FRCA Paper 04 - Electrical Safety Flashcards
electricity supply to an operating theatre has how many phases?
single phase
electricity supply to theatres consists of what wires?
LINE, neutral and earth wires
NOT live
electricity supply to theatres has what voltage?
approx 240V
Does the electricity supply to an operating theatre have its earth connection at the substation?
The substation earth (star point) creates the ‘Neutral’ wire. The earth connection is a local earth
Is current flow in a wire required for electrostatic interference from a piece of equipement?
no current required
Can an insulating layer stop electrostatic interference?
Insulation will not stop this, it requires a conductive casing as electrostatic fields will not pass through a conductive barrier.
Electrostatic interference from equipment can be reduced by?
- halved by doubling the distance between the signal and power leads.
- Prevented using screened cable.
Electromagnetic interference: must the source be using current flow?
Yes - When an alternating current is flowing an alternating magnetic field is generated around the power wire and this induces current in the signal lead.
Where is the earth connection?
The earth connection is the LOCAL earth and is NOT connected at or to the substation
What are the requirements for an earth connection in terms of:
- resistance?
- current capacity, earth surge test?
- voltage during surge test
- resistance less < 0.1 ohm
- Earth surge test requires that the local earth can carry a current of 25A for 5 seconds.
- Since the resistance of the earth must not exceed 0.1 ohm, the surge test would not require a voltage of more than 2.5 V (from ohms law)
What is the star point?
The earth connection at the substation.
Connecting one side of the 240V supply to earth at the electricity substation creates the neutral connection.
How does the star point protect users?
The substation earth (star point) provides a path for lightening to reach the ground
Why is a star point not used in a three phase electricity supply?
A true three phase supply does not use a neutral connection. Star point is connected to a neutral wire at the substation.
How does an earth free supply increase electrical safety?
The supply is not earth referenced.
The isolating transformer means that the 240 V supply is no longer connected to the Star point.
> > > current can therefore not flow through you, via the ground to complete the circuit.
Increases protection from electric shock.
A line isolation monitor is required for?
Required by an earth free supply to warn if a fault has compromised the safety of the system.
Alarms if the supply has accidentally become earth referenced as the consequence of a fault.
T/F: which ventricle is more sensitive to micro-shock?
The right ventricle.
Can a micro shock induce asystole?
No
In micro shock, what current is sufficient to have an effect?
Anything 50 mA and over
What frequency of electric shock can cause arrhythmias?
> 1000 KHz
T/F: In electric shock, electrons flow through the body.
False, the charge carriers in the body are IONS.
T/F: The earth connection at a substation must be tested regularly.
False, earth connection is LOCAL. its not connected to the substation.
Does the earth free supply have a local earth connection?
Yes - A local earth is still provided in case the ISOLATION fails because of a fault.
T/F: Considering electricity, electrons move at the speed of light.
False - Electron drift velocities depend on the current flowing and cross sectional area of the conductor (they are generally quite slow).
T/F: For a single phase mains supply the peak voltage is plus minus 250V
False. 250V RMS equates to +/- 339V
T/F: Direct current is easier to switch off than alternating current.
False.
It is harder to switch off direct current than AC because a continuous and therefore self sustaining arc is produced.
T/F: Current can be dissipated by an earth connection.
False. Current can not be ‘dissipated’ Electrons MUST FLOW AROUND A CIRCUIT.
What is the efficiency of an electrical transformer?
95%
In an electrical transformer, if the voltage is halved, what happens to the current?
If the secondary voltage is half that applied to the primary, then the secondary current is TWICE that drawn by the primary.
Electrical burns: caused by what type of current?
AC
Electrical burns are concentrated at which point in the body?
Are concentrated where the CURRENT DENSITY and resistance are greatest - usually at skin entry and exit points.
T/F: Electrical burns depend upon the voltage applied to the tissues.
True - Depends on current density (and therefore indirectly voltage) and resistance.
T/F: Electrical burns only happen if frequency is less than 100 KHz
False.
Frequency does not affect the direct dissipation of HEAT in the tissues. Diathermy works by producing local heating.
How does increasing paddle pressure effect electrical skin resistance to DC defibrillation?
Increasing paddle pressure increases contact area and therefore decreases skin resistance to DC defibrillation.
T/F - Direct current:
- Flows in one direction?
- Is of constant magnitude?
- Causes electrostatic interference?
- Can be produced from AC?
- True
- False, can vary in magnitude. Can be discontinuous, as long as it doesn’t reverse.
- False. Electrostatic interference requires a ALTERNATING voltage to be present.
- True - Of course by placing a diode (electrical one way valve) in the wire.
Electrical equipment:
Type II apparatus requires an earth connection? T/F
False.
Double insulated apparatus has TWO physical barriers between the electrically applied part (the part of the apparatus connected to the mains) and the user.
The maximum earth leakage current for Type B(F) equipment is 500uA? T/F
True. Type I equipment - leakage current must not exceed 0.5mA (500uA)
What type of equipment is internally powered? (Class)
Type III
For Type C(F) equipment, what does the F signify?
Is it earth free?
The F suffix states that the patient applied part (the bit of the equipment which is connected to the patient) is ‘FLOATING’,
i.e. is electrically isolated from the rest of the apparatus
It is not earth free.
Can Type III equipment be used whilst charging?
No - Type III equipment is ‘internally powered’, if connected to an external power supply whilst charging it is no longer internally powered and the classification would change to that of the charger.
If the fuse in the electrical supply to medical equipment is placed in the neutral wire is it safer?
No, it would be more dangerous.
In the electrical supply to medical apparatus:
- What colour is the local earth wire?
- What colour is the line wire?
- What should determine choice of fuse?
Local earth - yellow/green
Line wire - Brown
Fuse is chosen to allow the apparatus to draw the power it requires to operate.
Type III Apparatus: T/F
1. Requires three wires; line, neutral, earth
- earth leakage must not exceed 500uA
- must not be directly connected to pt
- cannot act as the source of leakage current to patient
- must have a fuse of less than 1 Amp
- False. It is internally powered
- False. Has no mains connection and therefore leakage current is not possible
- False. e.g. an internally (battery) powered nerve stimulator
- True. It has NO mains connection
- False. It has NO mains connection
What value of farad are the capacitors used in defibrillators?
Very low, usually micro-farad
How does increasing the distance between capacitor plates affect its capacitance?
Decreases capacitance .
For a simple capacitor, the capacitance is proportional to the area over which the plates overlap
And inversely proportional to the area over which their distances apart.
What is the formula for stored energy in a capacitor?
E = 1/2 x C x V^2
Which current can be used to charge capacitors?
DC
How does the introduction of a dielectric plate to a capacitor effect it’s capacitance?
Increases capacitance
Does the star point carry electric current back to the power station?
No, current flows between the LINE and NEUTRAL wires.
Is a free earth supply required for Type II equipment?
No
Can a D.C. cause:
- harm to tissue?
- electrostatic interference?
Yes it can be harmful to tissues as power is still dissipated into the tissue and ions still move.
No, electrostatic requires alternating voltage to be present.
What is a coulomb?
It is the unit of electrical charge, a derived unit, equivalent of 6.24 x 10^18 electrons.
What is the definition of 1 ampere?
It is the flow of 1 coulomb of charge per second through any cross sectional area of the conductor.
