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Thermal Overload Protection for cables, dry type xfr.

Thermal Overload Protection for cables, dry type xfr.

Thermal Overload Protection for cables, dry type xfr.

(OP)
Dear Colleagues.
Do we have some standard for the thermal overload curve in digital relays? It's some exponential curve.
I see in many relays eqv.
Teta (termal capacity)=
(I/1.05 x I (FLC/FLA))^2 x (1- e^(t/t(time constant)) x 100%
What is a source of this eqv?
I don't know why, but I think it's BS142 standard.
What is a ANSI/IEEE standard for 49 curves ( if any)?
Thanks for your help.
Regards.
Slava

RE: Thermal Overload Protection for cables, dry type xfr.

I believe that the equation originates from IEC 255-8 (1978). I've seen references to this standard in books illustrating the equation.

See http://library.abb.com/GLOBAL/SCOT/SCOT296.nsf/VerityDisplay/A5AEDE05CD6A6BDAC1256E54006DA68F/$File/1MRK509003-BEN-A-en_Thermal-overcurrent_relay_and_protection_RXVK_2H__RAVK.pdf

Also the Areva NPAG Chapter 19 on motor protection has some similar equations and curves illustrated.

Also, the following paper may be of interest:-
http://www.selinc.com/techpprs/6190_UnderSFTMO_SZ_20051020.pdf

Regards
Marmite

RE: Thermal Overload Protection for cables, dry type xfr.

(OP)
Dear Marmite.
Thanks a lot.
You are great!!!!!
Today it's IEC60255-8 (1990).
Best Regards.
Slava.

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