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Surgical Techniques & Safety > Electrical Safety > Flashcards

Electrical Safety Flashcards

1
Q

What is electricity?

A

Movement of charge through a conductor to produce current

Needs voltage and complete circuit

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2
Q

What does resistance lead to?

A

A large voltage drop

More power dissipation

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3
Q

What formula compares resistance and voltage?

A

V = I x R

Voltage drop = current x resistance

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4
Q

What does voltage mean?

A

Potential difference

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5
Q

What formula compares power to voltage?

A

P = I^2 x R

heat generated

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6
Q

What are the requirements for electricity to have any affect on the body?

A
  • potential difference must be present

- individual must be part of the circuit, so current will pass through them

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7
Q

Why are electrical current exposure hazards so common/unsafe?

A
  • patient is deliberately connected
  • patient often immobile or unable to detect pain
  • staff intentionally use devices that could never be considered electrically safe
  • body chamber is filled with salty fluid (conductive!)
  • nerve conduction & muscle function rely on movement of charged particles (these are then affected by electrical currents entering the body)
  • skin protects the body (higher resistance of skin layer minimises current getting to rest of body. However this means higher resistance = greater dissipation of heat = burns!)
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8
Q

What is the relationship between current and resistance?

A

Current is inversely proportional to resistance

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9
Q

What is the difference between DC and AC?

A

Direct current vs. Alternating Current
Direct current flows in 1 direction only, no varying of the current/potential
Alternating current so direction of current changes periodically and therefore so does the voltage (normally 50-60Hz, 270V), most household powers

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10
Q

What does voltage measure?

A

Amplitude

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11
Q

What effects can electricity at 60Hz current have on the body?

A
  • Burns, injury (Severity depends on resistance and voltage (at any current))
  • Nerve stimulation (0.1mA to 10mA)
  • Respiratory paralysis, fatigue and pain (10mA to 100A)
  • Cardiac arrest (1A - 100A)
  • Ventricular fibrillation (10mA - 1A)
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12
Q

How can nerve stimulation damage be portrayed at different currents?

A
  • threshold o perception changes at 200 microamps

- prevents you letting go at 10 milliamps

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13
Q

What is the most dangerous frequency range for electricity?

A

60 Hz

Lowest threshold for stimulating muscle & nerve

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14
Q

What do the effects of electricity on the body depend on?

A
  • frequency
  • duration
  • body weight
  • point of entry/contact area
  • gender
  • skin resistance
  • age groups include a few of these
    (- not really voltage)
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15
Q

What does ‘let-go current’ mean?

A

The lowest current possible that nerve stimulation can occur to lose control of your muscle and prevent you letting go
- So better if this is higher so less chance of smaller currents causing this damage

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16
Q

What are the most dangerous frequencies?

A

15-100Hz

  • Where let go current is the lowest
  • threshold is lowest here
  • Note this is what we use in everyday life (60Hz) = dangerous!
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17
Q

Where is let go current the greatest?

A

Very high frequencies

  • very high threshold
  • so these frequencies are used for medical applications
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18
Q

What is electrolysis? What is the effect on the body?

A

Movement of ions of opposite polarity through a medium

  • salt ions move though tissues when DC current gets passed through body
  • over a few minutes, can ulcerate skin, can be fatal, long time to heal
  • ulcerations as get concentrations of acids forming
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19
Q

Why do burns happen?

A
  • due to dissipation of power as current passes through a substance
  • Remember P = I^2 x R
  • power dissipation produces heat
  • heat produced is proportional to current density and amount of resistance
  • skin has very high resistance so burns are worse on surface layer of skin
  • internal burns are also quite common and can be fatal
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20
Q

Why/How does nerve stimulation occur?

A
  • nerves normally function through action potentials travelling through axons to surrounding cells
  • this effect can be stimulated by electrical currents
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21
Q

What happens if an electrical current affects a sensory nerve?

A

pricking sensation

- if it is intense, it becomes painful

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22
Q

What happens if an electrical current affects a motor nerve?

A
  • muscle cramps
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23
Q

How do muscle cramps occur?

A

When a muscle is subjected to a large electrical stimulus it will contract

  • called tetanus: involuntary contraction of a muscle
  • can’t let go feeling
  • severe contraction can be uncomfortable to very painful
  • if muscle contracting is vital it can rapidly kill
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24
Q

How can respiratory arrest occur?

A
  • prolonged tetanus of intercostal muscles
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25
Q

How can cardiac arrest occur?

A
  • tetanus of the heart muscle prevent beating = arrest
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26
Q

How can ventricular fibrillation occur?

A
  • most common cause of death after an electric shock
  • disrupted signals -> uncoordinated twitching of ventricles = no movement of blood out of the heart
  • unless corrected rapidly = will result in death as no blood pumped around body
  • due to 60-120mA from hand to hand across chest
  • 20microA directly through the heart
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27
Q

Why does the effect vary so much?

A
  • primary variable is the amount of current and path it takes
  • current is inversely proportional to area of contact (V=IR)
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28
Q

What does the current flow depend on in the body?

A
  • part of body
  • water on skin
  • amount of natural oils present
  • skin condition
29
Q

What body natural protection factors can minimize the impact of shock?

A
  • automatic reflex reactions = muscle contraction = push body away
  • intentional reflex reactions = intentionally pull away
30
Q

What happens if a patient is lying on a hospital bed, what protection factors do they/don’t they have?

A
  • connected to a machine= cannot pull away quickly
  • anaesthesised/unconsicous = unable to detect shock
  • no skin protection/skin resistance intentionally reduced?
31
Q

Why is point of entry of the current significant?

A
  • major factor in level of damage
  • hand to hand (will be sent through heart) but less damaging
  • catheter to the heart much more damage! smaller current will do more damage
  • macroshock vs. microshock
32
Q

Define macroshock

A
  • accidental contact with surface of body
  • e.g: electric shock (every day hazard in the home)
  • contact with exterior of the body
  • person becomes part of electrical circuit
  • often hand to hand across chest
  • results in severe burns, can lead to cardiac arrest or resp failure
  • 100-300mA = fibrillation
33
Q

Define microshock

A
  • currents applied internally within the body/low resistance path to the heart
  • particularly medical related hazard/hospitals
  • relates to current applied directly to the internal body
  • lead/electrical conductor/ catheter to heart/ internal organ extending out/ external pacemaker/ transvenous catheter
  • no skin protection/wet environment
  • low currents still fatal
  • 10-20microamperes = fibrillation
34
Q

What are leakage currents?

A

Non functional currents through or across insulation

  • any conductor at a potential above earth will have some leakage current
  • extent of leakage depends on resistance between conductor + earth AND voltage on the conductor (voltage needs to be connected to the ground before person touches it!)
35
Q

How are leakage currents classified?

A

According to path they take to earth:

  • earth leakage current
  • enclosure leakage current
  • patient leakage current
  • patient auxillary current
36
Q

Earth Leakage Current

A
  • from equipment through protective earth conductor
  • desirable path
  • very low impedance = easiest path
  • under normal conditions a person in contact with an earthed metal container of equipment should receive no leakage current
37
Q

Enclosure Leakage Current

A
  • from conductive part of enclosure to earth
  • through a path other than the protective earth
  • not desirable
  • could flow through you
  • value of enclosure leakage current is usually checked to ensure it is small enough not to be hazardous
  • if touch the external part of equipment will get shocked
  • or if ground is insulated then touch 2 parts of equipment with potential between them, will go from 1 hand to the other
38
Q

Patient Leakage Current

A
  • from either an applied part to ground or to an applied part from a power source
  • through the patient
39
Q

Patient Auxillary Current

A
  • from 1 part of equipment to another through the patient

- e.g between applied parts

40
Q

How do defibrillators work?

A
  • intentional currents between parts

- patient functional currents

41
Q

How to mimise risk?

A
  • either remove potential difference

- or remove individual being part of circuit so current does not pass through them

42
Q

How to protect against macroshocks?

A
  • general mains supply 110-120 or 220-240 V
43
Q

What is the 3 wire system?

A

Plug to the mains

  • ground (G) at ground potential
  • live wire (H) at 115 or 230 (carries current to equipment)
  • neutral (N) also connected to the ground (carries current away from equipment)

Current will flow from H -> N, G is not neccessary for a computer but is for safety reasons

44
Q

What is the ground wire?

A
  • first line of defence
  • providing a low resistance path so current will prefer this path
  • must be used to ground all external conductive surfaces
  • easier route to earth than through you
45
Q

What are circuit breakers?

A
  • intelligent device
  • detect expected currents
  • breaks/fuses circuit
  • failure = ground wire take current to earth
46
Q

How to protect against microshocks?

A
  • concerned about ECG electrodes, heart paddles, catheters
  • need to prevent electrical connection between two applied parts
  • need to prevent electrical connection between applied part and earth
47
Q

Earth Referencing

A

Method of protecting against microshock

  • no applied parts!
  • points of contact at same potential = prevent patient auxillary leakage current!
  • easiest = all points of contact earthed
  • type B device!
48
Q

Floating Applied Parts

A

Method of protecting against microshock

  • hold all applied parts as floating (electrically isolated)!
  • current can’t flow towards or away from that point
  • ensure applied parts are electrically isolated from earth so current flows safely between patient + applied parts
  • flow of potentially harmful leakage currents is blocked
  • F type applied part!
49
Q

How can electrical equipment be categorised?

A
  • according to method of protection against electrical shock it employs (CLASS)
  • according to the degree of protection that is provided against electric shock (TYPE)
50
Q

2 Methods of protection

A
  • Basic
  • Supplementary -if in the event of basic protection failure
  • Provides protection against single fault conditions to ensure it is still safe if there is a basic protection failure
51
Q

Class 1 protection

A
  • protection does not rely on basic insulation only
  • incudes additional safety precaution
  • normally a protective low resistance earth conductor OR a circuit breaker
  • normally has fuses to ensure earthing with function
  • no agreed symbol
  • Earth/protective earth/ equipotential symbols sometimes used
52
Q

What happens if there is failure of earth connection?

A
  • lowest resistance path is broken
  • current goes back through patient to other part of equipment
  • if there is only a basic protection this will be a problem
  • if there is class 1 protection, there will also be supplementary protection (will be another break in system of desirable current and break it down)
53
Q

Class 2 protection

A
  • basic + supplementary protection like class 1
  • refers to double/reinforced insulation
  • plastic laminates normally around class 2 equipment but not always!
  • 2 concentric squares is the symbol (double insulation)
54
Q

What is double insulation?

A

Basic + secondary level of protection to prevent contact with live parts

55
Q

What is reinforced insulation?

A
  • single layer of insulation

- offers same degree of protection as double

56
Q

Class 3 protection

A
  • not for mains powered equipment
  • normally battery powered equipment
  • voltages must be no higher than the safety extra low voltage (SELV) = 20VAC or 60VDC
  • if equipment is even capable of being operated when connected to mains (for battery charging for example), then must be safety tested for class 1 or 2
57
Q

Degrees of Protection

A
  • described by TYPE
  • B, BF & CF
  • usually related to applied parts which are attached to patient
58
Q

Type B

A
  • least stringent classification
  • used for equipment/applied parts that are not conductive and can be immediately released from patient
  • allows leakage current
  • reliability of the protective earth connection if present
  • applied parts may be connected to earth if they are class 1
  • symbol is a man
59
Q

Type BF

A
  • more stringent than B
  • for devices that have conduct contact with patent or applied parts that are fixed in medium/long term contact with patient
  • e.g. ECG electrodes
  • symbol is man in a box
  • basically type B + F (floating or isolated applied part)
60
Q

Type CF

A
  • most stringent
  • when an applied part is in direct conductive contact with the heart
  • allowable leakage currents are much lower in order to deem equipment safe to directly contact the heart
  • applied parts must be floating
  • symbol = heart in a box
61
Q

What kind of devices use Type B?

A

x-ray machines
operating tables
ventilators

62
Q

What kind of devices use Type BF?

A

endoscopes
blood warmers
electrosurgery devices
thermometers

63
Q

What kind of devices use type CF?

A

cardiac catheters
cardiac electrodes
intensive care monitoring
defibrillators

64
Q

Single Fault Conditions Define

A

A condition in which a single means of protection against a safety hazard is deficient

65
Q

What are the 2 types of safety tests?

A

Type AND Routine

66
Q

Type Tests

A
  • extensive
  • test equipment to its limits
  • check response under fault conditions
  • can damage equipment
  • under controlled conditions
  • specialist test centres
  • required to recieve certificate of compliance
67
Q

Routine tests

A
  • regular safety checks
  • within working environment
  • not to its limits = less destructive
  • indicate its operating safely
68
Q

What is Ohm’s Law?

A

V = IR

Surgical Techniques & Safety (5 decks)

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