Fault levels are crucial for ensuring electrical systems operate safely. The Wiring Rules mandate that protective devices must handle any overcurrent up to the prospective short circuit current at their installation point. Incorrectly rated equipment can be hazardous, and overrating can be unnecessarily costly. This publication explains a straightforward method to estimate fault levels effectively.
Fault Levels at the Point of Supply
At the point of supply, two fault levels can exist. The first is the actual fault level, determined mainly by the impedance of the distribution transformer supplying the installation. The second is that declared by the supply authority. This may be higher than the actual, to allow for flexibility when such a need arises later. No increase in rating of the installation will be made at a later date when the change in distribution transformer or its location is made and the installation is already designed for worst-case conditions. The following conditions are considered:Transformer Impedance vs. HV Supply Impact
In general, when estimating fault levels at the terminals of the distribution transformer, it is adequately sufficient to take into consideration only the transformer impedance. Generally, the HV supply is of much greater capacity, and that kind of supply can feed a short circuit on its output with little drop—5% is a typical figure—in voltage. When disregarding the impedance of the HV supply, it becomes very simple if the transformer impedance is known.Impact of Circuit Impedance on Fault Levels
The fault level at the transformer terminals may not represent the actual fault level available at the switchboard. The impedance of the connecting busbar or cables may cause a significant reduction. The effect of additional circuit impedance on the prospective short circuit level of the transformer can be determined. Calculating the circuit impedance can be very complicated, as it is dependent on the resistance and reactance of the conductors. The reactance, in turn, relies on the conductor shape and spacing.Impedance Considerations for Different Conductors
In small cables, the impedance is governed mainly by the resistance, while for large conductors it is the reactance. It is interesting to note that the impedance of busbars is higher than for cables, as the larger phase spacings produce higher reactance values. To use these tables, multiply the “impedance per metre” Z by the conductor length. The reduced fault level can then be determined.Main Switchboard Configuration for Fault Tolerance
The main switchboard shall be split into two sections and a bustie shall be provided to give the installation the facility to be fed with power from one transformer in the event of a failure of the other supply. This will be noted on the effect, seen by outgoing devices on fault prospective, when operating with the bustie closed.We design and manufacture high-quality switchboards. Contact us today to discuss your requirements and get started!