Chapter 11 - Backup Systems

All control systems using electronic and electromechanical equipment serving a critical life support function should include backup controls. Failures of any component in a primary control system can render the entire system useless and lead to catastrophic losses. Typical failures include those caused by power losses or surges, hardware, or software. Failures can occur in the computer, an Input/Output (I/O) device, an individual input or output module, or in sensors. Failure detection techniques include looking for computer lockup, toggling a symbol on the screen, setting up error messages, setting alarms for failed I/O boards, looking for sensor outputs that are significantly out of range, watching for toggling of the least significant bit on a sensor output, monitoring serial communication activity with another computer, and using an expert system.

In addition to the backup system, an independent and reliable alarm system is also essential. While a controller failure is fairly uncommon, it is possible. Equipment which may be used for backup control of critical systems includes uninterruptible power supplies, battery backed non-volatile memory, auto execute programs, manual switch over systems, automatic switch over systems, watchdog times, conventional controls, backup computers and connecting backup equipment in electrical parallel with the controller. Life support should be the primary focus of the backup systems. For example, some method of providing heat during brooding should be available even if the main control thermostat has failed. Similarly, during hot weather, a means to provide ventilation independently from the control system is essential.

In the case of central computer controlled systems, which provide many control functions in one unit, failure of a single component can have more serious impact than would failure of part of a distributed electronic control system or a single thermostat amongst many thermostats used to control fans or heaters. For this reason, designers of backup systems must take a ‘worst-case’ approach when selecting which support functions to backup. This is not to imply that centralized control systems are inferior, but rather it must be stressed that backup systems should be viewed as an essential component of these systems

For poultry facilities there are as many as four different critical backup functions that must be provided for short term life support. These include providing heat during cold weather for young broilers, providing minimum ventilation during brooding, providing minimum ventilation for older broilers, and providing maximum ventilation during mild to hot weather.

Note that these backup system schematics indicate that double-pole timers and relays should be used on all 240 VAC circuits. This is necessary because a properly installed controller should also switch both legs of 240 VAC circuits. If the controller were to fail and a single-pole backup thermostat were used (on one leg of the power circuit), it is quite possible that power would not provided on the outer leg of the power circuit.

If single-pole timers or thermostats are used, note that the same leg that is switched by the controller must also be switched by the backup device. This does not provide a safety disconnect because there is a power available at the fan motor via the in-switch leg.

Low temperature safety override

The most common backup system for young broilers is for heat in the event of control system failure, because young chicks cannot survive cold temperature. A low temperature safety override should be connected in electrical parallel with the control system wiring. Use two thermostats if more than one heater circuit is desired.

The low temperature override circuit provides power to heaters if air temperatures at the thermostat drops below the thermostat’s set point. Proper adjustment of this safety thennostat is crucial to proper operation. For example, setting the safety thermostat set point to a low temperature (e.g., 40°F) effectively removes it from the circuit and makes it worthless. For brooding it is common to include the backup thermostat set point adjustment in periodic control system adjustments.

High temperature safety overrides

During hot weather it is essential to activate several fans if a controller fails. In principal this is identical to the low temperature safety override, except that the mechanical thermostat is wired to activate on temperature rise. At least 2.5 CFM/broiler should be provided for emergency use, thus more than one high temperature override thermostat may be necessary depending on fan size and broiler density. For example, two 48-inch fans rated at a nominal 20,000 CFM each at 0.08 inch water column provides 20,000 broilers with about 2 CFM/broiler.

An alternative high temperature safety override is to provide an automatic drop curtain in the building. This system consists of a bimetallic thermostat which when activated, releases a safety catch on the cable holding a curtain running the entire length of both sides of the building. These drop curtains also must be adjusted properly and tested at regular intervals to ensure operation. Consideration should also be given to ensuring that curtains drop even if fans are still running, as can often happen if the drop curtain thennostat is out of calibration of incorrectly set.

Override systems using variable speed fans

Although fan speed controls are not commonly found in poultry facilities, they are quite common in HVAC systems used for office and industrial applications, and they can provide precise control of ventilation rate for conventional AC fan or blower motors. If variable speed fans are used, these should also be provided with a backup system. One approach is to provide a manual override switch which can be used to bypass the speed controller so that the motor will operate at full speed. With computer controlled systems, one approach is to allow a watchdog timer to switch over a proportional thermostat when the computer communication link is broken (due to a failure of the computer or one of the components between the computer and the output to the speed controller). If the speed controller itself fails, the controlled fan will usually be turned off which can be sensed by some loss of temperature or static pressure control. Most speed controllers also have alarm outputs which can be used for audible or visual alarms at the location or remotely sensed by a computer.