Can someone think of a way to REDUCE the HIGH ARC FLASH numbers for this design? 2

[bdn2004]

We've a customer that recently purchased a new MCC: Allen-Bradley 2100 Centerline. It is fed by a captive 2000kVA, 480V transformer that they own also. Per our software there are 65 calories at the incoming MCC terminals - Extreme Danger.

The customer bought a main circuit breaker for the MCC with a maintenance mode switch, but the MCC is not arc protected gear and the bus therefore shares the same air space as the incoming, rendering the main worthless as a protection of arc flash for the entire MCC. That's our understanding anyway.

It's been suggested to install 2000A fuses in a remote location between the transformer and the MCC. That's a costly option.

Does anyone have any thoughts on how to remedy this ?

 

[davidbeach]

A breaker ahead of the transformer. Or a breaker elsewhere between the transformer and the MCC.

 

[waross]

Have you included the impedance of 104 feet of cable in conduit in your calculations?
I have seen cable inductance used to limit Available Short Circuit Current at Power Distribution Centers.
A minimum of 100 feet of cable was required to reduce the ASCC at the PDCs to within the rating of the PDCs.
This may be a problem with conduit, but it is the cheapest and easiest way to reduce ASCC and the resulting arc flash levels.
If that is not sufficient then consider David's suggestion.
Relocate the breaker or install a second breaker adjacent to the transformer secondary.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mparenteau]

What about a SEL-751 w/ fiber optic sensing and CTs - actuate the main on detection? Overkill for a LV system though...

Mike

 

[oldfieldguy]

I had a similar issue. Chose @davidbeach's solution.

old field guy

 

[stevenal]

High speed grounding switch and relay.

 

[dpc]

I'm with David Beach. A breaker or fused switch on the LV side will be the best solution at this point. It is possible to mount the breaker at the transformer LV terminal compartment. Or mount just next to the transformer pad. If there is in fact a 12.47 kV circuit breaker somewhere upstream of the transformer primary, you could install a relay and transfer trip the breaker if the outage this would create is tolerable. But if not, adding a LV breaker or fused switch is the best option for reducing A-F levels at the MCC Main breaker.

You could investigate use of smaller primary fuses especially if the actual load will be less than 2000 kVA. You MIGHT be able to get the incident energy under 40 cal/cm2 before you start having issues with inrush. Expulsion fuses will be better for this than the current-limiting fuses due to the difference in the shape of the respective TCCs.

Or live with the 65 cal/cm2. I assume this is based on a maximum arc time of 2 seconds?

 

[bdn2004]

Thanks all...

Yes the conduit and wire is included in the impedance calculation per the software.

The circuit breaker or fuse between the transformer and the MCC have been discussed - but that's re-doing and re-working this whole installation. And those are options.

The less conventional techniques, that are cheaper, and less invasive is what I was asking about: the fiber optic AF detection, reducing that fuse size or type, high speed grounding switch...not sure how this is used ? I like. All these I'll pass on in addition to the fuse or breaker option.

 

[davidbeach]

By itself optical AF detection does nothing, you need something to trip.

The ground switch may be a good option. It accomplishes the desired outcome by shorting out the arcing fault with a bolted fault; no voltage phase-phase or phase-ground and thus no arc. It will then cause the transformer fuses to blow.

 

[stevenal]

Or in order to reduce the fault duty on the transformer, put the HSGS on the primary side allowing remote protection to operate. Trigger it with your 751/fiber-optic if you like.

 

[dpc]

I love grounding switches as much as the next guy, but no way will adding a 15 kV grounding switch be less expensive than adding a low side fused switch.

 

[stevenal]

Or communication based transfer-trip to remote protection.

 

[ThePunisher]

If the upstream transformer and main circuit breaker have spare protection CTs, why not consider differential relay (cheapest digital you will get) and will be arranged to trip the circuit breaker upstream of the primary fuse (assuming there is one and is a dedicated circuit). If ground fault is also an issue, consider high resistance grounding if there are no L-N loads.

 

[wbd]

Let me ask an obvious question which may impact the calculations. Are you using the available short circuit current obtained from the utility company?

I also see that you are using EasyPower. Why have a bus on the line side of the MCC breaker?

 

[dpc]

Why have a bus on the line side of the MCC breaker?

Not sure why the OP did it, but it allows a separate arc-flash calculation on the line side and the load side of the main breaker. If you name the bus "MCC Main Breaker" then it is always clear in the report what bus is being referred to. Preferable to using the "Both" option in EasyPower, IMO. If you don't need both line and load side results, then it isn't necessary. Also, between this bus and the main MCC bus there should be a short section of busway that the main breaker is inserted into. If you just connect the breaker between the line side bus and the load side bus, there can be unintended consequences.

 

[wbd]

Since most of the time you can't take credit for the main breaker in an mcc/panel, the breaker can be ignored by setting the arc flash to ignore main breaker. Saves a bus in you bus count and keeps the one line simpler.

Still like to know if the available fault current from the utility is being used. Also is the defaults used for the equipment dimensions?

 

[VTer]

This may not apply but another point to consider is code compliance. In the NEC land you are limited by how much you can run your secondary conductors without over-current protection. From that aspect you may need to provide some over current protection which will also help reduce your arc flash levels at the incoming terminals of the MCC. Current limiting fuses will do a good job but for MCC application be careful due to single phasing issues. Circuit breaker also may provide sufficiently good mitigation assuming the arcing current is in the instantaneous trip region of the CB.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[AppleJaxJap]

@waross lowering fault current does not necessarily reduce the incident energy levels. in some cases, it can raise it.

breaker or better yet fuse between the transformer and the MCC. but that really just moves the issue from the MCC to the lineside of the new Fused switch.

 

[oldfieldguy]

@AppleJaxJap -

True- there's still a point in your system where you're above the 40 kCal limit. I have the same case where I had to deal with this situation.

The point to make is that the disconnect/circuit breaker at the transformer will seldom need attention, but the MCC will possibly need more frequent attention. Lowering the level at the MCC makes working on it a much more palatable option.

old field guy

 

[AppleJaxJap]

@oldfieldguy Agree

Because I'm working from home and have too much time on my hands I looked up the referenced MCC

Link

CENTERLINE 2100 with ArcShield Technology

Helps to reduce arc flash hazards and provide you with increased protection against internal electrical arcing faults
Meets Type 2 accessibility requirements, as defined by IEEE C37.20.7
Includes arc-resistant latches on all doors for pressure relief to keep the door latched to the MCC during an arcing fault
Includes arc-resistant baffles in all MCC units, including variable frequency drives, to enable adequate heat dissipation and retain arc-containment


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