PV systems

[ericmark]

You have raised so interesting points. MCB may not mind which way around but RCD does. And any wire between inverter and mains supply would need protecting at both ends which would seem near impossible as with inverter in the loft you would need access into loft to reset. So I would assume cable is of a type not needing protection?

The grid tie inverter is clearly a special beast it has an auto shut down with over or under voltage and a minimum time before reconnect which in theroy should mean if the supply cable is damaged then even with a street full of inverters the voltage will either rise or fall causing the inverters to all disconnect.

But this safety system can also be triggered by high or low load and where firms fit the units on a hired roof space system they are likely to set their units just a little high so neighbours units will drop out first.

There seems to be some debate on maximum size of panels. Some say maximum output as fitted to roof others maximum output in ideal conditions? With a East / West facing roof with two sets of panels it will cut in sooner and cut out latter than on a south facing roof but you would need more panels to reach max output. There are special inverters made for the East / West roof as the voltage of both panels will not be the same.

It would seem you can get generators which connect to the mains in the same way. In the main sterling engines built into the central heating boiler which convert what heat is left into power so get the feed in tariff.

I have also seen some units sold for boats and caravans which supplement the supply so when drawing 2 amp the 4 amp left with 6 amp supply charges battery but when drawing 16 amp the 10 amp is supplemented from the battery. What was unclear was the safety aspect does the supply auto fail when unplugged?

 

[JohnD]

You have raised so interesting points. MCB may not mind which way around but RCD does.

if an RCD is comparing current flowing in two wires, how does it know which way the electrons are travelling? Especially when they reverse direction 50 times a second?

 

[JohnW2]

You have raised so interesting points. MCB may not mind which way around but RCD does.

if an RCD is comparing current flowing in two wires, how does it know which way the electrons are travelling? Especially when they reverse direction 50 times a second?

I wonder if eric was picking up on a point I made but expressing it wrongly?

As you say, it does not matter which way around an RCD is wired (i,e, which side is 'supply' and which is 'load'. However, the point I made was that an RCD cannot possibly function usefully unless the neutral on the 'supply' side of the RCD (inverter side, when inverter is supplying power) is connected to earth. If there is no such earth connection then an L-E fault (whether through a person or otherwise) or an N-E fault does not have any alternative path back to the supply neutral, so the L and N currents through RCD cannot possibly become imbalanced (different), so the RCD can never operate.

Kind Regards, John

 

[DetlefSchmitz]

These RCD considerations go beyond the usual ones in a domestic installation. So it is odd that the MCS, who are tasked with controlling such installations, should skate over the subject so easily.

http://www.microgen-database.org.uk..._Guide_to_installlation_of_PV_systems-MCS.pdf

As far as I can see the guidance is to decide whether BS7671 requires the use of an RCD, and to fit one accordingly. I may be guilty of skimming.

As we seem to have concluded, the question at least seems to need more thought than I suspect most installers give it.

 

[JohnW2]

As far as I can see the guidance is to decide whether BS7671 requires the use of an RCD, and to fit one accordingly. I may be guilty of skimming. ... As we seem to have concluded, the question at least seems to need more thought than I suspect most installers give it.

Quite. As I see it, whether BS7671 'requires' it or not, fitting an RCD will achieve absolutely nothing (when the PV is supplying power) unless the PV neutral is earthed on the PV side of the RCD. If the PV neutral is earthed, then the RCD should function 'as expected' (again, when the PV is supplying power).

If/when the PV system is 'receiving' power (the grid being the supply), the RCD will again function 'as expected', since the (grid) supply neutral (which then is on the 'supply' side of the RCD) will be connected to earth somewhere, whether at the cutout or the substation.

Kind Regards, John

 

[westie101]

Though by default does not existing RCD protection still provide protection.

Consider the case where the wiring is protected by an RCD which covers the wiring to the PV system.
A fault occurs to trip the RCD so that has ooperated correctly. But the PV system is designed to shut down on loss of volts, which it will see and promptly shut down, thuis removing all sources of supply.

So in effect the single RCD on the mains side is controlling the PV

 

[JohnD]

It would certainly be better, though, if it went via a small CU into a Henley, upstream of the house CU.

 

[JohnW2]

Though by default does not existing RCD protection still provide protection. ... Consider the case where the wiring is protected by an RCD which covers the wiring to the PV system. ... A fault occurs to trip the RCD so that has ooperated correctly. But the PV system is designed to shut down on loss of volts, which it will see and promptly shut down, thuis removing all sources of supply. So in effect the single RCD on the mains side is controlling the PV

Would that not depend upon 'where the power was coming from'? I'm really not at all clear as to how these systems work, but if the load(s) on the circuit which developed a fault were being 'totally supplied' by the PV if that is possible), then there is no obvious reason (well, not one obvious to me!) why the installation's existing RCD should operate. In that scenario, only a separate RCD in the feed from the PV could react to the fault - and, as I've been saying, only if the PV's neutral was connected to earth on the PV side of that RCD. ... or am I misunderstanding?

Kind Regards, John

 

[JohnW2]

It would certainly be better, though, if it went via a small CU into a Henley, upstream of the house CU.

As I've been saying, if that CU contained an RCD, I don't think it would serve any useful purpose unless the PV's neutral was connected to earth on the PV side of it.

Kind Regards, John

 

[JohnD]

as it would be if you had a PME installation.

But if the inverter is running, you must be connected to the public supply, so your fault-to-earth will affect the current detected by the RCD

 

[JohnW2]

as it would be if you had a PME installation.

No it wouldn't, unless one actually connected the neutral output of the PV inverter to the PME earth on the inverter side of the RCD in the feed from/to it.

But if the inverter is running, you must be connected to the public supply, so your fault-to-earth will affect the current detected by the RCD

Again, no - as I keep saying, the N-E connection of the main installation would be 'on the wrong side' of an RCD in the feed from/to the PV inverter for that RCD ever to be able to operate.

Kind Regards, John

 

[westie101]

Again, no - as I keep saying, the N-E connection of the main installation would be 'on the wrong side' of an RCD in the feed from/to the PV inverter for that RCD ever to be able to operate.

See my post above, The PV output cannot exist on it's own so the mains side RCD will still disconnect all supplies

 

[JohnW2]

Again, no - as I keep saying, the N-E connection of the main installation would be 'on the wrong side' of an RCD in the feed from/to the PV inverter for that RCD ever to be able to operate.

See my post above, The PV output cannot exist on it's own so the mains side RCD will still disconnect all supplies

But what about what I said/asked in response to your previous post? If all of current 'supplying' the fault (which, don't forget, only needs to be a '30mA fault' to operate an RCD) was coming from the PV, then there would not be any reason for the 'mains side RCD' to operate, would there?
(in any event, my comments were in relation to a postulated separate RCD in the PV connection, not the main installation's RCD).

Kind Regards, John

 

[ekmdgrf]

You have raised so interesting points. MCB may not mind which way around but RCD does.

if an RCD is comparing current flowing in two wires, how does it know which way the electrons are travelling? Especially when they reverse direction 50 times a second?

I got 99 changes a second.

 

[westie101]

If all of current 'supplying' the fault (which, don't forget, only needs to be a '30mA fault' to operate an RCD) was coming from the PV,

Er could you explain the circumstances when that could happen when the PV output and the mains are in parallel?