When system's X/R ratio is higher than OCPD's testing X/R ratio, the equipment will face derating on Aic rating. In low voltage system, for example 480/277V with 200kA fault x/r=8, What if your equipment and OCPD is already 200kAic, and you face derating. Due to the derating, your equipment will not have sufficient Aic rating. Then your equipment is not good at this scenario, what will be the solution? I think 200kAic is max Aic rating in the low voltage system.
Equipment/OCPD derating
- Thread starter binwork91
- Start date
- Status
Joethemechanic
Senior Member
- Location
- Hazleton Pa
- Occupation
- Electro-Mechanical Technician. Industrial machinery
I'm guessing you don't have a way to lower your X/R? Is this an existing installation?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
You need to look at the specifics of the AIC rating and what X/R ratio it was tested at. My experience has been X/R issues are likely to occur with lower AIC ratings on miniature molded case breakers.
David Castor
Senior Member
- Location
- Washington, USA
- Occupation
- Electrical Engineer
If you truly have 200 kA at X/R of 8, then there may not be a simple solution. I'm curious where the 200 kA figure came from? Is this a 480 V secondary network of some kind? Generally there will be cable limiters and network protectors.
The utility incoming short circuit is 200kA x/r:8. I am doing power system study.
It sounds like it is pretty hard to find out a solution.
Incoming short circuit at building's point of entrance is 480/277V short circuit 200kA x/r = 8 is provide by utility company.
The service switchboard is rate for 200kAic. Due to high X/R. This service switchboard and it's OCPD face derating.
David Castor
Senior Member
- Location
- Washington, USA
- Occupation
- Electrical Engineer
I would want to review how the utility came up with 200 kA - It's possible, but implies some type of spot network with multiple transformers tied together. When I am given a nice round number like 200 kA, I generally assume it is not based on the actual system data at the service. At 480 V, the magnitude of fault current will decrease pretty rapidly as the distance from the transformer secondary increases. The X/R will also decrease. I would consider these values somewhat suspect until I could verify the utility system configuration. and could calcuate the actual fault duty at the location of the protective devices in question.
That said, some current-limiting fuses are rated at 300 kA. Whether that would allow a higher rating on the switchboard is another issue. Fuses are tested at an X/R of 4.9 (generally)in the US, so some derating would still be required.
That said, some current-limiting fuses are rated at 300 kA. Whether that would allow a higher rating on the switchboard is another issue. Fuses are tested at an X/R of 4.9 (generally)in the US, so some derating would still be required.
JoeStillman
Senior Member
- Location
- West Chester, PA
The POCO should know better than to design a system with that high of an AFC. Nobody makes switchgear with higher than 200k. Here in the Philly region, Exelon keeps it soft just for that reason.
- Status