Fault Currents and Conductor damage
Fault Currents and Conductor damage
(OP)
Hi All,
What are your thoughts on this for Low Voltage systems? Is this a concern for phase conductors? Is it a concern for equipment grounds? Can anyone direct me to some papers concerning this? I looked in the Buff Book, but I'm not satisfied with the discussion. If there are any rules of thumb you could share...that would be great too.
Mike
What are your thoughts on this for Low Voltage systems? Is this a concern for phase conductors? Is it a concern for equipment grounds? Can anyone direct me to some papers concerning this? I looked in the Buff Book, but I'm not satisfied with the discussion. If there are any rules of thumb you could share...that would be great too.
Mike






RE: Fault Currents and Conductor damage
In small transformers with LV supply, cables usually have relative high resistance to damp the SC to a level that in most cases this may not be too much of a concern. However, in high capacity installation such as large metropolitan cities or downtown areas with network-connected transformer, very high SC levels may be a concern.
The type of protective devices plays an important role to clear the fault in a timely manner. CL fuses are typically faster than regular MCCB.
I hope the enclose figures could help.
RE: Fault Currents and Conductor damage
I'm familiar with these charts. From my perspective, there is a debate as to whether or not faults can damage low voltage phase conductors, and whether or not equipment ground damage can even be evaluated.
Mike
RE: Fault Currents and Conductor damage
RE: Fault Currents and Conductor damage
What you have to watch out for is if you have a relatively large short-circuit current source feeding small conductors, as might happen if you have a small motor fed from a large switchboard or unit substation. In that case, you might need to introduce a panel and feeder to increase the impedance to that circuit or add current-limiting fuses in that circuit.
RE: Fault Currents and Conductor damage
Upto 300mm2, PVC, Copper= (103 x S)/ [sqrt(t)]
Alum = (76 x S) / [sqrt(t)]
Over 300mm2, PVC, Copper =(103 x S)/ [sqrt(t)]
Alum = (68 x S) / [sqrt(t)]
THWN/THHN Copper = (104.48 x S)/ [sqrt(t)]
XLPE/EPR Copper = (114 x S)/ [sqrt(t)]
Alum = (92 x S)/ [sqrt(t)]
Where t = Total Fault Clearing Time
S = Conductor Cross Sectional Area
The cable withstand value may be compared to the maximum short circuit value.
As you may see, the OCPD (over current protective device) fault clearing time is a critical factor. The faster the OCPD, the greater withstand the cable will have...as stevenal has indicated I^2T.
For typical interrupting time of various OCPDs you may refer to Table 41 of ANSI/IEEE Std. 242-1996.
Aloha!!!