Monitors are single function devices that only look at one abnormal condition and either alarm or provide a means to remove power. Visual indication can also be used. The purpose of a monitor is to provide a low-cost solution to a dedicated problem. Monitors are typically added to existing protection, such as fuses, circuit breakers, or protection relays.
The single most common reason for electrical system failure is insulation breakdown. Insulation monitors can be installed at any point in the system to detect a problem with the insulation. The monitor is connected to one phase and injects a dc signal to continuously measure the system’s insulation resistance. The monitor is typically installed on de-energized feeders or motors and is cycled with feeder’s circuit breaker or motor starter. When the circuit breaker is open, the monitor is energized and begins to monitor the de-energized cables and motor windings. In ungrounded systems, the monitor will continuously monitor the insulation resistance to ground regardless whether the system is energized or de-energized.
Ground Continuity Monitors
Ground check monitors are used to detect problems in equipment ground conductors. Mobile equipment typically has an extra wire, or pilot wire, routed with the phase conductors. A monitor uses this pilot wire to send a signal down to the equipment to a terminating device, where the signal is sent back on the equipment ground conductor to the monitor. The monitor continuously monitors this loop for open or short circuits, indicating that a problem has occurred. The monitor provides an alarm for this condition.
As an example, portable loads are grounded via single or multiple conductors in a trailing cable. A ground fault on a portable load will cause fault current to flow through the ground conductors and all other ground-return paths. A hazardous touch voltage can develop when the ground conductor opens and a ground fault develops, assuming there is not enough current to trip a ground-fault relay. If the portable equipment has rubber tires or is not in good contact with earth, then the next person to touch the equipment under fault conditions will become part of the ground-return path.
As discussed in the resistance grounded systems section, a failure in the neutral to ground path will lead to a dangerous situation. Some examples of failure are stolen wires, loose connections, corrosion and broken resistor elements. The resistor monitor continuously monitors the path from system neutral to ground for a problem. When a problem occurs, the monitor provides an alarm.