DEVICES TO PROTECT DEVICES TO PROTECT AGAINST ELECTRICAL AGAINST ELECTRICAL

HAZARDSHAZARDS Mohd Yusof BaharuddinMohd Yusof Baharuddin

ObjectivesObjectives Explain Explain several devices available to several devices available to

protect patients and healthcare protect patients and healthcare workers from hazardous electrical workers from hazardous electrical current. current.

DiscussDiscuss devices to protect against devices to protect against high voltage microshock and high voltage microshock and macroshock hazardsmacroshock hazards

Line Isolation Monitor (LIM)Line Isolation Monitor (LIM)

Line Isolation Monitor (LIM)Line Isolation Monitor (LIM)

LIM place a relatively large LIM place a relatively large impedance from either secondary impedance from either secondary lead through an ammeter to ground lead through an ammeter to ground of the isolation transformer.of the isolation transformer.

If the conductive part through the If the conductive part through the equipment, the meter in LIM will read equipment, the meter in LIM will read a currenta current

The meter on LIM is calibrated to The meter on LIM is calibrated to read what current would flow read what current would flow through a short circuit fault if it through a short circuit fault if it should occur from either secondary should occur from either secondary to groundto ground

This value vary on the leakage of the This value vary on the leakage of the equipment or any fault exist between equipment or any fault exist between secondary leads and groundsecondary leads and ground

An alarm in LIM is usually set off when An alarm in LIM is usually set off when it calculated short circuit fault it calculated short circuit fault between secondary lead and ground between secondary lead and ground would draw 2 to 5 mA.would draw 2 to 5 mA.

This alarm indicates the backup This alarm indicates the backup system has failed and the equipment system has failed and the equipment is no longer isolated.is no longer isolated.

However, this doesn’t mean that However, this doesn’t mean that dangerous current are already flowingdangerous current are already flowing

Ground Fault Interrupter (GFI)Ground Fault Interrupter (GFI)

GFI protects against a shock that GFI protects against a shock that occurs if person occurs if person touches Htouches H with with one hand one hand and Gand G with the other with the other

GFI GFI opens power leadopens power lead if if H lead H lead current differs more than 2 current differs more than 2 mA from N lead currentmA from N lead current for for duration longer than 2 ms. duration longer than 2 ms.

GFI consist of a magnetic coil on GFI consist of a magnetic coil on which H and N lead are wound which H and N lead are wound with the same number of turn with the same number of turn but in different directions.but in different directions.

When normalWhen normal

IIHH = I = INN

and cancels magnet flux in the and cancels magnet flux in the coilcoil

Sensing coil does not have Sensing coil does not have voltage induced in itvoltage induced in it

However, when hot lead faults or However, when hot lead faults or is touched by a person, fault is touched by a person, fault current, Icurrent, IFF is shunted to ground is shunted to ground

IINN = I = IH H + I+ IFF

What will happened ???What will happened ???

Under this fault condition, Under this fault condition, corresponding flux are unequal and net corresponding flux are unequal and net flux exists in the coil.flux exists in the coil.

This induce a voltage into sensing This induce a voltage into sensing amplifieramplifier

If IIf IFF exceeds 2 mA for 2ms, relay opens exceeds 2 mA for 2ms, relay opens the line and prevents macroshockthe line and prevents macroshock

GFI can be mounted in power GFI can be mounted in power receptacle and is required in wet areareceptacle and is required in wet area

Safety AnalyzerSafety Analyzer

Safety AnalyzerSafety Analyzer

Primarily used to test medical Primarily used to test medical equipment for leakage current equipment for leakage current that can cause microshock to that can cause microshock to patientpatient

Safety analyzer is calibrated by Safety analyzer is calibrated by placing S5 in the calibrate placing S5 in the calibrate position while adjusting the position while adjusting the meter for 1 µAmeter for 1 µA

The equipment under test (EUT) The equipment under test (EUT) is plugged into safety analyzeris plugged into safety analyzer

To measure the To measure the chassis to chassis to ground leakage on EUTground leakage on EUT, S4 is , S4 is opened while S1 is in position 1, opened while S1 is in position 1, S2 is in position 2 and S3 is S2 is in position 2 and S3 is opened.opened.

Ground leakage is measured with H Ground leakage is measured with H and N leads reversed by mean of and N leads reversed by mean of switch S1.switch S1.

Switch S2 may then moved to position Switch S2 may then moved to position 1 to measure leakage current in 1 to measure leakage current in patient lead P1. patient lead P1.

Leakage current between patient lead Leakage current between patient lead when the H lead is applied to one of P when the H lead is applied to one of P lead is measured when S3 is closed.lead is measured when S3 is closed.

Receptacle TesterReceptacle Tester

The LIM and GFI are permanently The LIM and GFI are permanently attached to the power line in order to attached to the power line in order to warn of potential hazard or to open the warn of potential hazard or to open the circuit when hazardous current flows.circuit when hazardous current flows.

Receptacle tester maybe inserted in Receptacle tester maybe inserted in the power receptacle to check for the power receptacle to check for defects in wiring defects in wiring – Such as polarity reversals, shorts or opensSuch as polarity reversals, shorts or opens

Light emitting diodes (LED) is series Light emitting diodes (LED) is series with a directional diode and resistor with a directional diode and resistor R are used to give CODED indication R are used to give CODED indication of a receptacle fault.of a receptacle fault.

If receptacle If receptacle NORMALNORMAL, L1 and L3 will , L1 and L3 will be ON, blinking 60 times per second be ON, blinking 60 times per second when H is positive and ALL LEDs OFF when H is positive and ALL LEDs OFF when H is negative. when H is negative.

When H to G short,When H to G short,

ALL LEDs OFFALL LEDs OFF

When H and N reversed,When H and N reversed,

L1 OFFL1 OFF

L2 & L3 ONL2 & L3 ON

SummarySummary

There are few device that can help to There are few device that can help to prevent hazardous current or acting prevent hazardous current or acting as alarming system such as:as alarming system such as:– Ground Fault Interrupter (GFI)Ground Fault Interrupter (GFI)– Line Isolation Monitor (LIM)Line Isolation Monitor (LIM)– Safety AnalyzerSafety Analyzer– Receptacle TesterReceptacle Tester