FIRE APPARATUS ELECTRICAL OVERLOADS: A REAL BALANCING ACT
The firefighters were exhausted when they returned to quarters following a multiple-alarm fire in a warehouse that housed bailed paper. Many hours of arduous overhauling, searching for hidden fire in the cold, dark remains of the burnt-out building, took its toll on them.
The pumper outside had been supplying not only sporadic bursts of water but also illumination for the fire scene as well as a cab heater, the only place the “watch line” crew could get a respite from the cold and thaw out frozen fingers and toes.
Everyone had been happy to see a fresh company arrive on the scene for relief. Upon return to quarters, the firefighters cleaned tools, loaded hose, and readied the apparatus for its next response.
As fate would have it, moments later an alarm was received for a nearby location. As the company members boarded the rig, the driver climbed into his seat and depressed the starter button. Instead of hearing the 300+ horsepower diesel engine come to life, all they heard was the dull clicking of the solenoid, which followed the straining grunt of the starter motor that was incapable of cranking up the engine.
Even though a fire department faithfully performs proper operator and fleet maintenance, a failed response such as this one should come as no surprise. From the start, a tremendous load had been placed on the apparatus batteries and charging system, from which they could not sufficiently recover.
TAXING THE ELECTRICAL SYSTEM
Let’s examine the conditions during the first response that placed the apparatus electrical system at such a great disadvantage:
- The initial cranking of the starter motor placed a major load of 500 to 600 amps on the battery system for a short time.
- As soon as the engine started, the “master” light switch was depressed, illuminating every warning light on the apparatus. This placed a sudden and continuing load of up to 120 amps on the system.
- Since the response was at night, headlights, markers, and step lights all were in use.
- While responding to the heavy smoke and bright glow in the sky, the large mechanical siren (motor-driven) was in continuous use, consuming 90 to 100 amps.
- The captain was receiving and acknowledging radio messages and instructions en route.
- While on the scene, illumination was provided by the two 500-watt quartz lights mounted on the apparatus and powered by a 110-volt inverter. This configuration alone consumes 100 amps of current. In addition, the front and rear spotlights also were in use.
- Due to the cold weather conditions, the motor of the cab heater was operating at high speed to supply the only available heated rehab area for the firefighters.
- While retrieving equipment, members left compartment doors open with the lights on.
- As the operation continued, the alternator was providing maximum output, causing the temperature to increase. When exceeding 200°F, alternator output begins to decrease sharply.
- As the fire went into the overhaul stage, the need for water significantly was reduced, as was the engine rpm. This decrease in rpm directly affected the alternator’s ability to supply the needed power.
- When the second alarm was received at quarters, the diesel engine was still hot from the operation, A hot diesel engine (coolant over 14()°F) can require more than twice the amperage to turn over than is needed to crank the engine when cool. As an example, a 700-cubic-inch diesel that typically uses 525 amps to start cold could demand as much as 1,400 amps when hot (Dennis A. Litchenstine, Design Considerations—Vehicular Electrical Systems, 1990, p. 5).
From the moment the starter button was depressed until the apparatus returned to quarters, an excessive load overwhelmed the apparatus electrical charging system and thus depleted the reserves in the storage batteries. Consequently, when maximum output was needed for the second response, the system could not respond effectively.
COMMON ELECTRICAL LOADS
It is quite obvious that in the previous example the electrical load of the apparatus far exceeded that which the charging system could supply. This may appear to be an exaggeration, but you might be surprised when you examine the amp ratings taken from manufacturers’ electrical load charts (see box at right).
When writing specifications for new apparatus, if it is impossible or inconvenient to reduce the number of options requiring electrical power to match the alternator’s capacitv, then a program that selectively reduces this demand manually is necessary.
LOAD SHEDDING
AMP RATINGS
j * Light bars vary, depending on | number of rotators, type of bulbs. and additional accessories sped* i tied.
The procedure known as load shedding involves simply switching off unnecessary electrical loads to balance the system. Obviously, you would not consider shutting down headlights, warning lights, or the radio while responding. Once on the scene, however, you can make some intelligent choices. For example, if you choose to operate the two 500watt quartz lights from the vehiclemounted inverter, you might consider eliminating the headlights (doing this also improves visibility for on-coming drivers). If the operation is being conducted on a closed street or in a remote area such as a parking lot or field, there is little need for the 46amp light bar that warns of your presence. If there is any chance of traffic in the vicinity, by all means leave on the most effective warning lights, but eliminate those that are not needed.
The rating of reserve minutes of capacity for apparatus batteries is calculated at a discharge rate of 25, not 250, amps. Should a charging system fail, you might have to shed the entire load except for the circuits needed to keep the engine running and to ensure that the apparatus will continue operating and be able to return to quarters. This precaution is especially critical when electronically controlled engines and transmissions are powering the apparatus.
Activating the master warning light switch w ith the entire load connected creates a sudden surge that can damage the alternator. This effect has been likened to an “electrical water hammer.” A device called a sequencer can be used to avoid this problem. It activates and deactivates all of the connected loads in sequence and at half-second intervals. You can program the unit to include many or all of the circuits at your discretion.
An advanced model of this sequencer monitors the battery voltage and automatically begins to shed loads to keep the system balanced. This feature also is programmable. You can decide which circuits are the least important (i.e., compartment lights, cab lights, and so on), and the sequencer will eliminate them first. An override is provided to cancel this feature, when warranted, such as during an emergency response.
PREVENTING ELECTRICAL OVERLOADS
The following strategies can help you avoid overloading your apparatus electrical system:
- NFPA 1901, Standard for Pumper Fire Apparatus (1991), recommends as minimum a cold-rated, 130-amp, 12-volt alternator. The standard states that the alternator shall have an output adequate to meet the continuous anticipated load of the apparatus as manufactured, at 200°F, ambient temperature under the hood. A minimum capacity of 130 amps is recommended, but realistically the proper procedure is to specify an alternator and related equipment (regulator, wiring, and drive) that adequately will supply the calculated load of the apparatus as well as return the batteries to a fully charged condition.
- Consider an automatic throttle device, capable of increasing engine and alternator speed. An appropriate interlock to prevent activation while in pump or gear other than neutral should be provided.
- Investigate the various configurations of battery combinations and starter circuit wiring. The rationale for the old dual battery system was to hold one set of batteries in reserve. This reasoning is no longer valid, as the modern high-compression diesel engine requires the use of both banks of batteries to crank the engine. The split system actually creates more problems than it solves by adding unnecessary components and wiring.
- When writing vehicle specs, evaluate and compare the current draw of the electrical accessories available from the manufacturer. If you must reduce loads, consider new more efficient electrical devices rather than traditional high-draw equipment. Most catalogs for vehicle warning devices now list the amperage draw for the products shown. Some new bar lights use considerably less current than older models. An electronic siren uses only about 14 amps, compared with the large mechanical siren, which uses 100. If the added effectiveness of the motor-driven siren is necessary, combine it during the response with the electronic unit, and use it only at intersections.
- Properly maintain the electrical system by regularly inspecting the battery water levels, terminal condition, and alternator drive-belt adjustments according to manufacturerspecifications. (Be careful when working around storage batteries, as explosive gas might be present.)
- Use a battery conditioner (charger) mounted on the apparatus. Several manufacturers produce units that take shore-line, 110-volt power and continuously charge the vehicle batteries in quarters. These units are self-regulating and taper off when the charge is complete. As an alternative, a small automotive charger with polarized plug can help charge the batteries between responses.
- If you carry many items of auxiliary equipment such as rechargeable hand lights, meters, computers, or portable radio chargers, consider a separate 12-volt power supply. This unit receives 110 volts from the shore-line connection and converts it to 12 volts with an individual circuit powering each accessory. When the apparatus responds and the shore line is disconnected, the unit automatically transfers the accessory load to the vehicle’s 12-volt system.
- When replacing batteries, specify a battery that is equivalent to or exceeds the original battery installed by the apparatus manufacturer. Avoid the low-bid, low-quality battery that might work in ordinary vehicles; it will not stand up to the punishing duty cycle of a fire apparatus.
When writing specifications for new apparatus, specify a charging system that will meet the amperage requirements of the anticipated load as well as the battery and starter configuration that will best serve your department’s needs.
Maintain the batteries and charging system on existing apparatus according to manufacturer recommendations. Consider adding on-board chargers or auxiliary power supplies to help maintain the batteries in fully charged condition.
Finally, instruct your operators to be aware of developing low-voltage conditions and to take the proper steps to shed unnecessary loads to balance the system.
Following these recommendations will help to ensure fast, reliable starts when you receive an alarm.
