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Presentation on theme: "ELECTRICAL DISTRIBUTION SYSTEMS IN HOSPITAL"— Presentation transcript:

Prepared by: Dr. Nur Farahiyah Mohammad, Sept 2017

2 This Week Hospital Electrical System: Introduction
General power and lighting system Distribution of electrical power system Electrical Hazards in Hospital Macrocshock Microshock Leakage current Electrical Safety Cord and Standards Protection Against Shock Isolated Power distribution Ground fault circuit interrupters (GFCI) Equipment design Electrical safety analyzers / Testing electrical systems

3 Distribution of Electric Power: Introduction
Electric Power is needed in health-care facilities not only for medical devices but also for any other electrical equipment like lightning, air condition, telephone, television etc. BUT Medical devices underlie special safety regulations as they might stay in special contact to and with patients, applicants and third persons Overvoltage protection Special ground Example A lightning causes an overvoltage at the public power supply. The overvoltage is transferred directly to the patients heart by applied ECG-Electrodes. => Over voltage protection

4 Distribution of Electrical Power
(230 V) Simplified electric-power distribution for 115 V circuits. Power frequency is 60 Hz

5 Distribution of Electrical Power
High voltage enters the building via underground cable. The secondary stepdown transformer develops 240V. This secondary transformer has a grounded centre tap to provide two 120V circuits between ground and each side of the secondary winding. Heavy duty devices (air conditioner, electric dryers and x-ray machine) that require 240V are placed across the entire secondary winding by making connections to the two ungrounded terminals. Ordinary wall receptacles and light operate on 120V obtained either one from ungrounded hot (black) transformer terminals and neutral (white) grounded centre tap. For healthcare facilities, the National Electrical Code (NEC) for 1996 requires that all receptacles be “Hospital Grade” and be grounded by a separate insulated (green) copper conductor (Article ) (230 V) Simplified electric-power distribution for 115 V circuits. Power frequency is 60 Hz

6 Distribution of Electrical Power
High voltage enters the building via underground cable. The secondary stepdown transformer develops 240V. This secondary transformer has a grounded centre tap to provide two 120V circuits between ground and each side of the secondary winding. Heavy duty devices (air conditioner, electric dryers and x-ray machine) that require 240V are placed across the entire secondary winding by making connections to the two ungrounded terminals.

7 Ordinary wall receptacles and light operate on 120V obtained either one from ungrounded hot (black) transformer terminals and neutral (white) grounded centre tap. For healthcare facilities, the National Electrical Code (NEC) for 1996 requires that all receptacles be “Hospital Grade” and be grounded by a separate insulated (green) copper conductor (Article )

The factors to be considered in the design of an electrical power system include safety, reliability, adequacy, distribution, grounding and quality. The quality of electrical power is concerned with voltage viability, frequency, stability and waveform. Each of these factors is important but with different priorities on where in the hospital you are. Example: Reliability and quality become equally important in surgery so that cardiopulmonary bypass machines function continuously and at constant speed. Reliability – kebolehpercayaan Adequacy= kecukupan

The primary source for standards in wiring is the: National Electrical Code NFPA 70 is a regionally adoptable standard for the safe installation of electrical wiring and equipment in the United States. It is part of the National Fire Codes series published by the National Fire Protection Association (NFPA), a private trade association. In Malaysia : MS IEC (Electrical installations of buildings - Part 7-710: Requirements for special installations or locations - Medical locations (IEC :2002, IDT) IEC- International Electrotechnical Commission's


Should have 2 independents sources of power fed by separate distributions networks and substations. To increase the reliability of the power to the hospital Each source must have the ability to handle the entire load of hospital. Should have an automatic transfer from one source power to the other in the event of failure of the primary source of power. Hospital wiring diagram showing the distribution of electrical power

Must have an emergency generator on the premises (even though there is 2nd source of power), independent from the other sources of power to provide uninterrupted partial service to “essential” areas of the hospital. Emergency generator able to restore power to “essential” areas within 10 s after the interruption of the primary power source.

General power and lighting systems The majority load in the hospital is supplied by the general power and lighting system This system must be installed in such way that a fault (short circuit line to line or to ground) or failure in this system will not interfere the functioning of other electrical systems in the hospital.

Other electrical systems in the hospital: Essential electrical system Auxiliary power supply and its associate equipment, such as transfer switches and feeders. This system used during a disruption of the normal power supply Equipment system The division of the essential electrical system supplying heating system, selected elevators, and other devices necessary for hospital function. .

Emergency system The emergency system must be in full operation within 10 s after a fault in general power supply. This system serves the life safety branch, the critical branch, and life support branch Life safety branch : supplies power to the equipment necessary for patient and personal safety such as hallway and stairway lighting, exit signs and directional signs , smoke detectors, alarm systems. Critical branch: Serves the patient care areas as well as areas related to patient care such as nursing stations and pharmacy. Also supplies power to isolation transformer in anesthetizing locations. Life support branch: Provide power to those areas where electrical power is essential for patient survival. Such as respirator and heart lung machines.

16 Electrical Hazards in Hospital
Macroshock hazards “The undesirable effect of a current greater than 5 mA at 60 cycles applied to the surface of the body” How to prevent Macroshock in Hospital??

17 Electrical Hazards in Hospital- Protection Against Macroshock
Separate ground wire is used in hospital wiring systems. Local codes for home wiring do not require this separate grounding when metal conduit may serve as the ground return path. Hospital require the use of separate ground wire to keep the ground resistance as low as possible and to prevent breaks in ground path due to corrosion, which can happen with conduit. a channel for conveying water or other fluid. a tube or trough for protecting electric wiring. Hospital wiring Home wiring

18 Protection Against Macroshock

19 Protection Against Macroshock
Make sure that all outlets have a “good” ground, meaning that the ground wire is intact and is a low-resistance path for the flow of ground current. The grounding on the all electrical equipment is functional and is not circumvented by the use of two-wire extension cords or three-prong to two-prong. In the event of a short to the case of an electrical device, no harmful voltage will appear on the chassis or case because of the ground wire will provide a low-resistance path for the current to flow and will not allow the chassis voltage to rise to a dangerous level.

20 Protection Against Macroshock
Good visual inspection of the physical condition of the electrical devices. A good grounding system will not prevent macroshock if someone comes in direct contact with an electrically hot wire. This could the case if a person touched an exposed metal conductor where the insulation was worn or broken.

21 Electrical Hazards in Hospital
MICROSHOCK HAZARDS “Define as a effect where a low level current (μA) passes directly through the heart via a needle or catheter in artery or vein” Source of the low level current Leakage Current

22 Electrical Hazards in Hospital- Leakage Current
Exists in all power-line-operated equipment and is usually due to: Capacitive coupling – Capacitive leakage current Resistive coupling – Resistive leakage current Under normal circumstances, leakage current is conducted away by the ground wire.

23 Electrical Hazards in Hospital- Leakage Current
Capacitive leakage current Is developed any time two conductors that carry current are separated by a dielectric. In this case, the dielectric is the insulation on the wire When an alternating voltage is applied between the conductors, a current will flow that is given by the equation for current in capacitor:

24 Electrical Hazards in Hospital- Leakage Current
For grounded equipment, leakage current is conducted to ground. Figure 15.7: Leakage current = 100 μA Ground resistance = 1 Ω Chassis voltage = ? V = IR = (100 μA) (1Ω) = 100 μV

25 Electrical Hazards in Hospital- Microshock Hazards
If a catheterized patient, who is also grounded, comes in contact on the medical device, the chassis-to-ground voltage will be applied across the patient. Schematic of grounded catheterized patient, who comes in contact with medical device chassis

26 Current through the heart of the patients is:
This level of current is not known to produce ventricular fibrillation (a) Schematic of grounded catheterized patient, who comes in contact with medical device chassis, (b) equivalent-circuit diagram for the grounded catheterized patient,

27 If the ground wire is broken, the situation changes.
Microshock Now the chassis of the device has become a common node point connecting the device capacitance in series with the 500 Ω impedance of the patient. The current through the patient’s heart is much larger that it was in the previous case.

28 Electrical Hazards in Hospital- Leakage Current
Limits on leakage current are instituted and regulated by the safety codes instituted in part by the National Fire Protection Association (NFPA), American National Standards Institute (ANSI), Association for the Advancement of Medical Instrumentation (AAMI), and Emergency Care Research Institute (ECRI).

29 Microshock via Ground Potentials
Microshocks can also occur if different devices are not at the exact same ground potential. In fact, the microshock can occur even when a device that does not connected to the patient has a ground fault! A fairly common ground wire resistance of 0.1Ω can easily cause a a 500mV potential difference if initiated due to a, say 5A of ground fault. If the patient resistance is less then 50kΩ, this would cause an above safe current of 10μA Faulty electric polisher

30 Microshock via Ground Potentials

31 Microshock Hazards Conductive Path to the Heart
Ventricular fibrillation and pump failure thresholds vs. electrode area Threshold of ventricular fibrillation and pump failure versus catheter area in dogs. From O.Z. Roy, J.R.Scott, and G.C. Park, “Ventricular Fibrillation and Pump Failure Threshold Versus Electrode Area,” IEEE Transaction of Biomedical Engineering,1976,23,45-48.)

32 Electrical Safety Codes & Standards
Code – document that contains only mandatory documents Standard –also a document that contain mandatory requirement, but compliance tends to be voluntary, and more detailed notes and explanations are given. Manual or guide- is a document that is informative and tutorial but does not contain requirements.

33 Electrical Safety Codes & Standards
History: The process of development, adoption and use of standards and codes for electrical safety in health-care facilities began following tragic explosion and fire resulting from electric ignition of flammable anaesthetics such as ether. Lead to adoption of National Fire Protection Association NFPA and ANSI/AAMI ESI \-1985 standards.

34 Electrical Safety Codes & Standards
NFPA 99-Standard for Health Care Facilities – has evolved from 12 NFPA documents that were combined in a984 and revosed every 3 years. Current edition: 2015 In addition to electric equipment, this standard also described gas, vacuum, and environmental systems and materials. The primary document that describes the requirements for patient-care-related electric appliances used for diagnostic, therapeutic, or monitoring purposes in a patient-care area.

35 Electrical Safety Codes & Standards
The Association for the Advancement of Medical Instrumentation (AAMI) developed an American National Standard on “Safe Current Limits for Electromedical Apparatus,” ANSI/AAMI ESI This standard concern limits on chassis and patient-lead leakage currents, which are fixed from dc to 1 kHz and increase from 1kHz to 100 kHz.

36 Protection against shock
Method Protection 1 : Power Distribution Grounding System Isolated Power distribution Ground fault circuit interrupters (GFCI) Method Protection 2: Equipment design Electrical-Safety Analyzers

37 Basic Approaches to Shock Protection
There are two major ways to protect patients from shocks: Completely isolate and insulate patient from all sources of electric current Keep all conductive surfaces within reach of the patient at the same voltage Neither can be fully achieved  some combination of these two Grounding system Isolated power-distribution system Ground-fault circuit interrupters (GFCI)

38 Grounding Systems Low resistance (0.15 Ω) ground that can carry currents up to the circuit-breaker ratings protects patients by keeping all conductive surfaces and receptacle grounds at the same potential. Protects patients from Macroshocks Microshocks Ground faults elsewhere (!) The difference between the receptacle grounds and other surface should be no more then 40 mV) All the 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 grounding point that makes a single connection to the building ground.

39 Isolated Power-Distribution Systems
A good equipotential grounding system cannot eliminate large current that may result from major ground-faults (which are rather rare). Isolated power systems can protect against such major (single) ground faults Provide considerable protection against macroshocks, particularly around wet conditions However, they are expensive ! Used only at locations where flammable anesthetics are used. Additional minor protection against microshocks does not justify the high cost of these systems to be used everywhere in the clinical environment

40 Isolated Power Distribution
Not grounded ! In fact, in such an isolated system, if a single ground-fault occurs, the system simply reverts back to the normal ground-referenced system. A line isolation monitor is used with such system that continuously monitors for the first ground fault, during which case it simply informs the operators to fix the problem. The single ground fault does NOT constitute a hazard! Normally, when there is a ground-fault from hot wire to ground, a large current is drawn causing a potential hazard, as the device will stop functioning when the circuit breakers open ! This can be prevented by using the isolated system, which separates ground from neutral, making neutral and hot electrically identical. A single ground-fault will not cause large currents, as long as both hot conductors are initially isolated from ground!

41 Ground – Fault Circuit Interrupters (GFCI)
Disconnects source of electric current when a ground fault greater than about 6 mA occurs! When there is no fault, Ihot=Ineutral. The GFCI detects the difference between these two currents. If the difference is above a threshold, that means the rest of the current must be flowing through elsewhere, either the chassis or the patient !!!. The detection is done through the monitoring the voltage induced by the two coils (hot and neutral) in the differential transformer!

42 GFCI The National Electric Code (NEC ) requires that all circuits serving bathrooms, garages, outdoor receptacles, swimming pools and construction sites be fitted with GFCI. Note that GFCI protect against major ground faults only, not against microshocks. Patient care areas are typically not fitted with GFCI, since the loss of power to life support equipment can also be equally deadly!

43 Protection through Equipment Design
Strain-relief devices for cords, where cord enters the equipment and between the cord and plug Reduction of leakage current through proper layout and insulation to minimize the capacitance between all hot conductors and the chassis Double insulation to prevent the contact of the patient with the chassis or any other conducting surface (outer case being insulating material, plastic knobs, etc.) Operation at low voltages; solid state devices operating at <10V are far less likely to cause macroshocks Electrical isolation in circuit design

44 Electrical Isolation o Main features of an isolation amplifier:
CM CMRR SIG ISO Error Isolation barrier Capacitance and resistance - + Input common (a) *IMRR in v/v Output common o = IMRR Gain RF IMRR* ~ Main features of an isolation amplifier: High ohmic isolation between input and output (>10MΩ) High isolation mode voltage (>1000V) High common mode rejection ration (>100 dB)

45 Transformer Isolation Amplifiers
- + (b) + 15 V DC o Power return 25 kHz - 7.5 V +ISO Out SIG ISO + 7.5 V In com In In + FB 5 V F.S. Oscillator Signal Mod Rect and filter Demod ± 5 V AD202 Hi Lo

46 Optical Isolation Amplifier
Isolation barrier Input control Output -V +V +o - + o = i RK RG CR3 CR1 CR2 i2 i i1 AI AII i3 RK = 1M W (c) ~ 1 2

47 Electrical Safety Analyzers Wiring / Receptacle Testing
Three LED receptacle tester: Simple device used to test common wiring problems (can detect only 8 of possible 64 states) Will not detect ground/neutral reversal, or when ground/neutral are hot and hot is grounded (GFCI would detect the latter)

48 Electrical Safety Analyzers Testing Electrical Appliances
Ground-pin-to-chassis resistance: Should be <0.15Ω during the life of the appliance Ground-pin-to-chassis resistance test

49 Electrical Safety Analyzers Testing Electrical Appliances
Chassis leakage current: The leakage current should not exceed 500μA with single fault for devices not intended for patient contact, and not exceed 300 μA for those that are intended for patient contact. Appliance power switch (use both OFF and ON positions) Open switch for appliances not intended to contact a patient Grounding-contact switch (use in OPEN position) Polarity- reversing switch (use both positions) Appliance H (black) H To exposed conductive Internal Circuitry surface or if none, then 10 by 120 V N 20 cm metal foil in contact N (white) G with the exposed surface G (green) Insulating surface I Building ground Current meter H = hot N = neutral (grounded) G = grounding conductor Test circuit This connection is at service I < μA for facility Ð owned housekeeping and maintenance appliances entrance or on I > μA for appliances intended for use in the patient vicinity supply side of separately derived system

50 Electrical Safety Analyzers Testing Electrical Appliances
Leakage current in patient leads: Potentially most damaging leakage is the one with patient leads, since they typically have low impedance patient contacts Current should be restricted to 50μA for non-isolated leads and to 10 μA for isolated leads (used with catheters / electrodes that make connection to the heart) Leakage current between any pair of leads, or between a single lead and other patient connections should also be controlled Leakage in case of line voltage appearing on the patient should also be restricted.

51 Leakage current Testers
Test for leakage current from patient leads to ground

52 Leakage Current testers
Test for leakage current between patient leads

53 Leakage Current Testers
Test for ac isolation current Isolation current is the current that passes through patient leads to ground if and when line voltage appears on the patient. This should also be limited to 50μA

54 And this concludes…


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