thrown off quickly if liquid should spill

or splash into them. When handling liquids in open containers, high-top shoes should be worn

with trousers (cuffless if possible) worn outside them.



©2000 CRC Press LLC



©2000 CRC Press LLC



Stand clear of boiling and splashing liquids and its issuing gas. Boiling and splashing always

occurs when charging a warm container or when inserting objects into the liquid. Always

perform these operations slowly to minimize boiling and splashing.

Should any liquified gases used in a cold trap contact the skin or eyes, immediately flood that

area of the body with large quantities of unheated water and then apply cold compresses.

Whenever handling liquified gases, be sure there is a hose or a large open container of water

nearby, reserved for this purpose. If the skin is blistered, or if there is any chance that the eyes

have been affected, take the patient immediately to a physician for treatment (call for emergency

medical aid; normally rescue squads can be in immediate contact with an emergency room

physician by radio).

Oxygen is removed from the air by liquid nitrogen exposed to the atmosphere in an open

Dewar. Store and use liquid nitrogen only in a well ventilated place; owing to evaporation of

nitrogen gas and condensation of oxygen gas, the percentage of oxygen in a confined space can

become dangerously low. When the oxygen concentration in the air becomes sufficiently low,

a person loses consciousness without warning symptoms and will die if not rescued. The oxygen

content of the air must never be allowed to fall below 16%.

The appearance of a blue tint in liquid nitrogen is a direct indication of its contamination by

oxygen, and it should be disposed of, using all the precautions generally used with liquid oxygen.

Liquid nitrogen heavily contaminated with oxygen has severe explosive capabilities. In addition,

an uninsulated line used to charge Dewars will condense liquid air; liquid air dripping off the line

and revaporizing causes an explosive hazard during the charging operation.

If the cold trap mixture is allowed to freeze, and the cold trap becomes rigid, slight movement

in other parts of the apparatus could result in breakage of the trap or other glassware.

If a gas trap has to be lifted out of the Dewar cold bath for inspection, it will be difficult to

reinsert into the slush. Therefore, it is preferable to use a liquid that will not freeze at78.5"C.



ACKNOWLEDGMENT

Reprinted with permission of Rimbach Publications,

Instrument and Control Systems, vol. 36, pp. 109-111, July 1963.



Pittsburgh,



Pennsylvania,



from



REFERENCES

1.



Strong,



J.,



Neher,



H.V.,



Whitford,



A.E.,



Cartwright,



C.H.,



and



Hayward,



R.,



Procedures



in



Experimental Physics, Prentice-Hall, New York, 1938.

2.



Sax, N.I., Dangerous Properties of Industrial Materials, Reinhold Publishing , New York, 1961.



3.



Dushman, L., Scientific Foundation of Vacuum Techniques, John Wiley & Sons, New York, 1962.



4.



Kaufman, A.B. and Kaufman, E.N., Cold traps, in CRC Handbook of Laboratory Safety, 2nd ed., Steere,

N.V., Ed., CRC Press, Cleveland, OH, 1971, 510.



16. Care and Use of Electrical Systems

Some of the problems associated with electrical systems have been covered in previous

sections, such as Chapter 3, Section I.G. There may be some unavoidable repetition in this

section, which will be primarily concerned with the safe use of electricity rather than the

characteristics of individual items, although there will be a brief discussion of generic problems

associated with the design of equipment. Most of the hazards associated with the use of electri-



©2000 CRC Press LLC



city stem from electrical shock, resistive heating, and ignition of flammables, and most of the

actual incidents occur because of a failure to anticipate all of the ways in which these hazards

may be evoked in a laboratory situation. This lack of appreciation of the possible hazards may

be reflected in the original choice of suitably safe electrical equipment or improper installation

of the equipment. In some instances the choice of equipment may simply involve a continued

use of equipment on hand under conditions for which it is no longer suitable, so that safety

specifications are not really considered. Often, this is due to a familiarity with the existing

resources rather than a deliberate choice.

Part of the problem is a feeling that questionable electrical practices routinely followed at

home as well as in the laboratory are actually safe unless you do something “really bad,” such

as standing in water while in contact with an electrically active wire, or a similar feeling about

wiring, “Just hook it up with the extension cord bought on sale at the department store.” When

asked about a number of similar practices involving multiple connections to a single outlet or

the use of extension cords, most people will answer that they know that they should not do it,

but see no real harm.

Two major electrical factors need to be considered in the choice of most electrical items of

equipment. The equipment needs to be selected so that it will not provide a source of ignition

to flammable materials, and it should be chosen so as to minimize the possibility of personnel

coming into contact with electrically live components. This latter problem will be addressed first.

a. Electrical Shock

OSHA has included the relevant safety portions of the National Electrical Code in 29 CFR

1910, Subpart S. This regulatory standard, as are many other sections of the OSHA regulations,

is primarily oriented toward industrial applications, but it does speak directly to the problem of

preventing individuals from coming into contact with electricity. Live parts of electrical equipment

operating at 50 volts or more must be guarded against accidental contact. Indoor installations

that contain circuits operating at 600 volts or more, and accessible to electrically untrained

persons, must have the active components within metal enclosures or be located within a space

controlled by a lock. The higher voltage equipment also must be marked with appropriate warning

signs. Access points to spaces in which exposed electrically live parts are present must be

marked with conspicuous warning signs which forbid unqualified persons to enter.

The effects of electricity on a person depend upon the current level and, of course, on

physiological factors unique to the individual. Table 4.15 gives typical effects of various current

levels for 60-Hz currents for an average person in good health.

Several things affect the results of an individual incident. The duration of the current is

important. In general, the degree of injury is proportional to the length of time the body is part

of the electrical circuit. A suggested threshold is a product of time and energy of 0.25 wattseconds for an objectionable level. The voltage is important because, for a given resistance R,

the current I through a circuit element is directly proportional to the applied voltage V.

I = V/R



(1)



If the contact resistance to the body is lowered so that the total body resistance to the flow

of current is low, then even a relatively modest applied voltage can affect the body. The condition

of the skin can dramatically alter the contact resistance. Damp, sweaty hands may have a contact

resistance which will be some orders of magnitude lower than dry skin. The skin condition is more

important for low voltage contacts than for those involving high voltages, since in the latter case

the skin and contact resistance break down very rapidly. The remaining resistance is the inherent

resistance of the body between the points of contact, which is on the order of 500 to 1000 ohms.

As can be noted in Table 4.15, the difference in a barely noticeable shock and a potentially

deadly one is only a factor of 100. For an individual with cardiac problems, the threshold for



©2000 CRC Press LLC



Table 4.15 Effects of Electrical Current in the Human Body

Current (milliamperes)



Reaction



1



Perception level, a faint tingle.



5



Slight shock felt; disturbing but not painful. Average person can let go.

However, vigorous involuntary reactions to shocks in this range can cause

accidents.



6 to 25 (women)



Painful shock, muscular control is lost. Called freezing or “let-go” range*.



9 to 30 (men)

50 to 150



Extreme



pain,



respiratory



arrest,



severe



muscular contractions, individual



normally cannot let go unless knocked away by muscle action. Death is

possible.

1,000 to 4,300



Ventricular fibrillation (the rhythmic pumping action of the heart

ceases). Muscular contraction and nerve damage occur. Death is most

likely.



10,000 —



Cardiac arrest, severe burns, and probable death.



*



The person may be forcibly thrown away from the contact if the extensor muscles are excited by the



shock.



threshold for a potentially life-threatening exposure may be even lower. The major danger to the

heart is that it will go into ventricular fibrillation due to small currents flowing through it. In most

cases, once the heart goes into ventricular fibrillation, death follows within a few minutes.

Even if an individual survives a shock episode, there may be immediate and long-term

destruction of tissue, nerves, and muscle due to heat generated by the current flowing through

the body. The heat generated is basically resistive heating such as would be generated in heating

coils in a small space heater, with the exception that the resistive elements are the tissues and

bones in the body. The power, P, or heat is given by

P = I2R



(2)



The scope of the effects of external electrical burns is usually immediately apparent, but the

total effect of internal burns may become manifest later on by losses of important body functions

due to the destruction of critical internal organs, including portions of the nervous system, which

is especially vulnerable.

Several means are available to prevent individuals from coming into contact with electricity

in addition to exclusion of unqualified personnel from space, as mentioned in the introduction

to this section. These include insulation, grounding, good wiring practices, and mechanical

devices. Before addressing these latter options, it might be well to briefly discuss the concept

of a qualified person.

Certainly a licensed electrician would in most cases be a qualified person, and a totally

inexperienced person would just as clearly not be a qualified individual. There is no clear

definition of the training required to be “qualified” to perform routine laboratory electrical and

electronic maintenance. As a minimum, such training should include instruction in the

consequences of electrical shock, basic training in wiring color codes (so as to recognize correct

leads), familiarity with the significance of ratings of switches, wiring, breakers, etc., simple good

wiring practices, and recognition of problems and poor practices, such as frayed wiring, wires

underfoot, wires in moisture, overloaded circuits, use of too small or wrong type of conductors,

poor grounding procedures, and improper defeat of protective interlocks. This would not make

an individual a licensed electrician, which requires extensive training and experience, but would



©2000 CRC Press LLC



reduce the number of common electrical errors.

Insulation is an obvious means of protecting an individual against shocks. In general, good

wiring insulation is the most critical, particularly that of extension cords, which are often abused.

Insulation must be appropriate for the environment, which may involve extremes of temperature

or exposure to corrosive vapors or solvents. The insulation itself may need to be protected by

a metal outer sheath, or the wires may need to be installed in conduit.

As it ages, insulation may become brittle and develop fine cracks through which moisture

may seep and provide a conductive path to another component or to a person who simply

touches the wire at the point of failure. Many plastic or rubber insulating materials will soften

with heat and, if draped over a metal support, may eventually allow the wire to come into contact

with the metal, thus rendering the metal electrically active, if it does not first cause other problems.

Extension cords, as noted above, are particularly susceptible to abuse. They are often carelessly

strewn across the floor or furniture. On the floor, they may be walked upon, equipment may be

rolled across them, or they may become pinched between items of furniture. Extension cords

should only be used as temporary expedients, but if they are used, they should be treated as any

other circuit wiring, put out of harm's way, and properly supported on real insulators separated

by distances not to exceed 10 feet. Defective extension cords with badly deteriorated insulation

should be discarded. Insulation is not used solely to protect wiring. Insulation in the form of

panels supporting printed circuits may break if excessive force is applied. If an arc temporarily

flashes across an insulating surface, a carbonized conducting path may be permanently

established on the surface which could render an external component such as a chassis mounting

screw “hot.” Care should be taken with all electrical equipment, especially older items; to ensure

that the integrity of the insulation has been maintained.

Proper grounding of equipment is another requirement to ensure that components are not

electrically live. Most equipment for use with 120-volt circuits comes with a three-wire power cord,

which requires a mating female connector at the power source, many of which are designed so

that the neutral, hot, and ground connections can be readily identified and matched. The ground

wire, which is either green or perhaps green with yellow stripes, is always connected to the female

socket which accommodates the round prong on the male connector. The neutral circuit wire

w hich normally completes the circuit for the equipment is usually white or gray. The socket and

corresponding male connector are often wider than the connections for the hot wire. The hot wire

is usually covered with a black insulator, although red may also be used. Where there are both

red and black wires, usually both will be hot wires. Some equipment is double insulated, and does

not have the third ground wire in the power connector. Usually, these will have a polarized

connector, so that the neutral and hot wires will be properly oriented. Older circuits unfortunately

do not always provide the proper connections and should be replaced. If this is not feasible, the

third wire on the power connector, if one is present, should be directly connected to a good

quality ground.

Auto-transformers, which may be used to supply variable voltages to heating devices, may

be connected in such a way that either outlet line may be high with respect to ground. They

should be purchased with a switch which breaks the connection of both outlet sockets to the

power input line, or they should be rewired with a double-pole power switch to accomplish this.

The quality of all ground connections (and of all connections) needs to be good. This is often

taken for granted, but the connection may vibrate or work itself loose, or a careless worker may

fail to tighten a connection. In such cases, a significant difference of potential may arise between

two different items of equipment. This can be enough to give rise to a discernible shock for a

person coming into contact simultaneously with both pieces of equipment, and in some cases

can cause damage to the equipment if they are interconnected. A careful researcher should have

the electrical circuits checked periodically for the resistance to ground for all the wiring in his

facility. A ground with a resistance of 100 ohms will be at a difference of 10 volts with respect

to ground if a current of 100 milliamperes were to flow through the ground connection. Good



©2000 CRC Press LLC



quality grounds with resistances of a few ohms are easily achievable with care.

Adapters or “cheaters” can be used to allow power cords with three wires to be connected

to sockets providing only two wires, or used to avoid connecting the third wires to ground. There

are only a few exceptions which would make this an acceptable practice; one is where an

alternative direct connection to ground is provided for the equipment. Another woul d b e o n a

very few occasions in which even the difference of a few millivolts between the separate ground

connections would affect the experimental data signals. In the latter case, ground connections

can be made directly between the components, with a single connection being made to the

building ground. Except in clearly defined situations where their use is clearly made safe, these

adapters should not be used.

Simple devices such as fuses, circuit breakers, and ground fault interrupters are available to

cut off equipment when they overload or short out or an inbalance develops between the input

and output current from a device or circuit. More sophisticated devices can also be used to

determine a problem, such as a redundant heat detector used to deactivate a circuit serving a still,

condenser, or heat bath should the temperature become too high.

Fuses and circuit breakers are the simplest devices used to shut off a circuit drawing too much

current. A fuse inserted in one of the circuit legs functions by melting at a predetermined current

limit, and the breaker by mechanically opening the circuit. The latter device is more flexible in

that it can be reset while the fuse must be replaced. A ground fault interrupter (GFI), on the other

hand, specifically can protect an individual who comes into contact with a live component. The

individual*s body and the wires become parallel circuits through which a fraction of the total

current flows. The amount through the body makes the two normal halves of the circuit out of

balance, which the GFI detects, and causes it to break the circuit. A GFI can detect a difference

on the order of 5 milliamperes and can break the circuit in as little as 25 milliseconds. A review

of Table 4.15 would show that the contact might be barely noticeable but would cause no direct

harm because of the short duration of the current flow. Although GFI’s are generally used in the

construction industry they would serve a useful purpose in laboratories, such as, where moisture

would be a problem. Any laboratory containing equipment operating at high voltages should

have each electrical outlet protected by a GFI, supplemented by a master disconnect switch in

an obvious and easily accessible space. It is critical to remove the current source in as short a

time as possible.

The best defenses against electrical shock injuries are good work practices, as invoked by

using good judgment and exercising care appropriate to the risk. The basic principles embodied

in the OSHA lock-out, tag-out provisions of their electrical standard should be followed. These

are basic common sense. Maintenance of electrical equipment or wiring should be done only with

the system deenergized unless it is essential that the circuit be active for the required

maintenance. In the latter case, specific care is to be taken to come into contact (if neces s a r y )

with only one side of a circuit, so that the circuit cannot be completed through the body.

Procedures should be followed to confirm that power to the system has been disabled and

remains so during the duration of the maintenance activity or, alternatively, if the circuit must

remain powered, that a second person is available to disable the circuit and assist in the event

of an incident. Formal lock-out procedures are recommended where high voltage circuits are

involved. The tools used to perform maintenance should be in good condition. Barriers may be

needed to isolate live circuits in the maintenance area. Good judgment should be used to

determine safe distances, to not use metal ladders, or metal devices, where it would be possible

to contact a hot circuit. In some cases, it might be necessary to use rubber gloves and gauntlets,

insulating mats, and hard hats certified for electrical protection.

A relatively simple protective stratagem which should be followed by anyone working with

or handling live electrical circuits is to remove all conducting jewelry, specifically items on or near

the hands such as rings, watches, and bracelets, or to avoid wearing necklaces which may dangle



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from the neck and complete a circuit to the neck. If work activities involving direct contact with

electrical components are infrequent, removing these items at the time may suffice. If the work

is a normal activity, the practice of avoiding wearing metallic objects should be routine to avoid

having to remember to remove them. Avoiding the use of conductive items in the vicinity of

electricity should extend to any object which might come into contact with the circuit. Many tools

must be metallic, but any tool used in electrical work should have insulating materials in those

areas normally in contact with the hands.

Interlocks should never be bypassed by the average laboratory worker. If it becomes

necessary, the decision to do so must be done with the knowledge of all persons who might be

affected by the decision. Bypassing an interlock should not be a decision permitted for an

inexperienced graduate student or new employee. Whenever an interlock is bypassed, a definite

procedure, requiring positive confirmation that all personnel are no longer at risk, must be

adopted and in place. This may involve actual locks, for which only the responsible person has

a key, tags which cannot be removed without deliberately breaking a seal, an alarm, or a

combination of the above. No preventive procedure should depend upon the continued

functioning of a single device, such as a micro-switch, which may fail in such a way as to defeat

the alarm or interlock.

Each circuit should be clearly identified and labeled to correspond to a circuit breaker in a

service panel. Access to these service panels should be provided to most laboratory employees,

but they should not be permitted to remove the protective panel covers protecting the wiring.

No closet containing an electrical service panel should be allowed to be used for a storage closet

by laboratory personnel. Access to the panel should not be blocked by extraneous items, and

accidental contact with the wires should not be possible, especially for an untrained person

entering the space.

One of the most effective safety practices, as well as one highly conducive to productivity,

is a definite scheduled program of preventive maintenance. Each item of equipment should be

periodically removed from service, carefully inspected and calibrated, any faults or indications

of deterioration repaired, and tagged with the date of review and the name of the maintenance

person, if more than one technician could have been responsible. A permanent file or maintenance log on each major item of equipment is useful for identifying trends or weak components.

Finally, in a facility in which electrical injuries are a reasonable possibility, it is strongly

recommended that at least some permanent personnel be trained in CPR and the measures to be

taken should a person receive a severe shock. Individuals should also be trained to effect a

rescue without themselves becoming a casualty. If, for example, a live wire is lying across a

person and the circuit cannot be readily broken, they should be instructed to find a meter stick

or some other insulated device to lift the energized wire from the victim, or use rubber gloves or

other insulator in attempting to loosen a person from a circuit.

b. Resistive Heating

This is one of the two major electrical sources of ignition of flammable materials in a

laboratory; the other being sparks. Electrical heating can occur in a number of ways - poor

connections, undersized wiring or electrical components (or, alternatively overloaded wiring or

components), or inadequate ventilation of equipment. Equation 2 in the previous section shows

that the power or heat released at a given point in a circuit is directly proportional to the

resistance at that point. A current of 100 milliamperes through a connection with a resistance of

0.1 ohms would generate a localized power dissipation at that point of only 10 milliwatts, while

a poor connection of 1000 ohms resistance would result in a localized power dissipation of 100

watts. The former would normally cause no problems, while the latter might raise the local

temperature enough to exceed the ignition temperature of materials in the vicinity. Poor or loose

connections have, in fact, caused many fires due to just such localized heating. An alligator clip



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used to attach a grounding wire is a good example of a potentially poor connection. Similarly,

a contact which has been degraded by a chemical, a wire that has been insecurely screwed down,

or the expansion and contraction of a wire such as aluminum may in time result in this kind of

problem.

Table 4.16 Electrical Characteristics of Wire per 50 Feet

Wire

Size



Resistance



Maximum



Voltage



Power



Amperes



(D)



Drop (V)



Loss (W)



18



0.3318



7



4.6



32



16



0.2087



10



4.2



42



14

12



0.1310

0.0825



15

20



3.9

3.3



59

61



10



0.0518



30



3.1



93



8



0.0329



40



2.6



105



6



0.0205



55



2.3



124



4



0.0129



70



1.81



127



3



0.0103



80



1.64



131



2

1



0.00809

0.00645



95

110



1.53

1.42



138

156



0



0.00510



125



1.27



159



Overheating of switches, fixtures, and other electrical components due to electrical overloads

can be avoided very simply by reading the electrical specifications for the component, usually

printed or embossed on the item, and complying with the limitations. If a switch is rated to carry

7 amperes at 120 volts, it will not survive indefinitely in a circuit in which it is carrying 30 amperes.

Each size or gauge wire is designed to carry a maximum amount of current. This is based on

the voltage drop per unit length and the amount of power dissipated in the wire. The voltage drop

should not exceed 2 to 5% due to wiring resistance. A 5% drop in a 120 volt circuit supplying

an item of equipment would mean an actual voltage at the connection to the equipment of only

114 volts. Although many items of equipment will accommodate a drop of this amount, some may

not. The heat developed in an overloaded circuit may heat the wiring to a point where the

insulation may fail or in extreme cases actually catch on fire. Even moderate overheating,

continued long enough, will probably cause an eventual breakdown in the insulation. In addition,

any energy dissipated in the wiring is wasted energy.

Table 4.16 gives the maximum current for copper wire of various sizes, the resistance, voltage

drop, and power loss per 50 ft (about 15.25 m) of line (the latter two values computed for a wire

carrying the maximum rated current). Most inexpensive extension cords purchased at a

department store are made of either 16 or 18 gauge wire. As can be seen from the table,

inexpensive extension cords do not carry sufficient current to be useful for providing power to

more than a few instruments at most, when properly used. Overloading them will cause a larger

voltage drop and power dissipation (heating) in the wire. Although extension cords made of wire

which is too small will probably not immediately fail in most applications, they are not suitable

for continued use. The lower available voltages can result in damage to equipment or failure of

relays in control circuits, as their magnetic fields become weaker.

Additional electrical load is a problem for extension cords and for the permanent wiring as

well, if multiple outlet plugs are used in a socket. Virtually every safety professional has at least

one photograph or slide of several multiple plugs plugged into each other, all drawing current

from a single socket. The result will usually be an overheated fixture and wiring, as well as a lower



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