Buried ducts for medium voltage cables

The duct disposition will tend to reflect your selected arrangement for cables (grouping and/or use of single core cables as opposed to multicores) - in most cases, you would be looking for either flat layered ducts or trefoil arranged ducts. The duct grouping may well also need to reflect the drawing in arrangements. There is no point having four layers of ducts arrive at a chamber if you cant physically draw anything into the lower layers if the upper layers impede that with existing cabling


Backfill might well be influenced by the nature of the excavated material - so you may want to swap particularly peaty material (which is an excellent thermal insulator) for quartz material (sand or stonedust - which is a particularly goof thermal conductor. It might also be influenced by the fact that the excavated material is full of sharp and /or hard bits - you wouldn't want to bed cable (or ducts) in that without screening it


Think about the ground resistivity factor in your previous post regarding BICC Ratings


Don't be overly obsessed about ducting MV cabling - it is often better buried direct and ducting only used for areas where future excavation might be very difficult - unless you have contaminated soil of course


There are several "preferred" arrangements but nothing that you could specifically link to a "standard"  - for depths, the NJUG documentation is useful but it sounds like you have that anyway


Regards


OMS

Hi OMS. Your one are all valid considerations. 

I wondered if for multi layer arrangements of ducts - in addition to right space between ducts and right backfill materials to be used - there where also particular considerations to carry out in terms of mechanical stresses. 

For example, one duct could be buried at X m depth under carriageway, but in case of a multilayer arrangement (e.g.3 x 3) then the shallowest row of ducts should be deeper then X m? Hence my question about  typical arrangements..

Not really - you are concerned about minimum depth of cover (for avoidance of casual digging hazards) and maximum depth to lowest duct as it can quickly lead to some very deep chambers which rapidly need both proper access arrangements (step irons etc) and can often become "confined spaces" with all the attendant issues


Under carriageways, then you know the probable axle weight of typical road vehicles (if not you could assume a minimum of 400kN presumed from a D400 chamber cover) and calculate your risk of surcharging the duct at a range of depths. In other applications you might be assuming anything up to 900kN (typical of heavy plant yards or aircraft aprons)


I'm not aware of a "on size fits all" set of standards - depending on the contractual arrangements I would determine the electrical design and then pass the ducting problem to a civil engineer (just check he understands the equation that water and dirt equals mud) and you'll be good to go. Give him chamber sizes to aim for that meet your bending radii criteria, though  - they often fail to understand that as you approach the chamber edge, the radii get smaller. And ask for soakaways or positive drainage from the chambers - you don't want to lift a lid one day, to be presented with all your cabling submerged under filthy water


Regards


OMS

I'm interested in your use of the term "medium voltage cables".

I have searched in vain for a definition of this. The nearest I can get is that :
"High voltage systems are defined as those that operate at voltages exceeding 1000V ac or 1500V dc between conductors and/or 600volts ac or 900volts dc between conductors and earth." and low voltage below this. I suspect that this isn't what you mean.

I have also heard 132kV being referred to as medium voltage (with 250kV and 400kV as high voltage).

For clarity I have always referred to the voltage in question by a number, i.e.3.3kV or 11kV.

Here's Elland Cables' take on the 'definition' of MV https://www.elandcables.com/electrical-cable-and-accessories/cables-by-type/medium-voltage-cable - which seems (taking preferred grid voltages into account) to match what I've seen elsewhere that MV is basically 1kV - 33kV, and so everything 66kV and above is 'real' HV.


Official definition wise though it seems there is only LV and HV.


  - Andy.

strictly it is not a recognised term in the world of IEC harmonsied standards,

but  in the USA ANSI C84.1 defines medium voltage (MV) as being “a class of nominal system voltages greater than 1,000 V and less than 100 kV.” IEEE 141 (the Red Book) references ANSI C84.1 in recognizing the same voltage levels as the  MV range.

The standard MV distribution voltages used in the United States are 4,160 V, 12,470 V, 13,200 -13,800 V, 24,940 V, and 34,500 V  (phase to phase voltages) for four-wire systems and 69,000 V for three-wire systems.

Even more interesting that they use the "IEC Definition" of Medium Voltage, when IEC themselves state there is no agreed international definition.The definition that they do have on Electropedia (IEC 60050 online and probably their only free standard) is as follows:

You are quite right in your comment Andy since HV and LV are the only terms that are properly defined.

Did it not used to be that over 250 but below 1kv was considered medium voltage? I've noted a few 3ph service intakes with 'Danger Medium Voltage' labels on them.


And not a few that state only authorized persons are allowed to interfere with the motors <grin>. This is in domestic situations, where a 3p+n service head has only 1 phase in use. Very common in the PO postcode area

Of course, in law, there is no such thing as medium Voltage, in the U.K. 


Underground cable tables and construction details, if I remember correctly, are published by Electrical Research Association. https://www.amazon.co.uk/Current-Rating-Standards-Distribution-ERA69-30/dp/B0018LNT8A


russ

Did it not used to be that over 250 but below 1kv was considered medium voltage?


Almost there, the last time the Regs defined Medium Voltage was in the yellow 14th of 1974 where it was: "Normally exceeding 250 Volts but not exceeding 650 Volts, whether between conductors or any conductor and earth."


The last amendment of the 14th (Brown, 1976) only defines Extra-low and Low.


Regards


BOD (who really ought to get out more!)