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Contactor weld monitor makes redundancy redundant

08 June, 2007

Contactors used in safety applications are often connected in series pairs to protect against the risk of welded contacts, which can occur especially towards the end of contactors’ service lives. But using two contactors in this way can be costly, particularly for large devices.

Moeller claims to have come up with a less costly alternative in the form of a contact monitoring device (CMD) which checks whether a contactor’s main contacts have become welded. It compares the contactor control voltage with the state of the main contactors and indicates this using a mirror contact. If the contactor coil is de-energised and the contactor does not drop out, the CMD trips a backup circuit-breaker, motor protection breaker or switch-disconnector, via an under-voltage release.

Moeller CMD

The CMD (shown above) also monitors the functioning of the internal relay using an extra auxiliary make-and-break contact on the monitored contactor. The extra contact is positively driven and the break contact is designed as a mirror contact. The CMD is designed to be used with specific Moeller contactors and breakers.

Previously the series connection of two contactors was recommended for circuits of safety categories 3 and 4. Moeller suggests that one contactor and a CMD will now be sufficient.

The monitoring relay can be used for emergency-stop applications in compliance with EN 60204-1. It is also suitable for crane applications that use bar-mounted contactors.

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