Thermal Overload Protection for cables, dry type xfr. 1

[slavag]

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

 

[Marmite]

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/...urrent_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

 

[slavag]

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