Some basic questions

[the pict]

I want to make it clear I have no involement with installing PV systems nor have any plans to do so, however it is becoming more and more obvious that as an electrical contractor I will have to deal with installations that have PV installations,So I have to understand a bit more, So from the consumer unit a spare way is used or a MCB,RCD enclosure tailed off, this I take is the supply side to the converter ?, then, when the PV is generating does this then become the supply side ? then via the generation meter to the cut out, This may all sound pretty basic but I have not really given this much thought untill now, it a bit like when I thought computers were a fad and wouldn't last
Pict

 

[BruceB]

As background reading you could try:
- 7671 section 712
- the "New PV Guide" on this page Microgeneration Certification Scheme - Installer Standards

That latter might be more than you need but it has all you will ever want.

 

[Gavin A]

I want to make it clear I have no involement with installing PV systems nor have any plans to do so, however it is becoming more and more obvious that as an electrical contractor I will have to deal with installations that have PV installations,So I have to understand a bit more, So from the consumer unit a spare way is used or a MCB,RCD enclosure tailed off, this I take is the supply side to the converter ?, then, when the PV is generating does this then become the supply side ? then via the generation meter to the cut out, This may all sound pretty basic but I have not really given this much thought untill now, it a bit like when I thought computers were a fad and wouldn't last
Pict

key thing is that the cable from the inverter should run to the supply side of the protective devices for other final circuits, and there should be an isolator near the CU, as well as at the inverter location, although in some situations the MCB forms the isolator for the circuit at the CU. Just knock this off and work on the other circuits as usual.

Also remember the solar cable will need to be significantly higher rated per amp / Volt drop than standard, as the voltage drop down the cable should be limited to 1% rather than the usual 3% or so, plus they're liable to get hotter as they run at full wack for hours on end. So standard for a 4kWp / 16 amp inverter is 6mm2 cable, though 4mm2 can be ok for short runs - some companies have been using 2.5mm2 which is likely to get very warm, and probably result in inverters cutting in and out due to overvoltage problems caused by the higher resistance on the cable.

that's my starting points for you.

 

[the pict]

So with the AC Main switch off, the inverter cannot produce ?
Pict

 

[Gavin A]

with the ac isolator off any generation wouldn't get to the rest of the circuits, but yes as long as the inverter is functioning correctly, then they're designed to cut out within 5 seconds of a loss of mains power.

 

[shanky887614]

most inverters measure the incoming voltage from cu via mcb/rcbo.

then send the generated lecky through same cable.

if you think about it. some places have 230v at supply others 250v.


if you turn off mcb/rcbo the unverter will shutdown.


if someone is using it to charge batteries though you will need to physically go to the inverter and isolate the ac then dc isolators

(ac off first and on last to reduce arching from the dc side as a lot of inverters are powered from ac side)

 

[Toonlad]

Grid tied,being the operative words in this case. The invertor needs to detect both AC and DC voltages before it locks on. As an example that Gavin gave above.

 

[Worcester]

The problem when working on installations with PV installed is that you have twosupplies of power - Mains and the PV Inverter (there SHOULD be labels stating this, though we've visited about 20 other installers systems over the last 10 weeks, and not seen a single such label....)

So when working on the system for safety you should ensure that the system is isolated from both supplies.

Grid Tied systems are SUPPOSED to shut off when the grid is disconnected. the key word is supposed, so make sure both supplies are isolated and locked off, and then prove the supply.

 

[Earthstore]

Just to add, even if the AC power is off, and the invertor has shut down, the DC side of things can still be live (if it is daylight of course), this can be anywhere up to 1000VDC, not that you would ever need to touch that side of things, but just so you know not to..

I hope this helps.

 

[the pict]

My main corcern is when upgrading the board, with the mains switch off and the fused pulled, and in thoses circumstances when you need to go into the L & N blocks and perhaps even the meter connections, under what circumstances will the DC supply be live, My own opinion is that the PV generation side of things should be supplied into the DNO side of the cut out via whatever metering is required that way its dead when working on the customers installation, instaed of the situation where the supply is tailed into the L&N blocks
Pict

 

[Earthstore]

If the AC power is off then the PV system will not produce any AC power, so therefore the DC power will stop at the inverter but always check for AC power after 5 seconds that the system has shut down, just to be on the safe side.

The DC does not come into the AC side of things, but if you want to be extra safe then there should be a DC isolator, normally by the inverter or in the loft.

To shut down a PV system AC first then DC, and to switch it back on DC first then AC, or better still turn it off at night then there will be no DC.

 

[shanky887614]

My main corcern is when upgrading the board, with the mains switch off and the fused pulled, and in thoses circumstances when you need to go into the L & N blocks and perhaps even the meter connections, under what circumstances will the DC supply be live, My own opinion is that the PV generation side of things should be supplied into the DNO side of the cut out via whatever metering is required that way its dead when working on the customers installation, instaed of the situation where the supply is tailed into the L&N blocks
Pict

there is supposed to be three isolators for solar panels

one local to ccu, then a dc and ac local to inverter

 

[Worcester]

there is supposed to be three isolators for solar panels

one local to ccu, then a dc and ac local to inverter

Just to be an atagonist, and to question presumed requirements:

And where does it actualy say that in the guide?

It may be what we expect to see. If you take the latest edition of the PV Guide, what it shows in a picture is not definitive as it clearly states "They should not be used for a particular installation without taking into account the special circumstances of each individual installation." and the isolator at the CU (even if it is best practice) is not mentioned in the guide and only shown in the examples as "An additional a.c. isolator may be required by the D.N.O. in this position."

The only thing the guide says about AC isolation is that "
The PV system shall be connected to an isolation switch that fulfils the following conditions:
• Isolates line and neutral conductors
• Be securable in the OFF position
• Located in an accessible location

In its simplest form, for a single phase inverter, an unswitched fused connection unit mounted adjacent to the inverter may be used to fulfil this requirement ..."

Oh and where in the guide is the mention of securable in the off position ONLY..

So beware presumptions.

 

[shanky887614]

Just to be an atagonist, and to question presumed requirements:

And where does it actualy say that in the guide?

It may be what we expect to see. If you take the latest edition of the PV Guide, what it shows in a picture is not definitive as it clearly states "They should not be used for a particular installation without taking into account the special circumstances of each individual installation." and the isolator at the CU (even if it is best practice) is not mentioned in the guide and only shown in the examples as "An additional a.c. isolator may be required by the D.N.O. in this position."

The only thing the guide says about AC isolation is that "
The PV system shall be connected to an isolation switch that fulfils the following conditions:
• Isolates line and neutral conductors
• Be securable in the OFF position
• Located in an accessible location

In its simplest form, for a single phase inverter, an unswitched fused connection unit mounted adjacent to the inverter may be used to fulfil this requirement ..."

Oh and where in the guide is the mention of securable in the off position ONLY..

So beware presumptions.

when i say 3 isolators i dont just mean isolators but mcbs etc as well.

i know a couple guys who usually spur off incoming tails to another cu for solar as a lot of cxu have no spare ways.

 

[Worcester]

True, under BS7671, mcb's are considered a method of isolation, under EN GR83/1 they would need to be double pole to meet that requirement.

As you say a lot of people (ourselves included) would usually spur off the incoming tails to a dedicated CU. I was just pointing out that a number of installers may have ommited what a lot of people have assumed is required - an Isolator lockable only in the off position adjacent or near to the CU (dedicated or shared) - This is perpetuatued with a lot of suppliers supplying two rotary isolators in their standard 'kits'.

Getting back to the OP, the most important thing is to ensure that all supplies are properly isolated before working on the installation.