Fault Clearing Time

[timm33333]

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.

 

[ScottyUK]

Is there a credible failure scenario on the LV side of the transformer where the HV breaker is the only one capable of clearing the fault?

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.

 

[timm33333]

Yes that makes sense. So should we use the clearing time of secondary breaker in the calculation of incident energy, and ignore the primary breaker?

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?

 

[desrod]

Key words here is "protection coordination". If you are not familiar with the concept (and the questions you are asking tend to show that), I would suggest that you either ask experienced people to coach and review your work or subcontract the task. In some countries, arc flash studies are to be performed by professional/licensed eng. and there is a reason for that: safety.

 

[stevenal]

Where will the arc occur at the MCC? Ahead of the contacts or downstream? If on the downstream side, what will prevent it from flashing over to include the source side? The general philosophy is to calculate the highest incident energy at a location and label and protect accordingly.

 

[iceworm]

So should we use the clearing time of secondary breaker in the calculation of incident energy, and ignore the primary breaker?

Considering you are discussing the energy available at the MCC, both the xfm primary and secondary CB are in the circuit. So, yes, it depends on which one opens first.

...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. ...

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

 

[timm33333]

desrod, it looks that you don't know it either. It makes sense to coordinate the breaker of individual motor to upstream transformer primary (or secondary) breaker. But as you are focusing on the keyword "protection coordination”, then there should be no point to coordinate the transformer primary breaker with the secondary breaker because they are in the same exact series circuit and tripping of either of them will shut down the circuit and clear the arc at the phase to phase bus of downstream MCC.

 

[iceworm]

I'm not clear on your last comment. Do you have one xfm secondary CB or are their multiple secondary CBs.

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

 

[VTer]

There is specific answer to your questions. You simply have to calculate all possible scenarios and use the worst case one.

"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

 

[VTer]

Correction to above statement: There is "no" specific

"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

 

[timm33333]

iceworm, 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.

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.

 

[iceworm]

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.""" ...

There should not be any confusion. I don't have access to the document, "SOMEWHERE", but I would say the contest of the statement is concerning a fault upstream of the secondary CB and includes the xfm. The secondary CB is not in this circuit.

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

 

[jghrist]

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."""

It's confusing only because the "example of distribution transformer" assumes that there is no secondary breaker, which is common for utility distribution transformers. If there is a secondary breaker and it acts faster than the primary breaker, then it is the one that will clear the fault. You are making things more complicated that they need to be. The important thing is the time it takes to clear the fault.

 

[iceworm]

You are making things more complicated that they need to be.

Let me see if I got this. I'm making it more complicated because of a quote you attributed to me that was actually made by timm33333?

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

 

[desrod]

Timm,

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.

 

[timm33333]

Ok got it, thanks everybody for help!

Iceworm, jghrist was talking about me (not you).

 

[jghrist]

Let me see if I got this. I'm making it more complicated because of a quote you attributed to me that was actually made by timm33333?

I don't know if I should laugh or frown. I think I'll laugh, that's too silly to frown.

Let me make this perfectly clear!!

 

[iceworm]

jg -
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

 

[LionelHutz]

What you're asking is a physical access or exposure question. You typically have to use the primary TX protection when calculating the arc flash energy in the secondary side breaker cell since the incoming cables or bus or breaker stabs or whatever are exposed inside that cell and a fault on them can't be cleared by the breaker in the cell. This same concept applies to any enclosure or cell throughout the plant.

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.

 

[jghrist]

LionelHutz,
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.