Protecting life safety circuits in high rise buildings.

Staszewski, Zygmunt

Sat, 1995-07-01 12:00

Fire damage to emergency circuits in high rise buildings must be prevented. When combined with smoke and heat spread, such damage may result in far greater loss of life and destruction of property.On February 26, 1993, the confidence of numerous people in building integrity was shaken, for many, more so than the effect of a Calif. earthquake. On that day, a garage-made bomb was loaded into a van in

Fire damage to emergency circuits in high rise buildings must be prevented. When combined with smoke and heat spread, such damage may result in far greater loss of life and destruction of property.

On February 26, 1993, the confidence of numerous people in building integrity was shaken, for many, more so than the effect of a Calif. earthquake. On that day, a garage-made bomb was loaded into a van in N.J., driven to New York City, and parked in an underground garage under the World Trade Center complex in Manhattan. It exploded at 12:18p.m., killing six people and injuring more than 1000. Smoke spread throughout the building. The explosion caused structural damage to the garage, started a fire, and directly or indirectly caused various emergency circuits to fail. The event, with subsequent smoke contamination, disrupted operations of hundreds of businesses located in the World Trade Center towers. In addition, it caused the adjacent Vista Hotel to close for 18 months.

The explosion and a subsequent power shutdown interrupted most of the normal and emergency circuits. The event disabled all water-cooled emergency generators, fire alarm and voice communication systems, elevators, fire pumps, and emergency lighting in staircases.

The explosion severed key emergency circuits, including a circuit to water circulating pumps designed for the engine-generator sets. As a result of the explosion, the engine-gen sets ran for a few minutes until they automatically shut down because of overheated.

Most of the complex's 150,000 occupants were forced to immediately evacuate the building. The situation was finally brought under control almost 12 hours after the explosion happened.

Recently, a fire in a major New York City hotel caused damage to its fire alarm riser, disabling the initiating, signaling, and voice communication capabilities of the system. This, in turn, made evacuation more difficult due to lack of one-way paging and alarm-tone transmission functions. This hotel had its life safety systems updated, including the emergency circuits, just two years ago.

Most recently, the bombing of the Oklahoma City Federal Building pointed out some disturbing system failures.

Learning from these disasters

Can we learn anything from these and other similar disasters? At the least, they should force us to reflect on some life-and-death questions. And we should consider how we can avoid alarm system failures in the future.

* Will the life safety systems in a building you're involved with survive the damage caused by a fire, explosion, or flood?

* Will these systems help the fire department during evacuation of the occupants?

* Will these systems assist the fire department personnel during their fire fighting attack?

* Do these systems at least meet, or possibly exceed, minimum local and nationwide code requirements?

* Will these systems defend themselves against the scrutiny of plaintiffs' attorneys?

* Can anything else be done to avoid a risk of life safety systems failure due to an explosion?

There's not much we can do to prevent a bomb explosion. Very little can be done to prevent flooding in a basement electrical room during a flood or hurricane. And, even though structural engineers are advancing the design of buildings to withstand the effect of an earthquake, damage often occurs.

However, when it comes to the harmful effects caused by fire and smoke, you can control the extent of damage by using improved installation procedures, new control techniques, and certain products.

Site survey. Recognizing that various types of disasters can occur anytime, a survey of a building's life safety systems should be performed. The results of this survey should be carefully analyzed in regard to proper design and installation of electrical equipment, including the circuits and the type of equipment used. Then, if required, the life safety systems should be redesigned in order to improve their reliability and survivability.

Some emergency scenarios

When emergency circuits are damaged in a high-rise building, particularly when there is extensive heat and smoke spreading throughout the building, many occupants will find evacuation difficult and there may well be fatalities. Damage to elevator power feeders or control circuits may stop an elevator car between floors, with people trapped inside and the entire shaft filled with smoke and carbon monoxide. Damage to emergency lighting circuits, which were designed to illuminate horizontal paths of egress and staircases, will make evacuation almost impossible.

If fire alarm and voice communication circuitry is damaged, there will be no detection signals coming into the fire command station to allow the firemen to monitor the spread of smoke and heat. In addition, means of announcing evacuation commands by the fire department chief will also be lost.

Damage to fire pump feeders will shut down the water supply for the sprinkler heads and for standpipe fire hoses. The latter may put fire fighters at additional risk because the water streams serve as protection to them.

If the smoke control system feeders or controls are damaged, there will be no means of mechanical removal of smoke from the building.

Protecting life safely electrical systems

The primary areas of concern are the main and reserve power feeders, batteries, and engine-generator sets and transfer switches. These systems are designed to supply power to various life safety networks, including fire pumps for sprinkler and standpipe systems, emergency lighting, fire detection, alarm and voice communication, smoke control systems, designated elevators, etc.

With the exception of hospitals, where NEC Sec. 517-30(c)(3) requires metal race-ways to enclose the emergency circuits, the Code does not mandate specific wiring methods for the emergency systems. A building can be wired using whatever wiring method that is used generally within that structure. Some occupancies, however, do have wiring method restrictions, but they are generally based on the type of occupancy. For example, wiring in places of assembly usually must comply with NEC Sec. 518-4, which limits the wiring methods to metal raceways, nonmetallic race-ways encased in at least 2 in. of concrete, mineral insulated metal-sheathed (Type MI) cables, or metal-clad (Type MC) cables.

Even hospitals now allow rigid nonmetallic conduit in 2 in. of concrete, or non-metallic conduit of a certain wall thickness (Schedule 80) without encasement. Many design engineers, some local codes, and some related standards do impose additional restrictions on the allowable wiring methods for these circuits, and these must be followed as applicable.

NFPA 20-1993, Installation Of Centrifusal Fire Pumps, in Para. 6-3.1.1, states that:

electrical feeders for fire pumps, where the feeders run outside the switchgear room, shall be either routed outside the building, or embedded in 2 in. of concrete, or protected by an electrical circuit protective assembly with a minimum 1-hr fire rating.

Type MI cables, which are listed by UL for 2 hrs of fire exposure, are frequently used for fire pump feeders.

Electrical circuit protective systems are specifically tested for the survivability of the circuits within. They are derailed in the UL Building Materials Directory under the above heading. These protective systems are not the spray-on compounds often used to protect steel beams. The latter compounds are designed to prevent the beam from heating to the point of softening, which is far beyond the point where the circuits could survive. Many engineers have erroneously specified these compounds, particularly after being cited for failure to maintain a required fire separation. Because of this, one state, Mass., in its electrical code, now includes the following fine print note (FPN):

"Many techniques intended to prevent the deflection of steel members at high temperatures will not materially increase the survival time of circuits in electric race-ways."

In regard to emergency circuits, the Las Vegas electrical code requires a 1-hr fire rating. The Massachusetts electrical code requires a 2-hr fire resistive rating. The 1990 edition of the National Building Code has a requirement for a 2-hr fire performance. The New York City building code currently requires no fire protection of fire pump feeders. A code change request has been submitted to the Department of Buildings; it will be reviewed by a new committee to be established in the near future. In addition, the NEC is poised to include (in the 1996 edition not yet fully adopted) a 1-hr fire protective requirement for fire pump feeders and for emergency generation feeders in occupancies with a higher than usual hazard, including places of assembly over 1000 persons and in most high-rise buildings over 75 ft.

Regardless of whether some applicable codes or standards require enhanced protection for emergency circuits, it's a good engineering practice to provide fire protection of emergency feeders for all new and major retrofit construction. This can be accomplished by concrete encasement or by use of a UL-listed type of electrical circuit protective system. These include metal-sheathed MI cable with special support and termination rules that go beyond the requirements in Article 330 of the NEC. You can also use other systems, typically involving large rigid conduit with special wrapping, all such systems being detailed in the UL Directory.

To avoid the possibility of flooding, emergency engine-generator sets and transfer switches should be located above ground level. Provisions should be made to use emergency power for engine auxiliary systems, such as lube pumps, water circulating pumps, control systems, etc. Emergency feeders that go to critical loads should be run remotely from the main feeders.

Other types of life safety systems have specific criteria regarding fire protection of electrical feeders. NFPA 72-1993 National Fire Alarm Code, in Para. 3-12.3.2, requires that:

"Where the fire command station control equipment is remote from the central control equipment, the wiring between the two shall be installed in conduit or other metal raceway that is routed through areas whose characteristics are at least equal to the limited combustible characteristics as defined in NFPA 90A.... The maximum run of conduit or raceway shall not exceed 100 ft (30 m) or shall be enclosed in a 2-hour fire rated enclosure."

The last sentence is one of the most overlooked fire alarm installation requirements. The days of hard-wired fire alarm and voice communication systems, where the "central intelligence" was located in the fire command station panel, are over. Most new multiplex and addressable systems feature distributed intelligence and a fire command station performs mere functions of a status display panel and of a manual interface allowing activation of sounding devices, warden stations, etc. Software codes and electronic modules containing such codes, which actually control the system functions, are typically distributed throughout the building and are physically located in data gathering panels, transponder boxes, or equivalent equipment clusters. These modules are interfaced together via power and data riser cables.

If the control equipment of the fire command station is physically distributed throughout the building, how do you interpret the above NFPA 72 rule? Should all wiring linking such equipment be enclosed in 2-hr fire rated enclosures? If so, fire alarm power risers of most multiplex and addressable systems should be run in conduit embedded in 2-in. thick concrete. Or, these power risers should consist of one of the other electrical circuit protective systems, such as Type MI cable.

It should be recognized that fire alarm control equipment is not fire-rated because of ventilation requirements for electronic components and the relatively poor performance of semi conductors in elevated temperatures. Note that control equipment may be located only every 3 to 10 floors of a building; therefore, protecting the links between the panels may be more important than protecting the panels themselves.

Protecting signal and communication wiring

While metallic rigid threaded conduit, electrical metallic tubing (EMT), and intermediate metal conduit (IMC) provide mechanical protection, they do not increase the fire resistance of wiring installed within them. Results of Steiner Tunnel testing performed by various cable manufacturers actually indicates that conduits tend to act as heat sinks, thereby decreasing the time required to damage insulation and to cause cable failures.

Fire pumps. When a fire pump uses an emergency generator as a secondary power source, loss of normal utility power at the pump controller must start the generator and activate the transfer switch. Control circuits that are provided to perform such functions require the same degree of fire protection as feeders. Type MI cables are therefore frequently used for control circuits of fire pumps.

Fire alarm, voice communication systems. In fire alarm and voice communication systems, data riser cables that link various intelligent components to the fire command station should also be enclosed in 2-hr fire-rated enclosures to meet the intent of the NFPA 72, Para. 3-12.3.2. Then, not only the power risers but also the signal data risers of most multiplex and addressable systems would be run in conduit embedded in 2-in. thick concrete, or run in one of the electrical circuit protective systems such as type MI 2-hr rated cables. Such installations are to meet the requirements in the UL Directory.

The NEC, in Sec. 760-51, provides various requirements for wiring of fire protective signaling systems. Most of these requirements are related to conductor materials, number of strands, voltage ratings, and resistance to spread of fire. General purpose Type FPL (power-limited fire alarm cable), with certain exceptions as called for by the NEC, must be constructed so as to resist the spread of fire. According to the NEC, Sec. 725-51 (f),(FPN):

"One method of defining resistance to spread of fire is that the cables do not spread fire to the top of the tray in the 'Vertical Tray Flame Test' in Reference Standard for Electrical Wires, Cables and Flexible Cords, ANSI/UL 1581-1985. Another method of defining resistance to the spread of fire is the damage (char length) not to exceed 4 ft, 11 in. when performing the Canadian Standards Association (CSA) 'Vertical Flame Test - Cables in Cable Trays,' as described in Test Methods for Electrical Wires and Cables, CSA C22.2, No. 0.3-M-1985."

The NEC, in Sec. 760-51, lists several types of cables, their allowable use, and their restrictions. It's worth reading carefully. For example, Type FPLR (power-limited fire alarm riser cable) must have fire resistant characteristics capable of preventing the spread of fire from floor to floor. One method of defining such fire-resistant characteristics as noted in NEC Sec. 760-51(e)(FPN)is:

"that the cables pass the requirements of the Standard Test for Flame Propagation Height of Electrical and Optical-Fiber Cable Installed Vertically in Shafts, ANSI/UL 1666-1986."

The most restrictive requirements apply to Type FPLP (power-limited fire alarm plenum) cable allowed for use in plenums and ducts. This type cable must have adequate fire-resistant qualities and low smoke-producing characteristics. When tested, these cables cannot exceed an acceptable value of produced smoke in accordance with NFPA/ANSI262-1990. The fire-resistant feature is verified by establishing a maximum allowable flame travel distance of 5 ft when demonstrated in accordance with the same NFPA/ANSI procedure.

Unfortunately, the UL listings of such cables do not automatically guarantee a reasonable level of operating temperatures and mechanical properties. In fact, the reason for these UL listings has absolutely nothing to do with the survivability of these cables. These listings only exist to assure the user that these cables have passed certain tests showing the cables will not exacerbate fuel loading and smoke generation in the presence of fire. We are discussing how emergency circuits must survive fire and other disasters to help assure the safety of personal and property. Because UL listings do not necessarily meet this end, cables that are UL listed may not meet the survivability requirements being looked for.

For example, some FPLP-rated cable jackets are rated for only 75 [degrees] C, substantially below the temperature of boiling water (100 [degrees] C). Such cables may get damaged by hot water or hot smoke forced into HVAC ducts or plenums by a smoke control system. Some of the FPLP jackets are so soft, they may be stripped without tools. Sharp edges of ductwork or conduit frequently strip the jacket, damaging the wiring insulation, and causing failures. Unfortunately, UL does not have an established testing method of mechanical properties of fire alarm cables.

New York City Local Law No. 5 of 1973 mandates that all fire alarm signal wiring to be used in high rise commercial buildings, regardless whether in general purpose, riser, or plenum applications, be listed for plenum applications and have 15 mil (.015 in.) of primary insulation and a 25-mil jacket. (Both insulation and jacket must be made of "Teflon or equivalent" materials.) Such wiring is allowed to be run exposed when installed higher than 8 ft above finished floor (AFF). Otherwise, such wiring must be protected from mechanical damage by using conduit, tubing, or building structure when installed below 8 ft AFF. (When above 8 ft AFF, the wire can still be exposed; therefore it will still be subject to mechanical damage. However, the wire in such cases is usually hung above the ceiling or it is in the plenum cavity.)

After several fires in New York City adopted a new law that required the installation of fire alarm and voice communication systems in all new and existing high-rise hotels and new and existing low-rise hotels over 15 rooms. Because of the success of using plenum-rated cables in high-rise office buildings, this application led to expanding their use for fire alarm and voice communication systems in hotels and high-rise department stores.

Up to the mid-1980s, the only insulating materials considered equivalent to Teflon were fluoropolymers such as Kynar, Halar, and Solef. All these materials were difficult to strip but their physical properties reduced the probability of mechanical damage during the installation process. Unfortunately, new soft-jacketed FPLP plenum cables were introduced, causing frequent cases of wire damage. Recently, the New York City Department of Buildings as well as the New York City Fire Department established even stricter criteria for fire alarm cables that include 150 [degrees] C minimum rating. It should be noted that only certain hard-jacketed fluoropolymers, such as Teflon, Kynar Flex 2850, Halar, and Solef, are presently rated for 150 [degrees] C and these products may obtain special New York City certification by the UL through a new program established for the city.

Redundant wiring configurations

Sec. 700-9 of the NEC requires that wiring for emergency systems be kept entirely independent of all other wiring and equipment and not installed in the same raceway, cable, box, or cabinet with other wiring. Therefore, if the regular wiring system is damaged, the emergency wiring will not be effected. The 1993 revision of the NEC introduced the following paragraph:

Emergency wiring circuit(s) shall be designed and located to minimize the hazards that might cause failure due to flooding, fire, icing, vandalism, and other adverse conditions.

The same requirement applies to various sources of power covered in Sect. 700-12, which includes storage batteries, generator sets, uninterruptible power supplies, separate services, etc.

In smoke control applications, two or more purge fans connected to separate power sources, rather than one big fan, are recommended for much needed redundancy. The same guidelines apply to stair pressurization systems and engineered smoke control systems.

While all elevators serving fire floor(s) must be automatically recalled down to the designated evacuation level (so-called "Phase I" function), certain elevators also must be equipped with a "Phase II" feature that allows the fire department to use them for transportation of gear and evacuation of disabled people. If there are several remotely located banks of elevators, the required number of Phase II cars must be evenly distributed between all the elevator banks. These dedicated elevators must be fed by separate emergency power circuits (typically local utility power and an emergency generator with an automatic transfer switch). Since there is usually only one feeder from the transfer switch to the elevator machinery room, this feeder should be protected with a 2-hr rated enclosure or a mineral-insulated cable should be used. Both methods tend to accomplish another important goal: protection of elevator power feeders from water damage due to operation of a sprinkler system or fire hoses.

Emergency lighting fixtures and illuminated exit signs should be designed to be operational at all times for safe evacuation of building occupants and for safe access of fire fighters. Such fixtures should be served by both normal and emergency power sources. Battery packs with trickle chargers may be used as emergency sources of power if they provide at least 90 min. of illumination, with a maximum allowed output reduction of 40%. Good engineering practice requires that emergency lighting fixtures, especially those in staircases, be powered by normal and emergency power sources and be equipped with battery packs. This way, in the event of a feeder failure or if the main building circuits are shutoff by the fire department, there will be 90 min. of staircase illumination.

The next areas of concern are various types of fire alarm and voice communication systems. The provisions of NFPA 72-1993, such as Sec. 3-2.4(a), state certain performance criteria that require that the system be so designed and installed that attack by fire in an evacuation zone, causing loss of communications to this evacuation zone, shall not result in loss of communications to any other evacuation zone. Configuration details are usually left to the consulting engineers.

The New York City Building Code requires two separate speaker circuits per floor for high-rise buildings and redundant (hot backup) amplifiers to provide limited audibility if one of the circuits is damaged. Unless a microprocessor-based fire command station has an identical backup unit, or at least a backup central processing unit (CPU), a special Class "A" ("looped wiring") configuration is required for data and power wiring. This means the circuits must be installed in separate and remote vertical shafts or conduits to maintain a "degraded mode" of system operation. In such a mode, the alarm tone sounds throughout the fire floor, all of the locks and door holders connected to the fire alarm system release automatically, all fans serving the fire floor automatically shut down, elevators are recalled to a designated floor, etc.

Keep in mind that building and fire codes identify minimum requirements for the configuration and performance of life safety systems. Even the best codes cannot address the individual needs of each building. Electrical circuits for life safety system, when carefully designed, will certainly reduce chances of complete system failures caused by explosions, fire, or floods. Well designed life safety systems, even partially operational during such emergencies, will save lives, reduce property damage, and minimize legal exposure.