entc 4350 electrical safety. hospital electrical safety begins with the principles that we have...

ENTC 4350

Electrical Safety

Hospital electrical safety begins with the principles that we have discussed. • An electrical shock is always unpleasant, but

it can be lethal in the intensive care unit.

It is extremely important that all hospital personnel be constantly on the watch for manufacturing defects or wear and tear of critical parts. • There are documented cases where equipment from

reputable manufacturers was delivered with ground wires disconnected, cords broken, and improperly installed plugs.

• In the meantime, there is still the patient; it is your patient and your responsibility.

• You are the one who must be suspicious and check the equipment when it comes from the factory.

Even if equipment is in perfect condition when it arrives from the manufacturer, it is subject to the normal wear and tear of daily hospital use. • This type of deterioration may be very severe

if the equipment is dragged around, in a great rush, from one room to another in response to emergencies. • Once again, the part of the system that is most

likely to be damaged is the cord and plug assembly.

Quite often, the damage is not visible on a mere surface examination; you have to get out your VOM or continuity tester and test it. • Connect the continuity tester or VOM between the

ground plug on the end of the cord and the metal case of the instrument.

• If the test light goes out when you wiggle or pull on the wire, or

• If the resistance measured by the VOM is erratic when you move the cord, then the appliance is defective.

If your hospital has a red tag service that allows defective equipment to be marked for immediate pickup and repair, all is well. • However, if there is any danger that the

equipment might be used in patient service before the repair is done, the best thing to do is take your handy bandage scissors and cut the plug off. • That may sound like a drastic measure (surgery is

always drastic), but in this case, it is quite justified.

Another thing to watch for is someone else’s ‘home repair.” • This is particularly apparent in hospitals

where one sees cracked cords or broken plugs repaired with adhesive tape.

• That cord or plug cracked for a reason:• either age or

• misuse is usually to blame.

If the insulation is cracked, most of the conducting wires may be broken, too. • Just suppose that the last strand of ground

wire broke when it was being used on your patient, and reflect upon the results of our computations with the current divider equations previously.

In this regard, you have to watch the other staff members—i.e., the orderlies, aides, and so on—since the natural human tendency is to put the broken item back on the shelf and take one that looks all right. • Quite often, an aide will hesitate to report defective

equipment for fear of being thought to be a troublemaker.

• Only endless repetition, and possibly a cash prize for reporting defects, will alleviate this situation.

It should he clear to everyone that if any defective equipment is noted, or if a tingle is sometimes felt when using a piece of equipment, this is a signal to stop using the equipment and report it.

The patient is truly at your mercy, and equipment that comes near to him or her must be in proper condition. • At this point, you might be wondering just what proper

condition is and how leakage occurs.

• The specifications on electrical leakage are complex and subject to change; however, two good points to keep in mind are the leakage to the chassis of hospital equipment and the leakage through any patient-connected leads.

With the ground wire disconnected, the chassis leakage is limited to 100 A, and the patient-lead leakage must not exceed 50 A. • There are many causes for leakage:

• defective insulators,

• damaged wire,

• dirt,

• water, and

• the radiation leakage.

Figure 14.3 Let-go current versus frequency Percentile values indicate variability of let-go current among individuals. Let-go currents for women are about two-thirds the values for men. (Reproduced, with permission, from C. F. Dalziel, "Electric Shock," Advances in Biomedical Engineering, edited by J. H. U. Brown and J. F. Dickson IIII, 1973, 3, 223-248.)

© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.

Figure 14.1 Physiological effects of electricity Threshold or estimated mean values are given for each effect in a 70 kg human for a 1 to 3 s exposure to 60 Hz current applied via copper wires grasped by the hands.

They all add up to a problem for the hospital.

The danger of having a single hospital appliance with a defective three-wire cord is illustrated below. • Here we show a patient in an electrical bed with a three-

wire cord that is good.

This means that when the patient puts his hand on the bed rail, he is grounded. too. • There is nothing wrong with that until someone brings

over a second appliance, like an ECG or an apnea monitor, which has a defective three-wire cord. • The manufacturer designed the appliance with the idea

that the three-wire cord would be operational and that stray leakage in his unit would be grounded off to the case and removed by the ground wire.

• Unfortunately. in this case, the third wire is broken, and the leakage current goes back to the powerhouse via the patient with disastrous results.

Every time you wheel a piece of electrical equipment up to a patient, you have to ask yourself, • “Am I sure that the ground wire is OK?”


Figure 14.5 Effect of entry points on current distribution (a) Macroshock, externally applied current spreads throughout the body. (b) Microshock, all the current applied through an intracardiac catheter flows through the heart. (From F. J. Weibell, "Electrical Safety in the Hospital," Annals of Biomedical Engineering, 1974, 2, 126-148.)

UNDERWRITERS’ LABORATORIES

STANDARDS

There have been some gaps in the design of medical equipment, but these holes will be closed as more hospitals require that all new equipment meet the Underwriters’ Laboratories Standards for Medical and Dental Equipment (UL 544). • The important thing about UL 544 is its

marking code for guidance in equipment application.

Type A apparatus, the highest grade, is suitable for electrically susceptible patients. • This means that the leakage current has been

held to the lowest possible value, and the greatest measure of safety has been provided for patients in intensive care, cardiac care, or catheterization units.

Type A equipment is very costly, and for this reason, a somewhat lower standard is used for type B, which applies to equipment not suitable for electrically susceptible patients. • This equipment is not defective or poorly built.

• The designation is simply a recognition that the precautions needed in the CCU, for example, are not appropriate for the general medical patient.

The last, or type C, equipment label is intended for laboratory apparatus where patient contact is unlikely. • In some cases, no marking will be used on

type C equipment, but the hospital may want to have stickers saying not for use outside the laboratory area or not for use on patients. • The UL 544 code designation is one more item to

be checked when new apparatus is brought in for patient use.

Regulation of Medical Devices

In 1976, the U.S. Congress passed what are known as the Medical Device Amendments (Public Law 94-295) to the Federal Food, Drug, and Cosmetics Act.• Further amendments were made in 1990 in

the Safe Medical Devices Act.

Medical devices are defined as:• any item promoted for a medical purpose that

does not rely on chemical action to achieve its intended effect.

Medical devices are classified in two ways:1. The division of such devices into Class I,

Class II, and Class III.• Based upon the principle that devices that pose

greater hazards should be subject to more regulatory requirements.

2. Seven categories• Preamendment,

• Postamendment,

• Substantially equivalent,

• Implant,

• Custom,

• Investigational, and

• Transitional.

Software used in medical devices has become an area of increasing concern.• Several serious accidents have been traced

to software bugs.• Increased requirements for maintaining traceability

of devices to the ultimate customer, postmarketing surveillance for life-sustaining and life-supporting implants, and hospital reporting requirements for adverse incidents were added to the law in 1990.

Class I: General Controls

Manufacturers are required to perform registration, premarketing notification, record keeping, labeling, reporting of adverse experiences, and good manufacturing practices.• Apply to all three classes.

Class II: Performance Standards

These standards were to be defined by the federal government, but the complexity of procedures called for and the enormity of the task have resulted in little progress having been made toward defining 800 standards needed.• The result has been overreliance on the

postamendment “substantial equivalence” known as the 510(k)process.

Class III: Premarketing Approval

Such approval is required for devices used in supporting or sustaining human life and preventing impairment of human health.• The FDA has extensively regulated these

devices by requiring manufacturers to prove their safety and effectiveness prior to market release.

WALL RECEPTACLES

There are times when it is important for you to be able to find the neutral, hot, and ground jacks of the wall receptacles or outlets. • Suppose. for example. that you move into a new (or

new to you) facility with three-wire grounded receptacles.

• All is well until you begin worrying that the contractor might have forgotten to hook up the ground wire at some ot the receptacles. • It has been our experience that many contractors do not

install the receptacles correctly.


Figure 14.6 Simplified electric-power distribution for 115 V circuits. Power frequency is 60 Hz.

The protection that you think you have is not there, and you might not want to find that out the hard way. • When the hospital is new, the contractor will

fix a little thing like this when the building is 10 years old, however, he may be somewhat hard to find.

We suggest that when you move into a facility, you whip out your VOM or outlet tester and check the receptacles. • A typical outlet tester is shown below.


Figure 14.16 Three-LED receptacle tester Ordinary silicon diodes prevent damaging reverse-LED currents, and resistors limit current. The LEDs are ON for line voltages from about 20 V rms to greater than 240 V rms, so these devices should not be used to measure line voltage.

Its dimensions are those of a large spool of thread. • These gadgets are not perfect: a poor ground—i .e..

one with high resistance—might check out all right, because the neon bulbs inside the tester only require a small current to light up (a neon bulb operating at 65 volts and 0.25 watt will light up even when the ground resistance is as high as 13,000 ohms).

• However, that is no excuse to avoid using a gadget of this type, because something is always better than nothing.

A better method for checking the wall receptacles involves the use of your VOM.

The right-hand opening should be ‘hot,”

The left should be “neutral,” and

The other opening should be “ground”


Figure 14.18 (a) Chassis leakage-current test. (b) Current –meter circuit to be used for measuring leakage current. It has an input impedance of 1 k and a frequency characteristic that is flat to 1 kHz, drops at the rate of 20 dB/decade to 100 kHz, and then remains flat to 1 MHz or higher. (Reprinted with permission from NFPA 99-1996, "Health Care Facilities," Copyright © 1996, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the National Fire Protection Association, on the referenced subject, which is represented only by the standard in its entirety.)

To exposed conductivesurface or if none, then 10 by20 cm metal foil in contactwith the exposed surface

Insulating surface

Current meter

I

Test circuit

Input oftest load

Leakage currentbeing measured

1400

100

Millivoltmeter

15

0.10 F

900

Open switchfor appliances not intended tocontact a patient

Grounding-contactswitch (use inOPEN position)

Polarity- reversingswitch (use bothpositions)

Appliance power switch(use both OFF and ON positions)

This connectionis at serviceentrance or onsupply side ofseparately derivedsystem

Buildingground

(a)

(b)

H (black)

N (white)

Appliance

G

N

H

H = hotN = neutral (grounded)G = grounding conductor

I < 500 A for facility Ðowned housekeeping and maintenance appliancesI > 300 A for appliances intended for use in the patient vicinity

120 V

G (green)

mV


Figure 14.17 Ground-pin-to-chassis resistance test

Any hospital that hopes to avoid lawsuits should have a regular program for checking the outlets.• One of the things that we have found to be

important is the periodic testing of the actual resistance of the hospitals ground circuit. • This requires a special electrical gadget that the

Maintenance Department will have to buy and use regularly.

Another good rule requires the replacement of all cracked receptacles, even when they test out “OK.” • A cracked receptacle is like high blood

pressure; it is a signal for you to do something about it or expect to pay the penalty.

ELECTRICAL BEDS

THE ELECTRICAL BED

Electrical beds are great gadgets; they have saved thousands of nurses millions of hours of cranking up and down. • Like most gadgets, however, they have their

bad aspects.

• A patient in a standard hospital bed on the usual rubber tiled floor is effectively “floating” in the electrical sense. • The term floating simply means that he or she is

not electricaIy connected to the ground.

The conventional electrical bed has its frame and motor case connected to electrical ground via a three-wire cord. • This could be hazardous if the patient came in contact

with an appliance that had an electrical ‘leak” while he was touching a grounded, metal bed rail.

• To avoid this hazard, manufacturers have introduced a variety of systems ranging from insulated bed rails to doubly insulated motors.

Three points to keep in mind are:

1. Bed-related injuries to patients, such as falls and shock, are the leading cause of patient and visitor trauma. Such injuries outnumber, by a factor of five,

any other type of in-hospital injury.

2. The conventional electrical bed has its frame and motor case connected to electrical ground via a three-wire cord. • One hand on the grounded bed rail and the other on

a defective bed lamp can be a formula for disaster.

• Other problems can occur if any conductive liquids, such as water, blood, or urine, spill on the bed and leak through to the motor. • This might allow current to flow from the motor to the

patient via the grounded bed rail.

3. If the bed rails are ungrounded or are not connected to the grounded frame, the above hazards might be alleviated but you had better check this with your handy VOM to make sure no electrical path exists. • Ungrounded bed components, however, can

act as antennas and pick up signals that interfere with in-bed ECG or EEG studies.

A doubly insulated system has the usual layers of functional insulation in the motor. • In addition, the motor itself is isolated from the

bed frame by a layer of insulating material. • Mechanical power is transmitted from the motor to

the bed via a nonconductive coupling.

This does provide an added measure of safety, provided we recognize that even the best insulation will lose its insulating qualities if it gets dirty, wet, or both. • Dirt holds moisture, and the combination of

dust and water, blood, or urine could be a pretty good conductor.

The Double Insulation System

The doubly insulated system can be purchased with several different arrangements:• Double insulated with the motor electrically isolated

from the bed frame. 1.If there is a two-wire cord, the bed and the motor are

“floating” in the electrical sense.

2.Double insulated with a three-wire cord that grounds the motor but not the bed, because the motor is electrically isolated from the bed.

• Here again, the bed is “floating” rather than grounded.

3.Double insulated with a three-wire cord that grounds the bed and the motor. In this case, the bed is grounded.

The Use of Isolated Power

To appreciate isolation, we have to introduce a new electrical gadget: the transformer. • A transformer has two coils of wire;

• One is called the primary, and

• The other the secondary coil.

When an AC voltage is applied to the primary coil, a voltage appears at the secondary coil. • The important point here is that the transfer of

power from the primary to the secondary occurs without any physical connection, such as a wire, between the two coils. • All transfer occurs by means of electromagnetic

waves.

The circuit diagram for a transformer is shown below. • Note the primary and secondary coils and the

input and output voltages.

A transformer for an isolated electrical bed has the dimensions of a cube about one foot on each side and weighs about 50 pounds.• The important thing to note in the figure is that

there is no voltage between either output “A” or output B” and ground.

This can provide an important measure of patient protection; • For example, if the bed is grounded and the

patient puts one hand on the bed rail, he or she will be grounded, too,

• If something should go wrong with the bed and if either wire A or wire B should touch the patient, there would be no chance of injury, because there is no voltage between either A or B and ground.


Figure 14.7 Power-isolation-transformer system with a line-isolation monitor to detect ground faults.

The next question might be, “Which one is best?” • Our choice might be 1 or 2 above, because

the “floating” bed provides an added measure of patient protection. • However, it may well be that NFPA will stick to their

ruling that all beds must be grounded, in which case you may as well purchase the third type and save money.

If the patient, or anyone else, touches hot wire A and wire B at the same time, however, they have just grabbed 115 volts. • To maintain isolation of power, it is most

important that neither wire A nor B touch the bed or patient, because once contact is made with one wire, the next accidental contact with the other wire could be fatal.

It thus is a system that gives you one error for free, in that contact of either wire A or wire B with the patient does not produce any shock, but, if one wire remains in contact, it sets things up so that when the other wire makes contact, zap. • Notice that this may also occur if wire A or B contacts

any grounded object.

• Once the isolated power system is grounded, all protection is lost and the next contact will have serious consequences.

The Use of Ground Fault Detectors and Ground Fault Interrupters

The consequences of loss of isolation are so serious that most isolated beds will provide either a ground fault detector (GFD) to signal when a loss of isolation occurs or a ground fault interrupter (GFI) to shut off the power if the isolation is lost. • Most isolated power systems for electrical beds use

the GFI, because it does not require that a nurse or aide be present to notice something is wrong and manually disconnect the circuit.

Both the GFD and GFI operate by comparing the current in one line with that in the other line.


Figure 14.13 Ground-fault circuit interrupters (a) Schematic diagram of a solid-state GFCI (three wire, two pole, 6 mA). (b) Ground-fault current versus trip time for a GFCI. [Part (a) is from C. F. Dalziel, "Electric Shock," Advances in Biomedical Engineering, edited by J. H. U. Brown and J. F. Dickson IIII, 1973, 3, 223-248.)


Under normal conditions, current flowing to the load is equal to current returning from the load.


• The current flowing toward the load is exactly equal to the current returning from the load. – Current in both directions is made to pass through the

center of a detection coil (L1).

– Current passing through the detector coil produces a voltage at the output terminals of the coil, but because the outbound and inbound current is exactly equal and opposite, the two currents together exactly cancel and produce no output voltage from the detector coil.


• The differential current is sensed by the upper coil (green). – The lower coil injects a signal into both conductors

to detect a grounded neutral. – The "hot" wire is the insulated conductor shown

passing vertically through both coils, and the "neutral" wire is the bare wire.


• The coils are wound on a torroidal (donut-shaped) core made of a material called "ferrite", which is an efficient conductor of magnetic flux. – The arrangement, along with the many turns of wire wound

around the core, is called a torroidal transformer.

– The very large number of turns produces a useful detection voltage from the magnetic field of the line conductors passing through the center.


When a ground fault occurs, part of the current returns via the ground path.


• When some fault in the insulation of the conductors or the appliance, or accidental contact with a person causes leakage of current to ground, that part of the current does not flow in the intended path, so the current in one conductor does not quite equal the current in the other. – In this condition, the magnetic fields of the hot and

neutral conductors do not exactly cancel, and the detection coil produces a voltage.

– The ground fault detector circuit senses the voltage from the detection coil and sends a pulse of current through the trip solenoid.


When the GFI is tripped, the load contacts open to stop all current.


• The solenoid unlatches the interrupter contacts, which spring open to disconnect the protected wiring. – The GFI will remain tripped until it is manually

reset, so that the cause of the trip can be corrected safely.


When a ground fault occurs in the neutral wire, neutral fault detection signal current flows.


• Neutral-fault detection is an additional feature of the GFI. – If no load is connected, a ground fault in the

neutral conductor could escape detection. • This situation is not nearly so dangerous as a fault in the

hot conductor, but could become dangerous if further faults develop.


• To detect the condition, the GFI generates a signal, which is induced into both conductors (i.e. it is a common-mode signal). – If no ground fault exists, there is no closed path for the

signal to follow, hence it causes no current and is not seen by the detection coil.

– When a fault develops, the signal returns to its source via the grounding system, producing a current which is detected, causing the GFI to trip.

• The voltage and current levels at which this signal operates are very small, and have no effect on electrical equipment.

If the two currents are the same to within about 3 mA, the GFI or GFD just sits there. • If the difference is greater than that value,

then a current “leak” is occurring somewhere in the circuit, and the circuit is opened or the alarm is triggered.

The problem with any gadget of this type, however, is the usual story:• They have to be checked, calibrated, and

repaired on a regular basis if they are to be of any value.

• They are a severe burden to the group that is responsible for the training of new staff members and we suggest that isolated beds are not worth the extra cost.

It seems simpler, at least to us, to use the best electrical beds you can afford in regular patient areas and to stick to mechanical cranking in the cardiac care or intensive care areas. • These latter locations are usually well staffed,

and the requests for changes of bed position can usually be handled without undue difficulty.

General Safety Suggestions for the Use of Electrical Beds

If a patient is in an electrical bed, special care should be exercised when liquids, such as blood, plasma, or urine, are present.

If you have a patient in an electrical bed and an ECG or other electrical procedure is required, there are two important things to remember:

• Do not use any machine that has a grounded patient Iead.

• In many cases, these leads are not really at zero potential and might pass current through the patient to the grounded bed frame.

• Do not let the patient touch any metal part of the bed during the measurement.

• Isolate him or her with some pillows, or unplug the bed if necessary.

If the current National Fire Protection Association (NFPA) regulations require that the bed be grounded, you will have to do just that. • You can, however, try to isolate the patient

from the bed with pillows, blankets, and the like. • This is a case where a regulation may have been

written with the best of intentions but the worst results.

IN-BED ELECTRICAL EQUIPMENT

Such appliances used in hospital beds include • heating pads,

• thermal blankets,

• vibratory pads,

• hair dryers,

• hot combs, and

• so forth.

There is no question about banning the usual hair dryers and hot combs. • Most of them are so poorly manufactured that

they constitute a hazard to a well person, to say nothing of someone who is sick and in a grounded electrical bed.

Heating pads and thermal blankets are a different case: • Many patients may really need them, and

• The hospital often provides this sort of equipment. • There are so-called hospital grade heating pads on

the market, and they cost more than the type you buy at the drugstore, but they provide only a slightly reduced risk.

The problem is that a heating pad. by its very nature, is made of cloth, rubber, and plastics in which electrical heating wires are embedded. • The insulation materials deteriorate when

exposed to the ravages of time, high temperatures, or sharp objects, and patient perspiration can provide all the conductivity that may be needed to create a calamity. No good solution exists for this problem.

The pads with plastic covers and three-wire cords that are offered to hospitals are no help, because there is nothing on the pad to hook the ground wire to, in any case, a break or cut in the cover can allow liquid into the pad. • If someone invents a heating pad in a grounded metal

case, it would represent an improvement, but until then, heating pads can only be used with caution.

We suggest that a patient who has wet dressings or is perspiring will be safer with an old-fashioned hot-water bottle or chemical heating pad.

Many thermal blankets use electrical heating wires. They are therefore just big heating pads and are susceptible to the same problems discussed previously. • Some thermal blankets, however, have a network of

tubes through which heated (or cooled) water is pumped from a bedside console.

• These units are inherently safe as long as the console gets its power from a properly connected, three-wire cord and the tubes in the blanket do not leak.

• Water is a good conductor, and there is no need to make things worse by adding salt to the solution that the console will pump through the thermal blanket.

While we are on the subject of water and saline solutions, it might be worth mentioning that any solution that is being passed into a patient will be a very good conductor if it spills on the bed and soaks through to a bed motor. • Some of the stands that are used for intravenous

saline bags are pretty shaky affairs, and we suggest that they be fixed firmly to a bed or table to preclude their tipping over.

entc 4350 electrical safety. hospital electrical safety begins with the principles that we have...

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