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 Post subject: Technician Work Benches
PostPosted: Thu Sep 19, 2013 7:12 am 
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Joined: Tue Apr 17, 2012 8:19 am
Posts: 241
Location: Charlotte, NC
I am trying to determine the HRC at the technician work benches in our repair section. I have an Arc Flash Analysis for our facility. The analysis gives the Incident Energy to the bus ducts above our repair center work benches. Each bench has a tap to a 230V bus duct and a 460V bus duct that each feed through a fused disconnect, and in some cases through another set of fuses to the work bench.

I attached the study I did on each bus duct. There are three lines for each bus duct with a tA for the fuse, the tA for 1 cycle and the tA for 2 cycles. There also is a comparison on 4BD-6 showing the difference between RK5 (the fuses currently in the disconnect) versus Type T fuses and Semiconductor fuses.

My question is regarding the higher Incident Energy after the fuses at the disconnect versus that at the bus duct. I can see how the current-reducing fuses can lower the incident energy as shown on 2BD-2 and 2BD-5, but what about the increased Incident Energy on the 460V bus ducts?


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PostPosted: Sun Sep 22, 2013 4:12 am 
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Location: North Carolina
Two issues. The fuse specific equations are often used as fuse modelling has issues, but some software just uses the breaker equation. Second it is often the case that reduced let through increases opening time and thus increases incident energy substantially. Look at your opening times.


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PostPosted: Mon Oct 07, 2013 1:56 pm 
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Joined: Sat Nov 21, 2009 10:23 pm
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Location: Ohio
A different view:
1. Do not use the fuse equations, use the actual tcc that the software mfg has embedded into the software.

2. With 600V class current-limiting fuse if the minimum melt is less, your clearing will also be less on AC , therefore, the incident energy will be less.

3. The RK5 fuse is the worst fuse to reduce incident energy, however, you would be hard pressed to ever find a case above 1.2 cal/sq cm if the RK5 fuse is 60A or less.

4. The Class RK1 and Class J fuses virtually have the same tcc for a given rating. These are the best fuses for incident energy mitigation. The Class j fuse is 50% the size of the Class R, that is the rejection feature.

A. Through 200 amperes, either the Class RK1 time-delay Mersen A6D-R or RK1 time-delay Buss LPS-R will give you less than 1.2 cal/sq cm 99% of the time. The Class J counterparts are Mersen AJT and Buss LPJ. All the aforementioned are general purpose fuses and can be used at 125% of circuit size except for motors, where they should be sized at 150-175%. The second exception is transformers and you should then size these fuses at 175% on a transformer primary.

B. Once you exceed 200 amperes, the non-delay version of the Class RK1 and Class J can be used to mitigate the incident energy to less than 1.2 cal/sq cm. If you use the Mersen A6K-R or the Buss KTS-R in sizes less then or equal to 400A, in over 95% of your calculations you will end up with less than 1.2 cal/sq cm. In sizes of 450, 500 and 600 amperes, you will probably see less than 1.2 cal/sq cm in over 75% of the cases. The Class J version of the non-delay fuse is the Mersen A4J and the Buss JKS. These fuses cannot realistically be used on a single motor circuit or transformer primary circuit. They are perfect for a feeder protection to a MCC, bus duct or panel.

C. Once you exceed 600 ampers, the Mersen A4BQ and Buss KTU are the fuses of choice for mitigation. In these sizes each case must be evaluated upon its own characteristics.

PaulEngr wrote:
Two issues. The fuse specific equations are often used as fuse modelling has issues, but some software just uses the breaker equation. Second it is often the case that reduced let through increases opening time and thus increases incident energy substantially. Look at your opening times.


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