Thermal overload relay, TeSys Deca, 37-50A, class 10, box terminals LRD350

Medium-power motor protection in demanding installations

The LRD350 overload relay from the TeSys Deca series is a thermobimetallic device designed to protect electric motors from the effects of overload. The current setting range of 37…50 A makes it suitable for protecting drives with power ratings from approximately 18 to 25 kW at a three-phase 400 V supply - typically found in fans, water pumps, compressors, or belt conveyors in industrial installations.

The device works directly with contactors LC1D40A, LC1D50A, and LC1D65A - it is mounted underneath them as an expansion module. This ensures the installation occupies minimal space in the control cabinet. Built-in 1NO + 1NC auxiliary contacts allow for integration with the control system - the normally open (NO) contact signals the trip state, while the normally closed (NC) contact prevents re-closing until reset.

Operating parameters and construction

The maximum operating voltage is 690 V AC at a mains frequency of 0…400 Hz, allowing for use in both standard 50 Hz industrial installations and more demanding environments. The trip threshold is set at 1.14 ±0.06 Ir according to the IEC 60947-4-1 standard - the device reacts when the operating current exceeds approximately 114% of the set value.

The power circuits are equipped with EverLink BTR screw terminals, which allow for the connection of single-core or flexible wires (with or without ferrules) with a cross-section of 1…35 mm². The tightening torque for the power circuit is 5 N·m, ensuring a durable and secure electrical contact. Control circuits are implemented on screw terminals accepting wires of 1…2.5 mm² - here the torque is 1.7 N·m.

The overall dimensions are 70 × 55 × 123 mm with a weight of only 0.375 kg. The compact design facilitates placement in space-constrained cabinets.

Protective and operational functions

The 10A trip class indicates a time-delay characteristic adapted for squirrel-cage motors - the device will not trip during normal startup (up to ~10 seconds) but effectively protects against long-term overload. Additionally, the LRD350 detects phase loss: if two phases exceed the trip threshold (130% Ir), the third phase causes an immediate trip even at a lower load.

Resetting can be done in two ways - manually via the blue button on the housing or automatically after the thermobimetallic element has cooled down. Automatic reset allows for a quick return to operation without operator intervention.

Temperature compensation operates in the range of -20…60°C, so the characteristic does not drift with changes in ambient temperature - which is important for installations in production halls or engine rooms.

Standards, certificates, and operating conditions

The product meets the requirements of international standards:

Standard	Scope
IEC 60947-4-1	Low-voltage switchgear and controlgear - Contactors and motor-starters
IEC 60947-5-1	Control circuit devices and switching elements
UL 60947-4-1 / CSA C22.2 No 60947	US/Canadian certification
EN/IEC	European certification

Mechanical resistance includes shocks up to 15 Gn for 11 ms and vibrations up to 4 Gn - IEC 60068 standard. The enclosure protection rating is IP20, meaning protection against finger contact and against solid foreign objects >ø12.5 mm; however, it is not intended for operation in heavy dust or splashing environments without an additional enclosure.

The relay holds marine certificates (DNV-GL, RMRS, LROS) and ATEX INERIS approval, allowing it to be used in explosion-hazardous zones.

The product is covered by the Green Premium label - a declaration of compliance with RoHS and REACH directives and the absence of mercury and heavy metals, confirmed by manufacturer documentation.