Series rated system?

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Psychlo

Psychlo

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Melissa, TX
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Master Electrician
I am finding it difficult to come to a clear understanding of what the difference is between a fully-rated and series-rated system. I realize that it has to do with available fault current levels, but what is the practical difference to the installer? In other words, how does the installation differ on a series-rated system from that of a "regular" system? Is the actual equipment different? Or is it just how the equipment is installed?
 
Fully rated means that each and every part of the equipment is fully rated to clear the listed fault current.
Series rating means that there is a tested combination of two or more protective devices that end up with an overall rating that will clear the listed fault current. Some of the individual components are typically rated much less than the overall rating, making the cost much cheaper.
 
In short,

A fully rated system is one in which all the OCPD's and associated electrical equipment is rated to handle the available fault current to the system.

A series rated system is one in which testing has been done by a NRTL to determine that an OCPD with a rating that matches or exceeds the available fault current to a system can effectively "protect" lower AIC rated devices downstream.

Pete
 
Thanks guys for the replies. I'm understanding it a little better now, but still a bit fuzzy on application. Is it possible to bug you for an quick example of each system? Sorry for being dense, but I'm the visual type. If I can get a picture of this in my head, I'll be able to comprehend it.
 
The actual equipment will pretty much look the same, and be installed in essentially the same fashion. These are just circuit breakers and panelboards and the like.

The difference is the interruption rating of the particular devices installed.

In a power distribution system, you will generally have OCPD (fuses, circuit breakers, etc.) which feed other OCPD, which feed other OCPD, and so on down to your final branch circuit. Each of these OCPD will have ratings, eg the 'trip rating' , the 'voltage rating', and the 'ampere interrupting' rating. This latter rating is where 'series rating' comes into play.

Circuit breakers are rated to open against a rated current flow; exceed this current flow during a fault condition, and the breaker will _try_ to open, but may fail in the process. An arc might form which the breaker does not extinguish, mechanical components may fail, etc. Literally, the thing could blow up. The fault current is set by the voltage and impedance of the supply (transformer) and all of the conductors leading up to the fault. The available fault current from a transformer will normally be 10-30x the full load current of that transformer, so you can see that even in small systems (say a 100A residential service) the fault currents can easily be in the kiloamp range.

In a series rated system, a particular combination of upstream OCPD and downstream OCPD is rated to operate with potential fault currents that exceed the individual rating of the downstream device. It is still just an upstream circuit breaker and a downstream circuit breaker, but this particular _combination_ has been tested to deal with the series rated fault current.

-Jon
 
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series-ratings

series-ratings

Lou,
From an economic point series-rating is a must.
Compare the price of a 22k or 10k aic breaker to one of the same amperage rated 65k. Imagine the cost effect if you had to install every breaker at
full rating on a 65,000 amp AIC system.
I've seen no negative input concerning a properly designed and maintained series-rated system
 
Augie,
The problem I have is that it is often not maintained correctly.
If the series rating is based on an upstream fuse for example, the first time the fuse blows, the replacement is almost never the one that has a series rating anymore.
 
ron said:
The problem I have is that it is often not maintained correctly.
If the series rating is based on an upstream fuse for example, the first time the fuse blows, the replacement is almost never the one that has a series rating anymore.
Click to expand...
That's why labeling the equipment with proper replacement requirements are part of the series-rating specifications.

The advantage of fully-rated equipment is that no thought is required; it's just expensive. Series-rating of equipment requires a little brain work, but it's well worth the time.
 
Is it possible to bug you for an quick example of each system? Sorry for being dense, but I'm the visual type. If I can get a picture of this in my head, I'll be able to comprehend it.
Click to expand...

As to your 'dense' statement, you're not alone.

Perhaps a quick example would be nice. This is made up, I haven't even bothered to remember a 'real' example, but it gives the idea. You're putting a new, larger service on a small strip mall store, which means a new meter main. The utility says you need 14K equipment. You call the friendly distributor, who says there are two options.

Option 1, new panel, main, etc., with all the breakers rated 22k. Costs $2200 for this fully rated option.

Option 2, new panel, main, etc. But this is a factory series rated panel. So only the main circuit breaker is 22k, the branch breakers can now be 'regular' 10k rated. Costs $1600.

Another page with a nice explanation would be in the Siemens online training series, take a look at Load Center Ratings in part 3 of 3 load centers at
http://www.sea.siemens.com/step/downloads.html
 
Ron wrote Augie,
The problem I have is that it is often not maintained correctly.
If the series rating is based on an upstream fuse for example, the first time the fuse blows, the replacement is almost never the one that has a series rating anymore
Click to expand...
Ron, I agree the owner replaces with whatever is cheaper or in stock. They don't take the time to replace "like-with-like" to maintain the tested rating.

Lou wrote:Is series rated to be avoided?
Click to expand...
Absolutely not. It is a means to depend upon especially in a renovation or add-on.
Series-Rating simply means that 2 specific overcurrent protection devices "go" at the same time, sharing the fault current. It is a UL approved means of design help to gain a higher fault rating, with a lesser AIC OCPD.
Just my $.02
 
ron said:
Augie,
The problem I have is that it is often not maintained correctly.
If the series rating is based on an upstream fuse for example, the first time the fuse blows, the replacement is almost never the one that has a series rating anymore.
Click to expand...

Ron, I certainly don't disagree. But with the cost difference, I doubt we will ever see many fully rated systems, especially at high fault currents.

I wish every electrician and plant or commercial electrical maintence person was required to watch one of the "fault current" films the fuse folks have in order to see the safety issue associated.
 
One thing to remember is that you can never use "classified" breakers in a series rated system. All breakers must be OEM and if that breaker is no longer available you will have to replace the panel and all of the breakers.
Don
 
In my opinion series ratings in residential and commercial facilities are less likely to be invalidated in the future than are fully rated systems. Most violations of SCA ratings that I have seen are because a branch breaker was installed without ever considering the rating of the fully rated existing devices. If these had been series rated systems the cheaper more readily available replacement breaker would not have caused a problem.

Almost every series rating using fuses is based on a UL "umbrella" fuse so it does not matter which manufacturer's fuse is used as a replacement. As long as the fuse clips prevent a different class of fuse from being installed the series ratings is still valid. I often suggest class J fuses because they are more difficult to substitute.
 
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