GE relay 735 and IEEE Std C37.112-1996 1

[Birch]

Hi

The GE relay 735/737 defines ANSI inverse curves using a different formula than the IEEE standard C37.112-1996 for overcurrent inverse-time characteristics.

Is this perhaps because the standard is newer than this relay?
Why do then GE call the curves with name "ANSI"? Does such pre 1996 ANSI/IEEE standard really exist?

Although the IEEE and GE formulae are different, the resulting curves do roughly match at small current ratios. At high currents the match is not good. Especially extremely inverse curves differ a lot.

Link to GE 735/737 manual:

http://www.geindustrial.com/products/manuals/735737/735man-de.pdf

I would be grateful of any response.

 

[dpc]

I got bit by this once. Since the relay is what it is, I didn't worry too much about the reason, I just made sure I was using the correct equation. But there is definitely a difference between the old "ANSI" equation and the "IEEE" equation in the GE/Multilin relays.

 

[stevenal]

According to the introduction of C37.112 no previous standard existed. Yes, I believe the Multilin line of relays predate the standard. Perhaps the "ANSI" designation refers to the fuse curves the relay curves are meant to coordinate with. The fuse standards are older.

 

[cuky2000]

We should keep in mind that the early approach of the microprocessor relay was to emulate the electromechanical counterparts producing similar curve and physical shape.
On the other hand, the compliance with the standard by the manufacturer is optional. As far the curve resemble approximately and adjusted from moderately to extremely Inverse the manufacturer may be allow to called ANSI curve.

An interesting note how the equation provided by Multilin is related with the standard C37.112 is by observing the equation 1 of the enclose article.

http://ascc2004.ee.mu.oz.au/proceedings/papers/P205.pdf

Notice that the terms a_n and b_m are called coefficient of Sachdev model. Happens that Dr. M. S. Sachdev is an active member of the IEEE Standard C37.112

I hope this note could help to clarify some of your questions.