Need help decreasing the arc flash rating at the switchboards using SKM PowerTools

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donghan82

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Electrical Engineer
I currently modeled my riser diagram into SKM PowerTools. However, I am having difficulties of finding a solution to lower the arc flash ratings at three switchboards.

From the riser diagram, there is a 15.5 kV Loop switch with 2 inputs, kirk key interlock, and 4 outputs. Then 3 of the 4 outputs are then connected to the switchboard.

In the SKM model, I specified S & C, 15 kV, air-break, mini-rupter as the switches for the loop switch. The fuses at the primary of the transformers are the S & C SM5, standard speed, power fuse.

As you can see the attached arc flash report from SKM, the arc flash rating at the switchboards are very high.

However, I was unable to upload the riser diagrams since the pdf of the diagrams were too big. If you need additional documentation, feel free to contact me
(MODERATOR'S NOTE: email address removed. Please use Private Message feature).

If anyone can think of anything to help lower the arc flash rating at the switchboard, I would appreciate your opinion on resolving this issue I am having. Thank you.
 

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It is not uncommon to have the "dangerous" category in the main switchboard. There is not much you can do within SKM itself. There are features you can include in the specifications to reduce the arc flach hazard. For example, you can add a "maintenance mode" to the switchboard's main breaker. That switch will disable the long time trip, which will allow the breaker to trip more rapidly. As a result, there will be less time for the arc to continue, before the event is terminated, and the total amount of energy released will be lower. During the time that the switch is in "maintenance mode," you will lose selective coordination. A fault in a downstream panel could cause a trip of the main breaker. That is the risk the owner would have to accept, if they want to allow live work on that switchboard.

There are other methods of reducing arc flach hazards as well. But I am out of time, so someone else will have to chime in.

Welcome to the forum.
 
charlie b said:
For example, you can add a "maintenance mode" to the switchboard's main breaker. That switch will disable the long time trip, which will allow the breaker to trip more rapidly. As a result, there will be less time for the arc to continue, before the event is terminated, and the total amount of energy released will be lower. During the time that the switch is in "maintenance mode," you will lose selective coordination. A fault in a downstream panel could cause a trip of the main breaker. That is the risk the owner would have to accept, if they want to allow live work on that switchboard.
Click to expand...

LT is not disabled, enabling the maintenenace mode just inserts a fast trip that is similar to an INST trip. Nothing gets disabled. Think of it as a poor mans ZSI.
 
I agree with Ron and Bob.

Something isn't right if it takes a OCP 1,000 seconds to trip under a fault.

Edit: I probably should have just said it looks suspicious. I guess with low fault currents, or high impedence grounds it could be correct and possible. But the very long durations are probably where the high engery values are coming from.
 
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Thank you Charles, Ron, Steve, and Bob for your swift responses. These help me find solutions with the issue I am having with the SKM model. I will make adjustments incorporating these responses and hope that it has resolved the arc flash hazard issue I was having at the main switchboards.
 
OP,
Understand that you can just go the arc-flash options page and set the timeout value to 2 seconds, which is pretty much industry standard in cases where adequate egress is possible. IEEE 1584 Informative Annex B (not in a bucket truck)
But there are cases where the load requirements are such that you can't eliminate the high incident energy levels, and there's really nothing wrong with a switchboard having a Dangerous HRC classification. It's not like it's a bomb waiting to go off. It just has to be treated appropraitely - no energized access.
Too much focus on reducing arc-flash levels can compromise system reliability by limiting the amount of instantaneous available to satisfy the load, thereby losing selectivity.
John M
 
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