Overdutied Electrical Equipment

[pgonski]

There have probably been tons of back & forth on similar items,but here it goes.

The fault level on the utility primary is 24,000 A. A new enclosed service main breaker was installed that is rated 65kA and is thus fine.

However, after that breaker, the cables branch off to two main lug only panelboards, with 10kA breakers. All of the breakers are a mix of 1969 vintage GE & Square D. The fault level here is 21,000 A.

These two panels have branch breakers which feed about 20 other panels downstream, all of which are 10kA rated, and are calculated to be overdutied. Some of these panels have main breakers with 42kA, but I can not find any visible evidence that these were series rated. The panels are a wide variety of manufacturers and go back 50 years.

I'm stumped on the next step of ideas to solve this. Am I correct that placing a current limiting fuse at the main entrance would have to be series rated with all of the breakers in the panels through tests? I don't see how anything like this could guarantee that the fuse would clear the fault before any of the other devices start to open.

 

[petersonra]

There have probably been tons of back & forth on similar items,but here it goes.

The fault level on the utility primary is 24,000 A. A new enclosed service main breaker was installed that is rated 65kA and is thus fine.

However, after that breaker, the cables branch off to two main lug only panelboards, with 10kA breakers. All of the breakers are a mix of 1969 vintage GE & Square D. The fault level here is 21,000 A.

These two panels have branch breakers which feed about 20 other panels downstream, all of which are 10kA rated, and are calculated to be overdutied. Some of these panels have main breakers with 42kA, but I can not find any visible evidence that these were series rated. The panels are a wide variety of manufacturers and go back 50 years.

I'm stumped on the next step of ideas to solve this. Am I correct that placing a current limiting fuse at the main entrance would have to be series rated with all of the breakers in the panels through tests? I don't see how anything like this could guarantee that the fuse would clear the fault before any of the other devices start to open.

I never heard the term overdutied before.

Is the available fault current at the utility primary or secondary? In any case, is it real or based on some infinite bus idea.

I do not believe you can just add a CL fuse unless it has been tested with the downstream combinations.

Are these 208/120 panels? The 10kAIC rating suggest that to me. Might be cheapest just to replace them with new panels that are properly rated. Could probably be done in a few hours each if prepared for. Not the cheapest solution.

Sometimes there is not a cheap answer.

It has been there all these years and not blown up yet. Chances are it will be OK.

You might be able to add reactors to reduce the fault current, but I doubt you can reduce it by half in any practical way. Might be able to protect some of the downstream panels that way. Might even be able to just add some wire into the feeder to a PB to get the fault current down. I would start with a closer look at the calculations. Many times there seem to be a lot of conservative estimates and assumptions made that skew the fault current higher than it really is.

When you say the breakers are a mix of different brands do you mean the PBs are a mix of different or that there are mixtures of brands of CBs within the same PB.

 

[pgonski]

The fault level is on the secondary side of the utility transformer, they only provided infinite bus. Sadly, since the transformer is 25kV-120/208, with a 2% impedance, even with a 100 A fault on the primary, we are still way over the ratings.

I did explore reactors, but that would introduce a huge voltage drop in the facility. I don't think there is a cheap solution, outside of calculating which panels can be replaced with 22kA rated breakers.

My hunch is the original installation had a series rating as it was all GE equipment with a main. This has all been replaced over the years, so any series rating is voided.

I never heard the term overdutied before.

Is the available fault current at the utility primary or secondary? In any case, is it real or based on some infinite bus idea.

I do not believe you can just add a CL fuse unless it has been tested with the downstream combinations.

Are these 208/120 panels? The 10kAIC rating suggest that to me. Might be cheapest just to replace them with new panels that are properly rated. Could probably be done in a few hours if prepared for correctly. Not the cheapest solution.

Sometimes there is not a cheap answer.

It has been there all these years and not blown up yet. Chances are it will be OK.

You might be able to add reactors to reduce the fault current, but I doubt you can reduce it by half in any practical way.

 

[petersonra]

can you install another transformer 208 to 208 after the main?

it would not surprise me one bit to find out that in the early 70s whoever installed the original system just did not care about the available SCC.

 

[templdl]

There have probably been tons of back & forth on similar items,but here it goes.

The fault level on the utility primary is 24,000 A. A new enclosed service main breaker was installed that is rated 65kA and is thus fine.

However, after that breaker, the cables branch off to two main lug only panelboards, with 10kA breakers. All of the breakers are a mix of 1969 vintage GE & Square D. The fault level here is 21,000 A.

These two panels have branch breakers which feed about 20 other panels downstream, all of which are 10kA rated, and are calculated to be overdutied. Some of these panels have main breakers with 42kA, but I can not find any visible evidence that these were series rated. The panels are a wide variety of manufacturers and go back 50 years.

I'm stumped on the next step of ideas to solve this. Am I correct that placing a current limiting fuse at the main entrance would have to be series rated with all of the breakers in the panels through tests? I don't see how anything like this could guarantee that the fuse would clear the fault before any of the other devices start to open.

What did you say the system voltage was? And what was the amp rating of the main breaker again? I may have missed that.
Without having the need to shoot at shadows this information would be nice to know and then series ratings may be considered.

 

[kwired]

You may have trouble with series ratings, would depend on exactly what you have, and age of it could have some impact. Otherwise if it were all new and all from same manufacturer you very well may have some 10kA rated equipment that is fine if downstream of other equipment it is series rated with. These ratings are generally not something you will find a cross between brands though, the breaker manufacturer wants to sell their gear and not promote a competitors gear.

You see this all the time in smaller "loadcenters" a 22kA main and 10kA branch breakers, it works as long as the supply is not over 22kA.

 

[pgonski]

The overall system is a 120/208V system. Main Breaker is 1200A.

Several of the panels that are overdutied do have a main breaker that is 42kA with 10kA branch breakers. However, there is nothing to indicate that these were part of a series rating as it is from the mid-1960s.

The only solution i can think of is replacing all of the panels that are 10kA with 22kA.

The panels themselves are a mix of Square D circa 1990, and GE vintage 1967 breakers

You may have trouble with series ratings, would depend on exactly what you have, and age of it could have some impact. Otherwise if it were all new and all from same manufacturer you very well may have some 10kA rated equipment that is fine if downstream of other equipment it is series rated with. These ratings are generally not something you will find a cross between brands though, the breaker manufacturer wants to sell their gear and not promote a competitors gear.

You see this all the time in smaller "loadcenters" a 22kA main and 10kA branch breakers, it works as long as the supply is not over 22kA.

 

[petersonra]

The overall system is a 120/208V system. Main Breaker is 1200A.

Several of the panels that are overdutied do have a main breaker that is 42kA with 10kA branch breakers. However, there is nothing to indicate that these were part of a series rating as it is from the mid-1960s.

The only solution i can think of is replacing all of the panels that are 10kA with 22kA.

The panels themselves are a mix of Square D circa 1990, and GE vintage 1967 breakers

Just curious how you became involved in this situation. It appears like this has been there a long time. How did it manage to become an issue at all?

I do not see any inexpensive solution either, although one might expect the 1990 vintage Square D breaker boxes might have a less expensive answer as you might be able to just add a MCB to the PB.