Buried SWA De Rating

[pactrol]

Hi

Im wondering why the ratings for buried direct swa has been reduced in the 17th ed regs compared to the 16th given the dramatic rise in cable prices. it seems strange what are they saying ? are all previous installations are non compliant. as they have de rated the cables also just where in the uk is the ground temperature at 0.7 mts 20deg c ? also its interesting to note that The ERA don,t agree with it see ERA 69-30 is it a case of Bs7671 being over cautious

 

[Engineer54]

Just use a bit of commonsense when burying cables, ...at a depth of 0.6/0.7m they shouldn't need de-rating, i never have and i've been responsible for laying in thousands apon thousands of metre's of SWA in the ground, mostly in far hotter climbs than the UK... lol!!

If in doubt, always go by the manufacturers data, and not the holy BS7671!! lol!!

 

[brav]

Thts a good point E54, BS is a guide so they go with the worse case sineario and errs on the side safety.

 

[telectrix]

derating only applies when buried < 0.7m deep. or soil with Km/W < 2.5, or ground temp.>20deg.C. don't you just love BS7671/.

 

[Engineer54]

derating only applies when buried < 0.7m deep. or soil with Km/W < 2.5, or ground temp.>20deg.C. don't you just love BS7671/.

Think you'll find that de-rating kicks in >0.6 / 0.7m. It does with our cable manufacturers data!!!

 

[telectrix]

just looked in bgb, and it states derating if buried up to 0.8m. ie derate if shallower. if>0.8m, then ccc is higher

 

[Engineer54]

Here you go then... Another reason not to make too much of what is stated in the Holy BS7671 and it's associated guides, when there is far better manufacturers data available out there!! lol!!


Burial Depth DeratingFactor

Cables Cross Section
Depth of laying mt	up to 70 mm sq	95mm sq - 240 mm sq	300 mm sq and above
0.60	1.00	1.00	1.00
0.70	0.99	0.98	0.97
0.80	0.97	0.96	0.94
1.00	0.95	0.93	0.92
1.25	0.94	0.92	0.89
1.5	0.93	0.90	0.87
1.75	0.92	0.89	0.86
2.00	0.91	0.88	0.85

SOIL THERMAL RESISTIVITY DERATING FACTOR

Soil thermal resistivity in [SUP]o[/SUP]C cm / watt	80	90	100	120	150	200	250
Rating factor	1.17	1.12	1.07	1.0	0.91	0.80	0.73

 

[telectrix]

well, it's pretty close 0.99 ( your figure ) and 1.0 ( BS7671 ) at 0.7m.

 

[Engineer54]

Yep, ....Which is why i disregard depths of 0.6 / 0.7 / 0.8m.... I'm pretty sure that over a couple of hundred or more metres of cable trench, the depth's are never going to be a uniform depth. The only time i even look at de-rating, is when a cable is going deep into a lower basement, as will a few of our MV SWA cables on this project.

Another snippet of info, a part of this data sheet stated that SWA cables should never be direct buried under 0.4m and preferably have a minimum depth of 0.55m... Which makes a lot of sense to me!! ..lol!!

 

[telectrix]

i assume that the deeper you go, the cooler the ground becomes ( in moderate weather conditions, disregarding frozen ground in winter ) , so deeper is best. that is of course untill you get below the earth's crust, when i believe it starts to get warmer.

 

[Engineer54]

i assume that the deeper you go, the cooler the ground becomes ( in moderate weather conditions, disregarding frozen ground in winter ) , so deeper is best. that is of course untill you get below the earth's crust, when i believe it starts to get warmer.

Errr, not according to the manufacturers data, at 2m they want to de-rate to 0.85 for cables 300mm and above, and a little less for 95 to 240mm... lol!!!

These are German manufactured cables by the way, not Asian...

 

[telectrix]

but aren't these factors that you divide the rating by, so that a factor below unity means a higher ccc?

 

[Engineer54]

No they are de-rating factors, so these are minus figures, plus figures would be say 1.22 for the 0.88 factor!! Same as the table below for ''soil thermal resistivity'' table.... which shows both plus and minus re-rating factors...

 

[brav]

times by the rating factor i.e .... x 0.85

 

[SirKit Breaker]

This is another one that could run and run. Although BS7671 is non stat, the stat EAWR says that BS7671 is the requirement. BS 7671 has now developed well beyond the point of being a useful document to an Electrician. It is now a design document, in more ways than one. It is designed and written in a way to be able to stand up as evidence in court. And it is designed as a guide for designers of electrical installations. A lot of it we would not refer to day to day as an Electrician, hence we have the OSG. When all said and done, i much prefer common sense to application of the regs. Any half decent spark will be more than competent without referring to the regs on every job they do. In fact most of the contributors on here could rewrite the regs and make them far easier to understand and comply with.

Cheers..........Howard