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Siemens modernises thermistor motor protection relays

21 October 2016

Siemens is bringing a new product family of thermistor motor protection relays onto the market.

The Sirius 3RN2 replaces the preceding 3RN1 series, and with widths of only 17.5 and 22.5mm these relays require less space in a control cabinet. A standardised wide-range supply voltage reduces the number of versions. With just one device, machine manufacturers and users cover all supply voltages throughout the world. Sirius 3RN2 thermistor motor protection relays reliably protect motors against overheating by measuring the temperatures at points at risk of overheating within the motor. PTC (positive temperature coefficient) temperature sensors are integrated into the motor windings to monitor critical overheating.

Right from the start, the Atex versions of the new generation of Sirius 3RN2 thermistor motor protection relays were developed with the focus on functional safety. Verification of their suitability for SIL 1 applications fulfils an important requirement of DIN EN 50495. The verification of suitability for Atex is valid for both gas-laden and dust-laden atmospheres. This makes production safe for machine manufacturers and users, even under difficult conditions, such as those found in the oil, gas and chemical industries, and in mills and sawmills.

Commissioning of the new Sirius 3RN2 devices is simple. On the one hand, no parameters have to be assigned, which reduces the possible errors for the user. On the other hand, commissioning does not require any detailed knowledge of the motor, such as the maximum temperature of the particular motor type.

Background information:

Thermistor motor protection relays are used particularly where circuit breakers or overload relays do not provide adequate protection. In these cases, the motor may overheat, if, for example, the fan provided as standard fails or the ambient temperature exceeds the maximum defined in the assumed boundary conditions. Typical applications for thermistor motor protection relays are for a heavy start of, for example, centrifuges, braking procedures, frequent switching, such as in jogging and reversing operations, and where there is a risk of the motor cooling being impaired by contamination, for example in the paper and textile industries and in cement works.

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