Circuit protection

[Keitu]

Greetings

I have a 6600/550, 250KVA transfomer that I will like to provide protection for at the secondary(550V) side. I was thinking of using a MCCB to protect it. What are the limitation of these breakers.
This transformer is currently supplied via the ring switch on the 415KVA mini sub from the main sub.
Everytime I get a fault on the 525 circuit I loose the mini sub as well as the feeder at the main substation trips.

 

[gepman]

I assume that the "525 side" is the secondary of your 250 kVA transformer. It sounds as if you have a coordination problem. A MCCB may or may not solve your coordination problem, you will have to check the Time Current Curves for your new MCCB and the feeder to the mini sub, and the mini sub. Obviously you want your MCCB to trip first. I would purchase an electronic trip unit to make it adjustable.

I am from 480V land so I haven't used any at 550V but since most are rated at 600V I wouldn't see a problem at this low fault level (on a 250kVA xfmr). I assume that this is a solidly grounded system. MCCB's definitely are not rated highly for corner grounded delta systems and if you have an ungrounded wye or delta then you want to be sure that you fix your faults quickly (it sounds as if you don't) so you won't have a single pole fault condition (which is the problem with MCCB's on ungrounded or corner grounded systems).

Most of the major MCCB companies have brochures on the application of MCCB's to go into detail on their limitations.