POWER SUPPLY CIRCUIT FAULTS

The common components in low-voltage control units that may require occasional replacement or maintenance are relays, resistors, capacitors, diodes, transformers, fuses, switches, lamps or LEDs, meters, and wiring. In addition, a modular control unit has replaceable modules. The modules plug into the main control unit assembly. The modules vary in construction but usually contain solid-state devices mounted on one or more printed circuit boards (PCBs). Sometimes the modules are sealed, but more often they can be disassembled for repair. Each module may represent one zone or a group of zones, or it may perform a nonzoned function, such as one of the following:

Providing a time delay (such as shutting off bells after 15 minutes)

Providing output contacts for a remote auxiliary function (such as fan shutdown)

Transferring power (from commercial power to standby power and back)

Sounding a local trouble buzzer

Controlling audible signal devices

Providing a reverse polarity alarm output (for remote station connection)

Use the manufacturer's diagrams and servicing information to narrow down any problems to a small area. If a problem can be isolated to one of these modules or if a problem appears to be related to a zone module, the most immediate repair is to replace the module. If the module is not sealed, inspect it for a condition such as an overheated resistor or transistor, a poorly soldered connection, a bent connector pin, or a malfunctioning relay. Repair or replace the parts, resolder the connection, or straighten the connector pin. For other conditions more difficult to analyze, replace the module. (Keep spares on hand.)

CAUTION

Any soldering that is performed, especially in replacement of solid-state devices on printed circuit boards, must be performed with care, following good commercial soldering practice.

Grounded and Short Circuits

A ground fault in the power source wiring will typically cause the building circuit breaker for the fire alarm system to trip. The equipment will continue to operate on standby battery, if one is provided. If the battery is discharged or if no battery is provided, the equipment affected will be out of service, and fire alarm protection will be nonexistent. Because battery capacity is limited and complete discharge should be avoided to prevent permanent damage to the battery, repair the fault immediately.

Figure 7-20.\Troubleshooting chart for a power circuit ground fault.

Figure 7-21.\Troubleshooting chart for a short circuit fault in a power circuit.

Figure 7-20 is a power circuit ground fault troubleshooting diagram. Figure 7-21 is a similar diagram for a short-circuit fault in the same wiring.