The circuit involves a 1500kVA transformer feeding a 480V swichboard. The main is a Pringle with KRP-C 2000A fuses. The fuses are protecting an MLO 2000A MCC with over 60 buckets. Most of the buckets supply small HP motors (5 - 25HP) and have a combination of breakers and RK-5 fuses. The IE is over 100 cal/cm^2 on the load side of the fuses and at the MCC. 99% of the electrical maintenance is performed at the MCC bucket by qualified electricians as there are very few disconnects in the facility..

The long term solution is to replace the Class L fuses with a breaker and solid state trip unit which will lower the IE to Cat #1. The problem is that the requisition, procurement, and installation is going to take months to complete. Other than shutting the entire plant down to service the equipment, or buying 100 cal flash suits, any ideas on how to address this situation with an interim procedure?

Get rid of the KRPC fuses, instead use a high speed, like a KTU or KLU. I often find locations do not have enough arcing fault current to get these Class L "yellow/orange/green" fuses into their current limiting range.

JBD... I agree. But all the Class L fuses yield the same IE. Even the A4BY & A4BQ Amp Trap. However, what should be done over the next 6 months while waiting on retrofit of the gear?

6 months?

Six months was the estimate I was given. It doesn't matter if it's 6 months or 6 days. The situation is the same... what kind of interim policy should be adopted? Shut it down, 100 cal flash suits, or keep doing what they've been doing for 25 years?

What type of work are they doing that requires them to be working energized?

Typical MCC work... reset overloads, replace fuses, troubleshoot. This facility has very few disconnects at the motor so most motor troubleshooting is done at the MCC.

I would evaluate the MCC load and see if you can get by with a smaller fuse. I had 3 plants with similar situations, 1500 kva xfmr, 2000 amp pringle switch, feeding MCC. When we added up the load it turned out we did not need 2000 amp fuses. I was able to reduce 1 to 1600 amps and the other 2 to 1200 amps. I also changed the fuses to KTU type at the lower ratings which greatly reduced the arc flash calculations.

Also, are they KRP-C or KRP-CL. The current limiting version may make a difference, and use suggestion by tish53 to reduce fuse size, it should lower incident energy.

Sometimes. Current limiting fuses also sometimes increase incident energy because they lengthen the trip times.

Thanks for all the comments. However, most of you addressed the mitigation itself. We have the strategy in place (retrofit the gear and install breaker with SST unit and adjustable Instantaneous setting). The questions was NOT "how" to mitigate, the question was "what" to do in the interim while the procurement process gets approved, equipment ordered, and retrofit completed.

Let me give you some options:
1 - Shut down the entire facility (deenergize the MCC) to reset a tripped overload relay on a 1/2HP pump motor.
2 - Implement an interim policy that restricts interaction with the MCC and directs employees on specific tasks.

Now, in the real world. Option #1 is not an option at all as it would cripple the company and they would not be able to produce. Plus repetitive opening/closing on the main Pringle switch may actually increase the likelihood of catastrophic failure and the employees would have to wear a 100+ cal flash suit. SO, perhaps they should call the utility company and them drop the service while the overload is reset.

Surely I'm not the only one that has seen this situation. Maybe people are reluctant to say what they did in the interim.

The NFPA 70# Article 90.1 states " The purpose of this standard is to provide a practical safe working area for employees relative to the hazards arising from the use of electricity." How "practical" is shutting down the facility?

Remote switching

Zog... I agree that remote switching is an excellent long term mitigation strategy. However, remote switching of 60 overload relays on size 1 motor starters? Replacing an occasional 20 amp fuse in the bucket? Still, what do they do while they retrofit and install remote switching devices?

Don't your overload relays have external reset buttons?

Have you performed a risk analysis to go with your incident energy calculations?

Some of them do; but not all. You may be throwing me a curve here. The IE is >100cal/cm^2... are you saying that the electrician can wear a lower arc rated PPE dependent on the task? Let's just say the overload keeps tripping. There is no disconnect in the field. The electrician opens the MCC bucket to check current. What do they wear?

There is no "retrofit", you attach the actuator with a magnet, go outside the arc flash boundary, and push the button on the wireless remote.

There is no reduction in the maximum AFIE based on the task. But all tasks may not result in an arc flash.
Resetting an overload relay using an external operator (and which does not cause the contactor to pick-up) could be determined to not cause an arc-flash incident. However there is almost no way to justify opening a bucket door, and inserting metering equipment into it, as being incident free.

This is where it gets interesting. I used the example of resetting an overload which is the most common task in the MCC. Let's change the situation to replacing a 20A fuse. If there is no way to justify opening the bucket, then somebody has to open the 2000A Pringle, then replace the fuse, and then close the 2000A Pringle. Plant and production are down because of a 20A fuse. I suppose the electricians can wear a 100cal flash suit that weighs about 11 lbs.

How many times do you estimate you will need to replace fuses in your MCCs in the months it will take you to replace your fuses? Fuses blow, usually due to mis-sizing, in which case you were going to eventually lose some production, or most commonly due to a short circuit.

I believe you are being argumentative, rather than trying to understand other positions. I said there was no way to justify opening the door and inserting meters as being incident free.

It would be no different if you put the the word "Circuit breaker" in for fuse. You don't merely reset a circuit breaker if it opened during a short circuit. At least the fuse would do a far better job of equipment protection and there is less possibility of equipment damage below the fuse. Size the fuse at 175% of the NEC Table 430.250 and it would be unlikely that you would ever have to replace the fuse unless it were a major short circuit. For those that size at less then 175%: on a fully loaded motor when sized at 125%, the fuse has a max life of less than 100 starts; when sized at 175% it is close to a million starts.

As far as reducing the IE below the 2000A switch, it is as easy as mounting pressure/optical transducers within the mcc and using the mating I-Gard relay. The clearing time for the switch is in the 6-8 cycle range. I will be involved in testing this combination in the near future so I can provide some empirical data - I think the IE will show far less than the 8 cycle clearing since the switch will start to limit the voltage immediately. For those of you that use software, draw the 2000 amp fuse (A4BQ) and a 8 cycle relay in a project you will see what the that combination provides. Depending on short circuit current, Class L fuses don't do much good above 1600 amperes.

A couple of comments on additional fuses. When working at 480 volts look at the Ferraz/Mersen A6K and A4J product. In most cases they provide a HRC #0. Another fuse family that is well under utilized is the Ferraz/Mersen 9F60HMH65E, 9F60HMH80E and 9F60HMH100E products. These are 15 kV fuses that are respectively sized for 750, 1000 and 1500 kVA transformers. Unlike other 15 kV fuses these have a more vertical clearing curve, hence, they can be used on the primary side of the transformer and will reduce the IE below 40 cal/sq cm on the secondary.

Lastly, when and if you use a Bolted Pressure switch to reduce IE or sectionalize there are only two full line manufacturers. Eaton bought Pringle about five years ago and there is Boltswitch. Square D does make bolted pressure switches, however, I do not believe they have a full product offering. Lastly, it you buy a bolted pressure switch from anyone except Eaton it will probably be a Boltswitch with a different name. That might be good, I have seen a lot of field issues with Pringle, but, none with Boltswitch.

Lastly, I have some great short circuit and arc flash data that I generated for an IAEI Chapter presentation - is there anyway to post it on this site. I could post: screen shots, Powerpoint or individual tables - any suggestions.

Sort on interested, do you drive a Viper?