maximum through fault current
Can anybody explain me the difference between point number one and three. I know regarding number one but number three is a new word for me.
i. maximum fault current
ii. minimum fault current
iii. maximum through fault current
Not knowing the context, I believe the first refers to the maximum fault current available. The last refers to the maximum fault current that actually flows through the OCPD before it opens. The device may be current limiting.
For iii. do you mean let-through fault current?
That would be the fault current that a current limiting protective device passes when acting it is current limiting activation region.
Regular max fault current is the calculated value if a fault should occur at the point of analysis.
What book is this a copy from?
I guess this is what you are studying, as the reference is exact.
I believe it may be referring to amount of fault current that will be let through before the protective device trips - this will also depend on the reaction time of the device. It would be helpful to know the project you're looking at. Also I don't remember exactly but I believe I heard this term used in the context of transformer damage curves but I cant remember where and when I heard it (which means I may very well be wrong!).
please see this link
Originally Posted by ron
Chapter-4 fault current calculation page-1 pdf format.
It is regarding to protection system and I found some explaination regarding this in google book please check below link.
From the Fundamentals of Power System Protection iii is defined as the maximum fault current that can flow without causing a malfunction in the selective coordination of the system. The example given shows how it is possible for a differential scheme to trip on a external fault that is passing through the differential relays zone of protection and should have been ignored.
yeah, I had no idea regarding this before.
Originally Posted by SG-1
Me either, but I now see the possibility that it can happen. Actually it does happen in low voltage where molded case circuit breakers are used. I just never considered a cause beyond overlapping trip curves. Remember, I am a technician not an engineer.
Originally Posted by Hameedulla-Ekhlas
Through fault current usually refers to fault current that flows through an unfaulted portion of the system to a fault in another location. You need to check that the protection for a certain zone will not trip on through fault current to a downstream zone.
Usually this comes up in bus or transformer differential relaying. The CT signals measuring current at the inlet and outlet of the protected zone are compared by the differential relay to determine if the fault is in or out of the zone. Since fault currents are large, the CT's might saturate and cause false tripping.
Example: A facility is fed by 4000A, 480V switchgear. To reduce arc flash hazard the engineer installs a full bus differential protection system with 4000:5 A CT's on the Main and every feeder breaker. The bus differential relay monitors the currents in and out and trips the Main quickly for a fault in the switchgear. Fault current is 50,000A at a downstream MCC fed by a feeder breaker. For a fault on the MCC bus, through fault current will flow through the Main and the feeder breaker CTís. Only the feeder breaker should trip on its short circuit protection. The bus differential should not trip the Main Breaker (or any other breaker) because the fault is not in the switchgear. The engineer has to check the CT circuit and relay characteristics at that maximum through fault current to verify that a false trip will not occur due to CT saturation.