Adjustable trip

[augie47]

I have an 800 amp circuit breaker protecting some 500 kcmil conductors.
The equipment supplier advises the breaker is adjustabel to an "Ir x .5 which would provide 400 amp protection. The install seems to meet the provisions of 240.6(C), IF the "Ir" is (long time pick up setting)

I am not familiar with the terminology on breaker trip settings,
Does "Ir x .5" effectively give us 400 amp protection ?

 

[zog]

Typically yes but every OEM uses different terms, what is the breaker and trip unit type?

 

[augie47]

From the equipment mfg (CAT) trying to solve the problem:

"I have an 800 Amp frame on the shelf.

PG800 with the Micrologic 3.0 adjustable trip.
This is the same basic breaker but with an 800 Amp sensor.
At the Ir X .5 will provide the 400 Amp trip setting you are looking for. "

Sound to me like SqaureD, but I have not seen the unit.

 

[kingpb]

Yes, the value they are giving you would be the overload setting, or long-time pick-up.

 

[Jraef]

I_r = current (I) rating (_r)

So if you have an 800A trip unit, 800A is the I_r and therefore .5 I_r becomes 400A.

It's more clear when you can see the subtext, but a lot of systems cannot properly display it, or people typing it get lazy, or web graphics people don't understand that it's important, etc. etc. It can get confusing, especially when you get into things that can be confused with real words, such as In instead of I_n to mean "normal" current. These type of notations are actually specifically defined in IEC standards, but are bleeding over into our world because of globalization. Get used to it.

 

[zog]

From the equipment mfg (CAT) trying to solve the problem:

"I have an 800 Amp frame on the shelf.

PG800 with the Micrologic 3.0 adjustable trip.
This is the same basic breaker but with an 800 Amp sensor.
At the Ir X .5 will provide the 400 Amp trip setting you are looking for. "

Sound to me like SqaureD, but I have not seen the unit.

That is correct but it sounds to me like they are trying to make something work Ir is the trip rating which is In (Rating plug) x LTPU setting. So if you have a 800A plug and set LTPU to 0.5 it will trip at 400A. Note 0.4 is as low as the 3.0 version goes so you may be limiting yourself. If I am reading between the lines correctly you need 400A trip? Better to use a 400A rating plug. Also, the 3.0 is the most basic version of that trip unit with no metering or other features and only provides long time and INST protection. Not sure if that is what you want?

 

[augie47]

From an inspectors standpoint, I had a breaker with what looks t be a non-interchangable trip unit marked 1200 amps protecting one set of 500m kcmil.
CAT advised that with the Micrologic 3 adjustable trip the minimum protection would be 480 amps.
They propose to change the breaker to a 800 amp (to maintain the same frame and eliminate mounting problems) and use the M3 adjustable trip to obtain a 400 amp
rating.
With my limited knowledge of all the bells and whistles on modern breakers
I simply was trying to confirm that reducing the " Ir " setting to .5 satisfies 240.6.
They are asking me to sign off on that plan and I do not have the knowledge to
confirm that changing the "Ir" setting as opposed to other settings meets the Code.
From the answers above, it appears that the plan is viable,

 

[zog]

From an inspectors standpoint, I had a breaker with what looks t be a non-interchangable trip unit marked 1200 amps protecting one set of 500m kcmil.
CAT advised that with the Micrologic 3 adjustable trip the minimum protection would be 480 amps.
They propose to change the breaker to a 800 amp (to maintain the same frame and eliminate mounting problems) and use the M3 adjustable trip to obtain a 400 amp
rating.
With my limited knowledge of all the bells and whistles on modern breakers
I simply was trying to confirm that reducing the " Ir " setting to .5 satisfies 240.6.
They are asking me to sign off on that plan and I do not have the knowledge to
confirm that changing the "Ir" setting as opposed to other settings meets the Code.
From the answers above, it appears that the plan is viable,

Yes it is viable, but isn't there some new (2011) article about the adjustable trip units? Think it is only if they are accessable? In that case a 400A plug is better answer. What is the info off the existing 1200A breaker?

 

[augie47]

I will attempt to get more info on the existing.

 

[jim dungar]

Yes it is viable, but isn't there some new (2011) article about the adjustable trip units? Think it is only if they are accessable? In that case a 400A plug is better answer. What is the info off the existing 1200A breaker?

There is nothing in the NEC that would prohibit this installation, based on th info provided.

While not common, we have been known to use the trick of a larger frame breaker with a dialed down trip point (e.g. Ir = .5). This provides a relatively high magnetic point (which is often based on In) for selectively coordination, especially with high transformer inrush.

 

[templdl]

All said please don't overlook the application of NEC art. 240.6(B)Adjustable trip breakers.