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Edition 41
May 2010
 

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

 

 

 

 

 


Panel Firmware downloads available on the Kentec Web site

To allow 24-hour access to the current panel firmware files, the “Software” section of the Kentec web site has been updated to provide download links for all UK panel firmware files.

Kentec web site software downloads :

 

 

 
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