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

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

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!

(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.

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.

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.