1/2 Cycle fuses and Arc Flash Energy Levels 3

[rockman7892]

When looking at Arc Flash incident energy levels I have seen that any equipment immediately protected by a fuse clearing in 1/2 cycle has a low IE level and in some cases can be as low as a category 0.

What about equipment that is much further downstream of the fuse such as equipment that has 2 or 3 busses and circuit breakers between it an the upstream fuse? Does this downstream equipment also have a very low IE level because of the upstream fuse? In other words does every device downstream of the fuse have a very fast clearing time and therfore a low IE level or only devices that are immediately protected by a fuse?

Any good references on the impact of fuses on Arc Flash results?

 

[LionelHutz]

Looking at the TCC its obvious to see that the the MV bus has a higher IE because of the intentional delay on the breaker curve of about .09s. But what if this delay was removed and the instantaneous pickup was carried straight down? Would the bus then have the same IE level as the motor since the fault on the bus in now cleared with no delay. Or will the IE level on the MV bus still be a good bit higher on the bus since the breaker may take 3-5cycle to open upon fault pickup whereas the fuse only takes 1/2 cycle to clear at this point?

I'll try to address this one. I suspect that this curve was created by combining the protection relay and breaker operation to take the breaker opening time into account. In other words, the internal protection relay curve continues towards 0 time just like the fuse curve. However, it takes 0.09 seconds from the time the relay instantly detects the fault until the breaker has opened and cleared the fault. So, the combined curve for the protection relay and circuit breaker can never take less than 0.09 seconds to clear any fault. So, the curve takes a hard right turn at 0.09 seconds.

 

[rockman7892]

LionelHutz

Although it is necessary to take relay time and breaker operation time into consideration I do not believe that is what is being represented here with the .09sec delay on the breaker trip curve.

In looking at the rest of the TCC's for this MV bus it appears that the .09sec delay on the breaker relay curve is an intentional delay in order to coordinate with other fuses (fused feeders) on the same MV bus. If the instantaneous portion of the breaker curve was brough straight down with no delay it would intersect and interfere with (2) other fuses on this bus. For that reason it appears that the intentional delay of .09 sec was added to help coordinate this area better.

The notes in the report state that adding this .09sec delay did not effect the Arc Flash IE levels.

I have a copy of IEEE1584 that I am now reviewing. Although I am reviewing it in its entirity can anyone point out what section will address my confusion with these fuses and times?

 

[rbulsara]

I agree that 0.09s (in fact 0.1s as the printed settings on the TCC indicates) is the intentional time delay for the INST.

Rafiq Bulsara

http://www.srengineersct.com

 

[LionelHutz]

Rafiq - yup, looking at the curves again I see that a 0.1s delay is called out in the description.

 

[rockman7892]

I had the Arc Flash simulation for the busses shown on this TCC this time with the .1s delay removed from the instantaneous function of the breaker. The new attached curve shows the fuse and revised breaker curves. Using the same fault current of 22.2kA at the ER-2 MV Bus we can see that this current falls completely to the right of the instantaneous pickup and that by removing the delay on this pickup we are able to redude the IE level at this bus from 16.4 cal/cm^2 to 9.1 cal/cm^2.

This new reduced IE level still corrosponds to a PPE category of 3. I was expecting to see with the instantaneous delay removed that the IE at the ER-2 MV Bus would drop down signifigantly lower to a vaule that was close to that at the E18M bus and thus lower the PPE category to 1. I guess even though the bus fault currents lie to the right of both the fuse and the breaker pickup the IE at the ER-2 MV Bus does not reduce as much due to the 5 cycle operating time of the breaker. So in other words because the breaker has a longer operating time it prohibits this bus from obtaining a lower IE energy level than is achievable by the faster acting fuse.

This leads me to think then in this case that we are stuck with this category 3 rating at this bus with no further ways to reduce it. My initial thought when taking out the delay on the breaker pickup was that we could use a maintnance switch to remove this delay when working on this gear however this new simulation shows that that although it will reduce the IE level it will still require the same PPE. I guess in a sense this is good, however is there anything further that could be done in such a case to acheive a lower PPE level?