Circuit Security

"Ceramifiable" insulation offers a convenient, cost effective and capable cable circuit integrity

09/01/2001


Every day, engineering design professionals face the challenge of creating a cost-effective scheme that meets client goals, including long-term performance, and that is, of course, delivered with a reasonably high level of quality assurance. Nowhere does this tax the engineer more than systems that must provide life safety and property protection. But fire-protection engineers have a new UL-certified weapon in this continuous battle: circuit integrity (CI) cable. CI is alarm and system wiring treated with a new technology dubbed by manufacturers as the 'ceramification' process.

As defined by the 1999 edition of the National Electrical Code (NFPA 70), CI is the ' cable used in fire-alarm systems to ensure continued operation of critical circuits during a specified time under fire conditions.'

The problem today is that many existing fire-alarm systems are wired with cable insulated by thermoplastic materials that melt when exposed to heat or fire. Thus, they are unable to maintain electrical integrity when exposed to such conditions over a long period of time.

Critical conduit

NFPA has long recognized the need for maintaining the integrity of fire-alarm wiring and with the issuance of 72-1999, National Fire Alarm Code , NFPA put forth a number of regulations in the hope that the industry might produce a product capable of addressing this problem (see 'A Look at NFPA 72 Revision Highlights, p. 56).

While wiring may not be as obvious of a major component as say a fire-protection system's sprinklers, sensors and control systems, it is an integral part of a proper fire-alarm system. In fact, if the connection between the fire-alarm system initiating devices and the fire-alarm system control unit fails during the first few moments of a fire, the system's ability to detect the presence of a hostile fire will be impaired. Likewise, severing the link between the control unit and fire-alarm notification appliances will leave some portion of the building without adequate warning. In each case, valuable protection is lost. The lives of the occupants are placed in greater danger, and the possibility that the fire will spread to a greater portion of the property exists.

NFPA, in the fire-alarm code revision, acknowledged this by toughening standards. For example, take the instance where a designer desires to use a Class-A initiating-device, signaling-line or notification-appliance circuit, so that a single open or ground fault will not prevent the connected devices from operating. The code requires (parenthetical added): All styles of Class-A circuits using physical conductors (for example, metallic, optical fiber) shall be installed such that the outgoing and return conductors, exiting from and returning to the control unit, respectively, are routed separately. The outgoing and return (redundant) circuit conductors shall not be run in the same cable assembly (that is, multi-conductor cable), enclosure, or raceway (3-4.2.2.2) (see adjacent sidebar for exceptions).

The revised code goes on to spell out a number of other new requirements, including the need to have 2-hour rated cable. But in the past, to achieve that 2-hour fire-protection rating, the specifier typically had to turn to mineral insulated (MI) cable. Type MI cable, usually, is constituted of wiring insulated with magnesium oxide and then sheathed in a metal jacket. MI is impervious to most fire and many water conditions, but is costly. Additionally, as any electrician who has worked with Type MI cable can testify, it is difficult to install and will add significantly to the cost of a fire-alarm system.

Another alternative, outside of redundant wiring techniques or cable installed with rated fire barriers, is mica tape wrapped around cable conductors. Performance, however, varies with the type of mica used, its application and the cable construction itself.

Anatomy of CI

When compared with these other options, CI with ceramifiable insulation appears to be the best value. CI is not totally new either. The technology existed in Europe as far back as the early 1980s, and in the wake of the fire-alarm code revision, a number of manufacturers attempted to bring it to the United States. The European product, however, did not meet the NEC and was not UL-certified. After about five years of testing, at least two manufacturers now produce UL-listed CI cables ready for specification.

The cabling's insulation material-typically silicone-hardens when exposed to high temperatures or fire, producing the fire resistance required to maintain circuit integrity.

According to one CI manufacturer, ensuring circuit integrity with wiring that has undergone the ceramification process requires that the hardened material maintain a uniform space between the conductors and the ground during and after fire and water (sprinkler) conditions.

Rather than employing armor, the insulation and jacket thickness are increased, resulting in a larger cable. As a side note, the heavier insulated cable typically has lower capacitance, higher insulation resistance, greater voltage breakdown and increased installation toughness, according to the manufacturer.

CI-classified fire-alarm cable can provide a cost-effective means of keeping the critical information flowing between fire-alarm system components and the fire-alarm system control unit. This critical information will make certain the fire-alarm system can properly detect the presence of a fire, expeditiously warn the occupants and properly notify emergency responders so they can take action to limit the spread of the fire.

Communications open

Beyond code-specific requirements, fire-alarm systems may also merit circuit integrity when the needs of a client dictate it. For example, when the fire-alarm system that serves a facility oversees the operational integrity of other fire-protection systems, the circuits that connect the supervisory initiating devices to the fire-alarm system control unit may benefit from the added safety of CI cable.

In many high-value or high-hazard facilities, the fire-alarm system serves as the central nervous system of a fully integrated fire-protection network. It is important that this network continues to operate during an emergency so that it can supply those managing the emergency with up-to-date information on the progress of the fire and the operation of the fire-protection systems. Using CI cable to connect the system components to the fire-alarm system control unit helps assure operational integrity of the fire-protection network.

When the fire-alarm system provides notification to an in-plant fire brigade or other emergency response group, the use of CI cable can help make certain the system will continue to provide notification during a serious fire. Likewise, when the fire-alarm system actuates extinguishing systems, the circuits that initiate the discharge of fire-suppression agent can benefit from the added security that CI cable affords.

Safe bet

It seems likely that future changes to the various codes that impact the design and installation of fire-alarm systems will take advantage of the availability of CI cable. Until these changes occur, designers can still specify the use of CI cable for any circuit that will benefit from the added performance.

Ease of installation also must not be overlooked. Compared with current alternatives such as compartment construction or Type MI cable, CI cable provides a cost effective and easy-to-install alternative.



NFPA 72-1999 2-Hour Rated Cable Exceptions

Outgoing and return (redundant) circuit conductors shall be permitted to be run in the same cable assembly, enclosure or raceway under any of the following conditions:

a) For a distance not to exceed 10 ft. where the outgoing and return conductors enter or exit the initiating device, notification appliance or control unit enclosures.

b) Where the vertically run conductors are contained in a 2-hour rated cable assembly or enclosed (installed) in a 2-hour rated enclosure.

c) Where looped conduit/raceway systems are provided, single conduit/raceway drops to individual devices or appliances shall be permitted.

d) Where looped conduit/raceway systems are provided, single conduit/raceway drops to multiple devices or appliances installed within a single room, not exceeding 1,000 sq. ft. in area, shall be permitted.

A Look at NFPA 72 Revision Highlights

The 1999 revision of the fire-alarm code raised the level of standards, as a whole, to improve circuit integrity. Following are noteworthy changes.

In section A-3-4.2.2.2, the goal was to address adequate separation between outgoing and return cables. This separation is now required to help ensure protection of the cables from physical damage. The recommended minimum separation to prevent physical damage is 1 ft., where the cable is installed vertically, and 4 ft., where the cable is installed horizontally.

Another concern NFPA officials had in revising code was to make certain that fire-alarm systems remained operational for a significant period of time. Such systems include those that notify occupants to selectively or partially evacuate a building, or to relocate to an area of refuge during a fire emergency. Such emergency voice/alarm communications systems must continue to function normally during the fire (see 'Survivability' sidebar, p. 58 for details).

Protection of circuits as they pass through fire areas, other than the ones they serve, is addressed in A-3-8.4.1.1.4. The change was made to delay possible damage to such circuits. This is done to increase the likelihood that circuits serving areas remote from the original fire will have the opportunity to be activated and serve their purpose. Note that the protection requirement would also apply to a signaling line circuit that extends from a master fire-alarm control unit to another remote fire-alarm control unit where notification appliance circuits might originate.

Elsewhere, the code also provides requirements for interconnecting fire-alarm control equipment with building fire command centers. In fact, if the fire command center control equipment is remote from the central control equipment, specific requirements apply (see 'Fire Command' sidebar p. 58).

Fire Command Center Control Requirements

If the fire command center control equipment is remote from the central control equipment, the following requirements shall apply (NFPA 72.3-8.4.1.3.3.3):

The interconnecting wiring shall be provided with mechanical protection by installing the wiring in metal conduit or metal raceway.

The interconnecting wiring shall be provided with resistance to attack from a fire by routing the wiring through areas whose characteristics are at least equal to the limited-combustible characteristics defined in NFPA 90A, Standard for the Installation of Air Conditioning and Ventilating Systems.

If the interconnecting wiring exceeds 100 ft., additional resistance to attack from a fire shall be provided by doing either of the following:

a) Installing wiring in metal conduit or metal raceway in a 2-hour fire-rated enclosure.

b) Enclosing the wiring in a 2-hour fire-rated cable assembly and installing the cable in metal conduit or metal raceway.

NFPA 72-1999 Survivability Requirements

Survivability requirements for fire-alarm wiring are as follows:

A single notification appliance circuit shall not serve more than one notification zone (3-8.4.1.1.2).

Systems shall be designed so that equipment failure or a fault on one or more of the installation wiring conductors of one notification appliance circuit shall not result in a functional loss of any other notification appliance circuit.

Installed equipment must operate in a certain manner during fault conditions. For example, it is necessary that a fault such as a short circuit does not open a fuse or damage components common to other circuits (A-3-8.4.1.1.3).

Notification appliance circuits and any other circuits necessary for the operation of the notification appliance circuits shall be protected from the point at which they exit the control unit until the point that they enter the notification zone that they serve using one or more of the following methods (3-8.4.1.1.4):

A 2-hour rated cable assembly.

A 2-hour rated shaft or enclosure.

A 2-hour rated stairwell in a building, fully sprinklered in accordance with NFPA 13, Standard for the Installation of Sprinkler Systems.

*(Paragraph 3-8.4.1.1 applies only to systems used for partial evacuation or relocation of occupants.)



No comments