Fault Current at the Final Circuit

There is no need to determine a minimum short circuit current at any point in the power system. It can be less than normal load current in some cases. You need to determine the maximum current so your device is capable of interrupting the worse possible fault.

Your protective devices are set to protect the conductors and components for a given long term current rating. The components don't care if you are melting copper or iron downstream, as long as the amperes won't ruin the equipment. That is why arc detecting breakers have recently been developed, trying to anticipate faults that are less than the trip rating of the breaker.

If you suspect that there could be a fault where there is not enough current to trip the protective devices, then you need to adjust the protection to detect such a fault. For example, when protecting a power system suddenly islanded with a single generator, overcurrent relays with voltage restraint are applied, to reduce the tripping current of the generator if the voltage is low, indicating a short circuit. These devices can help detect a fault developing under abnormal conditions.

The threshold for fault current is often taken assuming you can protect down to the bottom 10% of the windings, so 10% of the voltage is available. Below that, you cannot reliably distinguish between load and fault current.

Ground loop current is different for different power system configurations. Since ground faults involve conductors that normally do not carry current, the threshold for fault detection can be much lower, really dependent on the actual ground sensing equipment sensitivity, and reliability. Again, ground loop resistance is of little interest, you just plan to detect based on the available technology, and plan to clear a fault at the lowest possible level, realizing there should be NO ground line current on a healthy system.

If you realize that at 480 volts, an arcing fault is self sustaining at 400 amperes, then you adjust your ground fault protection to assure a positive trip below 400A of ground fault current. The load current could normally be above 1000 amperes, but most phase to phase arcing faults will eventually devolve into a ground fault, by design, so it will get cleared without an enormous increase in damage. If the equipment value is high, then more expensive and sensitive protection such as differential current can be employed.

The <...method...to determine the short circuit current...> is given by the electrical standard applicable at the location. Were it to be in the UK, for example, British Standard 7671 applies.

If in doubt, consult a qualified local Electrician. These people are used to carrying out the calculations, selecting appropriate materials for the installation, installing it, testing and certifying as safe at the end.

A light fixture could have a metal body - or be changed by the installation owner to a retro style with metal body. Current regulations require that every lighting point has an earth wire to it, in the past lighting circuits just had live and neutral. Most manufacturers would require that a metal fitting was earthed as a primary safety requirement [unless it was certified "double-insulated", like an electric hand drill].

Usually, a short circuit calculation with earth fault loop impedance would give a lower current than live-neutral, less likely to trip a breaker, especially if the earth path involves cable armour or an "earth rod". Here, earth wires may be 60% of cross-section of Live/neutral, so short to earth is expected to be less current.

The earth fault calculation must include the source impedance and any other impedance in the loop back to the source from the live with the protective fuse or circuit breaker.

So the answer is 1) you must do both "conventional" [live-neutral??] and earth fault calculations to find the lowest S/C current 2) They do not find the same fault current.

I feel, for safety reasons, that I should warn that "rwilliams" comment #1, first paragraph, suggestion that it is not necessary to calculate a minimum earth fault and that breakers are there just to protect the plant and rated to clear the greatest short circuit current is misleading.

The post is clearly about lighting in a single phase 115 or 230V installation. At such voltage it is a normal requirement for a breaker to trip for live-neutral or live-earth faults. The minimum live-earth fault current [usually at circuit end, remote from source] must exceed the the breaker trip current at the required maximum earth fault time duration.

My experience is with 3 phase industrial systems, up to about 100 MVA. Well over 30 years of practical and applied experience. Single phase circuits and lighting have no distinction in this matter. There is very little technology available to distinguish between heavy load current and a short circuit fault that does not involve a safety ground or earth connection. If the fault does not cause enough current to trip the breaker, it just won’t trip. Little to nothing to be done about it. You have no control as the the severity of a short circuit. When the fault eventually involves a protective earth conductor, then it is easily cleared with the proper protective equipment. A line to neutral fault that is indistinguishable from load cannot be tripped with typical protective equipment applied in the lighting and low voltage circuits you cite. In no case whatsoever, can you select protective equipment to operate based on the minimum short circuit available, as there is NO minimum, ever.

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rwilliams

Sorry if I have "ruffled your feathers".

" In no case whatsoever, can you select protective equipment to operate based on the minimum short circuit available, as there is NO minimum, ever."

The wiring regulations here are emphatic that a 230V 1∅ building circuit is designed so that a short circuit between conductors [a zero ohm connection] line-neutral or line-ground at any point in the circuit will cause the protective fuse or breaker to open.

Obviously, the minimum short circuit current is at the extremities of the circuit.

The electrician can for a given cable size/breaker look up the maximum run length which complies [including volt-drop limit @breaker rating] in his On-site Guide.

For instance, with 6 amp type B breaker & 1.5/1 sq.mm [phase/ground] wire you can have 106 metres. When testing circuit there is a table of maximum compliant earth loop resistances for breaker type/rating.

67model

There can be no minimum fault current. The ability of a protective device to clear a fault and still function as an electric service depends on its ability to distinguish between a fault and load current. Some can, some can not.