Depth of burial for 132 kV Cable 1

[krisys]

Is there any specific criteria which dictates the depth of burial of cables. Specifically with respect to its rated voltage.

Also any safety code which is existing on this? or is it only the matter of standard installation practice and the capacity to de-rate?

 

[Marmite]

Some countries have regulations governing the minimum burial depth of cables, others do not. It depends on the jurisdiction. Where is the project?
Regards
Marmite

 

[7anoter4]

First of all I agree with Marmite, of course. In my opinion for USA NESC/2007 it is the recommended
standard.
For underground direct-buried cable of rated voltage above 50 KV the minimum depth could be 1070 mm [42”].
See NESC/2007 Rule 351 and 352:
Table 352-1—Supply cable or conductor burial depth

 

[7anoter4]

According NEC Art.300-50 Table 300.50 Minimum Cover Requirements for above 40kV the minimum recommended it is also 42”.

 

[k2panman]

I consider the wording to be vague in the USA NESC 2012 regarding depth that high voltage cable must be buried. A minimum depth is NOT stated, even though Table 352-1 does give depths. The rule that references Table 352-1 is 252D2, which states "Burial depths as indicated in Table 352-1 are considered adequate for supply cables, conductors or duct except as noted in a, b, or c following"

NESC states in 322D2.b.:

"Lesser depths than indicated above" [referencing the 42" depth for over 50kV cable from Table 352-1] "may be used where supplemental mechanical protection is provided. The supplemental mechanical protection shall be sufficient to protect the cable or duct from damage imposed by expected surface usage."

The NESC rule that DOES GIVE TEETH to Table 352-1 is Rule 012C which requires accepted good practice to be used. this IMPLIES that Table 352-1 is accepted good practice. However, with adequate "supplemental mechanical protection" these depths (1070mm or 42" for 50001kV phase to phase and above) are not required. Conduit is considered to be supplemental mechanical protection. So is concrete.

So you are really just required to use "accepted good practice". Our utility uses 36" as the minimum depth for our 25kV primary cable (we always use conduit - either sch 40 PVC or SD13.5 HDPE bore pipe) and 24" for our secondary (<600V) conductors. We only allow lesser depths where there is no practical alternative, then we require truck mixed red dyed concrete to be encase conduits 6" radially around the outside of the conduits.

I would suggest your 132kV cable be buried at 42" minimum, unless you encase or cover your conduits in concrete.





K2ofKeyLargo

 

[krisys]

Thank you k2panman for your detailed input.

Our 132 kV cable will be installed as per IEC practice. But in this part of the world, though they follow IEC practice, I see 1070 mm is the burial depth commonly specified. I was wondering the origin of this 1070 mm. I guess, it must be comming from NESC.

All said, the depth of burrial is to ensure that the cable will not be exposed or damaged during the normal life time of the cable. Specifically, due to soil errosion, or if the cable is installed below the farm land, to avoid any damage to the cable etc. As long as the bottom line is met, it should be safe.


But going beyond 1.5 mtr also posses Iinstalllation limitation. Such as working below the depth of 1.5 mtr is considered to be working in a confined space. Thus, additional safety measures for the workmen is required.