Cable segregation in fire detection
systems
Most modern fire alarm control
and indicating equipment products utilise microprocessor
technology to provide simple yet flexible configuration
options. Analogue addressable control panels also
employ low current digital communications between
the control panel and the analogue addressable
detection devices. Digital data signalling is
also used in networked systems between panels
and for expansion card communications.
The use of such technologies means that the effects
of electromagnetic interference can have a detrimental
affect on the system performance. Whilst the Kentec
control panels are designed, tested and certified
to the relevant EMC requirements and EN54-2 /
EN54-4 and have protection measures to limit the
effect of electromagnetic interference, careful
system installation will also help limit the possibility
of interference affecting the overall system performance.
BS5839-1:2002 clause 26.2 gives recommendations
for the installation of cabling and interconnections
on fire systems. Clause 26.2.l acknowledges that
there is potential for electromagnetic interference
between fire alarm cables and other low voltage
cables and that suitable segregation in accordance
with manufacturer’s recommendations should
be followed.
Some signalling circuits are more vulnerable to
the effects of electromagnetic interference than
others. For example, relatively high current D.C
circuits, such as the monitored sounder circuits
are less likely to be affected compared to the
analogue detection loop or the low current, RS485
transmission paths.
Two simple measures should be employed to minimise
the effect of electromagnetic interference induced
on the fire system cables:
Screened
cables –
Screened cables should be used
on all signalling paths whenever possible. The
effectiveness of the screen depends on the screen
technology used and the frequency of the electromagnetic
interference. Generally, helically wrapped foil
screens with a drain wire will provide a higher
level of protection across the frequency spectrum
than copper braided screens.
In order to ensure optimum protection, the screen
must be maintained throughout the length of the
transmission circuit and must be securely bonded
to earth at one end. When using drain wires in
foil wrapped screened cables, ensure a good electrical
connection is made at each cable joint.
Multiple earth bonding points of the screen along
the cable should be avoided – in particular
on detection loop circuits.
Cable
Segregation –
The level of electromagnetic interference induced
onto the fire alarm system cables depends on many
factors. The most significant is when the fire
alarm cable is running in parallel with other
cables. In this case the degree of interference
will depend upon the voltage of the cable, the
distance between the cables, the length that they
are running in parallel and any screening on each
cable.
In order to limit the effect of electromagnetic
interference induced onto the signalling cables
from low voltage power cables, the following minimum
segregation distances are recommended:
Unscreened fire alarm signal cables – 150mm
Foil screened fire alarm signal cables –
100mm
Mineral Insulated Copper Conductor (MICC) fire
alarm signal cables – 50mm
The fire alarm signalling circuits
must not be installed near to medium or high voltage
cables.
Summary
- Wherever possible, use screened cables and
avoid running these cables in parallel with
power cables.
- Ensure screened cables are correctly bonded
to earth in accordance with BS5839-1:2002 clause
29 requirements
- Limit the effects of electromagnetic interference
by good installation practice in accordance
with BS5839-1:2002 clause 28 requirements
- For analogue detection loop circuits, perform
loop calculations to ensure that overall cable
length, volt drop, inductance and capacitance
does not also affect the signalling between
control panel to detection device signalling
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