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Direct Buried cables within a controlled area and what constitutes mechanical protection

Former Community Member
Former Community Member
Hello 


I have a  query raised by a client stating that a PV solar Generation site built on agricultural land is non compliant due to direct buried cables. 

Some of the DC string cable is direct buried roughly at 600 depth laid within  cable sand and protection tape over. 

The cable spec is EU and states is suitable for direct burial. 

The protection system constantly monitors the  insulation resistance and dis engages the inverter upon fault. 

The client has stated a non compliance due that no mechanical protection has been installed underground but all cables above ground are mechanically protected. 

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    Obviously real world generators do not generate at 132kV, they would have stepup transformers; I was not trying to suggest otherwise. But as you have said a medium scale generator (>50MW, by Ofgem's definiton) would likely have multiple sets each with high voltage windings (yes likely each with a trafo etc), and the point is that if trying to claim exemptions (as well as obligations) under ESQCR, the definitions thereuder ought to be applied (in that context) and this specifically says generating at high voltage, not connecting at high voltage.


    Irrespective of whether ESQCR gives an exemption, you have yet to say where in BS7671 it says that it is excluded from scope. 1.5kVDC is still Low Voltage, and it's entirely feasible - likely even - that we'll see it on, if not domestics due to physical space, small community buildings, schools etc. To say that this is not anticipated by Section 712 is quite the leap. Given the prevalance of solar parks at the time of the last revision perhaps it would be expected that the authors would have been clearer in this exclusion?


    I'm not sure why you think I'm suggesting that the concern is persons off-site, and I don't have a problem per se with IT arrangements suitably protected. It is of course the norm (in the UK) for the PV DC wiring, and is adequately covered by BS7671.


    I am not trying to suggest that the installation is suddenly likely to erupt in a ball of flame. But the OP has posed the question of whether a particular arrangement conforms to a particular standard. This is a pretty binary question. It can be answered, and if negative, one can then address whether the arrangement does offer an equivalent level of safety accounting for other measures in place etc, and also consider the contractual implications (if any).


    However I have yet to see where the Regs say they do not apply to PV installations larger than XYZ, or to low voltage circuits supplied by a public network at HV via a private HV network (obviously the HV network itself is out of scope), or indeed to "generators" (legal person, not physical plant) as defined by ESQCR.
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    The only law which applies to BS7671 is contract law, that which is written into the contract. Whilst one may attempt to say a PV array farm is not a HV generator, this is not true if the connection to the Grid is a HV one

    Real generating stations operate with many generation voltages, there are numerous efficiency constraints that may suggest a particular generation voltage, but the power station transformers which connect to the metered grid often deliver at 132kV or more. This is the point, a generator would be very difficult to design which actually made power at 132kV, the first being the level of insulation required. Take a 600 MVA unit, it would need to deliver only 4545 amps at 132kV, but this is not a big current and would need a lot of turns. Much better from an insulation point of view would be 45 kA at 13.2 kV, and perhaps a single turn per phase. Cooling is the least of the problems here, insulation is both thermal and electrical, and a lightly insulated bar of a significant cross-section would cool best, or perhaps a number of smaller ones in parallel, giving more surface area. The voltage is then sorted by the transformer, which we need anyway for fault handling and protective purposes. At 1MVA the preferred generation voltage may be 3.3kV, but 400V is not unknown at this power level. If connected to the grid, this is still considered a generator, and G59 would apply, and the grid voltage would probably be 11kV. This kind of installation often has "differences" to BS7671. You seem to be implying that the reference to BS7671 is to use it as a safety case against the wiring. If one reads BS7671 it is quite clear that the safety is for consumers and users of wiring systems. To use it for 1.5kV DC wiring is pushing the scope quite a long way from its normal purpose, if it ended up in court a skilled Engineer would drive "a coach and horses" through the argument you are putting, simply asking why an IT system with inbuilt protection could be dangerous to anyone off-site, to which you control access and presumably maintenance operations? On-site, you as the operator simply need to set safe rules for staff, including such items as permissions to work.


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    (xviii) solar photovoltaic (PV) power supply systems

    Which part of BS7671 states that this doesn't apply to larger installations? If I operate a private HV network on an industrial estate and one of those substations feeds a solar park, is that in scope?


    I certainly agree that some items with LV inside do not come in the scope of BS7671. The point was that the generating units are generating at low voltage so the site is not a generator in ESQCR terms. And before anyone chips in with "yes but they connect at HV", first of all ocnsider the aforementioned private HV network with an embedded PV array, and then further the DTI guidance note on ESQCR:
    Generator

    For  the  purposes  of  the  Regulations,  generators  operate  high  voltage   equipment   and   are   particularly   associated   with   the   provision  of  electrical  energy  to  consumers  via  networks  (see  definition  of  network).

    The  intention  here  is  to  include  medium and  large  scale  generating  plant  incorporating  substations  and  electric lines to which the public may have access. Operators of any sort of generation in domestic situations are not expected to comply with the requirements placed on generators.  However, all operators and consumers should note that they may still be caught by the requirements of Part VI Generation.

    Generators  should  note  that  they  are  exempt  from  implementing  certain  requirements  within  generating  stations  (see  regulations  12 and 16).

    (My emphasis)

    Medium and large scale generating plants were at the time well defined and would have required a licence to operate, unlike solar parks which being SSEGs even on the multi-MW scale at 132kV do not require one as long as G59/G99 is followed. (As an aside, G59 and G99 also make reference to BS7671)


    The relevance of the contract is that BS7671 is only required to apply by law in particular cases, and in others it is de facto required as the easiest means to demonstrate the required level of safety. However, if the design or inspection contract say that BS7671 shall be applied then except for the parts excluded from scope either by other terms or by the standard itself, it should be held to apply.
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    Jam, 110.1.1 does NOT include generating stations. A PV system referred to in line with all the other things is a small domestic one, similar to (vi). The context here is the consumer, not the Grid as I said above.

    BOD, Hi, BS7671 is not and never has been a document that is subject to contract terms. In scope it is the standard, out of scope, do as is necessary following whatever,

    Jam, all items with LV inside do not come under BS7671. The connection to the Grid at HV is the bit that is important. Your invertor for PV may well be LV, but are the insides covered by BS7671, certainly not.

    BOD, your truism is indeed relevant, the result is always a fact of history! 110.2 is especially relevant, it is the consumer side of the meter!
  • Former Community Member
    0 Former Community Member
    Thank you Jam. On the basis that my simple question has been ignored and the OP's responses, time for this quote:

    When we ask for advice, we are usually looking for an accomplice.

    Marquis de la Grange 


    Regards


    BOD
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    Sadly I disagree with some of the above sentiments regarding scope. Leaving the not irrelevant question of contract (which BOD is right to question) to one side for a momeent...
    This is not an installation that comes under BS7671 is it?

    Au contraire: See the list of scope-inclusive items of BS7671 110.1.1, particularly (xviii), which is pretty unambiguous, and in that light I would suggest that the onus is on others to say how it is excluded by the list of 110.2.

     
    IET code of practice for grid connected solar PV systems 2015 section 8.2.  states in blue mandatory text General BS7671 requirements are for AC only


    This is well and truly out of context. Section 8.2 is titled "AC system requirements: Low Voltage" so "The A.C. circuit [...] shall be designed and installed so as to comply with the requirements of BS 7671" doesn't have any bearing on DC. There are many references to BS7671 for the DC design.

     



    BS EN 62446 covers the PV DC side


    Yes it does, for inspection, testing and maintenance, not design. However the same does require verification to IEC 60364, the national implementation of which is BS 7671 (though some other national version could conceivably be used if allowed in the contract). Depending on the vintage of the installation, (BS) IEC 62548 may also have applied to the design.

     



    “generator”  means  a  person  who  generates  electricity  at  high  voltage  for  the  purpose  ofsupplying consumer’s installations via a network

    ... But PV installations generate at low voltage (<=1500Vdc). They connect at HV. If you don't accept that, I don't see what there is to differentiate between that and a factory supplied at 11kV with a 20kW system on the office roof; after all from the DNO's perspective they're both now SSEGs (or PGMs in the new G99 lingo). So I would argue that they're not generators for the purposes of ESQCR. Indeed I would suggest that classing them as generators creates more problems than it solves (random example do any solar parks take any precautions to ensure continuity of the HV supply neutral?). They may however be persons operating a source of energy in parallel with a distributor's network, which sounds the same, but isn't by definition. Odd as it sounds absent further guidance (which to be fair I would welcome) I would go out on a limb and suggest the best classification in terms of ESQCR is that of consumer. (So no TN-C for you!)



    The above all considered, I would return to BOD's very pertinent question and add that in all the design projects I have seen for solar parks, the client has specified BS7671 in the contract specification and often the Connection Agreement with the DNO often stipulates the same.
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    Grammarly....
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    That'll be gnaw.

    Grumpy. BSc. MSc. Pedant etc.
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    The idea that BS7671 covers generating stations is not correct. Even the ESQCR is rather vague on this. I suggest again, the "consultant" is wrong. Suggest this to him and ask for evidence, that usually works for me! BS7671 does not cover generation, the Grid, or anything else except after the meter. Where are the metallic sheaths on EHV overheads? You say not buried, I suggest you look at the superconducting connections.

    Added as edit. The scope of these things is set out, it is much more important than the content!


    David CEng etc.
  • 0
    RE Elltec:

    IET code of practice for grid connected solar PV systems 2015 section 8.2.  states in blue mandatory text General BS7671 requirements are for AC only 

    BS EN 62446 covers the PV DC side 

     


    That sounds very peculiar to me - BS 7671 section 712 is full of requirements for the d.c. side and written in the usual section 7 way of modifying the general requirements - they'd hardly made sense as 'stand alone' special installation requirements.


    As far as I can tell BS EN 62446 only covers "Requirements for testing, documentation and maintenance" - so nothing about design or installation - was the quote perhaps taken from a section concerned only with ongoing maintenance and testing perhaps?


       - Andy.


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