additional cu cable size

[SolarCity]

Seems there are a lot of practices within the PV industry that is taken directly from Germany, another is the rod procedure for earthing an array frame if the earthing system is TNC-S. Here in the UK there is no problem with our stable PME system and I personally can not see an advantage of fitting a rod when you have a much more stable earthing system available to you.

Is the German PME system less stable than ours? I didn't know that.

The grey area with PME/PV systems is a nightmare. I can see both sides of the argument but currently we are advised to install a seperate earth rod and it's what we've been doing until we get told otherwise.

 

[Graeme Harrold]

The G83 test sets limits on DC injection back into the AC system and is set at 20mA, which is aptly detailed on the type test report.

 

[edexlab]

Is the German PME system less stable than ours? I didn't know that.

The grey area with PME/PV systems is a nightmare. I can see both sides of the argument but currently we are advised to install a seperate earth rod and it's what we've been doing until we get told otherwise.

The German grid is considerably better than in the Uk the inbalance limit is 4.6kw over there

 

[Mark Davey]

We have installed a Fronius IG35 PLUS today (8 x HIT Sanyo 235W) does anybody happen to know whether this also has built in protection that means a type B RCD is not required??

Also I read and hear differing views on the bonding of the array mounting system & module frames.
In no uncertain terms the tutor on the Eco-skies course I attended stated that only ground mount systems require the bonding and that this is achieved using a seperate CPC to a dedicated earth rod, he suggested that roof mount systems do not require bonding due to "placing out of reach", a definative answer on this would be great.

My final question is a very simple one, we have designed the installation in 2 strings and would like to know if the string fuses required or do the individual modules have satisfactory protection so that the solid links supplied from the factory OK.

I felt that my training was really not comprehensive enough and left so many basic questions unanswered so I have enroled on the C&G2399, but until such time a big thanks in advance to all the other forum members and site administer as your information which is much appreciated

 

[Worcester]

Check the DTi Guide, with only 2 strings it would be very unusual to need to fit string fuses so long as the cable is rated high enough.

 

[SolarCity]

A type B RCD would not be required as the inverter already offers electrical seperation between the array and the AC side of the installation due to the transformer in the IG35 PLUS. You need only worry about type B RCDs when using transformerless inverters. Generally, the same is true when considering the bonding of an array. An array installed with a transformerless inverter must be bonded - using an earth rod if the incoming is a TN-C-S system.

 

[Ramjam]

I just mined my way through this issue and this is what I discovered, I can only speak for the SB4000TL......

The Germans mostly don't earth array frames....

All panels induce some form of current in the array frame and in a transformerless inverter this current leaks over the the AC side.

The total leakage from a 4kWp system in never likely to exceed 80mA

The SB4000TL inverter has a 100mA type B RCD contained within it, should the DC leakage to the AC side exceed 100mA then the RCD will trip with a fault code on the inverter which can be inspected by an installer using Sunny Explorer.

The DTI guide demands that the array frames of systems with transfomerless inverters be earthed, in the case of PME systems this means to a dedicated earth spike rather than the MET (for me this has always been the case).

BS7671 requires that additional protection is provided by a 30mA RCD, in germany 100mA is acceptable, not here!

If you fit a 30mA RCD or connect to a CU with a 30mA RCD you run the risk of leakage from the array frame making it's way through the inverter without tripping the type B RCD in the inverter and nuisance tripping the 30mA RCD. To avoid the nuisance tripping fit a good earth rod and connect it well to the array frame (every rail).

SMA struggle to accept that their inverter should be protected by a 30mA RCD as required by BS7671 as their solution to nuisance tripping is to fit a 100mA RCD instead. They are just learning about the UK market and this is one of their grey areas.

The SB4000TL is a brilliant inverter, get a 1m SDS drill bit, an earth rod, inspection pit and some 10mm earth and lugs and way to go. Remember there are some simple ways to test earth rods, I shan't mention them because TT experts want you to go to Hogwarts before you can fit earth rods but it's pretty easy once you get the hang of it and with a dedicated PV earth rod the cost of TT failure is nuisance tripping not death so you don't need to be terribly scared.

 

[malcolmsanford]

Great post Ramjam and it outlines what I'm saying, the BS 7671-2008 tells us that the manufacturers instructions takes precedence, but for PV inverters most of these instructions are geared at the German electrical system not our UK one.

The 30mA RCD is one and the use of a rod is another. Because of the use of the 30mA RCD for additional protection then in the UK I would be very wary of using an Inverter that actually feeds DC current back into the AC system and if they leak in excess of 20mA again I don't think we should be using them, unless you can ensure that the installation that as been erected does not need 30mA protection. I can see now why the DTI are insisting Inverters be installed on a dedicated circuit, but surely if you use TX type there is no need for this as the separation will not allow DC currents back into the system. So I gather that have blanketed the installation method.

The point I want to also raise with the earthing is the use of a rod for a PME system. There is no concern these days with the exporting of the earth in a PME situation in the UK. Many years ago when the Electrical safety Regulations were in force it was advised to contact the DNO and seek permission, but today if you called the DNO to ask they would most likely be incredulous about your request. With the advice now on also earthing TV aerial masts, granted it is recommended more for communal installations, but many people are advocating all masts to be earthed, you could get a very dangerous situation of an earthed mast of a PME/TN-S internal system and an earthed array of a TT system, within touching distance of each other and IMO that is introducung a far more dangerous scenario.

 

[SolarCity]

Many years ago when the Electrical safety Regulations were in force it was advised to contact the DNO and seek permission, but today if you called the DNO to ask they would most likely be incredulous about your request.

We phoned the DNO on one of our first TL installs to ask them about exporting the PME onto the roof. They had no idea what I was talking about and one guy advised me to ask an electrician's advice!

 

[dansk]

Malcolmsanford are you saying its ok to take the earth bond to the MET? On a PME?

 

[malcolmsanford]

Yes but seemingly no according to the DTI.

I can only relate to normal electrical practices and if you were in a domestic installation and taking a sub main out to an out building, if that building have metallic parts to it either a metal service such as a water pipe, or the construction of the building was metal, then you would need to bond the extraneous conductive part back to the MET if you were exporting the PME earth. If you were not exporting the earth perhaps because the building was a long distance from the origin of the supply, then most likely you would TT system.

There is obviously a reason the German electrical industry do not export earths on a PME, to which I don't know, and so it seems they insist on the array being bonded via a separate rod. I have no problems with this as such, as long as you are not having other metal work at a different potential that can be touched while your working or touch the array frame, and I used the example of perhaps an aerial mast that is earthed via a PME/TN-S from within the equipotential zone.

 

[Ramjam]

I'd suggest that the most common earthed metalic object on a roof is the metal flue of a central heating boiler.

 

[dansk]

I find I a bit of a headache trying to the do the right thing, I spent hours last night researching pme and bonding, it seems a very grey area to me, but my thoughts are if it's roof mounted take the bond to the met rather than a rod, let's say somehow you could reach the array from inside the zone it would have to be bonding to the met anyway. But did come across this on my travels last night Geosolar DC Earthing which explains in simple terms

 

[dansk]

What if you was holding onto the scaffold and the array at the same time and it was bonded to pme? Would that cause a shock?

 

[SolarCity]

The issue is, as I see it, that in the event of a fault which took the neutral out at supply, the array would become 'live' and potentially lethal to anyone working outside of the property.

 

[dansk]

The issue is, as I see it, that in the event of a fault which took the neutral out at supply, the array would become 'live' and potentially lethal to anyone working outside of the property.

Sure - But isnt PME "multiple earth" so the neutral conductor is earthed in stages along the line from the supply? But if broken neutral in CU then is a problem

 

[SolarCity]

Sure - But isnt PME "multiple earth" so the neutral conductor is earthed in stages along the line from the supply? But if broken neutral in CU then is a problem

Exactly. And this is what creates the potentially lethal situation.

 

[malcolmsanford]

In section 9 of the ESQCR-2002 covers PME in LV installations. The DTI impose stringent guidelines that the distributor must adhere to when supplying the PME system, and one of these is the loss of a neutral. The distributor take measures against the loss of the neutral conductor, and it is therefore seen as a low risk occurrence, which is why exporting an earth is not seen as potentially dangerous as it once was.

 

[dansk]

:dizzy2:

 

[Mark Davey]

[h=2]

Re: additional cu cable size[/h]

The issue is, as I see it, that in the event of a fault which took the neutral out at supply, the array would become 'live' and potentially lethal to anyone working outside of the property.​

not only the array but also all bonded exposed conuctive parts?