Long time Pickup settings (lr)

[Ithinkimstillhere]

I am doing a breaker coordination study for a new unit substation. I have a single set of 500MCM (good for 380A) protected by an 800A static adjustable trip breaker. Do I need to set the long-time pick up settings 0.5 (400A) to properly protect the feeders?...I do not have the current loads

 

[templdl]

I am doing a breaker coordination study for a new unit substation. I have a single set of 500MCM (good for 380A) protected by an 800A static adjustable trip breaker. Do I need to set the long-time pick up settings 0.5 (400A) to properly protect the feeders?...I do not have the current loads

If the long time is capable of being field adjustable up to 800a then the cable must be rated at lease 800a.
If the long time adjustment can be locked or sealed in the 400a position being accessible by onle qualified people the you could protect a 500mcm cable with it. Does that make sense? You just don't want a situation where some thinks that they can just wall up and cranks the long time adjustment up from the 400a setting.

 

[George Stolz]

Do you have a code reference for that? If so, I'm in trouble.

 

[jumper]

Do you have a code reference for that? If so, I'm in trouble.

The adjustable trip rating? 240.6(B) and (C).

 

[jim dungar]

The adjustable trip rating? 240.6(B) and (C).

A plastic cover (standard on most adjustable trips) which 'requires a tool to open' is usually deemed sufficient to provide restricted access. Most of these covers have a method of being sealed by a wire or cable tie. I encourage the sealing to be done with 'bright pink' ties as they allow an easy visual check that the breaker has been commissioned with my settings.

 

[GoldDigger]

And that you support breast cancer awareness.

Sent from my XT1080 using Tapatalk

 

[BostonEE]

As long as the circuit breaker is located behind locked doors accessible only to qualified personnel (240.6(C)(3)), i.e. located in an electrical room, you're okay. But the long-time pick up setting should be 0.4 (320A) if the conductor is only rated for 380A.

 

[don_resqcapt19]

... But the long-time pick up setting should be 0.4 (320A) if the conductor is only rated for 380A.

Why? As long as the calculated load is 380 amps or less, you can protect a 380 amp conductor with a 400 amp OCPD.

 

[topgone]

I am doing a breaker coordination study for a new unit substation. I have a single set of 500MCM (good for 380A) protected by an 800A static adjustable trip breaker. Do I need to set the long-time pick up settings 0.5 (400A) to properly protect the feeders?...I do not have the current loads

Setting your long time pickup at 0.5 (400A) means you will get warnings when the load nears 380A and will surely trip when 400A is reached. Keep in mind that the 380A rating is based on cables installed with 75 deg C rated lugs and an ambient temperature of 30 deg. C. You could be good at 0.5 LTPU setting; if your ambient temp is higher than 30 deg or you have limitations on your terminal lugs, you could choose the next lower setting, IMO.

 

[weressl]

Setting your long time pickup at 0.5 (400A) means you will get warnings when the load nears 380A and will surely trip when 400A is reached. Keep in mind that the 380A rating is based on cables installed with 75 deg C rated lugs and an ambient temperature of 30 deg. C. You could be good at 0.5 LTPU setting; if your ambient temp is higher than 30 deg or you have limitations on your terminal lugs, you could choose the next lower setting, IMO.

Look at the trip curves a little more carefully and you will see that a 400A setting will result in the breaker 'potentially' tripping after 100-10000 seconds, but due to the deadband and repeatibility the value could be anywhere 0-10% of setting.

 

[mayanees]

Do you have a code reference for that? If so, I'm in trouble.

The other section being referenced here is 240.4(B), which allows protection of <800A conductors at the next highest standard setting over their ampacity, with the standard sizes being listed in 240.6(A).

 

[BostonEE]

Why? As long as the calculated load is 380 amps or less, you can protect a 380 amp conductor with a 400 amp OCPD.

I thought he or she was only doing the coordination study and wouldn't know what the calculated load is. If I were doing a coordination study I would limit my assumptions.

 

[kingpb]

The rating of the terminals is a mute point, it is an 800A breaker, which means it can take 800A at 75 deg C. Being able to handle 400A is not an issue.

If I was doing the settings, I would set it at 400A. Remember the cable is actually good for 430A at 90 deg C. At 430A on the cable, I doubt the terminals will be anywhere near 75 deg C, since they are capable of 800A at 75 deg C.
at 400A, which the trip device will initiate you should be well within all the ratings.

 

[don_resqcapt19]

I thought he or she was only doing the coordination study and wouldn't know what the calculated load is. If I were doing a coordination study I would limit my assumptions.

Why would you be doing a coordination study if you don't know the load? You have to know the load to pick the OCPDs and conductors.

 

[BostonEE]

Why would you be doing a coordination study if you don't know the load? You have to know the load to pick the OCPDs and conductors.

The OCPD and conductors are already selected, the original post is only establishing the settings - perhaps as a third party review. Either way, you're right, I forgot about 240.4(B) and the ability to exceed the ratings of the conductors.