Fault Currents and Conductor damage

[mpparent]

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

 

[cuky2000]

In general, the maximum SC current depend upon many factors including protective device clearing time, conductor resistance, thermal capacity, initial temperature and fusing temperature for equipment grounding conductor. In extreme situations, SC current can cause damage of cable insulation to melt, break down chemically, initiate a fire or produce toxic smoke. Therefore, this lead to the need verify the installation particularly if the available SC is high and the fault is close to the source.

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.

 

[mpparent]

Hi cuky,

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

 

[stevenal]

It's an (I^2)Rt thing. As long as there is sufficient voltage to provide the current through the R, voltage doesn't enter into it. Phase or ground doesn't change it either. Low voltage services from large primary fused transformers will commonly burn down before the fuse operates.

 

[tommom]

The question for any insulated conductor is not whether the conductor itself can be damaged, the question is whether the insulation can be damaged. That is what is shown on the curves (on the left side) posted by cuky2000.

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.

 

[nightfox1925]

Each type of cable are having their respective Thermal Withstand Short Circuit Ratings and are normally published by each respective cable manufacturers. These Thermal ratings depends on the value of short circuit current and the fault clearing time of the associated protective device. Here are some formulae taken from BICC handbook as supplemented by IEC 724 for estimation purposes:

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!!!