Fault Current Calculation

Gee, you can't get much more of a self defining phrase than "fault current calculation." What do you think that means?

Once you grasp that phrase , you should understand the idea of earthing.

The fault current is the supply voltage divided by the earth loop impedance.

Fault current calculations is usually a shortened form of "three phase bolted fault current calculations." There are tremendous amounts of energy available from the electrical utility company in the event that someones screwdriver drops across all three phases of a bus duct. That's why you see 200,000A interupting ratings on some fuses, even if they are only 40A fuses. This is not a subject to fully discuss here, as it involves knowing the potential input from the service drop and is calculated throught the various conductor impedences and transformer impedences to the point where you wish to know how much is available and must be protected for. In our plant, we have a potential on our 480VAC bus ducts of around 43,000A. Pretty scary, isn't it? I did these calculations for all projects for an engineering firm in the early 1970's, and no longer have a good knowledge of sources for info. I suggest you Google the full term and see what you get.

I once worked with a "local" network (downtown) that was fed by 4 different power plants. The facility that I was working with was a bank data center.

The feeds were all connected to individual transformers that were hooked in parallel.

The rated fault current was 200,000 Amps.

Being their usual paranoid selves, the data system operators were convinced that they needed a UPS system, even after I explained how stable the system was.

Interesting note- when one of the plants had a lighting arrester blow and take out about 35% of the city, the lights blinked in the building but nothing went off-line or lost any operating sequences. They still thought that a UPS was needed UNTIL the salesman from the UPS company mentioned his parallel-redundant system.

They asked what that was. He said that, since all things electronic eventually fail, the system had two equally sized systems that could each run the entire plant so that when one failed, the other would pick up the load. They said- FAIL?? End of discussion re- UPS.

Elleraj888,

You don't mention why you ask the question, so our answers may not be what you want to understand. The other posts have mostly addressed fault currents and their calculation, but said little about how this is related to earthing.

Under normal conditions, the electrical supply can provide you with a relatively modest amount of power. The calculations for this all are based on a time frame of minutes to hours, and follow the general forms in Ohm's law and the related equations. You should have easy access to them. The practical and experience side of electrical distribution systems imposes limits on the amount of current based on the unwanted heating of the cables and equipment supplying the current (transformers, generators, etc.).

When you switch from the normal conditions to a sudden (much-much less than 1 second) load on the supply system, you now are no longer dealing just with the energy that is being continuously transmitted from the generating equipment to you (the consumer). You are mostly dealing with the instantaneous energy that is stored within the distribution system and its customers. This is in the form of magnetic fields inside transformers and motors and electrostatic fields inside capacitors.

A short circuit presents an opportunity for this stored energy to be discharged very quickly. Thus, the short circuit current is dependent on the size and internal resistance (impedance) of the transformers (and motors and capacitors) instead of the size and voltage. Generally, the larger the transformer is, the lower its internal impedance and the greater the fault current it can instantaneously supply. It is not unusual to find a residential area with available fault currents of 2000-6000A on a 100A service. Nor is it unusual to find an industrial area with available fault currents of 30,000 to >150,000A on a 3,000A service.

Your fuse or circuit breaker ("overcurrent protective device") must be able to sense and open the circuit when its rating is exceeded for a specified amount of time (such as 125% for 30 seconds or 300% for 5 seconds). This relationship between the time and the amount of current is the typical time/current curve for the device. In addition to all this, if the fuse or circuit breaker attempts to open when the current is flowing into a fault, this device must be able to successfully open and break the short circuit without damaging the box it is within. Depending on the design of the fuse or circuit breaker, it may only be able to break a 100A fault, or it may be able to break as much as a 200,000A fault. It is the responsibility of the equipment designer and the electrical contractor to know how much fault current is available at all points in the electrical system that he/she is wiring and to select and install components capable of handling this fault current.

A good source for some simplified equations and study guides on this can be found buried in the technical papers of the Bussman web site.

Up to now, I have said nothing about the relationship to earthing. Many faults are between two incoming power lines (phase-to-phase). Usually they are in equipment that has some connection to earth/ground and caused by something that is comparatively small (such as a screwdriver or corner of a piece of metal). Since vaporized metal is a very good conductor, most phase-to-phase faults become phase-to-earth/ground faults. Other faults begin as phase-to-earth/ground faults and then increase in energy if they become phase-to-phase faults. I said "increase" because of the higher voltage and typically lower circuit resistance in phase-to-phase faults.

Any electrical system must have a way for fault currents to be sensed by and interrupted by the equipment supplying the current, and to limit the voltage to ground/earth from the circuit conductors. Therefore, you will see earthing/grounding of the system in the vast majority of all installations. In those for which the system is not earthed/grounded there is usually a very good reason and good supplementary safety.

If you are dealing with a 2-wire single-phase system or a 3-wire delta three-phase system, the transformer doesn't care which wire is earthed/grounded (if any). If none is earthed/grounded, an accidental connection between one of the circuit conductors and earth/ground will conduct a very low amount of current. In this case, a phase-to-earth/ground fault is insignificant. If it is followed by another phase-to-earth/ground fault, however, the second one acts like a phase-to-phase fault. Therefore, in such ungrounded systems, it is common (and desirable) for the installation to include equipment that senses the voltage imbalances created by an accidental phase-to-earth/ground connection. This allows the fault to be corrected without costly interruption of the process or user.

How much energy is released in a phase-to-phase or phase-to-ground fault? That is determined by the current and the time in the form of a mathematical integral of I²t. With properly chosen overcurrent protective devices, the total current that is "let through" the fuse or circuit breaker can be limited (thus their description as "current limiting"), and the time to clear the fault minimized. The cover of a box where the fault occurred may be deformed or blistered. Without proper overcurrent protective devices, the energy in the fault could blow the box apart, destroying anything within a number of feet/meters, and start a major fire.

It all goes back to safety and experience. Its not nice if things go "boom", particularly when people are around. Nor is it nice if people get fried. The electrical codes have evolved over the last 120 or so years of experience, as a means to take these bad experiences and describe ways to avoid their repeat.

I hope this helps. Do some searches with Google or other search engines on the topics you are looking at and learning about. Keep reading, keep studying, keep asking questions. With time, you will become the one who can supply answers and reasons for them. I have been an electrician for over 35 years and still am learning things.

--John M.

thanks john, thanks for your explaination its good for me to insist more to learn.

AWESOME EXPLAINATION. THANK YOU FOR TAKING THE TIME