Electrical Safety

Understand the concepts of patient safety as it applies to monitoring involving electrical devices

Electrical Principles

• Charge is the property of a subatomic particle which causes it to experience a force when close to other charged particles
  Charge is measured in coulombs (C).

• Current is the flow of electrons through a conductor
  Current is measured in amps (A).

• Voltage is the strength of the force that causes movement of electrons
  By tradition, voltages are quoted relative to ground (or earth). If a potential difference exists, a current will flow from that object to the earth via the path of least resistance. If this path contains a person, an electrical injury may result.

• Resistance describes to what extent a substance reduces the flow of electrons through it
  Resistance is measured in ohms (Ω).
  • Substances with high resistance are insulators
  • Substances with low resistance are conductors

• Inductance is the property of a conductor by which a change in current induces an electromotive force in the conductor, and any nearby conductors

• Capacitance is the ability of an object to store electrical charge
  Measured in Farads (F), where one farad is when one volt across the capacitor stores one coulomb of charge.
  • A capacitor is an electrical component consisting of two conductors separated by an insulator (called a dielectric)
  • When a direct current flows, electrons (a negative charge) build up on one of these conductors (called a plate), whilst an electron deficit (positive charge) occurs on the other plate
    • Current will flow until the build up of charge is equal to the voltage of the power source
    • Current can be rapidly discharged when the circuit is changed
  • An alternating current can flow freely across a capacitor, and causes no buildup of charge

• Impedance describes to what extent the flow of alternating current is reduced when passing through a substance
  Impedance can be thought of as 'resistance for AC circuits', and is a combination of resistance and reactance.
  • Reactance is a function of two things:
    • Induction of voltage in conductors by the alternating magnetic field of AC flow
    • Capacitance induced by voltages between these conductors

Electrical Injury

Potential electrical injuries can be divided into:

• Ventricular Fibrillation
  Likelihood is a function of:
  • Current density
  • Frequency
    Lowest current density required is at 50Hz.

• Burns
  Function of current density. Burns typically occur at the entry and exit point as this is where current density is highest.

• Tetanic Contraction
  Flexors are stronger than extensors, which may maintain grip on live wire. Death may result from either VF or asphyxiation from sustained respiratory muscle contraction.

Electrical Shock

Electrical shocks are divided into two types, based on their ability to induce VF:

• Microshock
  Current required to induce VF when applied directly to myocardium.
  • Typical current is 0.05-0.1mA
  • This requires skin breach
    Potential causes:
    • Guidewire
    • Pacing lead
    • Column of conducting fluid
    • CVC
    • PICC
• Macroshock
  Current required to induce VF from surface contact.
  • Typical current is 100mA
  • This is much higher because most of this current is not going to the ventricle, and so the total current must be greater to achieve sufficient current density in the myocardium to induce VF

Other detrimental effects seen at lower currents include:

Principles of Electrical Safety

Power points contain three wires:

• Active
  240V. Measuring voltage for AC current is not intuitive, as the voltage will be negative half the time. The root mean square (RMS) is used instead - each value for the voltage is squared (giving a positive number), and then divided by the number of samples to give an average.
• Neutral
  0V, relative to ground.
• Earth
  Direct pathway into ground.

An electrical circuit is completed between an appliance and the power station by returning current to the station via the earth. This is an earth referenced power supply.

Electrical Dangers

• Active wire shorts to equipment casing
  • Principle of earth wire, which provides path of least resistance for current to travel if an individual touches the case
• High current drain through a wire generates heat and starts a fire
  • Principle of fuses which trigger when current drain is >15A

Methods of Electrical Safety

• Insulation
  Conductors are coated by a high-resistance substance, preventing current flowing where it shouldn't.

• Fuses
  Safety devices which cease all current flow when current exceeds a certain threshold (typically 20A). If there is a fault which greatly lowers resistance (i.e. insulation breaks, causing a device to become live and drain via the earth wire), a high current will flow and the fuse will be triggered.
  • A fault requires:
    • A fault that causes a high current flow
    • The fuse to work correctly

• Residual Current Devices
  An RCD measures the current difference between the active and neutral lines.
  • In an non-fault situation, these will be equal
  • In a fault situation, current will be being delivered by the active line but not returned via the neutral
    Current will instead flow to ground via faulty equipment/through the patient.
    • The RCD will detect if there is a >10mA difference between the active and neutral lines, and disconnect power within 10ms if it does so
  • A fault requires:
    • Current to flow
    • A single fault will turn off the circuit
  • Pros: Safe
  • Cons: Will shut off power to the device, which is bad for ECMO/CPB/ventilators without battery backup

• Line Isolation Supply, with a line isolation monitor
  A line isolated supply is a 'transformer' with an equal number of windings, such that the voltage produced is the same on each side. However, the powerpoint is not physically connected to the supply, creating an earth-referenced floating supply.
  • A fault requires:
    • Two faults
      This makes a failure with potential for shock much less likely.
      • Active wire must be connected to ground
      • Neutral wire must be connected to ground
      • A circuit then exists: active wire - ground - neutral wire, and a current could flow
  • A line isolated supply is paired with a line isolation monitor
    This monitor states how much current could flow, if a second fault completed the circuit.
    • This is called a prospective hazard current
    • The line isolation monitor continuously checks the hazard current by evaluating the impedance between the active wire and ground, and the neutral wire and ground
      • In a no-fault situation, both impedances should be the same and close to infinite
        (Impedance won't be absolutely infinite as there will always be a small current leak from devices).
      • In a single-fault situation, the calculated impedance for the affected line will be significantly lower, and therefore the prospective hazard current will increase
        • An alarm will sound when the prospective hazard current exceeds 20mA
  • Pros: A single fault is not dangerous and will not result in a power loss (important for vital equipment)
  • Cons: Two or more faults are dangerous, and will still not result in a power loss

• Equipotential earthing
  This is the only method which prevents microshock.
  • Ultra-low resistance earth cables are attached to electrical devices and the patients bed
  • These cables are then attached to special wall earth connectors
  • This ensures all equipment is referenced to a common ground, minimising the risk of leakage currents between devices and the patient

Classification of Electrically Safe Equipment

These classifications are designed to limit macroshock:

• Class I: Earthed
  Any part that can contact the user is earthed to ground.

  • If a fault develops such that parts of the device that the user can touch are live, then there is a risk of shock
  • If the case is earthed, the path of least resistance should be via the earth wire
    This will cause a large current to flow, and should blow a fuse, ceasing current flow.

• Class II: Double-insulated
  All parts of the device that the user can touch have two layers of insulation around them, reducing the chance of the device becoming live.

• Class III: Low-voltage
  Device operates at less than 40V DC/24V AC, limiting the severity of shock a device can deliver.

Classification of Electrically Safe Areas

• B areas: Protection against macroshock
  • Residual Current Devices
  • Line Isolation Supply

• BF areas: Cardiac (microshock) protection
  • Equipotential Earthing
    All devices, and the patient, are earthed to each other by thick copper (i.e. low-resistance), such that any potential difference between devices will be equalised via the path of least resistance (the wire, not the patient).

• Z areas: No particular protections

Electrical Devices which Attach to Patients

Devices such as ECG and BIS require an electrical connection to the patient. Risk of electrocution by these devices is reduced by:

• High resistance wires

References

1. Electricity and Electrical Hazards.
2. Alfred Anaesthesia Primary Exam Tutorial Program
3. Aston D, Rivers A, Dharmadasa A. Equipment in Anaesthesia and Intensive Care: A complete guide for the FRCA. Scion Publishing Ltd. 2014.