Presentation on theme: "ELECTRICAL DISTRIBUTION SYSTEMS IN HOSPITAL"— Presentation transcript:
1 ELECTRICAL DISTRIBUTION SYSTEMS IN HOSPITAL Prepared by: Dr. Nur Farahiyah Mohammad, Sept 2017
2 This Week Hospital Electrical System: Introduction General power and lighting systemDistribution of electrical power systemElectrical Hazards in HospitalMacrocshockMicroshockLeakage currentElectrical Safety Cord and StandardsProtection Against ShockIsolated Power distributionGround fault circuit interrupters (GFCI)Equipment designElectrical 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.BUTMedical devices underlie special safety regulations as they might stay in special contact to and with patients, applicants and third personsOvervoltage protectionSpecial groundExampleA 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 )
8 HOSPITAL ELECTRICAL SYSTEM 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 – kebolehpercayaanAdequacy= kecukupan
9 HOSPITAL ELECTRICAL SYSTEM 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
11 HOSPITAL ELECTRICAL SYSTEM Should have 2 independents sources of power fed by separate distributions networks and substations.To increase the reliability of the power to the hospitalEach 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
12 HOSPITAL ELECTRICAL SYSTEM 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.
13 HOSPITAL ELECTRICAL SYSTEM General power and lighting systemsThe majority load in the hospital is supplied by the general power and lighting systemThis 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.
14 HOSPITAL ELECTRICAL SYSTEM Other electrical systems in the hospital:Essential electrical systemAuxiliary power supply and its associate equipment, such as transfer switches and feeders.This system used during a disruption of the normal power supplyEquipment systemThe division of the essential electrical system supplying heating system, selected elevators, and other devices necessary for hospital function..
15 HOSPITAL ELECTRICAL SYSTEM Emergency systemThe 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 branchLife 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 wiringHome wiring
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 currentLeakage Current
22 Electrical Hazards in Hospital- Leakage Current Exists in all power-line-operated equipment and is usually due to:Capacitive coupling – Capacitive leakage currentResistive coupling – Resistive leakage currentUnder normal circumstances, leakage current is conducted away by the ground wire.
23 Electrical Hazards in Hospital- Leakage Current Capacitive leakage currentIs developed any time two conductors that carry current are separated by a dielectric.In this case, the dielectric is the insulation on the wireWhen 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 μAGround resistance = 1 ΩChassis 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. MicroshockNow 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 10μAFaulty electric polisher
31 Microshock Hazards Conductive Path to the Heart •Ventricular fibrillation and pump failure thresholds vs. electrode areaThreshold 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 documentsStandard –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: 2015In 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 ESIThis 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 DistributionGrounding SystemIsolated Power distributionGround fault circuit interrupters (GFCI)Method Protection 2: Equipment designElectrical-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 currentKeep all conductive surfaces within reach of the patient at the same voltageNeither can be fully achieved some combination of these twoGrounding systemIsolated power-distribution systemGround-fault circuit interrupters (GFCI)
38 Grounding SystemsLow 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 fromMacroshocksMicroshocksGround 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 faultsProvide considerable protection against macroshocks, particularly around wet conditionsHowever, 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 GFCIThe 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 plugReduction of leakage current through proper layout and insulation to minimize the capacitance between all hot conductors and the chassisDouble 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 macroshocksElectrical isolation in circuit design
44 Electrical Isolation o Main features of an isolation amplifier: CMCMRRSIGISOErrorIsolationbarrierCapacitanceand resistance-+Input common(a)*IMRR in v/vOutputcommono=IMRRGainRFIMRR*~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)
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 applianceGround-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 switchfor appliancesnot intended tocontact a patientGrounding-contactswitch (use inOPEN position)Polarity- reversingswitch (use bothpositions)ApplianceH (black)HTo exposed conductiveInternal Circuitrysurface or if none, then 10 by120 VN20 cm metal foil in contactN (white)Gwith the exposed surfaceG (green)Insulating surfaceIBuildinggroundCurrent meterH = hotN = neutral (grounded)G = grounding conductorTest circuitThis connectionis at serviceI< μA for facilityÐowned housekeeping and maintenance appliancesentrance or onI> μA for appliances intended for use in the patient vicinitysupply side ofseparately derivedsystem
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 contactsCurrent 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 controlledLeakage 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 currentIsolation 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