Download devices to protect against electrical hazards

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 = I H + I F
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.
 When
H to G short,
ALL LEDs OFF
 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