DEVICES TO PROTECT AGAINST ELECTRICAL HAZARDS Mohd Yusof Baharuddin

Objectives Explain 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 ground This 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 doesn’t 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 other GFI 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 normal IH = 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 amplifier If IF exceeds 2 mA for 2ms, relay opens the line and prevents macroshock GFI can be mounted in power receptacle and is required in wet area

Safety Analyzer

Safety Analyzer Primarily used to test medical equipment for leakage current that can cause microshock to patient Safety analyzer is calibrated by placing S5 in the calibrate position while adjusting the meter for 1 µA

The equipment under test (EUT) is plugged into safety analyzer To 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 Tester The 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.

ALL LEDs OFF L2 & L3 ON When H to G short, When H and N reversed, L1 OFF L2 & L3 ON

Summary There 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 Analyzer Receptacle Tester