Domestic consumer unit rating with PV and battery storage.

I think that is wrong Graham, unless one wants additional protection. As I pointed out the inverter cannot provide excess current above its rating, and unless the fault is of a very high resistance cannot provide significant output voltage either above its rating. The problem is with the wording of 419.2, which is you suggest is requiring ADS when it is not necessary because the output voltage is completely limited by the output power rating and possible fault impedances. You are translating this as "we don't know" when this should be included in the specification, and in fact is by the maximum power output rating and time for which this may be provided. 419.2 clearly has thought of this in the part about reduced output voltage, and for that reason should be readily available from a manufacturer. Adding an inverter RCD is as Andy says unnecessary as the connection I suggest to the supply leaves all the normal RCDs to isolate faults elsewhere in the installation, and an Earth fault on the inverter cable, either SWA or steel conduit is exceedingly unlikely and not dangerous due to the limited output voltage. Due to the nature of the inverter, many people would probably suggest a type F RCD which is very expensive, and in my view unnecessary.

At the start the discussion was about 16 amp battery storage, however the V2G vehicle to grid inverters may have an output of 6 kilowatts, around 26 amps, so with that in mind what’s the short circuit current likely to be from a V2G inverter?

https://www.indra.co.uk/v2g

Agreed

Sparkingchip said:

so with that in mind what’s the short circuit current likely to be from a V2G inverter?

The "rule of thumb" is 120 % of (1.2 times) the nominal maximum output of the inverter, so 19.2 A for a 16 A output inverter, or 31.2 A for a 26 A output inverter.

Hence, you might just get away with B6 meeting the disconnection times, but a B10 would not be considered to comply with BS 7671 for disconnection times (where you have to consider 5 times nominal rating for ADS).

This is why the IET CoP for EESS tends towards recommending RCDs at the inverter output and RCD / RCBOs protecting final circuits in island mode.

Of course at short circuit, the voltage has fallen to zero, so there is no need for any rapid  ADS at all, so the MCB is irrelevant. The inverter output stages either folding back to current limit or going bang is an adequate and far faster safety mechanism than any MCB fuse or RCBO, happening in a fraction of a half cycle.

Mike

mapj1 said:

Of course at short circuit, the voltage has fallen to zero, so there is no need for any rapid  ADS at all, so the MCB is irrelevant.

The time in which the voltage drops is definitely important. Have a look at 1st para of Regulation 419.2.

419.2 For installations with power electronic convertors with nominal voltage U0 greater than 50 V AC or 120 V DC and where automatic disconnection is not feasible, the output voltage of the source shall be reduced to 50 V AC or 120 V DC or less in the event of a fault between a live conductor and the protective conductor or Earth in a time as given in Regulation 411.3.2.2, 411.3.2.3 or 411.3.2.4, as appropriate (see BS EN 62477-1).

mapj1 said:

The inverter output stages either folding back to current limit or going bang is an adequate and far faster safety mechanism than any MCB fuse or RCBO, happening in a fraction of a half cycle.

So, that would put the time factor to bed, but if the manufacturer does not provide the relevant information on the operating times, and also the further information necessary, namely adequate methods for initial verification and periodic inspection and testing, it can't be relied upon (2nd para of Regulation 419.2):

The power electronic convertor shall be one for which the manufacturer gives adequate methods for the initial verification and periodic inspection and testing of the installation.

well yes,  the inverter makers need to provide spec about long term load rating and behaviour in short circuit conditions.
For some milspec cases, we insist on being able to ride over a 100% overload for so many cycles, and then in effect they sell us the next size up with a dual time constant slugged current limit. And they charge the earth  for it. The standard offering is much simpler.
Mike.

Slowly getting more information and have asked the manufacturer for confirmation on some points of protection built in to their inverters.

Copy of Residual Current Device Protection Manufacturer's Declaration  https://kb.givenergy.cloud/article.php?id=63 this built in device is a RCM not RCD so where protection is required in island mode by the use of an RCD this would have to comply with 531.3.4.1 or 531.3.4.2. The declaration does recommend a type A RCD where one is required.

During my research I have also found one technical report which has helped me gain a better understanding of inverter operation so thought I would share, unfortunately it is a USA based paper but has still added to my knowledge.

https://www.nrel.gov/docs/fy10osti/46698.pdf

Extract from paper - Both inverters test results suggest that inverters designed to meet IEEE 1547 and UL 1741 produce fault currents anywhere between 2 to 5 times the rated current for 1 to 4.25ms

I shall wait to hear back from the manufacturer but my initial understanding is short circuit protection built in to the inverter operates much quicker than a fuse or circuit breaker, when operating in island mode no other short circuit protection will be required where the suggested minimum cable size of 2.5mm is used for connection to the EPS output on a 3.6kW rated inverter.

Agreed regards BS 7671 and fault protection, you are limited to a either Reg 419.2 for ADS (comprehensive manufacturer information), and where an RCD is either selected, or otherwise required for additional protection, it should be selected from the lists of standards in Reg Group 531.4.

Mspark said:

Extract from paper - Both inverters test results suggest that inverters designed to meet IEEE 1547 and UL 1741 produce fault currents anywhere between 2 to 5 times the rated current for 1 to 4.25ms

I shall wait to hear back from the manufacturer but my initial understanding is short circuit protection built in to the inverter operates much quicker than a fuse or circuit breaker, when operating in island mode no other short circuit protection will be required where the suggested minimum cable size of 2.5mm is used for connection to the EPS output on a 3.6kW rated inverter.

Unlike RCDs, when selecting overcurrent protection, other devices providing a level of protection not less than that given by the list of devices in 533.1.1 are permitted. So, if you could read-across from the inverter product standard (BS or BS EN preferably), or the manufacturer's confirmed data, there would be no problem relying on that for overcurrent protection. Back-up protection could also be provided.