Overheating of cables in trench 2

[sinhchoo]

We had a cable fault with the following findings:
8 out of 18 numbers of 630mm sq. XLPE/PVC copper cables were found shorted to earth in a 670mm x 500mm(D) Trench. Cables were laid in three layers, 9 cables in each layers. Cables were found laid touching each other.

According to design specification, the 18 cables are to carry a maximum rated amps of 3400A feeding from a generator the main switch board. Cables are to be laid in a 700mm x600mm(D) trench in three layers with spacing of one cable size in between two cables.
First layer:R, Y, B, N, R, Y, B
Second layer: R, Y, B, N, R, Y, B
Third layer: R, Y, B, N. with each layers spaced with 75mm of river sand.
With the above design specification and IEE wiring regulation, 5 cables should carry 5 x 1423A x 0.47(correction factor) = 3344A.

With the existing cable trench size (670 mm x 500 (D)) remain unchanged, the layout of the cables in three layers will still fulfill the IEE wiring regulation with one cable size spacing in between cables and at least 30mm spacing to the wall of the trench. To make sure cables contractors are doing the right job to space the cables according to IEE regulation, can we introduce hard wood timber at every 2 feet interval for the cables to sit on? Is there any risk of doing so? Is there any other options we can undertake to rectify this problem?

 

[Marmite]

Are the screens solidly bonded? If so you will have some hefty circulating currents which may be in excess of the earth bond rating on any straight joints, and will derate the cables in any case. Your trench seems too small. In my view, this needs a proper rating study rather than a crude empirical rating calculation.
Regards
Marmite

 

[waross]

We used to mount and support single conductor feeders in maple (hard wood) blocks to both space the cables correctly and insulate against circulating currents.
Today, I would look for of the shelf clip together plastic spacers. The cables should be jacketed for a buried installation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sinhchoo]

There is no joint in the cables. We are looking into any possibility of maintaining the trench size due to the site constrain. On the other hand, is there any doubt on the original design specification which is adopting the method 20 of cable installation of the IEE Wiring Regulation.

 

[sinhchoo]

Waross, I don't get what you mean by cables should be jacketed.

 

[alehman]

Just doing a quick check of rating tables, it seems like you may be overloading these cables. You may want to re-check your calculations. Also sand is good thermal insulator when dry, which may make matters worse.

As Marmite said, it's probably time to get some thermal conductivity tests of the fill and surrounding soil, and do a proper study.


Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[Kiribanda]

sinhchoo,

1)In my opinion you have used the correct tables in IEE Regulations.

Installation method 20 in Table 4A
Derating factor (0.47 for 630 sq. mm) in Table 4B3
Current Carrying Capacity for 630 sqmm XLPE/single core cable in Table 4E1A

Now per Table 4E1A, the current carrying capacity (1423 A)is for free air under certain standard conditions.Did you check the actual current carrying capacity of the perticular cable make you have used,has the same current capacity of 1423 A?(Because,in AEI cable catalogue, 630 sqmm cable is having only 1085 A in free air)

2)I donot agree to use wooden blocks to seperate out the cables, as we donot have any idea of how much heat is developed inside the trench under normal running conditions.I have seen cables sitting nicely in trenches seperated by diameter even after 10 yrs in "good sand".

3)Are these cables having non-magnetic armour?

 

[sinhchoo]

Thanks for all your replies. We are going to do a thermal check on the site condition as there is another identical installation side by side to this faulty installation.

These cables are non-armoured. I will ask for the cable catalogue to double check the rated amps.

From the investigation, cables were found laid without the one D spacing. Personally, I have not witness any cables laying work, can this one D spacing be achieve without much difficulty?

 

[waross]

That depends on the cables and on the skill of the cable crew. If they are sloppy unwinding the cables and have twists in the cables it may be almost impossible to avoid having the cables touch. A more experienced crew may be able to avoid twists in the cable in the first place. Cable on the reel may have twists but this is quite rare.
Bottomline: A good crew can avoid twists in the cable but a sloppy lazy crew may create a situation that is extremely difficult to correct.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

First of all I agree with Marmite and alehman you have to re-check your calculation [I think according to IEC 60287 will be better] and for the dry sand the thermal resistivity of 2 K.m/W could be possible.
If the soil thermal resistivity is 1-1.5 K.m/W it is possible to load 680 A per cable, in my opinion.
If the cable order will be R,S,T,N,T,S,R for the first and second layer the currents will be more equilibrated.
I agree with Kiribanda usually it will be not necessary to hold the cables in their place if the sand is well compressed. But, I think, if you need to do this, a plastic holder as Id-Technik Cable Clamp will be better. See:

http://www.idtechnik.de/download/katalog_en_2008.pdf

 

[sinhchoo]

Thanks everyone for the information provided.

 

[sinhchoo]

I have checked the IEE regulation (which is available to me), there is no table to include soil thermal resistivity derating factor for cables. Whereas, from a manufacturer specification, with dry sand thermal resistivity of 2 K.m/W, the derating factor is 0.79 for 630sq.mm cables.

If I am using IEE regulation the installation method 20 in Table 4A and derating factor of 0.47 for 630sq.mm in Table 4B3 (18 single core cables laid in horizontally flat with one diameter spacing), can I assume the soil/sand thermal resistivity factor is already included in the factor of 0.47?

Is there any reference available on cables R,S,T,N arrangement in a cable trench?

 

[Kiribanda]

sinhchoo,
As I mentioned in my post, all the tables are developed on certain "STANDARD" conditions. Even the current carrying capacities given in vendor catalogs are based on these "STANDARD" conditions. As far as I can remember, these "STANDARD" conditions are given in IEE 16 edition.

So if your soil resistivity is different from the "STANDARD" conditions you have to apply an additional correction factor too.

 

[7anoter4]

First of all, the permissible current of 1423 A is calculated for 1 cable in 30 oC free-open air -and as cable diameter is more then 30 mm- one cable dia clearance for flat 3 phases configuration.
The maximum permissible current for buried cable depends on Earth temperature, thermal resistance and buried depth, no.of cables and their arrangement.
Calculated according to IEC 60287 the permissible current for 0.1 m depth [first lay], buried in a soil of 1 K.m/W, at 20oC, of 3 phases-5 single-phase cables per phase 630 sqr.mm copper conductor ,47 overall dia.- will be 680 A. The shield, if it exists, will be grounded only at one end. The conductor currents in all parallel cables are equal, I suppose.
If thermal resistance will be 2 K.m/W then the maximum permissible current will be only 485 A.
In Handbook on BS7671-IEE Wiring Regulation- it is noted:
"Unless otherwise shown to be safe, cables connected in parallel shall be of the same type, construction, size, length and disposition, have no branch and shall be arranged to carry equal currents"
If the parallel conductors of the same phase are not symmetrically disposed in space with respect to the other phases, the current imbalance between these parallel conductors may reach up to 20%. 5 parallel conductors are difficult to be spread totally symmetrical but you could arrange the first and second layer more symmetrical.
See:
EPRI-Power Plant Electrical ref. series VOL.4-WIRE AND CABLES [EQ.A-1 FROM APPENDIX A]
"Calculation of current division in parallel single-conductor power cables for generating station applications"
Petty, K.A. Power Delivery, IEEE Transactions on Volume 6, Issue 2, Apr 1991 Page(s):479 – 487.
It is a sketch in Prysmian Cable & Systems [in Spanish but it is still very clear!] see:

http://www.cooprg.org.ar/documentacion/Errores en el trabajo con cables.pdf

 

[sinhchoo]

Thanks 7anoter4 for the information provided here.We are going to add one more cable to each phase to increase the current carrying capacity of the cables, meaning 6 cables per phase with three neutral cables. Instead of three layers of R,Y,B,N,B,Y,R configuration, is the following configuration a better choice?

R,Y,B,N,B,Y,R
Y,B,R,N,R,B,Y
B,R,Y,N,Y,R,B

Thanks.

 

[7anoter4]

I am afraid it is not the right configuration.
If I well translated from Spanish what is written under the Error#1 sketch:
"NOTA: Cuando los cables son tendidos en varias capas las disposiciones indicadas se repiten en cada estrato."
In English it sounds like:
"When the cables are laid in layers the disposition indicated is repeated in each stratum [layer]." So R,Y,B,N,B,Y,R has to be the same order in each layer.

 

[sinhchoo]

7anoter4, thank you very much for the translation. I can't read Spanish. So far I have not come across any other write up on this topic.