Transformer seconday side Breaker

[Humble2000]

a 750KVA ANN transformer secondary side breaker is ordered with a 1000AMp LTP and fixed LTD . This is 1.4 times more than Tx FLA at 600V. Breaker is rated for 100% rated name plate operation.
Two issues is first the primary S&C fuse at 25KV does not coordinate with a 25 AMp fuse ( they touch at LTD) and the bigger size would violate the ESA ruels for more than 1.5 times of primary amps.
Does the consulatnt pick a wrong size breaker?
I dont think the breaker would provide adequate transformer protection with that size breaker.

 

[Zogzog]

What type breaker?

 

[davidbeach]

Define "adequate transformer protection". If the breaker curve is below/to the left of the transformer damage curve you have protected the transformer against through faults even if you don't have overload protection. It is often difficult to coordinate primary and secondary protection, but if they overlap the outage is the same regardless of which clears (or if both clear). With fuses on the primary, however, it would be nice for the breaker to clear for anything on the low side to avoid the need for fuse replacement.

 

[Humble2000]

It's cutler hammer.

NDC,LSIG, Non adjustable Rating Plug 1000

 

[Humble2000]

Well , transformer is delta-wye. There is a portion of the single-phase and three phase curves ( three phase shifted to the left) which is not protected by the fuse , so we need the secondary protective device,nd even that a small portion of the transformer single phase curve is not covered.
The question is if that size breaker is aequte for transformer protection.

 

[Zogzog]

That breaker may or may not have adjustable trip settings, I cant tell from the partial type you listed. If it dosent you can swap the rating plug.

 

[Humble2000]

Well, How much overload the transformer can tolerate. I assume in this case 0%. No cooling on the transformer . I am not sure why consuktant picked 1000 amp rating for this breaker?

 

[unclebob]

First of all, NDC breakers are rated at 65 kA and 80% rated name plate at 600 V. For 100 %, you need a CNDC, witch is 65 kA too.

Is this a padmount transformer? If so, they're usually equipped with internal primary fuses. Standard here are
RTE Bay-0-Net 358C 25 A.

If you really want to lower the LT pick-up, you can replace the 1000 A rating plug with a 800 A, for a better overload protection.

 

[Humble2000]

unclebob,
you are right. it is rated at 80%. so does this mean in SKM I have to use a 800 plug instead of 1000 Amp plug to count for 800A ratings. by the way LTD is also fixed.

 

[dpc]

No, if it's a 1000 A plug, it's a 1000 A plug. The 80% rating just means it can only carry 800 A on a continuous basis. With "continuous" defined as three hours or more.

The 1000 A breaker will still have a time-current curve based on the 1000 A trip plug and related LT pickup settings.

 

[unclebob]

FWI

The Long Time Delay is fixed at 1 x rating plug and 12 sec delay.

BTW, your trip unit seems to be a RMS310. If you can adjust
both SDT pick-up and delay, then the SDT has a flat caracteristic. If there's only one knob to select STD
pick-up, then the delay is fixed at 100 msec and has a I2t caracteristic.

Those are tricky ones...