Electrical Concepts, Definitions and Standards

Revised and reviewed 2 August 2015

OVERVIEW

• You may have noticed that much of the equipment used in critical care runs on something called electricity… Hence, it is useful to know the basics!
• Critical care practitioners may be exposed to, and need to mitigate, electrical hazards
• Basic electrical concepts are also relevant to the treatment of victims of electrical injuries

DEFINITIONS

Current (I)

• charge per unit time (unit is A – ampere)
•  described in terms of the direction of current flow – alternating (AC e.g. mains power) or direct (DC e.g. a battery)
• simplistically, the ability of a material to conduct current depends on “how tightly the electrons in the outer atoms are packaged, with less conduction occuring when they are very tightly bound”

Conductor

• material that allows current to flow (e.g. metals)

Insulator

• material that doesn’t allow current to flow (e.g. rubber)

Semiconductor

• material that permit current flow intermediate to that of a conductor and insulator (e.g. crystalline inorganic solids)

Resistance (R) and Impedance

• Resistance measure of how much a material resists the passage of current (unit is Ω – ohm)
• called Impedance if it is resistance to AC current as opposed to DC current
• the inverse of resistance is Conductance (unit is S – siemens)

Potential (aka electrical potential)

• energy per unit time

Potential difference (Voltage)

• the difference in electrical potential between two points in an electric circuit or field (unit is V – volt)
• current will only flow through a circuit when a potential difference exist, from negative to positive
• sources of potential difference include batteries, generators and being plugged into the ‘electrical grid’

Ohm’s law

• describes the relationship between potential difference, current and resistance
• V = I x R

ELECTRICAL STANDARDS

• different electrical standards for electrical safety in hospitals apply in different countries
• In Australasia the relevant electrical standard is AS/NZS3003: Electrical installations: patient treatment areas of hospitals and medical and dental practices
• In Australia, like the UK and New Zealand, mains electricity is supplied as 50 Hz AC at a potential of 230V.

AS/NZS3003 divides treatment areas into:

• Cardiac protected electrical area
  • areas where patients may have electrical conductors near or inside their hearts with a risk of microshock
  • these areas have equipotential earthing and Residual current devices (RCDs)/Line isolation monitors (LIMs)
• Body protected electrical area
  • areas where patients are attached to equipment that lowers their natural resistance to current injury but are not at risk of microshock
  • RCDs/LIMs are used

Equipment

• classified by the risk of current leak
  — Type B have no restriction
  — BF items may only leak a maximum of 5 mA
  — CF items are allowed a maximum leak of 50 microA
• ‘Defibrillator proof’ refers to items that can safely remain attached to the patient during discharge of a shock

See Electrical Safety Devices for an overview of electrical safety equipement

References and Links

LITFL

• CCC — Electrical injury
• CCC — Electrical Safety
• CCC — Hands On Defibrillation

Journal articles

• Singh. Basics of electricity for anaesthetists. Contin Educ Crit Care Pain 2011;11(6):224-228 [Free Full Text]
• Boumphrey, S et al. Electrical Safety in the Operating Theatre.Contin Educ Crit Care Pain 2011;3(1):10-14 [Free Full Text]

FOAM and web resources

• HowEquipmentWorks.com — Electricity Basics
• HowEquipmentWorks.com — Electrical Safety