This month’s feature article is about devices used in the protection and monitoring of equipment. These devices are commonly named ‘Relays’ but are generally sensing devices with dry contact outputs or solid-state outputs for signaling a PLC input or alarm circuit. The discussion will briefly cover Phase Loss Relays, Phase Sequence Relays, Heater Burn-out Relays, Current Relays, and Ground Fault Relays.
Phase Loss Relays sense the loss of a phase in a 3-phase circuit. The loss of a phase can be costly if gone undetected. Downtime and equipment repairs can result. Three-phase motors and devices can be affected. Motor windings will overheat, as a motor will typically continue to run with only two phases. For example, a 3-phase motor operating at 70% of FLA (Full Load Amps) will then operate at about 120% of FLA after a phase has been lost. And it is not uncommon for motor overloads to be oversized in the real world. Phase loss can result from the utility side (downed lines or weather conditions) or on the equipment side (blown fuses or contactor failure). Phase Loss Relays are relatively inexpensive (less than $200) and are simple to install. They have a connection for each phase (A, B, and C) and a dry contact output for connection to a PLC input, a circuit interlock or an alarm circuit. They can be used on the line or load side. Phase Reversal/Sequence Relays prevent the operation of a motor if the phase sequence is not correct. Incorrect phase rotation will cause a motor to run in the wrong direction. In some applications this will cause damage to the driven device, for example a pump. Many times, the functions of phase loss and phase sequence are combined into one product. Heater Burn-out Relays can detect for the loss of a heating element for single-phase or 3-phase heaters. These sensors can work based on either voltage or current. Many times heating elements are in areas of machines that are very difficult to troubleshoot. Save on troubleshooting time and product quality issues for a range of manufacturing equipment, including packaging lines and molding machines. There are some heater burnout relays that also have the functionality to detect a shorted SCR. Current Relays / Current Switches monitor circuit current and have adjustable trip settings. There are a lot of devices that warn or trip on high current like motor overloads and circuit breakers. But sometime it is beneficial to detect for a low or no current condition – this is where a current relay will provide the necessary signal. Ground Fault Relays detect for current leakage in a circuit same as a GFCI receptacle does. They have practical application where there is a concern for the breakdown of insulation on electrical devices or wires. Chemical reactions (like cutting oil) or frictional wear are common root causes of insulation breakdown. Available options include a test button and sensitivity adjustment.
NFPA 70E – Electrical Safety in the Workplace
The standard places emphasis on training and re-training. Because of the potential for severe injury or death when working with electricity, Article 110.2 outlines training requirements. Part 110.2(D)(3) requires that retraining be performed at intervals not to exceed 3 years. For employees that rarely use safety tools and higher category PPE, retraining is important as they will forget proper procedures that are required very infrequently. Some customers are now requiring suppliers to train their employees in the hazards of electricity in the work place – NFPA 70E is a great resource for those requirements. Finally, for those who would like some practical experience in Arc Flash Boundary calculations, visit the Informative Annex D. You may find parts D.1 and D.2 interesting.