Why is sand filling required inside a concrete cable trench ?

[eletest]

As I understand , for directly buried cables , a layer of sand is provided before laying , and another layer is provided after laying for protection. However if I lay the cable inside underground concrete cable trench on cable trays , is it still required to fill the cable trench with sand. Does this requirement of sand filling change if the cable trench is passing through a hazardous area .

 

[davidbeach]

The sand is to provide bedding material that is free of rocks and to ensure good thermal conductivity. If you're installing on cable tray there's no need for a bedding material. Tray in a box may cause you thermal problems though.

 

[eletest]

Thanks @davidbeach for your reply.However the practice that I have seen in our refinery is that for cable trenches that pass through process units , the whole trench is filled with sand just after the cable laying is completed .I prezumed was that it must be related to probable hazard that a spark due to any cable fault might carry. However I have not been able to find any references in the standards such as IEEE-422 and IEEE-525 (Design and Installation of cable systems).

 

[ScottyUK]

Sand fill prevents - or at least restricts - the passage of hydrocarbons and the spread of fire from hazardous areas to safe areas and to other hazardous areas.

 

[LionelHutz]

Using sand might not be a specific rule in a code but it might meet the intent of a rule in a code. In other words, a rule might say the cables openings need to be sealed between hazardous and non-hazardous locations and your company has decided the sand fill was the best method to accomplish this.

 

[eletest]

Just continuing with this, another practice that I have seen is that no overhead cable trays are allowed inside the process units .All the cables to the equipments are brought through the cable trench (filled with sand)only.In fact , if a new cable is to be laid when the unit is in operation ,it becomes a huge task as overhead cable trays are not permitted.
I am not arguing with the intent behind the practice . My only query is that what it should be as per international standards.

 

[SK07]

It is common practice to fill cable trenches with sand and leave no space for hydrocarbon to get trapped in the trench, which otherwise could be a hazard. This practice is followed in Oil & Gas installations. Any person familiar with Hazardous Area Classification and preparation of related Drawings will be more aware of it.

 

[jghrist]

What is the advantage of using sand filled cable trenches over direct buried cables?

 

[ScottyUK]

The trenches are frequently cast concrete with load-bearing covers. The cast concrete provides a definite hard barrier to hydrocarbon movement, and in the heart of a plant the surface is usually concrete anyway so the trench makes it easier less difficult to deal with repair or adding new cables compared with breaking out 8" - 12" of reinforced concrete apron to expose the route. It has the further benefit that you actually know where the cables are rather than just having an idea of where they are. ;-)

 

[7anoter4]

NFPA 850/2010 Recommended Practice for Fire Protection for Electric Generating Plants and High Voltage Direct Current Converter Stations:
13.5.2.4.2 Electrical power, control, and instrument cabling required for safe shutdown of critical equipment during emergency plant shutdown should be routed outside the fire area. If routed through the fire area, it should be protected with fireproofing material in accordance with API Publication 2218, Fireproofing Practices in Petroleum and Petrochemical Processing.
API Publication 2218:
6.1.8.1 Electrical Power and Instrument Cable
a. Burying cable below grade. [it could be considered sand filled trench also?]
3. The use of cable tray systems designed to protect the cables from fire.
c. Cable trays encased with calcium silicate insulating panels with calcium silicate sleepers to hold cables away from bottom of the cable tray.
Anyway it is no mention of cable tray in sand filled trench.

 

[racookpe1978]

Ref that NFPA 850 above for electric power plants, I've never seen power plants that use sand filling on any below-ground cable runs - which are usually concrete lined outside of the plant buildings. But a power plant is not a refinery - much, much less flammable material exposed at high pressures and temperatures.