devices to protect against electrical hazards mohd yusof baharuddin
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DEVICES TO PROTECT AGAINST ELECTRICAL HAZARDS Mohd Yusof Baharuddin
ObjectivesExplain several devices available to protect patients and healthcare workers from hazardous electrical current.
Discuss devices to protect against high voltage microshock and macroshock hazards
Line Isolation Monitor (LIM)
Line Isolation Monitor (LIM)LIM place a relatively large impedance from either secondary lead through an ammeter to ground of the isolation transformer.If the conductive part through the equipment, the meter in LIM will read a current
The meter on LIM is calibrated to read what current would flow through a short circuit fault if it should occur from either secondary to groundThis value vary on the leakage of the equipment or any fault exist between secondary leads and ground
An alarm in LIM is usually set off when it calculated short circuit fault between secondary lead and ground would draw 2 to 5 mA.This alarm indicates the backup system has failed and the equipment is no longer isolated.However, this doesnt mean that dangerous current are already flowing
Ground Fault Interrupter (GFI)GFI protects against a shock that occurs if person touches H with one hand and G with the otherGFI opens power lead if H lead current differs more than 2 mA from N lead current for duration longer than 2 ms.
GFI consist of a magnetic coil on which H and N lead are wound with the same number of turn but in different directions.When normalIH = IN
and cancels magnet flux in the coil
Sensing coil does not have voltage induced in it
However, when hot lead faults or is touched by a person, fault current, IF is shunted to ground
IN = IH + IF
What will happened ???
Under this fault condition, corresponding flux are unequal and net flux exists in the coil.This induce a voltage into sensing amplifierIf IF exceeds 2 mA for 2ms, relay opens the line and prevents macroshockGFI can be mounted in power receptacle and is required in wet area
Safety AnalyzerPrimarily used to test medical equipment for leakage current that can cause microshock to patientSafety analyzer is calibrated by placing S5 in the calibrate position while adjusting the meter for 1 A
The equipment under test (EUT) is plugged into safety analyzerTo measure the chassis to ground leakage on EUT, S4 is opened while S1 is in position 1, S2 is in position 2 and S3 is opened.
Ground leakage is measured with H and N leads reversed by mean of switch S1.Switch S2 may then moved to position 1 to measure leakage current in patient lead P1. Leakage current between patient lead when the H lead is applied to one of P lead is measured when S3 is closed.
Receptacle TesterThe LIM and GFI are permanently attached to the power line in order to warn of potential hazard or to open the circuit when hazardous current flows.Receptacle tester maybe inserted in the power receptacle to check for defects in wiring Such as polarity reversals, shorts or opens
Light emitting diodes (LED) is series with a directional diode and resistor R are used to give CODED indication of a receptacle fault.If receptacle NORMAL, L1 and L3 will be ON, blinking 60 times per second when H is positive and ALL LEDs OFF when H is negative.
When H to G short,ALL LEDs OFF
When H and N reversed,L1 OFF L2 & L3 ON
SummaryThere are few device that can help to prevent hazardous current or acting as alarming system such as:Ground Fault Interrupter (GFI)Line Isolation Monitor (LIM)Safety AnalyzerReceptacle Tester