Fault Clearing Time
Fault Clearing Time
(OP)
For arc flash calculations of an MCC, should we only calculate the fault clearing time of the upstream breaker on the primary side of the transformer? Or should we calculate the fault clearing times for breakers on both primary and secondary sides of transformer, and then choose the faster fault clearing time to calculate incident energy? Thanks.






RE: Fault Clearing Time
If you believe that you could get a fault on the LV tails between the transformer and the MCC incomer then you may well have an awkward combination of very high fault level and slow clearance time. Bear in mind that a primary-side earth fault relay won't help you if you have a typical Dyn* transformer, so you're relying on the HV overcurrent element unless you also have a unit protection scheme to clear the fault more rapidly.
RE: Fault Clearing Time
I see that in ETAP, changing the clearing time of any of the two transformer breakers (primary or secondary) changes the incident energy for the MCC. I am not able to understand how they enter the clearing times of two breakers in the formulas? My understanding is that the clearing time of only one breaker (not two) can be entered in the formulas?
RE: Fault Clearing Time
RE: Fault Clearing Time
RE: Fault Clearing Time
Etap is based on IEEE 1584. I highly recommend getting a copy - and reading it. I personally believe it is best to understand the computer program algorithm.
Keep in mind not all faults are bolted. In fact, for grounded systems, most start phase to ground and don't trip until they go phase to phase (personal opinion based on anecdotal evidence). The maximum fault energy may well be at intermediate levels (arcing faults).
Consider plotting the two CB Trip curves. Your changing the clearing time may make the curves overlap - which could easily change the energy at the MCC. I've even been known to get log-log paper an plot by hand. I don't have a light table anymore - but I have taped a few to the window.
Yes, as noted, you could well need some knowledgable help. However, nothing is stopping you from doing your own reading and investigating.
ice
Harmless flakes working together can unleash an avalanche of destruction
RE: Fault Clearing Time
RE: Fault Clearing Time
If one:
Normally, the primary CB is sized somewhat larger than the secondary CB (taking into accound the XFM ratio of course). The Primary CB must stand inrush current - the secondary does not. It is not uncommon to have the primary at 250% FLA and the secondary at 125%. So faults at the MCC that don't get into the instantaneous range likely will trip the secondary first.
However, for smaller xfm, say 225kva and smaller, it is not uncommon for the primary and secondary to both be 125%FLA. In this case, a fault in the MCC could easily trio either - or both.
ice
Harmless flakes working together can unleash an avalanche of destruction
RE: Fault Clearing Time
"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
RE: Fault Clearing Time
"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
RE: Fault Clearing Time
What causes confusion is that I have read the following statement somewhere: """the protective device that will clear the fault at the arc flash analysis location must be determined. In the example of distribution transformer, this is the transformer high side (primary) fuse."""
But as you guys have suggested; taking the transformer secondary breaker as the one which clears the arc might possibly be a better option.
RE: Fault Clearing Time
You are discussing faults downstream of the secondary CB. Both CBs are in the circuit. You really need to lay out the curves - or let ETAP do it - and see which one trips first.
I certainly would not take the Secondary CB as the limit, without checking. Also, I'm uncomfortable with changing the clearing times of the CB. All these CBs have a trip curve and spec. That is what you should be using. If you are changing the clearing times, where are you getting the numbers from?
And, I still recommend you read IEEE 1584.
ice
Harmless flakes working together can unleash an avalanche of destruction
RE: Fault Clearing Time
RE: Fault Clearing Time
I don't know if I should laugh or frown. I think I'll laugh, that's too silly to frown.
ice
Harmless flakes working together can unleash an avalanche of destruction
RE: Fault Clearing Time
Usually you will have more than one LV feeder connected to the main transformer and therefore you want the LV breaker to operate before the main breaker (That's coordination). Interesting to note that you now have a fuse protecting the transformer not a breaker. Different configuration. There is a limit to how far your main MV device (fuse) will reach due to transformer impedance and connection wires between your transformer LV side and MCC. It might even not operate on a fault at your MCC (Utilities have multiple stories of electrical fires started because of a faulty LB breaker on the customer main pannel)
Please be careful and as suggested by Iceworm, read IEEE 1584. ETAP should be able to give you the proper clearing time based on the fault level but If you can't find the operating time of your protection, use the default clearing time suggested in the method. It will be conservative but safe. And for such a simple configuration I would even suggest that you do it by hand so that you understand how arc flash is performed.
RE: Fault Clearing Time
Iceworm, jghrist was talking about me (not you).
RE: Fault Clearing Time
RE: Fault Clearing Time
yes - My comment did not come out humorous
no body language available - sigh
Yes, sometimes I need the hammer - double sigh
ice
Harmless flakes working together can unleash an avalanche of destruction
RE: Fault Clearing Time
If you can't understand this concept of physical access or exposure then you should not be doing these calculations since you're putting someones life at risk.
And I certainly hope you're not just entering a clearing time without ensuring it's a reality.
RE: Fault Clearing Time
The question is the IE at the MCC. The secondary breaker is at the transformer not the MCC. There is no physical access to the source side of the secondary breaker because it is in a different piece of equipment.
RE: Fault Clearing Time
RE: Fault Clearing Time
You never specified where the secondary breaker is installed. There are many installations where the transformer feeds the main breaker inside the first section of a MCC. Does that not make the main breaker cell part of an MCC? If you understood what I posted then anything after the first cell can include the secondary breaker.
Desrod posted that you want to co-ordinate the protection for good reasons. There was no need for your insulting response to him. The co-ordination might be to prevent you from having to call in the utility to change your primary protection fuses. I'm sure there are other reasons to co-ordinate as well. You didn't provide enough specific information to know if that is the case or not but it's a possibility.
You're here posting question without ever contributing back and then insulting the members who try to help you. Nothing more then a borderline troll. I really hope you don't kill someone, but if it happens I hope you live in a part of the world where they use criminal prosecution in cases such as this.
RE: Fault Clearing Time
RE: Fault Clearing Time
"I have utility at the top, then transformer primary fuse, then transformer (only one transformer, not transformers in parallel), then only one transformer secondary breaker which is acting as the main breaker of the MCC, then MCC, then three motors each of which has its own breaker."
Maybe you should all have a beer and calm down before writing any more comments?
RE: Fault Clearing Time
RE: Fault Clearing Time
You have to open the doors and look. You can't consider the compartments isolated when there are big gaping holes between them or thin plastic barriers capable of only preventing touch access.
RE: Fault Clearing Time
RE: Fault Clearing Time
RE: Fault Clearing Time
RE: Fault Clearing Time
Almost always, I set the HV breaker to grade with the LV breaker if there is sufficient grading time and it is safe to do so. Agreed that for a trfr tripping of the HV or LV breaker makes no difference to the load but I deem it wise to always trip closest to the fault first. Sometimes it has the aided benefit of informing that the fault is not in the trfr. Useful if the trfr has no unit protection. Some trfrs also have auxiliary trfr at the LV side feeding station supplies.
Regards.
RE: Fault Clearing Time
I think you are reading what you want to see into that ETAP documentation. Unless the barrier between the main breaker and the rest of the equipment has been tested as arc-resistant, it is not 100% certain that the arc will not propagate. Since arcing faults have blown doors completely off enclosures, it is not possible to claim that a "sheet metal" barrier will always be sufficient. There is nothing in IEEE 1584 or NFPA 70E that is going to give you a certain answer either. The decision on whether to include or exclude the (local) main breaker from the arc flash calculation is a decision that you must make for every piece of equipment that you deal with. It is always conservative to exclude the main.
RE: Fault Clearing Time
RE: Fault Clearing Time
It will likely happen that way, but there is no way to be completely sure. Also, even if you include the main breaker for determining the bus arc-flash, you will still have a separate (higher) incident energy for the main breaker section.
RE: Fault Clearing Time