Chapter 4 Part 2 CENG Electrical Safety and Standards Flashcards
What is leakage current?
-Leakage current is the current that flows from either AC or DC circuit in equipment to the chassis, or to the ground, and can be either from the input or the output.
-All electrically operated devices have some low-value currents that flow from the energized electrical portions of the device to the metal chassis. This current is not a result of a fault but is a natural consequence of electrical wiring and components. This current is referred to as leakage current and has two components, capacitive and resistive.
- Small currents (µA) that flow between any adjacent insulated conductors that are at different potentials
What are the two leakage currents? (C and R)
1) Capacitive Leakage Current
2) Resistive Leakage Current
What is capacitive leakage current?
Capacitive leakage current results from distributive capacitance between two wires or between a wire and a metal chassis. For example, the “Live” copper wire forms one plate, the wire insulation forms the dielectric, and the metal chassis forms the other plate of a capacitor.
This capacitor is actually distributed over the entire length of the power cord (Length increases, capacitance increases, higher leakage currents). A 6m power cord of capacitance 500pF/m at 50Hz on a 230V power system gives 1.06MΩ of capacitive reactance and thus 217μA of leakage current.
Components that cause capacitive leakage currents are RF filters, power transformers, power wires, and any device that has stray capacitance.
What is resistive leakage current?
Resistive leakage current arises from the resistance of the insulation surrounding the power wires and transformer primary windings. Modern power cords are of very high resistance that the resultant leakage current is negligible compared to capacitive leakage.
What are the 2 purposes of Earth wire?
- Drain off leakage current
- Blow off the plug fuse in case of Live-to metal chassis short (thus preventing electrical shock to anyone touching the “live” chassis).
How do macroshocks occur?
A macroshock results when a person touches the live ungrounded chassis and any grounded object simultaneously.
What happens when the chassis and cabinet are not grounded?
An insulation failure or a short between the live wire and the chassis can result in a high voltage differential (230V) between the chassis and any grounded object.
What happens when ground wire is broken?
The chassis potential rises above ground.
How do microshocks occur?
If a patient who touches the chassis and has a grounded electric connection to the heart, he may receive a microshock.
What are some uses of electrical power in healthcare facilities?
- operation of medical instruments
- lighting
- maintenance appliances
- patient conveniences (TV, hair dryers, …)
- clocks
- nurse call buttons
What are the 2 ways to provide electrical safety?
- control the availability of electric power
- provide proper grounding in the patients’ environment
What are the 3 processes of electricity distribution? TSD
- Transmission: High voltage enters the building, usually via underground cables.
- Substation: The secondary of a stepdown transformer develops 240-V. This secondary has a grounded centre tap to provide two 120-V circuits.
- Domestic power supply: Ordinary wall receptacles and lighting are obtained from either one of the ungrounded hot transformer terminals and the neutral grounded centre tap.
How is a grounding system connected?
All receptacle grounds and conductive surfaces in the vicinity of the patient are connected to the patient-equipment grounding point.
Each patient-equipment grounding point is connected to the reference point that makes a single connection to the building ground.
What are the 3 different types of grounds?
- signal ground
- chassis ground
- earth ground
What are ground faults?
Ground faults are short circuits between Live & Earth injecting large currents and thereby tripping the circuit breakers
What are the several techniques used to protect clinicians, patients & visitors from electrical shock? (3 methods)
- Isolation transformer
- Line Isolation Monitor (LIM)
- Ground Fault Circuit Interrupter (GFCI)
How does an isolation transformer work?
The Isolation transformer offers electrical safety by converting grounded power into ungrounded power.
This is accomplished by grounding the primary winding of the transformer and not grounding the secondary winding.
What happens when single fault from either conductor to ground occurs?
The system simply reverts to a normal grounded system
(a second fault from the other conductor to ground is then required to inject high currents in the ground.)
A Line-Isolation Monitor (LIM) must be used with isolation transformers to detect the occurrence of the first fault from either conductor to ground.
When is a patient still safe?
If either Hot or Neutral of Isolation Transformer secondary is in contact with patient, patient is still safe.
What is a line isolation monitor (LIM)?
To monitor the degree of system isolation, a line isolation monitor (LIM) is employed. Its function is to continually analyze the entire isolated circuit and quantify its degree of isolation from ground, i.e. to monitor the leakage current to ground. The LIM provides visual and audible alarm signals when the predicted ground-seeking current exceeds a specified magnitude.
How does a LIM work?
This monitor alternately measures the total possible resistive and capacitive leakage current (total hazard current) that would flow through a low impedance if it were connected between either isolated conductor and ground.
When the total hazard current exceeds either 2.0mA or 5.0 mA LIM setting, a red light and an audible alarm are activated.
What happens when the LIM detects too much power and what should be done?
The LIM will alarm and indicate that there is an overload. The power used must be reduced immediately by moving some equipment to another circuit as soon as possible until the alarm stops sounding.
