Electrical Safety Flashcards
What is electricity?
Movement of charge through a conductor to produce current
Needs voltage and complete circuit
What does resistance lead to?
A large voltage drop
More power dissipation
What formula compares resistance and voltage?
V = I x R
Voltage drop = current x resistance
What does voltage mean?
Potential difference
What formula compares power to voltage?
P = I^2 x R
heat generated
What are the requirements for electricity to have any affect on the body?
- potential difference must be present
- individual must be part of the circuit, so current will pass through them
Why are electrical current exposure hazards so common/unsafe?
- 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!)
What is the relationship between current and resistance?
Current is inversely proportional to resistance
What is the difference between DC and AC?
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
What does voltage measure?
Amplitude
What effects can electricity at 60Hz current have on the body?
- 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)
How can nerve stimulation damage be portrayed at different currents?
- threshold o perception changes at 200 microamps
- prevents you letting go at 10 milliamps
What is the most dangerous frequency range for electricity?
60 Hz
Lowest threshold for stimulating muscle & nerve
What do the effects of electricity on the body depend on?
- frequency
- duration
- body weight
- point of entry/contact area
- gender
- skin resistance
- age groups include a few of these
(- not really voltage)
What does ‘let-go current’ mean?
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
What are the most dangerous frequencies?
15-100Hz
- Where let go current is the lowest
- threshold is lowest here
- Note this is what we use in everyday life (60Hz) = dangerous!
Where is let go current the greatest?
Very high frequencies
- very high threshold
- so these frequencies are used for medical applications
What is electrolysis? What is the effect on the body?
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
Why do burns happen?
- 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
Why/How does nerve stimulation occur?
- nerves normally function through action potentials travelling through axons to surrounding cells
- this effect can be stimulated by electrical currents
What happens if an electrical current affects a sensory nerve?
pricking sensation
- if it is intense, it becomes painful
What happens if an electrical current affects a motor nerve?
- muscle cramps
How do muscle cramps occur?
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
How can respiratory arrest occur?
- prolonged tetanus of intercostal muscles
How can cardiac arrest occur?
- tetanus of the heart muscle prevent beating = arrest
How can ventricular fibrillation occur?
- 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
Why does the effect vary so much?
- primary variable is the amount of current and path it takes
- current is inversely proportional to area of contact (V=IR)
What does the current flow depend on in the body?
- part of body
- water on skin
- amount of natural oils present
- skin condition
What body natural protection factors can minimize the impact of shock?
- automatic reflex reactions = muscle contraction = push body away
- intentional reflex reactions = intentionally pull away
What happens if a patient is lying on a hospital bed, what protection factors do they/don’t they have?
- connected to a machine= cannot pull away quickly
- anaesthesised/unconsicous = unable to detect shock
- no skin protection/skin resistance intentionally reduced?
Why is point of entry of the current significant?
- 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
Define macroshock
- 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
Define microshock
- 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
What are leakage currents?
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!)
How are leakage currents classified?
According to path they take to earth:
- earth leakage current
- enclosure leakage current
- patient leakage current
- patient auxillary current
Earth Leakage Current
- 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
Enclosure Leakage Current
- 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
Patient Leakage Current
- from either an applied part to ground or to an applied part from a power source
- through the patient
Patient Auxillary Current
- from 1 part of equipment to another through the patient
- e.g between applied parts
How do defibrillators work?
- intentional currents between parts
- patient functional currents
How to mimise risk?
- either remove potential difference
- or remove individual being part of circuit so current does not pass through them
How to protect against macroshocks?
- general mains supply 110-120 or 220-240 V
What is the 3 wire system?
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
What is the ground wire?
- 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
What are circuit breakers?
- intelligent device
- detect expected currents
- breaks/fuses circuit
- failure = ground wire take current to earth
How to protect against microshocks?
- 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
Earth Referencing
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!
Floating Applied Parts
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!
How can electrical equipment be categorised?
- according to method of protection against electrical shock it employs (CLASS)
- according to the degree of protection that is provided against electric shock (TYPE)
2 Methods of protection
- 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
Class 1 protection
- 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
What happens if there is failure of earth connection?
- 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)
Class 2 protection
- 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)
What is double insulation?
Basic + secondary level of protection to prevent contact with live parts
What is reinforced insulation?
- single layer of insulation
- offers same degree of protection as double
Class 3 protection
- 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
Degrees of Protection
- described by TYPE
- B, BF & CF
- usually related to applied parts which are attached to patient
Type B
- 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
Type BF
- 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)
Type CF
- 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
What kind of devices use Type B?
x-ray machines
operating tables
ventilators
What kind of devices use Type BF?
endoscopes
blood warmers
electrosurgery devices
thermometers
What kind of devices use type CF?
cardiac catheters
cardiac electrodes
intensive care monitoring
defibrillators
Single Fault Conditions Define
A condition in which a single means of protection against a safety hazard is deficient
What are the 2 types of safety tests?
Type AND Routine
Type Tests
- 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
Routine tests
- regular safety checks
- within working environment
- not to its limits = less destructive
- indicate its operating safely
What is Ohm’s Law?
V = IR
