ELECTRICAL & ELECTRONICS
POWER IN ELECTRICAL SAFETY
BENDER UK
EFFECTIVE CONTROL AND AUXILIARY CIRCUIT
MONITORING REDUCES DOWNTIME AND INCREASES
PROFITABILITY
Early identification of earth faults in
mission critical AC or DC control and
auxiliary supplies can prevent equipment
shutdown, increase production output and
improve profitability.
Unearthed IT Power Systems
An unearthed IT system as per BS7671
section 3 and IEC60364-4, is tolerant to first
earth fault and will not cut power, unlike
a TN system which would cause a power
failure - therefore an IT system makes it more
reliable and safer for mission critical systems,
protecting both production and people.
The ungrounded system has no direct
connection between live conductors and
earth. The system does not shut down when
the first low R earth fault occurs, which
means the power supply is maintained.
An IT system is used widely in critical
applications like Group 2 Medical locations
and AC/DC auxiliary circuit applications
for more effective protection, control and
tripping supply in substations/power
stations.
IT systems are particularly appropriate for
auxiliary circuits where high reliability is
required - see BS7671 557.3.4.1 Note 2.
However, the full benefit of an IT system can
only be realised by detecting development
of the first earth fault and repairing it
before it develops to a second earth fault
on a different conductor. That requires
an Insulation Monitoring Device (IMD)
which performs best when paired with a
fault location system (iFLS) which enables
maintenance teams to easily find the faulty
circuit.
Insulation Monitoring
Device (IMD) in Unearthed IT
Systems
BS7671 clause 411.6.3.1 states “Where an IT
system is used for reasons of continuity of
supply, an insulation monitoring device shall
be provided...”
Neutral conductor N and ground conductor
PE are connected via a bonding jumper. First
low R earth fault results in disconnection of
supply.
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PECM Issue 33
Effective Control and Auxiliary Circuit Monitoring
Can Reduce Downtime and Increase Profitability
conductive All parts
liable to cause malfunction
The governing standard
of the Insulation
The governing standard of the Insulation
IMDs must be labelled to highlight system of
monitoring
device
(IMD) is BS EN
as per inadvertent
use i.e AC,
DC or AC/DC. of the controlled
operation)
monitoring device (IMD)
is BS EN
61557-8
as 61557-8 (e.g.
IEC 60364-4 and BS7671 557.3.4.3. and 538.1.1
equipment.”
per IEC 60364-4 and BS7671
557.3.4.3. and
BS EN 61577-8 prohibits use of passive devices
538.1.1
in an IT circuits which cannot detect symmetrical
All IMDs must be labelled to highlight
faults. A symmetrical fault is particularly dangerous
to control
circuit
because it can lead to inadvertent
BS EN 61577-8 prohibits
use of
passive
system of use i.e AC, DC or AC/DC.
switching or false trippping without detection. The
below
shows a detect
double high resistance
devices in an IT circuits diagram
which
cannot
Insulation
monitoring technology combined
symmetrical insulation fault on DC PLC circuit.
The
symmetrical faults. A symmetrical
fault
is passive device and
fault is not detectable
by the
with earth fault location systems developed
can
therefore
cause
switch
to
be
seen
as
closed.
particularly dangerous to control circuit
Insulation monitoring technology combined
by circuit
Bender are
in a systems
wide developed
range by
of
with employed
earth fault location
BS7671 clause 557.3.201
states: “A control
because it can lead to inadvertent
switching
Bender
are employed
in a wide range
of critical
shall be designed, arranged and protected critical
to limit control
circuit
applications
within
control circuit applications within industry
dangers
resulting from a fault between the control
or false trippping without
detection.
sect ors that
cannot cannot
afford unscheduled
circuit and other conductive parts liable to
cause
industry
sectors
that simply
simply
afford
The diagram below shows
a double
high operation) of the
shutdowns or interruptions in production and
malfunction
(e.g. inadvertent
unscheduled
shutdowns
process
operations. or interruptions in
controlled equipment.”
resistance symmetrical insulation fault
production and process operations.
on DC PLC circuit.
The fault is not
detectable by the
passive device and
can therefore cause
switch to be seen as
closed.
BS7671 clause
557.3.201 states:
“A control circuit
shall be designed,
arranged and
protected to limit
dangers resulting
from a fault
between the control
circuit and other
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