Cable types, Fire Tests of Cables, and Installation Practice
Fire resistance or enhanced cables receiving increase attention. Cables are the communication pathways between components of the fire alarm system and comprise the following class:
~~Low voltage cable system, typically power supply cables to control panels (110V, 1-phase or 240V, 1-phase or 415V, 3-phase)
~~Extra low voltage (ELV) cable system, typically for data, signalling or device power-line cables at less than 50Va.c. or 110Vd.c.
With the notion of linking issues of fire safety and circuit integrity or survivability, fire rating of cables are now receiving increasing attention from both codes. Both codes though not providing detail explanation of fire ratings, cross-referenced other codes which have relevance to fire rating of cables. Information on fire rating of cables and the specification of fire rated cables bear some difference in opinion between the European standards (which is also the IEC standards) and the North American ANSI/UL standards. This section will include an explanation of fire rating of cables which are not found within BS5839-1 and NFPA72 themselves.
BS5839-1; prescription for cables
The 2002 edition of BS5839-1 contains major upgrade to the 1988 edition by recommending fire resistant cables. Fire resistant cables are now extended to include two types (1) Enhanced and (2) Standard. The following recommendations are included as follows:
~~Clause 26 now specify that all cables must comply with existing requirements of BS6387, EN50200 PH30 (standard) or EN50200 PH120 (enhanced).
~~All system cables including LV mains supply to the panel to be fire resistant.
~~Standard fire resistant cables should be considered sufficient to meet the effects of fire with suitable jointing and support.
~~Enhanced cables are recommended in:
(1) Non-sprinkler buildings with more than four phases of evacuation.
(2) Non-sprinkler buildings of greater than 30 metres in height.
(3) Where the critical interlinking paths might be affected in unsprinklered linked buildings with occupancy requiring supervised evacuation or some difficulty in evacuation e.g. hospital.
Notes: In case (3) above, standard cables may be used if network loops provide the 'interlink' and such loops have start and return routed separately. In such case, the network loop is said to declassify the interlink as 'critical' .
(4) Where following risk assessment enhanced cables are deemed necessary.
~~Cable support system shall match the fire rating and performance of the cables. In practice this may require examination of plastic ties, trunking or clips which may act as critical components of the support system and which may not be suitably fire rated.
~~No external joints shall be used. Where junction boxes are not avoided, they shall be labelled "Fire Alarm" and match fire resistance rating of the cables.
~~Standard cables installed below 2m height require mechanical protection unless surface clipped to strong construction in relatively benign environments e.g. offices shops etc.
~~All conductors should have minimum cross sectional area of at least 1mm² and if stranded a minimum cross sectional area of 0.5mm².
~~Segregation of wirings:
(1) fire alarms should be segregated from other services in separate conduit or trunking.
(2) Where multicore cables are used none of the other cores should be used for other purposes.
(3) Mains cables should be segregated from system cables outside and inside the panel. They should not enter the panel at the same point.
~~Fire cables should be a single common colour throughout a building to aid identification, e.g. red. Figure 3 – Fire Rated Cables to BS5839-1 2002 .
Fire rating of cables in NFPA72 is cross referenced to NFPA70 (NEC)
In NFPA72 only two paragraphs described wiring requirements with a cross-reference to NFPA70 (National Electrical Code). This cross referencing however opens a wide topic related to cable types and fire performance rating based on North American (NFPA, UL and ANSI) standards. The following tabulation list Articles in NFPA70 (NEC) which are relevant.
Table 1 – Cable types and Fire Tests (North American)
Article 760 – Fire Protection Signalling System Applications Fire Test FPL Power limited fire alarm cable for general purpose fire alarm use. UL 1581 Vertical Tray Flame Test FPLR Power limited fire alarm riser cable for use in vertical riser shafts UL 1666 Riser Flame Test FPLRP Power limited fire alarm plenum cable for use in ducts and air plenums UL 910 Steiner Tunnel Test Article 770 – Optical Fiber Cables and Raceways OFNP OFCP Non conductive and conductive optical fiber plenum cables suitable for use in ducts, plenums and other environmental air spaces. OFNR OFCR Non conductive and conductive optical fiber riser cables plenum suitable for use in vertical run in shaft or from floor to floor OFNG OFCG Non conductive and conductive optical fiber cables suitable for general purpose use except in vertical risers and plenums. OFN OFC Non conductive and conductive optical fiber cables suitable for general purpose use except in vertical risers, plenums and spaces used in environmental air. Article 800 – Communications Cables and Raceways CMP Communication plenum cable listed as being suitable for use in ducts, plenums and other spaces for environmental air CMR Communication riser cable listed as being suitable for use in vertical run in shaft or from floor to floor CMG General purpose communication riser cable listed as being suitable for general purpose use except in vertical risers and plenums. CM Communication cable listed as being suitable for use suitable for general purpose use except in vertical risers and plenums. CMX Limited use communication cable suitable for use in dwellings and raceways CMUC Undercarpet communication cables suitable for undercarpet use
Note: CSA C22.2 No. 0.3M (Canadian Standard Association) defines resistance to the spread of fire is for the damage (char length) not to exceed 1.5 m (4 ft 11 in.) when performing the vertical flame test for cables in cable trays.
Details pertaining to segregation of cables similar to BS4839-1 (section 5.2) and measures for the mechanical protection of cables are included in NFPA70. Articles 760, 770 and 800 however contain more details pertaining to such installation measures compared to BS5839-1.
Fire Tests to European / British Standards
An understanding of fire tests on cables is essential before designers and installers can select the correct type of cables in compliance with the code. As British Codes and Standards are harmonising towards European Codes (EN), a description of the fire test for EN and BS can be taken to be similar. Table 2 describes the test listed under the hierarchy cables specified under BS5839-1.
Table 2 – List of Fire Tests under British Standards
BS6387:1994 CWZ
Fire Resistance, with and without water and mechanical shock; Specification for performance requirement for cables required to maintain circuit integrity under fire conditions:
Cat. C Exposed to Fire @ 950ºC. for 3 hours
Cat. W
(1) Expose to fire @ 650ºC for 15 mins., then
(2) Expose to fire @ 650ºC with water for 15mins.
Cat. Z
(1) Expose to fire @ 650ºC for 15mins., then
(2) expose to fire @ 650ºC. with mechanical shock for 15mins.
EN50200 BS5839-1:2002 Fire Performance Cable
Standard Grade BS5839-1:2002 - PH 30
PH30 (30 mins)
(1) Exposed to fire @ 830ºC. for 15mins., then
(2) exposed to fire @ 830ºC. with water & mechanical Shock for 15mins.
The temperature may vary +40 / - 0 deg. C
(Test No 2. is not detailed within EN50200 PH30 but is covered in BS5839-1:2002, Clause 26.2-D)
Enhanced Grade BS5839-1:2002 - PH 30
PH120 (2 hours)
(1) Exposed to fire @ 950ºC. for 60mins., then
(2) exposed to fire @ 950ºC. with water & mechanical Shock for 60mins.
The temperature may vary +40 / - 0 deg. C (Test No 2. is not detailed within EN50200PH120 but is covered in BS5839-1:2002, Clause 26.2-E)
BS7629-1 BS7629-1 E1 BS7629-1 E2
Specification for 300/500V Fire Resistant Electric Cables.; Having low emissions of smoke and corrosive gasses when effected by fire (Multi Core Cables)
BS4066-1-15.5, Cat. S
Fire Performance; Test on Electric Cables Under fire Conditions
BS7622 Cat.S Replaced By BSEN50268-2:2000 BSEN50268-1:2000
Smoke Emissions; Common test methods for cables under fire conditions, Measurement of smoke density of electric cables burning under defined conditions.
BS 6387 Tests by fire, water and mechanical shock. This test is used to determine capability of cables to maintain circuit integrity under fire conditions. Additional conditions of water and mechanical shock are applied for grading of capability of cables. Code used to designate capability of the cables are as follows:
Resistance to fire | Symbol |
650ºC for 3 hours | A |
750ºC for 3 hours | B |
950ºC for 3 hours | C |
950ºC for 20 minutes | D |
Resistance to fire & water | Symbol |
650ºC for 15mins. then for 15min with fire and water | W |
Resistance to fire with mech. shock | Symbol |
650ºC for 15mins. with 30 seconds hammer blow | X |
750ºC for 15mins. with 30 seconds hammer blow | Y |
850ºC for 15mins. with 30 seconds hammer blow | Z |
Figure 4 – Fire Resistant Test; Figure 5 – Resistance to Fire and Water; Figure 6 – Resistance to Fire and Hammer Blows;
IEC 60331 Fire Tests This test is used to determine whether a cables can maintain circuit integrity during and after exposure to fire.
A sample of cable is exposed to fire for 3 hours at a temperature of between 750ºC and 800ºC while energised. After 3 hours the fire is extinguish and the circuit turned off. A duration of 12 hours is allowed before re-energising the cable and checking for circuit integrity.
IEC 60332-3 – Flame Propagation Tests
This test defines the ability of bunch cables to restrict flame propagation when laid in trunking, cable trays or conduit. The tests comprises 3 categories each determined by the amount of combustible material in a 1 m sample.
Category | A | B | C |
Litres of combustible material in a 1 metre sample | 7 | 3.5 | 1.5 |
Exposure to fire in minutes | 40 | 40 | 20 |
The cable sample are placed vertically next to one another on a vertical tray where they are exposed to fire from a ribbon gas for the duration of exposure. After burning, the samples are wiped clean to examine for char on the surface of the cable. Charring should not reach a height exceeding 2.5m above the bottom edge of the burner. Figure 7 IEC 60332-3 Flame Propagation Test
IEC 61304 Smoke Density Test
This test measures the smoke emission from cables during a controlled fire. The test sample is burn in a chamber measuring 3m cubed The amount of smoke emission is measured by a light beam-photocell which measures the opacity of the smoke.
Figure 8 – Smoke Density Measurements
North American Standards on Fire Tests
The main fire tests recognised by the North Americas are the following:
- UL VW1 – Single Cable Burner test
- UL 1581 Vertical Tray Flame Test
- UL 1666 Riser Flame Test
- NFPA262, UL910 Steiner Tunnel Test
UP VW1 test on a single cable. This is the lowest grade test for assessing the fire resistant ability of a single cable. It is also similar to IEC 60332-1. It applies a flame (500W) to a single vertical cable sample and assess flame spread capacity (pass or fail criteria).
UL 1581 Vertical Tray Flame Test This test is similar to the IEC60332- part 3 test for group of cables (Figure 9 – UL 1581 Vertical Tray Flame Test (CM rating))
The flame load is a 30kW burner with the vertical samples free standing (compared to IEC60332-3 which is installed against a wall). Optional smoke density measurements may also be made.
UL 1666 Riser Flame Test
This test address the need to assess fire performance for cables grouped in risers. In UL1666, cables are mounted in a vertical tray arrangement within a 19ft high concrete shaft divided into two compartments at the 12 ft level and with 1ft by 2ft opening between compartments (to mimic a cable riser). The ignition source is a gas flame of 155kW which is left to burn for 30 minutes. Cables pass the test if no "flame" appear at the top of the bottom compartment during the test. Char length and smoke obscuration, mass loss or heat release may (or may not) be measured. Results are based on flame height Figure 10 – UL 1666 Riser Flame Test (CMR Rating)
150kW burner tested for 30 mins. for cables grouped on vertical tray in a riser shaft 19' high with bottom compartment 12' high. Criteria for passing test is the absence of flame at the bottom of top compartment during fire test.
NFPA262 or UL910 Steiner Tunnel Test
is most the stringent test for plenum cables. Test samples of cables grouped are loaded into a horizontal tunnel 25ft long by 1 ft wide (Steiner Tunnel). A gas flame of about 88kW is applied for 20 minutes under a 240 ft/min air flow rate. Flame spread distance along the cables (from flame origin) and smoke optical density at the exhaust duct of the tunnel are measured. Cables are certified acceptable when flame spread is less than 5ft from flame origin and optical smoke density do not exceed 0.5 peak and 0.15 average. Figure 11 – UL910 or NFPA262, Steiner Tunnel
Cables certified under 'Steiner Tunnel' test are said to be fire resistant with low smoke characteristics and are suitable for use in plenums (space where services pipes and ducts are routed e.g. space above false ceiling).
Hierarchy of Fire Tests
A hierarchy of fire test as illustrated in Figure 4 above shows the fire performance rating rank by fire test. Figure 12 – Hierarchy of Fire Performance Tests
Note: BRE/FRS refers to the "Building Research Establishment/ Fire Research Station" at Bedford, England who set up full scale or scaled test rigs.
Next --> Figures; of Fire Tests
(1) Abstract and Introduction
(2) Overview of BS5839-1 and NFPA72
(3) Circuit Design and Survivability
(4) Appendix B – Circuits By Class and Style NFPA 72-2002
(5) Power Supply, Emergency Supply, Fail Safe Supply
(6) Cable Types, Fire Tests of Cables and Installation Practice
(7) Fire Test -Figure2 3 t0 12
(8) Conclusion & Trends
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