Phase-failure sensitivity [t-head2]

Overload relays Z offer, due to their design, an effective protection against phase failure. They have phase failure sensitivity to IEC 947-4-1 and VDE 0660 part 102 and therefore can also provide protection for EEx e motors (→ following diagramms).

When the bimetallic strips in the main current section of the relay deflect as a result of three-phase motor overloading, all three act on a trip bar and a differential bar. A shared trip lever switches over the auxiliary contact when the limits are reached. The trip and differential bars lie against the bimetallic strips with uniform pressure. If, in the event of phase failure for instance, one bimetallic strip does not deflect (or recover) as strongly as the other two, then the trip and differential bars will cover different distances. This differential movement is converted in the device by a step-up mechanism into a supplementary tripping movement, and thus accelerates the tripping action.

Design note → section Motor protection in special applications;

Further information to motor protection → section All about Motors.