When do loads NOT contribute to fault current?

[JohnMdd]

I'm working thru Graffeo's other book, PE exam primer - Three Practice Exams for the Power PE.

There 3-phase fault current question which shows a generator connected thru a transformer to a transmission line, which is faulted. The other end of the transmission line is connected thru a transformer to a load bus, we are given its voltage, KVA and Z-pu. I used the load bus as a source of fault current in addition to the generator, but in the key, Graffeo did not. Is this normal, without specifying in the question that the load does not contribute to fault current? How do you know when to consider the load as a source of fault current?

Thanks!

 

[bobbilly]

Some problems will state that the load contributes no fault current, it is also a load how can it contributed fault current? If the load is a motor then it is a different story and that will have a fault value contribution. 

 

[JohnMdd]

Right, I have seen problems that state the load does not contribute, or that it is a resistive load. In this example, that was not stated. I believe it was an oversight on Graffeo's part. As far as I know, any motor loads contribute fault current; and if not said otherwise, a KVA load can be assumed to consist of motors. I just wanted to make sure I wasn't missing anything.

 

[bobbilly]

I've so far assumed that loads do not contribute fault current, I haven't seen a problem that has had the load contribute fault current besides a motor. 

Do you mind posting the question and answer?

 

[rg1]

Generally synchronous motors are the loads which feed the fault. This is because they have energy of their inertia of rotation. This energy is dissipated as electrical energy into the fault, as the motor starts working as generator  once the terminal voltage of motor reduces due to fault. If you go deep into the subject, inductive and capacitive energy stored in the power system in various such loads also contributes to the fault.  Having stated this fact, if I am right in my prediction, I do not expect such concepts mixed with fault current questions in PE exams. Most of the fault current questions are asked from steady state point of view and what we are talking about ( synchronous motors, inductive and capacitive loads can only feed the fault till they exhaust of their energy which is for a small initial duration). And the Load bus is a passive element and will never contribute any energy  to the fault.