Cable Damage Curve Plot on a TCC 1

[rockman7892]

Why on a Time Coordination Curve (TCC) the cable damage curves are not a continuous line. They always start at the high current value and then go to about midway through the plot before they stop. Then at the top of the plot at the highest time the pickup of full load rating of the cable is shown as a small mark. Why are these not shown as a continuous line?

When plotting protective devices against these cable damage curves must you have both segments of the cable damge curve above or to the righ of the protective device curve? Should the protective device curve even be below the cable damge curve pickup point? I'm referring to breakers upstream of the cable, but how do downstream breakers come into play?

Also on a slightly different subject does the transformer inrush point shown of on a TCC need to be completely clear of a LV breaker tolerence bandwidth. What if the inrush point is not completely below the LV breaker band but rather falls righ in the middle of the band? Is this a potential problem?

 

[alehman]

The cable damage curves are intended for coordination during fault conditions. Lower currents are categorized as overload and the damage curves are not defined in this range. The NEC doesn't allow for overload conditions.

If the inrush point is within the band, then there is theoretically risk that the breaker may trip. Sometimes this is difficult to avoid. The default inrush points in most software only rule-of-thumb values.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[dpc]

Regarding the transformer inrush - I assume you are talking about a LV breaker on the transformer primary? If the inrush point is inside the breaker's tolerance band, you are at risk of it tripping on inrush. But as Alan said, this inrush point is a very gross assumption used as an aid in setting the primary protection.

I used to use 6X to 8X at 0.1 sec for xfmr inrush. Now I use 10X to 12X after dealing with some nuisance tripping problems over the years.

As far as cable damage curves, virtually all of the software packages I have seen use the old GE Short Circuit Heating Limit TCC GES-9503. It ends (or starts?) at 10 seconds. I suspect beyond that time frame the specific heat transfer conditions of a particular installation become the determining factor regarding insulation damage.

In my experience, cable damage for a downstream fault is almost never an problem. Long-term overloads are a different story and not really a coordination issue.

David Castor

http://www.cvoes.com

 

[rockman7892]

Alehman and dpc Thank You for your responses!

As far as the cable damage I saw somewhere in a book or paper that the cable damage curve should always be above the maximum avaliable fault current at 0.1s. Do you guys ever heard of this or have any experience with this? DPC I know you said that in your experience cable damage for a downstream fault is never a problem which would indicate to me that there is no need to worry about the cable curve being above the fault current?

Beside the item I mentioned above about the cable damage curve being above the fault current do you need to make sure that the protective devices are set below the cable damage curves at all points less than 10s? Does it matter which device upstream or downstream?

So it sounds like any time above 10s is considered a long term overload. For these cases I have seen software packages put a hash mark at the top of the plot representing the ampacity of the cable. Is it important than to make sure that the circuits OCPD is below or to the left of this mark? If so does it matter of it is the upsteream or downstream device in relation to the section of cable? DPC is this what you are referring to whey you mentioned long term overloads as not being a coordination issue?

 

[dpc]

For overload protection, you need to comply with NEC and other requirements. That trumps anything you might put on a TCC.

If the cable damage curve is "above" the maximum fault current, then it is not an issue, if I'm interpreting your statement correctly.

So it sounds like any time above 10s is considered a long term overload

I don't think anyone said that. But TCCs are intended for use in coordinating responses to faults, not so much for overloads.

In olden times, overcurrent relay TCCs did not even start until 1.5x the pickup setting, because response times for currents below this value were inherently unpredictable.

David Castor

http://www.cvoes.com

 

[rockman7892]

Thanks dpc!

I agree for overload protection NEC requirements must be followed and this would trumph anything on a TCC. So on a TCC I guess a LV breaker pickup could be above the cable rated ampacity shown on the TCC as long as it was compliant with the NEC.

For looking at overloads what about the case of MV cables where the NEC allows a breaker or fuse setting up to 300% and a breaker setting up to 600% for protecting a cable. Would these be cases where since the NEC allows much more flexibility in settings that you would want to look more carefully at a TCC plot especially in the overload region to verify propoer protection?

So as far as the fault region goes which as you said if the purpose of the TCC do we want to make sure upstream breaker is below cable curve in fault region or is this not necessary if damage curve is above max fault current?

I know the Buff book covers a section on cable protection but is there any other standard that covers this protection in detail as well?