Arc Flash Hazard? Yes or No? 3

[FEinTX]

According to our arc flash study, we have a maximum of 47,000 A available 3-phase fault current coming into a 600 V cabinet via some really big copper buswork and wires. It connects to a main circuit breaker which is rated at 50,000 A. However, there are also three 4 AWG wires that tap off the line-side of each phase (no fuses) and go to an adjacent cabinet to a much smaller circuit breaker which is rated at only 35,000 A. The arc flash study is treating this all as a single electrical node, hence, the 47,000 A fault current exceeds the maximum current of the circuit breaker in the event of a bolted fault. Case closed. Or is it?

Question - Can #4 AWG wire carry 47,000 A of electricity for any meaningful period of time? If not, what would be the limit for this size wire?

Seems to me, the #4 wire would serve as a fuse link and simply vaporize with this kind of current, but this is way outside my training (my background is microelectronics). Any help is greatly appreciated.

Thanks,

FEinTX

 

[FEinTX]

Stevenal,

Each leg of the #4 tap (this is 3-phase) is just 28" in length. See attached drawing. (If there are 2, use the 2nd one).

FEinTX

 

[FEinTX]

Figured out the attachment process... Here it is. -FEinTX

 

[jghrist]

With only 28" of tap length, the fault current will be close to 47kA and the breaker will be underrated. I would replace the #4 copper with something that will not fuse in the clearing time of the main CB.

 

[dpc]

You have 47 kA of fault current from a 600 V generator? It must be a large unit. Is this a wind turbine-generator?

I'm just a little skeptical about the 47 kA if the source is a local low-voltage generator.

 

[FEinTX]

Yes, wind turbines. The pad-mount transformer is rated at 1850 KVA. High side is 34.5 KV with cable up to 1250 kcmil going to a 345 KV substation. Low side is 600 V with four DLO (Diesel Locomotive) cables (DLO262, I believe). So a fault at a turbine is fed not only from the substation, but also as many 12 other turbines producing as much as 20 MW.

FEinTX

 

[dpc]

So you actually have fault current from both directions. Is the 47 kA the sum of all of the fault currents?

In this case the main breaker can not reliably clear a fault on the tap circuit regardless of the tap conductor size.

 

[LionelHutz]

dpc - original post says tap is on line side of main breaker meaning main breaker can not clear a fault in this circuit.

See my last post;
- install fuses right at the bussbars
- install larger size tap wire
- install new tap breaker with higher fault rating

This is the only way to address the fault rating problem and it should also lower the arc flash rating of the tap panel to a workable level.

 

[Zogzog]

What is the breaker type?

 

[FEinTX]

Breaker on the #4 tap is:

ABB S4L 3 Pole Unit