Cable Damage curves vs coordination
Cable Damage curves vs coordination
(OP)
I'm trying to coordinate breakers using SKM software. I don't understand how the cable damage curves come in. Aren't cables damaged in more of an long-term heating "overload" situation and not in the .1 or 1.0 second range? I actually have not considered cable damage before so I would love some insight. I could coordinate my bldg beautifully if I ignored the damage curves!






RE: Cable Damage curves vs coordination
The cable damage curve represents the maximum current that can flow through a cable for a given time without permanent damage to the insulation. It's a function of the insulation type and conductor size.
It's normally not a limiting factor in most coordination applications, since the fault should be cleared well before the damage curve is reached.
RE: Cable Damage curves vs coordination
With parallel cable runs use the N-1 option for the cable damage line. The N-1 line lets you determine if one faulted cable will damage the others as they feed the fault from the far end. The "1" line is way too restrictive when you have more than 2 parallel conductors and the "N" line doesn't look at faults in the parallel set.
Unless you have short, small circuits with high fault currents you shouldn't have any trouble with cable damage.
RE: Cable Damage curves vs coordination
I²t = K² S²
where :-
I = short-circuit current
t = short-circuit duration
K = constant depending on conductor specific heat, conductor continuous (initial) and short-time (final) permissible temp. eg for XLPE 90°C and 250° respectively.
Cable manufacturers and Standards will give values for K.
S = cable cross sectional area in mm²
When in the SKM-PTW library go General and click circular mils button. If the circular mils are not entered, the cable damage curve will not be draw. Even when PTW is configured for IEC standards and metric cables are being used, the cable libraries must also include the cable area in circular mils.
1 circ mill = area of circle 1/1000 inch diameter
The cable damage curve is drawn to ensure the cable is short-circuit protected.
RE: Cable Damage curves vs coordination
Your breakers Time Current Curves (TCC) when overlayed on your cable damages curves, should appear to the left of your cable damage curves. Below 0.01s is the short circuit time frame, usually not shown on breakers as they are typically slower to open under fault conditions.
Not as familiar with breakers as I am fuses, yet, the curves work in the same manner for protection of your cables and equipment.
RE: Cable Damage curves vs coordination
- open a tcc
- right click on a blank area of the tcc
- left click on TCC Settings...
- set the desired time axis ( and current axis ) ranges.
- click OK
RE: Cable Damage curves vs coordination
RE: Cable Damage curves vs coordination
RE: Cable Damage curves vs coordination