Square D Power-Style fusible Switchboard

[mayanees]

I'm working on a system that was originally installed in the late 60s or early 70s that has two 208V, 4000-amp Square D Power-Style switchboards with fused bolt-loc pressure switches (FBPS) as the service entrance devices. The gear is labeled: SUPPLY BUS BRACING 100kA on one piece of gear, and 150 kA on the other.
Since the original installation, a spot network was installed, and the Utility now reports 200 kA available fault current at 208 Volts.
Each FBPS has 4000-amp current limiting fuses (CLF) that reduce the let-through to 87kA.
In my opinion, the label on the front of the gear needs to say SUPPLY BUS BRACING 200kA in order for the FPBS to stay together and interrupt 200kA.
I called Square D Tech Support to see if I can upgrade the bracing to 200kA. The techs initial reaction was the system meets the fault current requirements because of the current-limiting fuse, and he's going to send me info in support (I think UL891? info applicable at the time) He said that if a CLF can be installed to reduce the level to below the 100kA that's written on it, then it's acceptable.
Maybe if the bolt-loc switch is rated for 200kA, then maybe that's how it will work. It's integral to the gear; however, it is a standalone switch.
Typing this out just made that click in my mind! Thanks!
Opinions are welcomed.
John M

 

[dpc]

Each FBPS has 4000-amp current limiting fuses (CLF) that reduce the let-through to 87kA.

The let-through current approach is generally not recognized unless supported by actual test data for the particular equipment involved. The old "up-over-down" method of determining "let-through" current was abandoned many years ago.

I'd lean on Square D for test data that will allow this equipment to used in a 200 kA fault current environment.

At the same time, I'd be highly skeptical of the claimed 200 kA available at 208 V. Even a few feet of cable or bus at 208 V will cause a significant reduction of fault current. It's pretty tough to have 200 kA at 208V unless it is right at the transformer secondary terminals.

 

[mayanees]

Thanks dpc for the reply.
Yes the Utility contribution is suspect at 200kA. However, what they've shown me certainly has the potential. There are (5) 40kA sources in a spot-network located 20 feet from the gear.
The Utility (Con-Ed) reports 200kA for the 480 and 208 Volt sources, as in 200,000 amps. It puts the responsibility on us to vary that magnitude for the arc-flash calcs since the 200kA source trips the main fusing super fast, resulting in a very low incident energy buildup. And the system could easily be running on 3 of the 5 available sources.
John M

 

[dpc]

But at 208V, the arcing current will be much less than the available (bolted) fault current. So you need to do the calculation.