Series Vs. Fully rated

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Mike01

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What are some of the advantages / disadvantages to fully vs series rated why select one over the other? Just curious on your opinions...
 

jim dungar

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ryan_618 said:
One disadvantage to series ratings is that they can't be selectively coordinated.

Not always true. For low fault currents, they are difficult but not impossible. For example, the coordination of a branch circuit which may have 2kA SCA at the end of its run may be possible even if the branch breaker is series-rated at 22/10kA.

Almost all residential panels are based on series ratings.

Series ratings have been used for over 25 years. Yes there are trade-offs that must be considered, but I don't know that one system is always better than the other. I have very little problem using series ratings for "lighting and appliance" branch circuit panels, but I am reluctant to use them on power panels.

A common complaint about series ratings is that the replacement equipment must be chosen carefully. However, I have seen many panels where "standard rated" breakers have been ignorantly(?) added after the original installation of "fully-rated" breakers.
 

ryan_618

Senior Member
Jim, if the fault current was that low, it wouldn't be a series rated system. The breakers would be fully rated if the fault current is only 2K. If the fault current one inch away from the breaker is over the fully rated value of the circuit, you have a series rated system. How do you selectively coordinate it if you are depending on the upstream device to open?

* I need to add a note here that I am not an expert on this, but what I am saying seems like logic to me.
 

jim dungar

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ryan_618 said:
Jim, if the fault current was that low, it wouldn't be a series rated system. The breakers would be fully rated if the fault current is only 2K. If the fault current one inch away from the breaker is over the fully rated value of the circuit, you have a series rated system. How do you selectively coordinate it if you are depending on the upstream device to open?

* I need to add a note here that I am not an expert on this, but what I am saying seems like logic to me.

One of the most common misconception is that series ratings always require the upstream device to open.

Every breaker needs to be short circuit rated for the fault current available at its line side terminals. But, a breaker needs to be coordinated only for the fault curent that can flow through the circuit. Rarely are these two fault currents the same value.

Imagine 200ft of #12AWG conductor fed from a 20A 1-pole branch breaker installed in a 400A 208Y/120 MB panel. For a coordinated system only the fault current through both the 20A branch and the 400A needs to be considered.
 

steve66

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Illinois
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ryan_618 said:
Jim, if the fault current was that low, it wouldn't be a series rated system. The breakers would be fully rated if the fault current is only 2K. If the fault current one inch away from the breaker is over the fully rated value of the circuit, you have a series rated system. How do you selectively coordinate it if you are depending on the upstream device to open?

* I need to add a note here that I am not an expert on this, but what I am saying seems like logic to me.


Im not an expert either, but I know Square D has some literature that says its not true that series rated breakers can't be coordinated. There may be some difference here between "fully coordinated" and "coordinated". In some cases the fault would introduce some additional impedence that would keep the fault current below its maximum calculated value.

For non-critical applications, I don't see any reason why someone would always require fully rated equipment. The cost just wouldn't be justified for a breaker that has a very small probability of ever tripping anyway. (That's not to say fully rated equipment is never justified, but I think you have to weigh the cost vs. the possible benefits.)

Steve
 

winnie

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Springfield, MA, USA
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Electric motor research
It is probably fairer to say that in a series rated system, selective coordination can be probabilistic but cannot be guaranteed.

In a series rated system, you are depending upon the _combination_ of the upstream device and the downstream device to open in the event of a fault which may exceed the capability of the downstream device. I don't believe that the upstream device is required to open first; simply that one of the two devices open.

In the example that Jim gave, if the fault downstream of the 20A breaker were to happen _inside_ the panel, and were to be some sort of bolted fault (the forklift driver smashes the panel, and the 12ga wire gets pinched into the neutral bus), then the fault current might approach the maximum available at that feeder. This would presumably exceed the rating of the 20A breaker, and IMHO all bets are off as to which breaker would open first.

But the likelihood is that the fault will occur somewhere down the circuit, which will greatly decrease the magnitude of the fault current, no matter what the available short circuit current at the feeder. It only takes 40 feet of #12 to reduce short circuit current to 2000A in a 120V fault.

If the actual fault current involved is less than the rating of the 20A breaker, then it may be possible to coordinate it.

-Jon
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
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Electrical Contractor
After all that's been said, in my opinion, the only advantage of fully-rated equipment is that you don't have to do any homework, but that's an expensive easy-out.
 

ryan_618

Senior Member
winnie said:
In the example that Jim gave, if the fault downstream of the 20A breaker were to happen _inside_ the panel, and were to be some sort of bolted fault (the forklift driver smashes the panel, and the 12ga wire gets pinched into the neutral bus), then the fault current might approach the maximum available at that feeder. This would presumably exceed the rating of the 20A breaker, and IMHO all bets are off as to which breaker would open first.

-Jon

And that would be a lack of coordination. This is what I was trying to say, but I am not as smart as Jon.
 

coulter

Senior Member
ryan_618 said:
And that would be a lack of coordination. ...
Let's see... Fork through the panel, and it only hits the one #12. Okay, that could happen. Add: the panel has critical equipment on it. Ok. The main trips along with the 20A branch cause it's series rated equipment.

Is this a design problem with series rated equipment and lack of coordination? Maybe, but I'm thinking if you have a critical panel venerable to forklifts, the answer is, "Full-rated equipment is fine, but not as important as concrete and steel".

carl
 

ryan_618

Senior Member
I don't recall reading anything about forklifts. What am I missing?

I was thinking more along the lines of a fault due to someone working in an energized panel creating a fault in the panel, or, more likely, a fault in a box close to the panel.
 

coulter

Senior Member
ryan_618 said:
I don't recall reading anything about forklifts. What am I missing?...
Uhhh... I don't know. Take a look at post 12 by Ryan Jackson, quoting winnie(Jon) in post 10.

carl
 

coulter

Senior Member
ryan_618 said:
... I was thinking more along the lines of a fault due to someone working in an energized panel creating a fault in the panel, or, more likely, a fault in a box close to the panel.
Okay. My, translation is: your concern is for lack of coordination for critical panels where the fault is in the panel or very close (low impedance) to the panel. Is that close to what you are thinking?

carl
 

davidr43229

Senior Member
Location
Columbus, Oh
In a series rated system, you are depending upon the _combination_ of the upstream device and the downstream device to open in the event of a fault which may exceed the capability of the downstream device
Jon is correct here. This is why the Breaker people and Fuse people submit these combinations to UL to achieve a higher AIC rating. Both Overcurrent devices are intended to open at the same time, which means you loose selective coordination.
One disadvantage to series ratings is that they can't be selectively coordinated
This is to reinforce Ryans point.
Truth be known, even with fully rated systems you can not achieve Selective Coordination, due to the instantious region within the breaker (.01)
People generally use a Series Rated system to save money, until they have a fault and the mains trip out and they fully understand what series rated systems really mean then. It's a trade-off, you get what you pay for.
Just my $.02
 

jim dungar

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PE (Retired) - Power Systems
davidr43229 said:
Jon is correct here. This is why the Breaker people and Fuse people submit these combinations to UL to achieve a higher AIC rating. Both Overcurrent devices are intended to open at the same time, which means you loose selective coordination.
This is to reinforce Ryans point.
Truth be known, even with fully rated systems you can not achieve Selective Coordination, due to the instantious region within the breaker (.01)
People generally use a Series Rated system to save money, until they have a fault and the mains trip out and they fully understand what series rated systems really mean then. It's a trade-off, you get what you pay for.
Just my $.02

There is no assurance ever that the upstream device will always open in order to protect the downstream device in series rated systems. It is not possible to predict in which order the devices will clear the fault, that is why they must be tested. Manufacturers can, and do, use the results of series ratings tests to confirm that breaker to breaker coordination does exist at specific maximum fault levels.

And, while expensive, it is possible to fully coordinate circuit breakers. I have helped with one system: from a 120V receptacle circuit, a 208Y/120 panel main breaker, a step down transformer feeder breaker, an ATS feeder, to the 3200A 480V main breaker.

edited: cleaned up my last statement, made it an example
 
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Mike01

Senior Member
Location
MidWest
Thanks...

Thanks...

Ok, I am starting to understand the difference, does anyone know where I can find some good literature comparing the two systems, from what it sound like series rated systems are more difficult to coordinate, but less expensive equipment to purchase because of the series rating of the breakers, however the maintenance staff needs to be aware of this when repairing, replacing, or adding new electrical equipment. A fully rated system is easier to coordinate, more expensive, but easier to maintain as it is not dare I say idiot proof but makes it easier for maintenance, repair. Also future additions would probably be easier with a fully rated system, if it is series rated without all the correct documentation there would be a lot of field investigations to verify existing equipment. For non critical installations ex. big box retail it might be acceptable but for labs, healthcare etc. I believe you would want a fully rated system. One last thing when looking at an existing installation in the field is there any easy way to tell if the existing system is series or fully rated??
 
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