by Ross George, Littelfuse
December 17, 2011

Yet another type of electronic device combines ground fault protection with a ground-check function, which monitors the integrity of an equipment ground. It is used if ensuring ground continuity is the foremost issue. These relays are also used with trailing cables, when the equipment may be a large distance from the starter or breaker.

By ensuring a ground connection, this prevents a potential voltage rise across the fluid or on the framework of the equipment during a ground-fault, which may represent a shock hazard to operating and maintenance personnel. Common application environments are surface mines, underground mines, quarries and submersible pumps. These devices are even used on the pumping equipment for golf course watering systems. They can also be used as a remote permissive for the pumping equipment.

In many electrical systems, a neutral grounding resistor (NGR) is used on the grounding system. This is sometimes called a high resistance ground (HRG), where a resistor is placed between the neutral of the supply transformer and ground.

Using resistance grounding can minimize the amount of damage caused by a ground fault. In some cases, it may allow operations to continue until the fault can be cleared. In addition, an NGR used in conjunction with an NGR monitor relay can provide a predictive maintenance function by alerting staff to the ground-fault before excessive damage is done. Moreover, resistance grounding also prevents a re-striking fault from elevating the system voltage relative to ground, which adds more stress to the insulation and can lead to a phase-to-ground-to-phase fault.

Pump manufacturers are being asked to incorporate ground-check relays into their equipment. This includes a zener diode termination assembly that is part of the relay's open and shorted ground-check loop functions for portable power cables. The relay feeds two different levels of current to the termination assembly and checks to make sure that the voltage drop is the same across the zener diode at both levels. This is a more desirable way of ensuring ground integrity compared to a resistance terminated loop, as a high-impedance fault with the right amount of resistance can provide an erroneous indication.

Benefits of Microprocessor-Based Protection

Microprocessor-based relays may incorporate several of these protective and predictive functions to guard pumps and motors from overload damage and some other fault conditions. Depending on the design and feature-set, they may include a limited functions or a wide range of protection and monitoring features. Some of the advanced features include protection in the event of phase loss, phase imbalance, improper phase sequence, jam and undercurrent protection. Other features may include digital inputs/outputs, internal data logging, and interfacing data communications. See Figure 2.

Figure 2. Microprocessor-based motor protection relay simplified circuit diagram

A useful feature for maintenance personnel is continuous real-time monitoring of an operating motor's thermal capacity. As mentioned earlier, this may prevent a motor from becoming overloaded, avoiding insulation damage.

Combined with a data communications interface, this monitoring allows trend analysis by a central control system computer or PLC and subsequent scheduled maintenance.

A pump jam will cause motor overload due to excessive current that can damage the motor windings' insulation. A motor protection relay will interrupt the supply power as it does for a ground fault. A pump running dry does not cause an overcurrent condition but a low current—a common occurrence in submersible pump applications. Mechanical floats may not be sufficient, as they may be prone to failure. In many cases, a pump that runs dry means a loss of lubrication, which can lead to bearing damage or other failure modes.

A microprocessor-based relay with a low-current setpoint can protect against a dry pump situation. During normal operation the microprocessor sees nominal operating amperage.

If the pump runs dry, this current drops to an “idle” amperage level, which is defined by the motor. When that occurs, the protective relay can send a signal to an interrupt device, or signal to a PLC for analysis, which may actuate the interrupting device.

Cost Considerations

Microprocessor-based motor protection devices can be sophisticated. In addition to current inputs, some devices have provisions for voltage inputs, which can calculate power, power factor and other parameters. These parameters are fed back to a motor control center or PLC where the data can be logged and/or analyzed.

Network communication capabilities may include protocols for Ethernet, DeviceNet or ProfiBus. With these, microprocessor-based motor protection can be used to help optimize a pump/motor system electrically and mechanically. Some decisions and actions that may be taken are load analysis, altering flow rate or changing power factor correction.

The cost of devices with this level of sophistication may be prohibitive for pumping systems operating with 50-horsepower or smaller motors. In these cases, simpler microprocessor-based devices may be a cost-effective way to upgrade an antiquated pumping system or add protection to a system.

Another consideration is the measurement devices required for use with the protection relays. For example, some microprocessor-based relays have built-in current transformers (CTs) for overcurrent detection circuitry with specified current ranges. In other cases, these CTs must be purchased and installed. In some cases, using separate CTs may be appropriate, allowing a protection relay to be tied into almost any power system.