Response of motor thermal overload relays and phase monitors to power quality events

Laboratory tests were carried out on seven voltage monitoring relays to determine their operational characteristics. The objective was to gain an understanding of the impact of identical power quality events on different voltage monitors commonly installed on utilization equipment. The tests focused on typical monitors found on a local utility's customers' equipment including device models that have previously encountered power quality issues. A similar experiment was carried out on nine thermal overload relays that are widely used for motor protection. The results indicate that both the thermal overload and voltage monitoring relays are prone to erratic response and may be responsible for some nuisance trips observed on utilization equipment.