electromechanical relays

[Shavner]

I'm trying to understand the differnece in operation between a 50/51 and a 67...

I understand that the 67 is a directional device and will only operate if current is in the reverse direction.

My question...what happens to a 50/51 relay when current is in the "wrong" direction?

 

[mvcjr]

A 50/51 relay is bidirectional. It will operate once the set threshold is reached without regards to current direction.

God bless!

 

[jbartos]

Suggestion: Reference:
IEEE Std 141-1993 (Red Book)
1.
67 is ac directional overcurrent relay. It functions on a desired value of ac overcurrent flowing in a predetermined direction.
2.
50 is instantaneous overcurrent relay that functions instantaneously on an excessive value of current in either direction.
3.
51 is ac time relay. It functions when the ac input current in either direction exceeds a predetermined value, and in which the input current and operating time are inversely related through a substantial portion of the performance range.

 

[busbar]

Aside — often there are electrical systems where power may flow in either direction at a particular node, and you may want to have varied tripping for different directions. Simply— busbars, cable, fuses, breakers and transformers do not care the direction of power flow. Typically, they operate unaffected by direction.

A 67 device will have an additional pair of terminals for a voltage- or current-polarizing source to establish directional characteristics.