LR3D** relays are used for the unbalanced loads : there can be different currents in the three phases of the overload relay ( according to its size ) . examples : distribution circuits which have not the same loads on each phases, or single phase motor. This relay will not trip if the currents in its phases are different.
The LRD** which is for balanced load : that means with the same current on the three phases ( e.g. a squirrel cage motor ).
This is the only difference between LRD & LR3D relay.

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