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farang1 (Electrical) (OP)
2 Jul 04 7:08
The Power Company contractor is installing an undergroung HV feeder, the ground is rocky and in spots they can only go 20-30cm deep. the cable is in black hddp conduit with red stripes and is coverde with cement blocks and cement. The customer is concerned that the installation is unsafe.
The cable is being installed along a public road with limited access. The feeders are protected at each end with fuses and the load is minimal (less than 3 amps)and there is only one load at the end of the 600 meter installation a 60 kva transformer.
Opinions please on safety and code requirements if avaliable but the power company is doing the job so the code doesn't apply!!!!!
Thanks
Farang
Helpful Member!  thorangle (Mechanical)
8 Jul 04 17:46
At my utility the following are trench depth standards:

600V - 50Kv  Min. = 76cm, prefered = 90cm, Max. = 122cm

Allowable exception due to rocky soil conditions are as follows:

All cable in schedule-80 conduit, allowable trench depth to top of conduit = 46cm.

All cable in DB 120 conduit with a 10cm thick concrete cap, allowable trench depth to top of conduit = 30cm.

All cable in schedule-80 PVC with a 10cm thick concrete cap, allowabletrench depth to top of conduit = 15cm.

Hope this helps.
alehman (Electrical)
8 Jul 04 22:45
In the U.S., the National Electrical Code requires 24 inches for PVC schedule 40 DB and 6 inches for schedule 40 steel direct burried. Non-DB PVC must have at least 2 inches of concrete cover. Cover for non-metallic conduit can be reduced by 6 inches for every 2 inches of concrete covering the conduit. Depths are to top of the conduit.
BJC (Electrical)
8 Jul 04 23:33
alehman

The NEC dors not apply to utilities.
Bung (Electrical)
8 Jul 04 23:37
We wouldn't be keen on giving anybody any kind of 60kVA supply at 33kV, never mind at the end of 600m of fuse protected cable.  But such an extension would be at the customer's cost, and we would not compromise on the depth.  Either the customer pays for the correct depth trench, or he goes without the supply, or he pays for some other solution.

But if it is necessary for some reason (eg political interference), it might be an idea to fill the trench with concrete.

Bung
Life is non-linear...

alehman (Electrical)
9 Jul 04 0:52
BJC - I agree the NEC doesn't strictly apply to utilities, but it may serve as a useful guide anyway. The NESC doesn't provide specific burial depths.

I agree with Bung. That seems like a very expensive way to get 60kVA.
BJC (Electrical)
9 Jul 04 10:58
farang1
IT's not safe.  20 cm deep is less that  8" deep.  Normal practice here would be 36".  What does the utility say?  It sounds like the contractor doesn't have the equipment to trench in rock and is trying to get someone to agree with his scheme for doing it on the chrap.  Trenching in rocky ground is not cheap - no way around it.  
A semi public road is a public road, Murphys law applies.  IT may be the gardner or some yahoo digging fishworms but sooner or later someone may get into it.
The cable in conduit is a good way to go.  Back fill with some red dyed flowable fill and that's about as safe as it gets.
robertb (Electrical)
9 Jul 04 12:42
farang1

Table 1, Case 8 of State of California Public Utilities Commission General Order #128 (Rules for construction of underground electric supply and communication systems") requires depth of 751-35000V supply cables and ducts to be 24" (minimum), above 35000V: 36".

Fig.1 of the referenced document shows the dimensions to concrete encasement

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