Series rated system question

[ryan_618]

I am reviewin plans for a large building that is using a series rated system. In one of the branch circuit panelboards, they have a TVSS. My question is, can 285.6 be satisfied through the use of series rating?

285.6 Short Circuit Current Rating.
The TVSS shall be marked with a short circuit current rating and shall not be installed at a point on the system where the available fault current is in excess of that rating. This marking requirement shall not apply to receptacles.

Thanks in advance.

 

[charlie]

Re: Series rated system question

IMHO any equipment must be applied within its withstand rating and a series rated system will satisfy that requirement. As an example, suppose a piece of equipment had a withstand rating of 25 kA and a panelboard had 40 kA in front of it. The main was rated at 42 kA and the branch circuit breakers are rated for 22 kA. Now suppose a fault were to occur that was downstream from the panelboard at this piece of equipment and the possibility for 40 kA was there. What would happen is that both the main and the branch circuit breaker would start to open at the same time. The dynamic impedance of both arcs (in the circuit breakers) would keep the fault current at the piece of equipment from exceeding 22 kA. Most likely, the fault current would be considerably less.

So, IMO the answer to your question is yes.

 

[ron]

Re: Series rated system question

Ryan,
If the calculated short circuit current available at the panelboard is higher than the short circuit rating of the TVSS that is installed in the panelboard alone, then it fails. There is no series rating for TVSS's. Nobody has ever tested the series combination between an upstream device and a TVSS, unless they are in the same enclosure and listed as a individual piece of equipment (OCPD and TVSS together). Although the panelboard itself may be ok, since it has a series rating with the OCPD's, the TVSS is no good.

Charlie,
In your hypothetical case of a downstream load being rated for a withstand of 25kA, and the upstream panelboard OCPD's have an adequate series rated combination, making the panelboard acceptable, you would still need to calculate the short circuit available at that load without considering any upstream current limiting.
If the calculated fault current at the load, that is rated with a withstand of 25kA, calculates out to be higher than 25kA, then it doesn't pass too.

[ October 07, 2004, 11:36 PM: Message edited by: ron ]

 

[charlie]

Re: Series rated system question

Ron if that is the case, then what would be the purpose of ever using a current limiting device?

 

[ron]

Re: Series rated system question

Current limiting devices are to accomplish a listed combination series rating. I also use them when calculating arc flash fault currents, to ensure I result in a low arc flash fault current value, which when applied to the OCPD characteristic gives a longer fault clearing time (depending on the device down stream) and adequate PPE category. It lowers the PPE requirement if the current limiting device is the device located where the energized work is occurring.

I also understand that in 2005 or a proposal for 2008 includes an opportunity for a PE to sign their life away, and under "engineering supervision" determine through analysis (up over and down method on the let through curves) that a series rating exists, even if it hasn't been tested. This was supposed to be for older devices that don't have a series rating currently (and nobody wanted to test the old device).
Hard for me to imagine who will sign a document saying this. Dynamic impedance seems too unpredictable to be really sure which will open first.

[ October 08, 2004, 07:47 AM: Message edited by: ron ]

 

[charlie]

Re: Series rated system question

Ron, you are correct on the engineered series rating . . . to a point. This method can not be used unless:</font>

• <font size="2" face="Verdana, Helvetica, sans-serif">A licensed professional engineer designs the system</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">This PE must be engaged in the design or maintenance of electrical installations</font>

<font size="2" face="Verdana, Helvetica, sans-serif">Even the IEEE Blue book suggests that this is a safe way to approach a protection strategy. This is not a method that can be used everywhere so it is not likely that you will see much of it.

Tell my why a current limiting device can't be used to protect a downstream device if the downstream device will stay latched during the first 1/2 cycle? You know that is the case with the old, large air frame circuit breakers and with modern circuit breakers with a delayed trip on them.

I do know that the IAEI doesn't like it because they have to rely on the PE's calculations. IMHO, that is no different than relying on an architect and engineer to design a tall building. I assure you that the inspectors are not double checking their calculations.

 

[ron]

Re: Series rated system question

Charlie,
I agree that that structural calculations of a engineer or Arch are not challenged in the way that electrical calculations are challenged.

I believe that a current limiting device (fuse or circuit breaker) cannot reliably be counted on to open prior to a downstream circuit breaker unlatching. Even for an old slow device.

Dynamic impedance, and the lack of concrete information regarding the amount of fault current that the current limiting device will see, make it impossible to know how long it will take the fuse to operate. We do a good job calculating the maximum fault current at a piece of equipment in a distribution system, but we don't know what the fault current will be during an actual fault. There may be some impedance during the fault, or just a low available fault current, that will result in the fuse operating outside of its current limiting range and the fuse taking several cycles to operate. This gives the old slow circuit breaker an opportunity to begin its unlatching and clearing operation, introducing further dynamic impedance to the mix.

IMHO, it is too unreliable to count on a non-tested series rated combination, and I would not at this time support the code permitted engineered series rating by any PE in my office.

 

[jim dungar]

Re: Series rated system question

The current limiting effect of overcurrent protective devices can always be taken into account. But they cannot ever be used to generate/engineer a series rating with most other "fault interrupting" overcurrent protective devices.

The up-over-and down method has never been "approved" for determining the performance of series connected dynamic devices. The only OCPDs it can be used with are ones with withstand as opposed to interrupting ratings

 

[ron]

Re: Series rated system question

Jim,
To apply the current limiting effects for OCPD's with such characteristics would require that no other OCPD in that portion of the distribution, would begin to clear the fault, thus changing the current limiting devices capability to open in a current limiting range.
Why is it different for application of a series rating for equipment with a withstand or AIC rating? I would think it is similar, unless the current limiting device is the only device on the entire circuit upstream of the fault, which I cannot imagine when that could ever happen.

 

[ryan_618]

Re: Series rated system question

I spoke wtih Chuck Mello of UL and CMP 5, and he spoke to one of his guys that he claims is very sharp on the subject. They are of the opinion that the TVSS can only be installed in accordance with 285.6, whether there is a series rating or not.

 

[jim dungar]

Re: Series rated system question

Ron,

Most "iron frame" breakers without instantaneous tripping are available with 30 cycle withstand ratings which allows full coordination with upstream OCPDs. Most molded case devices include a self protecting instantaneous tripping point which makes them incompatible with engineered series ratings.

 

[charlie]

Re: Series rated system question

I want to make it clear that I am not an engineer but I work with engineers that I trust. I also work with several engineers on Code Making Panel 10 that I trust as well. Vince Saporita is one of those, he is Vice President of Technology Sales and Services for Bussmann. I asked Vince about this and this is his reply:

"The current-limiting action of a current-limiting fuse or current-limiting circuit breaker can be taken to the bank. The let-through current will never exceed that value found from the let-through charts, no matter what device is downstream, whether the downstream device exhibits dynamic impedance in the first 1/2 cycle or whether it is totally passive. Take a minute and look at the let-through chart for a current-limiting fuse or a current-limiting circuit breaker. As long as the short-circuit current is less (goes to the left on the x-axis), both the peak let-through current and the RMS let-through current is also less. No matter how much the short-circuit current is decreased by the dynamic impedance of a downstream device, the let-through current is also decreased. It is Ohm's law. If the impedance is increased (because of the dynamic impedance of a downstream device), the current is decreased. I am amazed by the statements that try to say that the dynamic impedance created by the downstream device will actually cause the fuse to let-through more short-circuit current. That's trying to violate Ohm's law.

As for the TVSS short-circuit current rating, it will most generally have been short-circuit tested with a specific line-side overcurrent protective device, many times with a current-limiting fuse. If that is the case, the TVSS device can then only be applied where the available short-circuit current is equal to or less than the TVSS's short-circuit current rating. You can't use the current-limiting ability of more than one device at a time, and the first one is the one that was used to give the TVSS device its short-circuit current rating. Now, if the TVSS device's short-circuit current rating is not based upon a particular upstream overcurrent protective device, then the let-through chart may be utilized to determine if the device is being utilized within its short-circuit current rating. For example, if any passive device is rated for 5,000 amperes for three cycles, and the available short-circuit current is 25,000 amperes, a check of the let-through chart might show that the let-through current is 4,000 amperes. That would be a good application. If the let-through were 6,000 amperes, it would not pass.

As for the new Code allowance for series rated circuit breakers, let-through currents can easily be utilized by professional engineers for existing installations where the downstream device is a power (air frame) circuit breaker that takes 3 cycles to open (no dynamic impedance in the first 1/2 cycle). There are no currently acceptable methods to engineer an up-over-and-down series rating for molded case circuit breakers that exhibit dynamic impedance during the first 1/2 cycle. The circuit breaker and fuse manufacturers are in agreement over this. However, there are other engineering calculations that can be made by the consulting engineer for existing installations. For example the engineer might evaluate an existing underrated series rated system for use with specific commercially available current-limiting fuses. (All recognized combinations of series rated fuse-breaker combinations have been tested with special umbrella limiters that must let-through more I squared t and peak current than the UL standards allow. That way, in the field, it doesn't matter which manufacturer's fuses are utilized. They will always be better than the ones with which the circuit breakers were tested.) When real, commercially available fuses are utilized, they will be much more current-limiting than the umbrella limiters. Or, the engineer might witness testing of exactly the same breakers at commercial high current labs. Or, the engineer might hire any number of testing laboratories that would test the desired combination to the UL 489 requirements."

I edited this to put Vince's actual title into the post. If you want to know about some of the papers he has done, put "Vince Saporita" or "Vincent J. Saporita" in the Google search engine.

[ October 11, 2004, 08:19 AM: Message edited by: charlie ]

 

[ryan_618]

Re: Series rated system question

Thanks for your help guys, I really appreciate it. I wrote it up as a required correction on the plans I am reviewing and will try to remember to post here what comes out of it.