The power within
Although two-way and microwave technicians consider radio to be their primary area of responsibility, they must also understand and, in many cases, repair the support systems that power the radios. Most two-way base stations operate directly from ac power while microwave and multiplex systems operate from a dc source.
In either case, a failure in the power source will affect the equipment being powered. Radio technicians must be familiar with site power equipment and distribution systems to isolate trouble and minimize communications outages.
Ac power sources
Sooner or later a communications facility will experience an interruption in commercial power. Although the problem is often with the local power utility, it can also be due to equipment failure within the site. A radio technician is not usually expected to accomplish repairs involving ac power feeds. However, an employer will certainly value the technician who knows what checks to make and where to isolate the trouble. Once it is determined whether the problem is with the power utility or “in house,” the proper repair personnel can be dispatched to the site.
Facilities designed with reliability in mind are equipped with an emergency power generator that can run the site in the event of a commercial power failure. The transfer panel is a key component in the site electrical system and a point of termination for the commercial power feed, generator power feed and load (the communications and support equipment). Connections for each are available within the panel on the transfer relay, as shown in Figure 2., and are usually accessible for making measurements with a voltmeter. Larger communications sites have a three-phase power feed. Voltages can be checked between legs (phases), or from each leg to the neutral to verify proper values.
Under normal conditions, the transfer relay routes commercial ac power to the load. When commercial power is lost, the generator is signaled to start. Once the generator is up to speed, the transfer relay changes state, transferring the load over to emergency (generator) power. When commercial power is restored, the transfer relay returns the load back to the commercial feed (often after a preestablished time delay) and the generator shuts off. The transfer relay will often have auxiliary contacts for connection to an alarm system remote terminal unit. The RTU offers remote registration of ac power failure and generator running alarm and status conditions. The “load” side of the transfer relay connects to circuit breaker panels allowing ac power distribution within the site. If primary power to the site is 240Vac or 208Vac, there are often breaker panels for both that voltage and 120Vac. Generally, the 240Vac/208Vac breaker panel provides power to the larger items like HVAC systems and high-current battery chargers, while the 120Vac breakers distribute protected power to the rest of the site equipment, as shown in Figure 1.
The emergency generator has a control panel allowing it to be started manually for testing or taken out of service for maintenance. Under normal conditions, the generator controls are left in “auto.” This means the generator will automatically start if a commercial power failure is sensed. However, under certain conditions (such as when a mechanic is working on the generator) it is taken out of the “auto” mode and put into “manual.” This disallows the generator from starting automatically. It is common to have a red light on or near the generator that illuminates when the switch is taken out of the “auto” position. This serves as a visual reminder to the generator mechanic to put the switch back in “auto” before leaving.
Some generator control panels offer relay contacts for connection to an alarm system RTU. Probably, the most common alarms used are “not in auto” and “generator common failure.” The latter is a summary or “catch all” alarm triggered by various engine and electrical sensors such as those for low oil pressure, high coolant temperature and output voltage/frequency out of tolerance.
Generators must also have on-site fuel containment systems. Propane tanks can be equipped with a low-pressure sensor that activates a switch contact when the fuel level drops to a predetermined level. When connected to a RTU, the contact provides a “low fuel” alarm. Other fuel tanks may offer alarm outputs for detected leaks in interstitial compartments or secondary containment areas.
Dc power sources
Communications facilities often house one or more dc power sources that are common to various pieces of communications equipment throughout the site. For example, microwave and multiplex equipment may operate from the same –48Vdc plant while two-way equipment is powered from a common +12Vdc buss. Common power sources eliminate the need for each discrete piece of equipment to have its own power supply.
The typical communications site dc power source is a low-noise, ac-to-dc battery charger. The output terminals of the charger are usually connected in parallel with an array of batteries referred to as a battery plant. The battery plant is comprised of individual cells connected in series such that their combined voltage equals that of the charger output. Under normal operating conditions the charger supplies current to the load and maintains a charge on the battery plant. The plant provides dc backup power in the event of charger failure, or commercial power failure if no emergency generator exists. When determining the required capacity (expressed in A/hr) of a battery plant, the load current must be known as well as how long the batteries (by themselves) are to power the load. For example, if the required load current is 25A and the batteries must supply power to the load for 12 hours, the battery cells must have a 300A/hr capacity:
battery capacity (A/hr) = load current (A) X length of operation (Hrs)
Proper maintenance of the battery plant helps ensure that uninterrupted dc power will be there when it’s needed. Cell terminal connections should be torqued to manufacturer specifications and kept free of oxidation. Individual cell voltage levels should be checked annually to verify no cells within the battery plant fall below limits established by the manufacturer. Flooded cells should have their specific gravity checked at similar intervals.
The battery charger must have sufficient capacity to power the load, while at the same time recharge a depleted battery plant. To determine charger capacity requirements, the battery plant recharge time must be specified. For example, with a load current of 25A, battery plant capacity of 300A/hr and a recharge time of 8hrs, the charger must be able to supply 66.25A (see formula, below). The closest common charger capacity to that requirement would be a 75A charger.
charger capacity (A) = (1.1 X battery plant capacity / recharge time) + load current
The charger output voltage is adjustable and maintains a continuous surface charge on the plant. This is referred to as a “float charge.” The per-cell float voltage depends on the type of cell and should be specified by the battery manufacturer. Most commercial chargers are also capable of being switched into an equalize mode, which increases the per-cell voltage across the plant. An equalize charge is applied on a temporary basis to help minimize voltage variations between cells or increase the voltage of those that have fallen below manufacturer specifications. The per-cell equalize voltage should also be specified by the manufacturer.
Chargers are typically equipped with internal transformer tap options allowing them to be powered from 240Vac, 208Vac or 120Vac. The charger outputs are current-limited and protected by either an internal fuse or a front-panel circuit breaker. The output of the battery plant is also protected with a dedicated fuse or circuit breaker.
Redundant battery chargers offer an additional level of system reliability. Some chargers are designed to allow their outputs to be paralleled so that if one charger fails, the other will carry the load. Additionally, a load-sharing option ensures the load current is divided evenly between the two (+/-5%, typically).
Modular-type chargers are comprised of power modules that slide into a chassis. Each module has a specific rated capacity. If, with time, facility load requirements increase, additional modules can be inserted to meet the elevated demand.
Most commercial chargers offer dry alarm relay contacts for connection to an alarm RTU. Fault detection circuitry within a charger can provide a variety of alarm outputs such as “high voltage,” “low voltage,” “low current,” “ac fail” and “charger fail.”
When several dc voltage levels are required within a site, dc-to-dc power converters can be used to step the operating voltage up or down to the appropriate level.
Critical ac-powered devices can benefit from the presence of a dc plant through the use of dc-to-ac inverters. Because the inverter is powered from a battery-backed dc source, the load is not affected by commercial ac power variations and transients. Most commercial dc-to-dc converters and dc-to-ac inverters are also equipped with alarm relay contacts for connection to a RTU.
If, due to an ac power or charger failure, the battery plant is required to power the site equipment for a prolonged period, it can eventually become over-discharged. This results in cell hydration, a condition where the specific gravity drops low enough to allow the lead components within the cell to begin dissolving into the electrolyte. Compounds and salts develop, which clog the internal separator pores. If the cells are recharged in this state, lead begins to form within the electrolyte, effectively causing internal short circuits. The result is an increase in charging current and a decrease in storage capacity.
To prevent an over-discharge condition, a low-voltage disconnect panel is installed between the charger/battery plant and the load. The low-voltage disconnect panel continuously monitors the battery plant voltage. If, due to discharge, the plant voltage drops to a predetermined level, relay contacts within the disconnect panel isolate the load from the plant. This saves the cells from destruction, but at the same time disconnects dc power from all load devices.
Power is distributed from the load side of the disconnect panel through fuses or circuit breakers. Since dc-powered devices can be mounted in equipment racks located throughout the site, one primary fuse or circuit breaker is usually dedicated for each rack operating on that voltage. Secondary distribution within the rack is accomplished with its own fuse or breaker panel (see Figure 3). A blown fuse or tripped breaker will energize an alarm relay internal to the fuse or circuit breaker panel. The relay contacts are used by the remote alarm RTU to register a “blown fuse” or “tripped breaker” alarm.
Sooner or later, a communications facility power source will fail. The more devices operating from that source, the greater the panic when it does.
In such circumstances, radio technicians can offer effective and swift trouble isolation when equipped with a basic knowledge of common site power sources, their termination points and distribution methods.
Ashley is a communications technician and writer based in Ventura, CA.