TCC - Equipment Protectoin

[Mike01]

In looking at a coordination study I noticed the following:
Main switchgear consist of 480V ANSI Sw. Gear. [single ended] with LVPCB's and electronic trip units

Feeder No.2 is a 800A feeder with parallel 600's per phase to a 800A dist. panel

The dist. panel [main lug only] is listed with a 65kAIC rating [3-cycle bus bracing]

The instantenous pickup on Feeder No.2 is set above 65k amps [set to 70k]

Is the equipment still properly protected? the clearing time of the 65k rating would be into the short time setting on the breaker exceeding the 3-cycle rating of the dist. panel.

Should [I believe] the instantenous maximum setting be at 65k to match the dist. panel 3-cycle rating to provide proper equipment protection? wanted to get your thoughts?

 

[petersonra]

In looking at a coordination study I noticed the following:
Main switchgear consist of 480V ANSI Sw. Gear. [single ended] with LVPCB's and electronic trip units

Feeder No.2 is a 800A feeder with parallel 600's per phase to a 800A dist. panel

The dist. panel [main lug only] is listed with a 65kAIC rating [3-cycle bus bracing]

The instantenous pickup on Feeder No.2 is set above 65k amps [set to 70k]

Is the equipment still properly protected? the clearing time of the 65k rating would be into the short time setting on the breaker exceeding the 3-cycle rating of the dist. panel.

Should [I believe] the instantenous maximum setting be at 65k to match the dist. panel 3-cycle rating to provide proper equipment protection? wanted to get your thoughts?

by the time you figure in the cable impedance chances are there will not be 65kA at the MLO panel. there may not be 65kA available at the main regardless of the setting.

 

[ron]

Feeder No.2 is a 800A feeder ....

If it has an 800A breaker, then how can the instantenaous be set at 70kA? Generally the highest inst setting is 12x, which would be ~10kA

Maybe I'm misunderstanding the question? MAybe the TCC shows the 800A inst protection curve all the way out to 70kA and it isn't realized that it protects to the left of that too?

 

[Mike01]

LVPCB

LVPCB

If it has an 800A breaker, then how can the instantenaous be set at 70kA? Generally the highest inst setting is 12x, which would be ~10kA

Maybe I'm misunderstanding the question? MAybe the TCC shows the 800A inst protection curve all the way out to 70kA and it isn't realized that it protects to the left of that too?

Ron,
I will post the TCC when I have a chance the breaker is a LVPCB with 1600A frame and sensor, the LT setting has been adjusted to 0.5 [800A]. I understand the actual fault current [as designed] is lower [likely in the 50kA range], however it just did not make sense to me to provide a higher instantenous 3-cycle rating that the downstream panelboard is listed for?

 

[beanland]

Instantaneous is Optional

Instantaneous is Optional

I do protection for a living, there is nothing that requires the use of the instantaneous trip element. If this is an LSI trip unit, the S element may still operate fast enough to protect the panel even if the I element is set above the available fault current. Review of the TCC which shows the entire picture is required.

 

[ron]

I do protection for a living, there is nothing that requires the use of the instantaneous trip element. If this is an LSI trip unit, the S element may still operate fast enough to protect the panel even if the I element is set above the available fault current. Review of the TCC which shows the entire picture is required.

It becomes pretty complicated if you don't have instantaneous protection. Most equipment (not SWGR) in the US have a 3 cycle rating. If you don't pickup until short time, you need to be sure the equipment (not SWGR) can withstand the current and most equipment doesn't have >3 cycle ratings published.

 

[kingpb]

As Beanland stated, need to see the curve.

The 70kA that is indicated on the curve is going to be for the main bus where the feeder breaker is located, not the distribution board.

Also, there is nothing that requires instantaneous setting to be set. In actuality, it may have been set so high because it can't be disabled but the person making the setting didn't want it to trip.

But, need to see the curve.