SMA TL Inverters and type B RCDs

[gr7]

I have just read the document produced by SMA on the subject of type B RCDs with their TL inverters. Section 4 states: "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)."

http://files.sma.de/dl/7418/RCD-UEN103120.pdf

Have other member of this forums read this and what are your thoughts?

 

[SolarCity]

I've read it and take it that a type B RCD is not required. Same situation with Fronius TL inverters. Not sure about the other brands.

 

[BruceB]

http://files.sma.de/dl/7418/RCD-UEN103120.pdf

Have other member of this forums read this and what are your thoughts?

Yes that document has been cited a few times already on the forum. If you need an rcd in your pv circuit then it does not need to be a type B rcd for those SMA inverters.
Regards
Bruce

 

[greenspark]

Out of interest, is there a pre-determined value of earth leakage for the SMA TL Invertor Models or a way of calculating it relative to the model that is being installed?

 

[whlphil]

Out of interest, is there a pre-determined value of earth leakage for the SMA TL Invertor Models or a way of calculating it relative to the model that is being installed?

I think from memory two values, its 30ma for personal protection and 300ma for equipment protection. Those are certainly the values for Power One inverters.

 

[SibertSolar]

Guys,

I'll try to be as impartial as I can here. If the installation calls for an RCD to be fitted and there is no galvanic/electrical isolation involved between the SSEG and the Grid then a Type B RCD/RCCB should be used. As has been advised by others, both SMA and Fronius have released statements regarding their TL range of products and the fact that their inverters do not allow DC leakage currents to be imposed on the grid supply.

At the moment, there is a great deal of discussion going on between the DTI/BRE, the likes of SMA and Fronius (and others) and the major RCD manufacturers. The consideration/discussion is this - does the RCMU in the inverter (that prevents such leakage currents being transposed onto the grid supply) fulfil the total requirements of the legislation. In other words, just because the inverter complies, does that mean that the whole PV installation complies......

Doepke's stance is this "if an RCD is required and you are unsure as to which type to use, then fit a Type B". Using a Type B RCD (where there is a requirement to fit an RCD as an additional protective measure for a PV system that does not include simple separation), will ensure that the Contractor does not potentially have to make any last minute changes to the installation to obtain sign off/certification etc "IF" the legislation or the stance of the TL inverter manufacturers changes at some point in the near future.

Futher to this, and specific to Type B devices, it's not just a case of the level of leakage current protection in terms of mA limits etc. Each type of inverter will have different characteristics in terms frequency harmonics, and of the frequencies at which the maximum leakage currents occur. It's important then to understand the nature of the SSEG's leakage current frequencies, not just their amplitude. 30mA protection ("human" protection) is NOT normally a factor here as the PV installation "should" include Class2 devices, without any socket outlets/connections. More often, fire hazard consideration is the primary factor - therein lies further understanding in relation to the upper trip limits (300mA or 420mA, for example). To cover all scenarios, our (Doepke) products are available in 3 different model types for Type B RCD/RCCB protection. Selecting the right solution does require a certain degree of understanding of the nature of the installation and the inverters being used.

Hope this helps, more info can be found on our website and/or the Doepke UK website.

Andy

 

[BruceB]

Guys,

At the moment, there is a great deal of discussion going on between the DTI/BRE, the likes of SMA and Fronius (and others) and the major RCD manufacturers. The consideration/discussion is this - does the RCMU in the inverter (that prevents such leakage currents being transposed onto the grid supply) fulfil the total requirements of the legislation. In other words, just because the inverter complies, does that mean that the whole PV installation complies......

I suspect the answer to that is likely to be no, because if an rcd is required then it is probably required to provide:
- additional protection for buried cables; or
- additional protection from fire; or
- additional protection due to TT
(and no doubt a few others)
That protection would still have to be in place when the inverter was isolated at the inverter end, which leads me to believe the rcd would have to be at the consumer unit end.

Regards
Bruce

 

[SibertSolar]

Precisely Bruce.

High-resistance, "slow burning" DC leakage currents may be generated by a DC cable insulation rubbing on metalwork, for example, or from excessive moisture/dew especially in early mornings. (On a non-bonded/floating system...).

I've said before that I'm no expert on general wiring reg's etc but what I will add also is that if there are any buried cables (that are not armoured), then an RCD fitted should be 30mA rated, right? Most TL inverter manufacturers will tell you what their leakage current values are but I suspect that nuisance tripping would be a problem in most regards for 30mA RCDs. If TL inverters are an absolute must to be used then it could be said that all possible efforts should be made to "dial-out" any install methods that result in the requirement of a 30mA RCD.

One good idea would be to get to know your inverter manufacturers and their spec's well. They don't typically give out leakage current details (amplitude or harmonics/frequency info) on their standard spec sheets but when asked what trip limit RCD to fit, they should be able to advise based on their own knowledge of their product's leakage current characteristics. The least they should do is provide the info you need to make the decision. For TL inverters, your MCS inspector may also provide you with some guidance as to what would be accepted or not. Under the terms of the current guidelines, it could be inferred that ALL transformer-less inverter installations should be fitted with a Type B RCD, regardless of what the manufacturer says.

Hope this helps. At the end of the day, we (Sibert) can't dictate what type and rating of RCD should be fitted. With Doepke's assistance we can only support what the regulations state. However, with additional technical information provided by the inverter manufacturer, and the design/installation engineer's expertise and experience with the standards/regulations and installation environment, we can certainly help to find the best solution.

As a guide for anyone who is curious, a 16A, 100mA, 2-pole Type A RCD should cost around £45ea. A 16A, 100mA, 2-pole Type B RCD should cost between £300 and £350 (depending what model type needed/selected)

Andy

 

[new sparks]

so if i have a ground mount system ac wired with swa cable on a tt earthing system with sma tl inverter , are we saying i need a 30ma b rcd

 

[BruceB]

How do you get that?
TT requires an rcd, but the regs do not specify a value directly (it depends on Ra/Zs and disconnection time required). TL installation instrs say min 100mA. Does not need to be type B because SMA inverters cannot feed dc to ac side.
So an ordinary type A at 100mA or greater should be fine in the example you have quoted, given a satisfactory earth rod.
Regards
Bruce

 

[Worcester]

So if we are having this detailed discussion now and type B's only just appear to be coming available, what have people been using with Fronius TL inverters up to now?

 

[U2Singhy]

Does anyone have the document from Fronius stating no type B RCD is required for their TL inverters please?

 

[malcolmsanford]

It might pay to read what the regulations say concerning this and the IEC 60755. If the new invertors are providing seperation, and therefore not allowing DC fault currents into the electrical installation,without the use of a transformer then it adheres to regulation 712.411.3.2.1.2 and therefore not requiring RCD protection. Note also that the reg apart from wanting a type B does not advise what rating the device should be, so it can be anything from 30mA to 300mA depending on the use.

I was interested in the comment on the double pole protection devices, is this a manufacter recommendation??

 

[Worcester]

If I remember correctly it's because the inverter is a generator, so your current flow is the opposite way (hence RCD at the inverter end which is now upstream..) and if you don't disconnect both you still have a circuit. with earth as one of the current paths, and the 'neutral' as the other.

 

[malcolmsanford]

Worcester that would seem to colate to reg 551.7.2 (iii) I believe. Which in essence a main RCD in a 17th edition split board will give you.

 

[paulsamuel1984]

SMA have advised me that DC injection cuts off at 100mA for the STP range. So they are certain a type B rcd is not required. However, if this is the case, is a type A rcd required, or no rcd at all? what rating should the device be if required? Where would you locate the RCD, prior to the G59 relay in your PV switchboard?

 

[SibertSolar]

Interesting, what do they mean "cuts off at 100mA"? Are they talking about an inherent DC leakage current? Or are they referring to a potential leakage current fault?

I think that whether an RCD is needed or not is down to the nature of the installation and/or the type of inverter. The DTI guidelines state (section 2.3.1) that if at least a simple method of separation (DC side to AC side) is not incorporated into the PV power supply system (for example: no transformer in the inverter) then a Type B RCD must be used. SMA's statement regarding their TL range refers to not needing a Type B. They don't mention Type A because they don't need to - in Germany they don't even use Type AC RCDs so the base/default type of RCD they would use, if the electrical installation required it, would always be a Type A. The UK is different in that we still allow the use of basic, Type AC RCDs.

In terms of ratings, the TL inverter's inherent leakage current characteristics should be considered but (in Doepke's experience) a 100mA trip limit is the typical level required to avoid nuisance tripping by the inverter itself. If there are fire hazard considerations to be taken into account then a 300mA, or even a 420mA limit may be used.

As far as location is concerned, I'm not sure I'm best qualified to advise so I'll leave that to those on here who actually work in the real-world of installations... ;-)

Hope this helps, open for correction if my understanding is wrong/incomplete

 

[paulsamuel1984]

http://files.sma.de/dl/3427/EK_RCD-SBSMC-ZEN104814.pdf

attached link for discussion

 

[SibertSolar]

@sellsolar - thanks for this. What I believe they are saying is that a minimum 100mA trip limit RCD is recommended (to avoid nuisance tripping).

Point to note is that they state that the integrated RCMU is an additional safety device to an RCD. The tests they performed to determine whether a Type B RCD is needed were done without taking the integrated RCMU into account. The only reason I highlight that point is to avoid people possibly interpreting the SMA (or Fronius, or PowerOne etc) inclusion of an "RCD-type device" such as the RCMU as being "fitted already with a Type B RCD" - which is not the case, the RCMU is not an RCD, Type B or otherwise.....

I need to check on the list of "approved RCD Type A devices" and whether that has been updated recently as the document seems to be dated 26th Nov 2010.

In case anyone needs the other SMA document regarding selection of RCDs, a copy can be obtained here: http://www.sibert.co.uk/uploads/spare_document/filename/37/RCD-UEN103120.pdf

Thanks again for this,

Andy

 

[Worcester]

This may help explain why SMA TL's don't need an RCD : http://files.sma.de/dl/3491/TECHGRID-11-AE2306.pdf Is about being more than an RCD and embodying the requirements into a standard rather than prescribing a technology.