NCEES Power #137

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Hi,

It's a problem with time dial setting on 50/51 relay. The problem states that a minimum pick up for 50/51 relay is 420A. Looking at the graph, curves originate at roughly 1.5 of pick-up setting. Shouldn't 1.5 (on x axis) correspond to 420 A, since that's where curves originate, hence lowest pick up current relay senses?

At 1.0 (supposedly 420A) relay would never trip.

Thank you for clarification.

 

[rysmith2vt]

The minimum pickup correlates to 1.0. The curves should actually extend all the way to the top of the graph. If the curves were shown completely, they would extend and cross at the top of the graph at approximately 1.0.

I'm not sure this answers your question completely. Let me know.

 

[BamaBino]

This is my graph

http://i12.photobuck...lvis256/137.jpg

 

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The minimum pickup correlates to 1.0. The curves should actually extend all the way to the top of the graph. If the curves were shown completely, they would extend and cross at the top of the graph at approximately 1.0.

I'm not sure this answers your question completely. Let me know.

It does. I am a young engineer and haven't had much exposure to Time-Current Curves. My assumption was that relay would not be able to pick up anything once the curve ends.

Did not realize that I could "extend" them all the way to top. My assumption was that area on left was the "operating region" of device ( i.e. breaker) and relay should never trip there. Using your method relay would trip after 100 seconds @ .6 of 420A on one of the settings. Does it mean that this relay should be used on breaker that's rated below 250A?

For simplicity sake, if I see similar problem on test I will assume that 1.0 on X axis corresponds to minimum pick up setting.

 

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To answer my own question. IEEE 141 states that values below 1.5 of pick up setting cannot be accurately predicted and are not plotted as a result, but can be requested from individual manufacturers.

Looks like I'll have to expand my reference materials to understand each question in details.