SAFE CABLING SYSTEMS IN TUNNELS UNDER FIRE

Transcription

1 SAFE CABLING SYSTEMS IN TUNNELS UNDER FIRE Urs Welte Amstein + Walthert AG, Switzerland ABSTRACT The paper aims at a detailed analysis of the current know-how in the construction of tunnel cabling systems and shows the impact of fire and heat and the respective measures to be taken. Therefore the results of the test projects EUREKA Firetun and MEMORIAL are scrutinized and evaluated in view of the today's boundary conditions in typical cross sections. It will clearly be shown how the heat distribution is dependant on the cross section and the ventilation system. Further it will be shown how concrete will protect cables from heat and damage even if the coverage is low and how far open mounted installation channels are endangered. For typical cabling systems it shall be outlined which requirements and specifications regarding fire and heat resistance are reasonable and required. A short discussion of the applicable norms and standards shall provide further information and decision guidance. It shall be clarified that high safety standards re isolation and fire resistance are not generally required. 1. CABLES IN THE TUNNEL Cables are accommodated in road tunnels at different places: in lower floor panel troughing systems, containing tubing blocks and pits, on routes at the tunnel cover (lighting, communication, fire protection), on routes behind the wall lining, as well as in pits, ascents, air ducts, traversing and in accessible cable tunnels. cable channel lamp fire sensor cable radio cable ceiling tube fresh air exhaust air ventilator wall ascent signal wall ascent fire emergency lighting tubing block tubing block cable duct transfer tubing block LED-lighting cable duct Fig. 1: Cabling systems in typical cross section The different fire hazard of the cable systems can be judged due to the heat distribution in the case of fire.

2 FIRE IN THE TUNNEL A fire event in a tunnel is often combined with a rapid temperature rise, connected with a fast propagation of smoke gases. The fire scenario depends on most diverse factors, which are substantial: fire load: Passenger vehicles, bus, truck with/without charge ventilation system of the tunnel air flow conditions at the beginning of the fire The fire load can reach very high values, in particular if several vehicles are involved. It starts for individual vehicles at 5 MW (passenger vehicle) over 20 MW (bus) up to 100MW (truck). Additionally the air flow (partially affected by the mechanical ventilation) has an impact on the propagation of the hot combustible gases in the tunnel. 3. STANDARD FIRE CURVES In order to have a uniform basis for technical calculations as well as for practical tests, the temperature gradient is defined by so-called standard fire curves. Temperature [ C] Fire duration [min] Fig. 2: Standard fire curves The most common standard fire curve is the ISO standard 834. It forms the basis for the examination of buildings and construction units. The examination of cable systems (cables inclusive fittings and laying systems) is described in the DIN and led to the wellknown inspection certificates like e.g. "E30" (function during 30 minutes). However, the ISO standard was developed for above ground structures - not for tunnels -, and has therefore only limited relevance for this special field. Therefore, also the curve of the Netherlands authorities (Rijkswaterstaat) is used, the so-called RWS-curve. The temperatures reach in accordance with this fire curve over 1300 degrees after approximately 50 minutes. However, this curve is internationally not recognized.

3 FIRE TESTS Some time ago two series of fire tests in tunnels were executed. The results thereof are often consulted for the evaluation of fire risks. Truck Bus Car Temperature [ C] Distance to fire [m] Fig. 3: Maximum temperatures at the ceiling (EUREKA-Project 499 Firetun) The tests showed very high roof temperatures over the fire source, which were reached after a short time of only 20 minutes. However, the tunnels had a cross-section area of 30 m2 only. This corresponds with about the half of a modern road tunnel with two driving lanes. 5. MEMORIAL TEST USA 1993 The American Memorial tunnel has a 60 m2 cross section and is similar to the Swiss standard; thanks to ventilation systems also the simulation of air flows were possible.

4 Fig. 4: Memorial Tunnel Fire Ventilation Test Program: 10 minutes after fire start, 20 MW-fire Fig. 5: Memorial Tunnel Fire Ventilation Test Program: 10 minutes after fire start, 50 MW-fire The comparison with EUREKA shows that with attention of the flow effect and the cross section many lower temperatures are measured. Further, the higher temperatures were measured with relatively high fire loads only. 6. WHICH TEMPERATURES HAVE TO BE EXPECTED? A critical comparison of the fire curves and the test series leads to the clear result that the temperatures reach generally neither the values of the standardized fire curves, nor those of the EUREKA tests. Very close to the Swiss conditions are the MEMORIAL tests. The Memorial tunnel tests showed clearly that with an efficient smoke exhaust close to the fire source the temperature and smoke propagation can be reduced considerably (in accordance with the new Swiss guidelines for the project engineering of road tunnels the ventilation system must offer the possibility of a local smoke exhaust). The following illustration shows the temperatures which can be expected.

5 no temperature change 50 C small temperature change C high temperature change 150 C Fig. 6: Temperature zones during fire 7. CABLE UNDER FIRE: STANDARDS For cable systems the behaviour in case of fire (passive characteristics) as well as the fire resistance (active characteristics) are important to consider. In order to define the appropriate safety level, the standardization situation has to be regarded more closely. Properties International Europe flame retardant IEC EN halogen free IEC EN no corrosive gases IEC EN low smoke emission IEC EN low flame propagation IEC EN Fig. 7: Applicable standards for behaviour in case of fire For the active characteristics of cables the following standards are relevant. They define the requirements during fire. Properties International Switzerland Circuit integrity - Fire only - Voltage (<1kV) - Data cable - Fibre optic cable IEC Fig. 8: Applicable standards for circuit integrity The system circuit integrity of cable systems is mostly the most important issue. Thus the system function can be guaranteed during fires for a certain period. In the extensive standardization work DIN 4102 the system circuit integrity is treated for electrical cable systems in the part 12. The standard comprises the entire cabling system, i.e. cable inclusive carrying system. However, it is applicable only to low-voltage cables (< 1 kv), thus not to high-voltage cables and also not to fibre optic cables. System circuit integrity classes are defined (in minutes): E30, E60, E90. Furthermore, the requirements can be divided in three groups:

6 Requirements Group Requirements IEC-Standard Halogen free and flame retardant cables (FE 0) Circuit integrity ( FE 5) Circuit integrity (E30 - E90) Function Circuit integrity DIN X Isolation Circuit integrity IEC X Material flame retardant IEC X X X halogen free IEC X X X no corrosive gases IEC X X X low smoke emission IEC X X X low flame propagation IEC X X Fig. 9: Requirements groups 8. RECOMMENDED CABLE SAFETY CLASS Safety cable systems cause extra costs; it is therefore worthwhile to regard conditions: Basic Requirements Higher Requirements Additional cost cables: 1.5 mm 2 16 mm 2 FE 0 FE 180 (E 30) % 30-50% FE 5 FE 180 (E 30) 20-40% 10-20% FE 180 E 30 0% 0% Installation material no fire protection E % Cable trough Material: without fire protection E % Installation: without fire protection E % Fig. 10: Additional cost for circuit integrity cables In which cases are these extra costs justified? Generally, halogen-free and flame-retardant cables are mandatory for a tunnel. Also safety cables, which offer a protection against the consequences and the propagation of a fire. Safety cables with system circuit integrity are recommended if temperature and availability risk are relevant, e.g. installation cables in the tunnel cross section: Lighting, ventilation, signing. Cable systems in tubing blocks or in pipes with sufficient concrete cover (> 10 cm) usually require no system circuit integrity, since they are sufficiently protected by their specific situation. Safety relevant cables have to be considered especially, e.g. cables for uninterrupted power supplies (UPS): They should at least have the same system circuit integrity as the overall system they belong to. 9. CONCLUDING REMARK Fire accidents in road tunnels result often in high temperatures due to the limited cross section. The temperatures indicated in the standards always describe the values maximally arising during a fire event; however, these values have to be expected only within the upper tunnel area. At the lower level the temperatures are significantly lower. Due to safety considerations, the general demand for system circuit integrity cannot be justified for cable systems in accordance with DIN , since the risk reduction is only marginal in many cases. However, each project must separately be regarded due to the object-specific conditions.