Underground LV cables
Underground LV cables
(OP)
I need to connect a 400V generator to a main switchboard and I think it would be better to use parallel multicore 4x185mm2 or 6/8x95mm2 XLPE/SWA/LSF armoured cables in underground ducts - but I've had some feedback that this is not a good idea and it is better to use single core cables.
Can anyone give me a reason for this - my view is that parallel cables is OK as long as the cables are same size, type, length and use same installation method...
Thanks
Can anyone give me a reason for this - my view is that parallel cables is OK as long as the cables are same size, type, length and use same installation method...
Thanks






RE: Underground LV cables
Very large conductors are not economical - the ratio of ampacity to cross-section decreases as the conductors get very large.
Not to mention the difficulties in pulling and terminating very large conductors.
RE: Underground LV cables
RE: Underground LV cables
If the fault is downstream of the paralleled conductors, the fault current should still divide up between the conductors OK.
If the fault is in one of the paralleled conductors, it will see a high fault current, but it is already damaged, so the short circuit withstand capability isn't really an issue.
Maybe I don't understand your comment...
RE: Underground LV cables
The cable carrying the fault must be thermally protected. More often than not it is the minimum fault current that needs to be assessed with respect to cables ( A2secs will be more). With parallel conductors, as Wilkin points out, great care is needed to assess the thermal withstand capacity of each leg of the parallel arrangement.
Regards,
Lyledunn
RE: Underground LV cables
Thread238-31174
ausphil
RE: Underground LV cables
I guess I still don't understand why only one cable is carrying the fault current unless the fault is in that cable. If the cable itself has failed, its thermal capacity is a moot point.
RE: Underground LV cables
However, multicore is advantageous in less voltage drop and cancellation of the magnetic fields.
RE: Underground LV cables
Whilst it might not apply in this particular circumstance, thermal protection is still relevant in faulted cables in buildings.It is not unknown for insulation to burst into flames because of inappropriate fault protection.
Regards,
Lyledunn
RE: Underground LV cables
Two equivalent parallel conductors have less impedance than a single set of cable. To compare the SC stresses imposed in a single and parallel set of cable, the following rough estimation is suggested:
(1)- 4/C -185 mm^2(~350kcmil)….R=3.33, X=4. 07…… ~100 kA for 2 Cycles
(1)-4/C- 95 mm^2 (~ 4/0 AWG)….R=5.34, X=4.14 …… ~ 80 kA for 2 Cycles.
For SC purposes a rough approximation to determine the parallel conductor impedance is assumed to be half than a single conductor R/2=2.62, X/2~2.07. A single cable that meet this characteristics is close to 600 kcmil (~300mm^2).
(2)-4/C- 95 mm^2 (~1-600 kcmil). R=2.62, X~2.14?…….~120 kA for 2 cycles.
The above example appear to shows that the SC withstand capability of a cable insulation is higher for 2 equivalent parallel conductor than a single set for the same continuous current rating. A detail calculation is require to determine the extend of this statement.
REFERENCES:
1- Cable Resistance and Reactance from Table 10, ANSI/IEEE Std 242.
2-The graph enclose was used to estimate the SC withstand capability of insulated cable.
http://www.okonite.com/engineering/short-circuit-currents.html
RE: Underground LV cables