additional cu cable size

[BruceB]

If you don't fit an RCD B type as fault protection then you have to install an Inverter that will not allow DC fault currents back into the AC side.
If the SMA Inverters are transformerless, and do not need a Type B RCD protection, then they must have either another form of separation, or the capability of not to allowing those DC fault currents to pass through.

It's the latter with SMA inverters. The document the states it has been cited here a few times.
http://files.sma.de/dl/7418/RCD-UEN103120.pdf page 8

"All SMA inverters with transformer and all transformerless SMA inverters mentioned in the following are not
capable of feeding DC residual currents to the grid due to the circuit design. They fulfill this requirement in
accordance with DIN VDE 0100-712 (IEC60364-7-712:2002)."

Regards
Bruce

 

[malcolmsanford]

Well as said obviously the TL inverter must accomplish sepration by another means as I posted.

What worries me about this is that SMA are advising on German installation procedures in a UK environment. Though we have some harmonisation additional protection in the UK for instance cable that is buried and not to Reg 522.6.6 needs 30mA protection and if your installing as to reg 551.7.2 on the load side then again you need 30mA protection.

So though I won't go as far as to say that the SMA TL Inverter should still be protected by a B type RCD, I would rather be reading a manufacturers instruction that is based on UK installation methods and regulations.

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.

 

[BruceB]

Well as I have stated in another thread I do not believe you should be connecting PV on the load side.

I am not sure you should blame the Germans - it was our dear DTI that published their guide. Personally I agree with you and think connecting TL framework to TNCS MET is arguably less risky than putting an additional rod in. But I would not do it that way whilst I have MCS assessors looking at my work.

Regards
Bruce

 

[malcolmsanford]

The DTI seems to have literally copied the German practices and fair do's if that is the way they want to go and you need to dance to that tune you have no option.

I'm not blaming the Germans they are doing the installs to their standards, I expect if the German Department of Trade and Industry were forcing UK standards on German electricians there would be a couple of Germans berating this as we are, an such a forum.

It still seems to me that it is the wrong thing to do, unless of course we intend to adopt more European practices, or because the industry is still in it's infancy, they may review installation practices at a later date, and for now adopted their methods.

What is worrying me is that it is putting this magical 100mA RCD in installers minds as an accepted norm, where here in the UK additional protection is by a 30mA RCD. I'm just surmising that perhaps not so savvy installers will not realise that they are installing systems that require additional protection by 30mA devices and fitting just 100mA as the SMA catalogue advises.

 

[SolarCity]

If the SMA Inverters are transformerless, and do not need a Type B RCD protection, then they must have either another form of separation, or the capability of not to allowing those DC fault currents to pass through.

I think that's correct. Seperation isn't needed by some kind of mechanism to prevent DC fault currents certainly is.

 

[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?