Empower Yourself With the Knowledge of Power Supplies
One of the most important aspects of installation involves the source of operational power. Unless the power source is reliable and possesses enough capacity to operate the system, as outlined in Section 1-5.2.4, NFPA 72, National Fire Protection Association (NFPA), the fire alarm system may not perform as required when a fire is detected.
With that in mind, let’s take a closer look at power supplies; specifically, the primary power supply, secondary supply and external auxiliary supplies.
Using Power From Street, Generator
In most cases, the primary power supply utilizes nothing more than the public electric bus that enters the facility from the roadside (light and power service). Another acceptable source is an engine-driven generator, which requires full-time supervision by a trained operator.
On a new installation, the fire technician can pretty much be assured that the engine-driven generator installed meets the code requirements of Section 1-5.2.10, NFPA 72. However, when working in existing facilities, a review of the aforementioned section would be prudent to assure compliance (generators will be covered in a subsequent column).
Section 1-5.2.5 provides insight about what constitutes an adequate primary power supply using the public electric bus. The first approved method involves the use of 2-wire hook-ups utilizing 120VAC, 60Hz. (U.S.). In this case, the 120VAC light and service connection is used for both operating power and the signaling system’s trouble signal circuit.
Fire technicians can also use a 3-wire 240VAC, two-phase hook-up with an unfused neutral. When using this method, code stipulates that the disruption of one phase not interfere with the proper operation of the fire alarm system. Using this arrangement, one phase is used for operational power, while the second phase is used for the trouble signal circuit.
Breaker Mechanics Often Ignored
Probably one of the most ignored aspects of code in the area of primary power sources involves the breaker(s) that provides power to the alarm panel. Simply put, when using the public electric bus, always use a dedicated circuit.
By code, fire technicians cannot pick up primary power in a hodgepodge fashion. This means that the fire panel must be connected to a 120VAC or 240VAC breaker with nothing else connected to it. A qualified electrician should be employed to install a #14 or #12 AWG electrical cable from the fire panel to the public electric bus service panel.
When using a single-phase hook-up, for example, three conductors are required – one hot wire, one neutral and one ground. If the fire technician intends to utilize a two-phase hook-up, four wires are necessary – two for the hot wires, one neutral and one ground. When using #14 AWG, a 15A breaker is necessary, or a 20A when using #12 AWG.
The breaker used for this purpose must also be marked in red. It also must be labeled so there is no question as to what it controls. NFPA 72 stipulates that it is labeled “Fire Alarm Circuit Control.”
In addition, access to this breaker must be restricted in some manner. In most cases, the installer will merely use a simple breaker lockout device. In order to turn the breaker off, the lockout device must be removed with a screwdriver or some other means.
Secondary Power Used as Back-up
The mission of a secondary power supply is simply to provide back-up operating power whenever the primary supply has failed.
For example, when the power being supplied by the public electric bus wanes to a level where the fire alarm panel cannot maintain operation, the secondary supply will take over. In most instances, the secondary power supply consists of a series of batteries combined to provide the required voltage and power capacity. NFPA 72, Section 1-5.2.6(b)(c), also allows the fire technician to use a self-starting generator.
Through the use of capacitors within the control itself, operations can be sustained for a limited period of time. Secondary power, however, must be switched into full use within 30 seconds of the brownout or complete power outage.
Standby System May Be Needed
There are two exceptions to the rule concerning secondary power supplies. Both of them involve the use of an approved, legally required standby system and an optional standby system in accordance with NFPA 70, National Electrical Code, Article 700, and 701.
“Fire alarm installers must know whether the building they are installing in has a legally required emergency system. Although the battery pack is still used, the fire alarm panel’s primary power supply inputs must be connected to a dedicated circuit from the generator switch gear,” says Nick Markowitz, owner of Markowitz Electric Protection in Verona, Pa.
According to Markowitz, there are cases where 24 hours to 60 hours of battery standby is not enough. He cites cases such as high-rise buildings where natural disasters are likely to disrupt primary power longer than the fire alarm panel’s secondary battery supply is able to accommodate.
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