Arc flash energy on 208 V switchboard

[WiringBoy]

Hi all
My application is an installation in a substation. I bought a 120/208 v station service switchboard. I hired a consulting firm to do arc flash energy on the switchboard.

Switchboard is fed from a 750 kva 13.8kv/208v transformer. It has two cells. Right section has a main-tie-main transfer scheme with 208v molded case breakers rated at 1600 amps. This section has the main bus. One of the two breakers can be closed at the same time only.

Left section has the mcb panel 120/208 v which has 24 breakers and this section is fed of the Molded case breakers in the right section.

Study tells me that main bus (right section)has Cat 3 but mcb panel bus has cat.1.

Here is my question if some body has to turn of a 120 v 25 amps 1 pole breaker in mcb panel(left section), do they need to be dressed with cat 3 clothing????

Kind suggestion would be appreciated.

 

[WiringBoy]

Any response please .

 

[dpc]

I guess it depends on how lucky they feel. The sheet metal barrier between the two switchboard sections are not rated as arc-proof, so there is no guarantee that a fault in the main-tie-main section will not blow this barrier out.

But you need to distinguish between the hazard and the risk. If someone is operating a MCCB with the cover on the panel, the risk of initiating an arcing fault is very low. Coupled with the fact that this is a 208 V system makes the risk even lower.

The latest (2012) version of NFPA 70E has an informative note that says that the consensus of the committee is that "normal operation of enclosed electrical equipment ..... is not likely to expose an employee to an electrical hazard." (See Article 130.7)

For normal operation of a circuit breaker with covers on the panel, probably no PPE is required. We generally recommend that all electricians and operators wear 8 cal/cm2 shirt and pants all the time.

 

[WiringBoy]

Thanks dpc.

Let me see if I got it right. I will label the whole switchboard as Category 3, but mcb panel holding 25 Amps breakers can be operated with Cat 2 clothing.

So if a fault has to happen while operating 25 A breaker the main 1600 A molded case breaker in the right hand section will trip.

I just have to make sure that switchboard is never worked on live and hopefully it would suffice.

or I can have fused disconnect switches with current limiting fuses supplying the switchboard that can reduce the arc flash energy to Cat. 2.

 

[dpc]

Labeling the entire board as HRC #3 would be a conservative approach and perfectly acceptable.

If you read NFPA 70E, the informational note implies that no PPE is required for simply operating a molded case breaker in dead-front equipment. Just wearing 8 cal/cm2 shirt and pants does not meet all the requirements for HRC #2.

 

[cranky108]

I believe at those voltages the arc is not consitered self sustaining, and we list it as 8 cal/cm2 (We follow NESC, not NEC). But we consiter all personel at a 15 inch distance (The length of your arms).
All personal are invited to wear additional clothing if they feel the need, and they will be provided (except for underground vaults).