Underground LV cables

[daveR]

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

 

[dpc]

I agree with you. It should be fine, provided conductor length, routing, etc is similar. There was another thread on this not too long ago.

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.

 

[wilkin]

The short-ciurcuit rating for a cables in parallel is only that of a single cable and this can cause problems with protection settings

 

[dpc]

To wilkin:

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...

 

[lyledunn]

To DPC
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

 

[ausphil]

Previous thread covering most of the concerns on parallel cables (deals with HV cables but concepts similar) . . .
thread238-31174

ausphil

 

[dpc]

To lyle

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.

 

[Tuas]

Only reason for proposing single core is the ease of installation, large multicore cables are rigid and cannot be installed if too many bends in ducts.
However, multicore is advantageous in less voltage drop and cancellation of the magnetic fields.

 

[lyledunn]

DPC,

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

 

[cuky2000]

For most LV practical applications, the voltage drop is the limiting factor and not the SC. If the SC is the limiting factor, a remedial alternative is to use faster protective device, such as current limiting fuse (CLF) with 1/4 cycle clearing time.

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

 

[ItAintMe]

A common reason for preferring single phase cables in ductlines is simply that some older ducts are too small and the multicore won't fit.