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Tingles from lead flashing on house with solar array - anyone else seen this?

mapj1
A long phone call today with a good friend who is a Niciec Contractor, who is now facing a rather odd problem with a domestic solar array installed by others. The DC string cables from the panels on the roof to the inverter run along some distance under lead flashing, and now workers on the roof are reporting tingles from the metal flashing when standing on their scaffolding.

As part of the testing , the cables have been checked and are isolated from the metal and not damaged in any way. There is a rather variable AC voltage on the lead, relative to the scaffolding, which is at more or less the same as local earth potential. The measured voltage is considerably more when the inverter is on though does not fall completely to zero when it is not.

Now I have not seen the set up, as it is many miles away, but as the voltage is so variable depending on conditions, I am minded to suggest it is capacitive coupling between the DC string cables and the adjacent metal.

I'm also going to suggest earthing the flashing in any case.

However, has anyone else with more experience of modern domestic solar installation ever seen this sort of thing?

 And am I even right in assuming the inverter  action means DC strings are commutating at 50Hz relative to the mains, as would be needed to explain this effect?

Or am I going up the wrong tree altogether ?


The inverter suppliers are not much help, being more of a kit supplier than technical experts on what they stock, and this is not in their FAQ.

  • 0
    Broadgage is right that "DC" on a PV array will contain grid frequency (and HF switching) superimposed on it. The magnitude, and whether it is centred around the same point as the DC, depends upon the inverter construction (older inverters with transformers have somehat less). This does to capacitive voltages between the array frame (and by extension, anything in electrical contact with it) and system/true earth, which will also vary with weather conditions.


    The UK guidance used to be that PV array frames should be floating unless brought into the equipotential zone by other building components (amongst other reasons), because the equipment itself should be Class 2 and bonding to the MET a) Means exporting the PME earth outside the building envelope, if it's PME, and b) Provides a return path for leakage current, and hence fun with RCDs we're getting used to now. (Transformerless inverters were rarer when that guidance was written though). We were brought closer to European practice with the IET Code of Practice, which expects that the frame will be bonded in all circumstances.


    Jam
  • 0
    Thank you for that very helpful response Jam - you raise  a point I had missed - is the current advice then that there should already be an earth bond (well to the house CPC, if not to true earth) also coming outside to ground the frames holding the solar panels ?

    If so then I think the installation even though recent,  is perhaps not to the current standard, as unless it has not been described well, there is no such connection, just the DC 'string' cables.

    I am well aware that the coax of TV antennas should be earthed, according to the Confederation of Antenna Installers, but that the advice has not yet reached the man in the street or the leaflets with the antennas in B&Q Wickes etc..

    I suppose solar panels like the TV antenna are just another thing to catch unwary builders with stray voltages that may be non lethal as a shock, but make them flinch, which on a roof ladder is bad news( more on the TV antennas advice here ) It makes sense that TV antennas and solar panel frames should be treated similarly - on many houses they will be close enough to be  simultaneously accessible (or perhaps simultaneously in-accessible to anyone who is not a good climber.)

  • 0
    First of all a clarification/correction: MIS 3002 v3.5 is still apparently current according to Gemserv and this makes no mention of the IET Code of Practice, so strictly speaking installers working to that can still use the 2013 "MCS Guide v2" as 'tis known and controm to contractual obligations. MIS3002 is a pre-requisite for the FIT, and it's possible that a local installer didn't get the memo about the about turn that means (£69) best practice contradicts their go-to main (free) standard. I don't often get involved in domestics these days though so there may have been an advisory, although with MIS 3002 most recently being updated in 2019 and the IET COP coming out in 2016 (with the same lead authors - plus others - as the MCS Guide) I'd expect to see it written into the standard by now if it were going to be.


    A summary of the decision tree in the IET COP is visible on p15 of this BRE presentation.

    In the document is the following statement

    CHANGE FROM PREVIOUS APPROACHES IN THE UK

    This Code of Practice requires that the PV array frame is connected to earth in all circumstances. This differs from the approach taken in some previous UK guidance documents.

    The arguments for and against earthing an array frame are relatively complex and cannot be described in detail within this Code of Practice. However, the approach whereby PV array frames are earthed in all circumstances has the advantages of:

    (a) allowing the d.c. isolation fault detection systems that are now provided within inverters to operate correctly (i.e. earth insulation resistance, residual current monitoring);

    (b) removing the shock risk due to leakage currents on systems with transformer- less inverters; and

    (c) bringing the UK in line with the approach taken in most other countries around the world (and IEC standards/documents).


    In some circumstances an installer may need to work on or inspect a system installed to a previous standard that did not require array frame earthing:

    (a) where an old inverter is being replaced by a modern device that includes d.c. isolation fault measures, it is recommended that, where practical, the installation of an array frame earth be included as part of the replacement works;

    (b) where an installer is performing an inspection test on an older system, it is recommended that the lack of an array frame earth be noted on the condition report



    (For those interested in such things, the original decision to not bond the array goes back to the first edition of the "DTI Guide" as twere known in 2002. DTI published an interesting and open commentary, "Background Information to the Installers Guide for Small Scale Mains Connected PV", DTI S/P2/00355/REP/2.... they appear to have been aware of the hazard but took the other path)


    So the latest advice, from the IET of all people, would be that the array should be bonded, but both options appear to be current.


    As an aside on larger commerical installations the accumulated leakage current can be into the dangerous region, but the scope of the MCS guide is only up to 50kWp so hopefully above that everyone is referring to the IET COP by now.

  • 0
    If we're concluding that the problem is due to capacitive coupling between the "d.c." string cables and the flashing, would a better answer to be to screen the string cables rather than bond the flashing? If the cables could be screened within an insulating oversheath (or sleeving or conduit) we should hopefully have same cancelling effect, but without 'exporting' the installation's MET potential (PME or otherwise) to anything touchable outside.


      - Andy.
  • 0
    yes - if the dc strings had been something like SY with the braid earthed ,we would have the best of all words - sadly that is not an easy option to do, - all the available suitbly rated string cable and associated  connector options etc  assume unshielded cables. Although if we obey the IET COP, and other organisations advice and by the look of it, that is not as universal as it should be, the PME earth should be on the exposed frames anyway (as well as satellite dishes, TV antennas and outside taps.) so perhaps a bit more on the flashing is not so serious.
  • 0
    It's not so much the cables as the array itself, which is a very large surface with short distances to earthed/floating metalwork (the framing) area and no screen. If the "tingles" are coming from lead flashing I would suspect some form of contact between the leadwork and the array mounting system.


    SMA published a paper giving a means of calcuating the leakiage current - mainly for RCD considerations - many years ago. Surface area of the array and insulation / glass / backsheet thickness to the fame (and presence of water) being key variables.


    DC runs longer than 50m should be screened anyway, but I believe that's more for EMC (likewise + and - run together, failings so to do being a personal bugbear).


    Jam
  • 0
    Has anyone insulation tested the PV panels and the down leads that run under the lead flashing?



  • 0
    OK, what about some kind of filter (e.g. LC network) on the d.c. string cables as they enter the inverter to stop the "a.c. noise" from the inverter escaping into the d.c. cables (or as my head thinks of it, a smoothing capacitor on the end of the d.c. cables so that when the inverter tries to draw pluses of power the extra current comes from the capacitor rather than along the string cables, and when it's not the panels charge the capacitor).

      - Andy.
  • 0
    You could, and I wish they did, filter the RF components of the AC off the DC strings far more positively. The value engineers think that meeting the minimum EMC spec once in a test in the most favourable layout of cables in a test house is enough, to show it is good enough to cover all cases, and it really is not. (bring a medium wave radio near it if you do not believe me)

    Some big solar farms with lots of inverters are in effect long and medium wave jammers with an effective radius on weak signal reception of several miles.

    Equally simple changes, like not wiring the DC strings as a big loop, but as twisted pairs and some ferrites can have a big effect on the RF problem is, no one does.
    an American perspective on the interference issue


    However, the 300V p-p at 50Hz, which I think is a large part of the tingle is essential to the correct commutating operation of the inverter, and while we may wish to round the edges off a bit and limit the rate of rise, we cannot get away from the fundamental.

    Meanwhile my friend has been at another project. and has not come back to me with how this one has gone, so I have nothing to report.

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