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Rail-mounted double-pole electronic circuit breaker

21 October 2009

E-T-A Circuit Breakers has introduced a rail-mounted electronic circuit breaker, the E-T-A ESS22-T, that is compliant with the machinery safety standard EN60204-1. The device offers selective disconnection and double-pole physical isolation of loads, only disconnecting faulty load circuits in the event of an overload or short circuit. It provides electronic overcurrent protection for ungrounded 24Vdc power supply systems and comes with a choice of eight current ratings from 0.5A to 10A. Response to an overload or a short circuit is fast enough to ensure stability of the 24V control voltage for the other loads, such as PLCs, control systems and bus couplers, to prevent stoppages of the entire automation system.

Short circuit currents are limited to around 1.4 times the rated current and the faulty load is disconnected after approximately 100ms. Inrush peaks however, from switching on capacitive loads up to 20,000 microF for example, are tolerated, thus avoiding nuisance interrupts. In the event of an overload, the electronic circuit breaker will disconnect at typically 1.1 times the rated current after 3 seconds.

The type ESS22-T can be snapped easily onto symmetrical rails, and, with a width of just 22.5mm and height/depth of 90mm, it ensures space saving in compact control cabinets. Status indication of the connected load circuit is provided by a multicolour LED and the condition of the ON/OFF switch. In the event of a failure, two physically isolated ‘make and break’ contacts provide failure signalisation to a control system. The load circuit can be re-set using the manual ON/OFF switch, a switch which can also be used during maintenance.

For processes powered by 24Vdc power supplies without circuits protected by a device such as the ESS22-T, an overload in a single circuit will cause the power supply to shut down the output to all the circuits it supplies. On recovery it may go into ‘hiccup’ mode. Costly downtime, scrap and even danger to personnel may result. Traditional protection solutions do not provide the answer. Circuit breakers with fast operating characteristics would be likely to cause nuisance trips during current peaks and switching on of capacitive loads. On the other hand MCBs are too slow, and neither delayed operation, nor high-performance, circuit breakers will respond adequately in this situation to overcurrents. The power supply will therefore switch off completely, bringing all connected loads, i.e. the entire system, to a standstill.

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