Engineered Series rated equipment

Status
Not open for further replies.
I

Intrepid

Member
Location
Newburgh, Indiana
Can someone explain to me the idea behind engineered series rated and tested equipment as mentioned in section 240.86(A) and (B) of the 08 NEC? The labeleing requirements are understandable and clear, but I just finished doing an update class for a large facility and not a single one of the electrical PEs had a clue what series rated equipment even was. I did my best at explaining, but would like more insight for next time:confused:

What defines the difference between tested combinations and engineered systems?
 
The difference is that the listed tested combination have actually been tested under fault conditions and found to perform as required. The engineered system is one designed by an experienced power engineer based on the time current curves of the OCPDs involved and the fault currents. The issue with the engineered system is something called "dynamic impedance"...and this is very very difficult to account for in the engineering calculations. I am sure one of the engineers here will jump in and give a better explanation.
 
don_resqcapt19 said:
The difference is that the listed tested combination have actually been tested under fault conditions and found to perform as required. The engineered system is one designed by an experienced power engineer based on the time current curves of the OCPDs involved and the fault currents. The issue with the engineered system is something called "dynamic impedance"...and this is very very difficult to account for in the engineering calculations. I am sure one of the engineers here will jump in and give a better explanation.
Click to expand...

Sounds like a good start.
The reason why the electrical PEs may not know what you are talking about is because some areas do not allow series rated equip. I have seen man a doc's say "shall use Fully rated equip"

The reasoning may be to prevent the wrong use later down the line.
I have seen a hotel wired were the Main panel was rated at 65k with its branch breakers at 42k, the house sub was rated a 10k the sub fed from the sub was rated a 22k.
The EC installed the equip in the wron order. He did not know any better. If the equip is fully rated it should not matter what order.
 
Engineered series ratings are very difficult to create. If OCPDs are involved the engineered must confirm that they will not try to open (creating dynamic impedance) until the fault has been cleared by other devices. In existing installations engineered solutions are best left to determining equipment withstand (SCCR) capability instead.
 
don_resqcapt19 said:
. . .The engineered system is one designed by an experienced power engineer based on the time current curves of the OCPDs involved and the fault currents. The issue with the engineered system is something called "dynamic impedance"...and this is very very difficult to account for in the engineering calculations.
Click to expand...
Dynamic impedance is the impedance that is actually in an arc. As an arc grows in size, the actual impedance changes so as to make an overcurrent device change the amount of time it takes to interrupt a fault.

Don, the dynamic impedance is not something that can be predicted. The only way to deal with dynamic impedance is to use the tried and true method of testing.

The only way to use the engineered series rating is that the "experienced power engineer" has to be sure that the overcurrent protection that is being protected will remain closed for the first ? cycle of the fault. This will permit the current limiting device to limit the fault current to the protected device. There are very few situations where this will actually work and it is seldom used.

The proposal was made by George Ockuly and supported by several of the Panel 10 members. It was basically extracted from the IEEE Blue Book as a viable alternative to replacing large air-frame circuit breakers. The thing that is scary about this process is that the engineer must indeed be an experienced power engineer and be aware of the possibility of more let through fault current than down stream devices can handle. In other words, the entire plant distribution system needs to be looked at to see if this will work. :)
 
And as I recall the permission to use an engineered system was hotly debated between the fuse and breaker people in the ROPs and ROCs.
 
don_resqcapt19 said:
And as I recall the permission to use an engineered system was hotly debated between the fuse and breaker people in the ROPs and ROCs.
Click to expand...
Absolutely! The proposal came from the fuse people and the breaker people would rather sell a large piece of new equipment rather than protect a large air-frame circuit breaker. Additionally, most people would use a set of fuses and switch when used in areas that have a formidable amount of fault current.

Another thing that may be done is to use the settings on a circuit breaker to have it latch for the first ? cycle of the fault and then protect it with a fuse. Works well if you have an adjustable circuit breaker. :)
 
charlie said:
Absolutely! The proposal came from the fuse people and the breaker people would rather sell a large piece of new equipment rather than protect a large air-frame circuit breaker. Additionally, most people would use a set of fuses and switch when used in areas that have a formidable amount of fault current.

Another thing that may be done is to use the settings on a circuit breaker to have it latch for the first ? cycle of the fault and then protect it with a fuse. Works well if you have an adjustable circuit breaker. :)
Click to expand...

A UL489 breaker will always try to open when the current gets too high (they inherently self protect) an ANSI/UL circuit breaker will stay closed only if an INST function is not included.
 
Thanks a million!!!

Thanks a million!!!

:smile:

You guys are awesome! Thanks so much for your insight. I have sat in classes with an attempt made to explain these systems, and just got better insight from you all. Wow- this forum is GREAT!
 
Yes - I was in an arc flash training seminar with guest speakers from both the breaker industry and the fuse industry, and their comments were in direct opposition concerning arc flash reduction and breaker/fuse arc curves, etc. The audience was not pleased that the "data" proved both fuses and breakers to be the "best" at minimizing arc fault current. My question on this one: who do we really believe???

Ken
 
Intrepid said:
. . . who do we really believe??? . . .
Click to expand...
Actually, both. :roll:

The reality is that both have the best for particular circumstances.

For circuit flexibility, circuit breakers win. If it is coordination, fuses win. For less expensive installations, it is normally circuit breakers. High available fault current and larger size installations, it is normally fuses.

When dealing with these guys, you are looking at apples and oranges where both are giving you data but it is not the same data. :)
 
Status
Not open for further replies.
Top