Fault Level

You need to specify what type of "Fault Level".

Do you mean:

1. Fault level internal to an Alternator/Generator/Battery.
   
2. Maximum Short Circuit Current available at the input to a Main switchboard.
3. Maximum Fault level internal to a Main switchboard.
4. Fault level at input to a Distribution sub-board.
5. Fault level internally at a Distribution sub-board.
6. Fault level at the input to a load device = motor/welder/light fitting etc.
7. Fault level internal to a load device = motor/welder/light fitting etc.

You need to specify the following:

1. Supply type = DC or AC.
2. If DC the Internal resistance of the supply battery or Generator.
   
3. If AC the Voltage, frequency, number of phases, at all stages of the transmission network.
4. Resistances of cables if DC supply.
5. Reactance/impedance of lines/cables at supply frequency if AC supply.
6. If AC, then reactances of transformers etc. in the transmission network from the Supply.

There are other factors to consider, but the above are the main considerations.

Once these are known, the "Fault levels" at all parts of the network may be calculated...

Importance of fault level is that the protective devices installed should be capable to withstand the fault current and break the specified fault current level.Method to calculate fault level is .

The rated Volt amperes of transformer is taken as base VA in MVA.

fault MVA = Base MVA

--------------

Base Impedance

Suppose one Xmer is a 11kv/433V,1600KVA,impedance=5%

Then Base MVA = 1.6 MVA

BASe Impedance = 5

--- & Fault MVA = 1.6x100

100 -------- =32MVA

5

Fault current on LV side is 32x1000x1000

-------------------- = 42669.16 A

1.732 x 433

Fault current on HV side 32x1000x1000

-------------------- = 1679.61 A

1.732 x 11000

So LV breaker should be capable of breaking 43KA for a specified time(1 sec or 3 sec)

and bus bar should be able to withstand 43KA for the specified time.

The LV cables should be capable to to take the required fault current till the breaker opens(breaker opening time).

Fault level calculations are necessary to ensure that all the components in your power system can safely handle the maximum fault currents if they occur.

As a power system designer or maintenance person , you have to assume that during the life of the facility, faults will develop ( I guess Murphy's law).

When they do , your design must me able to detect the fault location and the protective devices must respond fast enough to isolate the fault to prevent significant disturbance to the system .

The components in the system must be capable of handling the maximum fault levels which will be generated without themselves suffering catastrophic failures.

CTs , circuit breakers , fuses etc can shatter if underated.

For this reason , earthing grids and circuit breakers and other components in the system must be properly earthed so as to ensure the development of fault currents that can be detected by the protection relays and trip the required circuit breakers to isloate the faults.

Hope this non-mathematical explanation was of some use.