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Topic Title: Array Switch-Disconnector.
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Created On: 06 December 2017 06:49 PM
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 06 December 2017 06:49 PM
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stuff

Posts: 23
Joined: 13 February 2014

Array Switch-Disconnector.

"BS7671 - 712.537.2.2.5 - A Switch-disconnector shall be provided on the d.c. side of the PV convertor."

"MIS3002 - 4.2.1 - Solar PV Microgeneration systems shall be designed and installed in accordance with the MCS/ECA publication: Guide to the Installation of Photovoltaic Systems."

"Guide to the Installation of Photovoltaic Systems - 2.1.12.4" - allows for a switch-disconnector to be omitted from a PV system, only where micro inverters have been installed.


Regarding the clause and statement below, would an inverter with a circuit board on/off toggle switch be classed as a switch-disconnector? The array would be connected by plugs to the inverter.
The inverter manufacturer is stating that the system is compliant and that there is no need for a d.c. Isolator.

I'm finding this hard to get my head around. My thoughts are no, as the toggle switch doesn't meet the definition of a Switch-Disconnector in BS7671 Part 2, the PV Guide 2.1.12.2 or the IET CoP for PV systems 5.7.2. Therefore a d.c. isolator would be required. Your thoughts are greatly appreciated.


"Guide to the Installation of Photovoltaic Systems - 2.1.12.4 - A switch-disconnector integral to the inverter, if the inverter includes a means of isolation (e.g. plugs) which can only be operated with a tool and is labelled with a readily visible warning sign or text indicating ("Do not disconnect under load").

Manufacturer information "ON/OFF switch: Turning this switch ON starts the operation of the power optimizers, enables power production and allows the inverter to begin exporting power to the utility grid. Turning it OFF reduces the power optimizer voltage to a low safety voltage and inhibits exportation of power. When this switch is OFF, the inverter control circuitry remains powered up."

Edited: 06 December 2017 at 07:10 PM by stuff
 06 December 2017 08:05 PM
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ashrow

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Joined: 14 May 2017

I am jumping the gun here as i am not an expert but stating my opinion
Is the toggle switch a safe way of isolation if it can not be "locked off " ?
 06 December 2017 08:13 PM
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stuff

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Thanks for the reply. There is no requirement for the PV d.c. isolator to be lockable. Best practice though...
 07 December 2017 08:56 AM
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statter

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Joined: 06 February 2013

Hello
I don't think this is compliant. One of the purposes of this isolator is to allow the inverter to be changed safely. If the isolator is part of the inverter then there is no way this can be done dead.
 07 December 2017 09:06 AM
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mapj1

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Except by working in the dark perhaps I'm inclined to agree, a toggle switch in the equipment may be handy for customer use, it is not really a formal means of making the system safe to work. Given that some arrangements generate lethal DC voltages when the sun comes out, it is quite a pertinent point to be sure it is really off when the system has to be maintained, and I'm sure that was the intention of the reg. It maybe the thinking that a 'micro inverter' cannot create a dangerous output. I'd be wary of tha, as some are hundred watt plus.
Only if the micro inverter is an integral part of the panel assembly and it all removes as one assembly, maybe, as then there is no DC bus to isolate.

-------------------------
regards Mike
 07 December 2017 09:28 AM
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ectophile

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My guess would be that the means of isolation would be to set the switch to OFF, to shut down the optimisers. Then pull the plugs out to do the actual isolation.

That's assuming you can get at the switch without the risk of touching any live (AC or DC) parts.

-------------------------
S P Barker BSc PhD IEng MIET
 07 December 2017 09:41 AM
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jammyc

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Regulation 712.537.2.2.5 hasn't really kept up with modern practice or technology.

IET Code of Practice for Grid Connected Solar PV Systems §5.7 discusses switch disconnectors for different sections of the array in different scale installations. For example, 5.7.4 describes how departures from BS7671 can be made for microinverters - yes Mike even if they're separate components.

In short, for small arrays, a switch disconnector built into the inverter is acceptable if it is possible to safely disconnect & isolate with suitable (touch-proof) plugs, such as MC4s. So your means of isolation is actually the plug/socket, but it's off-load only so you require the inverter for switching.

"Turning it OFF reduces the power optimizer voltage to a low safety voltage and inhibits exportation of power"
would suggest it's a DC optimiser system similar to SolarEdge. I would check the details for the specific scheme but for SE the "safety voltage" is 50Vdc and the array should be open circuit so safe to disconnect. Although if memory serves SE do have rotary switches too.

As an aside, I would always recommend taking the system off-load through the UI and then isolating the AC before doing any DC switching or disconnecting, partly due to the extra wear DC switching entails, partly indeed due to the fact that you have extra confidence it is off-load that way, and partly because I have seen enough installations where the fitter didn't know to check isolators were DC rated as well as AC or consider the capacitive load it was meant to switch...

None of which is to say it is compliant with BS7671, but do consider whether the safe function is still there - i.e. it's might just be a departure, but possibly a considered one.

Can we ask which device it is?
 07 December 2017 09:50 AM
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AJJewsbury

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My guess would be that the means of isolation would be to set the switch to OFF, to shut down the optimisers. Then pull the plugs out to do the actual isolation.

That was my thought too - the plugs/sockets do the isolation, the switch just allows you to remove the load first so you don't get any nasty arcing when they're separated. I could believe that such an arrangement would comply with the requirements, although personally I'd be happier with a nice grey/black rotary isolator - just to keep everything obvious.

- Andy.
 07 December 2017 08:13 PM
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stuff

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I'd rather not mention the manufacturer, their technical guidance wasn't very helpful when the question was asked, "yes it's compliant!!", was their only response - no explanation on how compliance was achieved....

Thanks again for the replies. I see it now - the switching and isolation are provided separately.

The problem has been, that although the physical installation may be compliant, there has never been a label fitted to explain the isolation procedure. I'd be surprised if any optimiser system, with separate switching and isolation, has been labelled correctly.

From the IET Code of practice for PV systems: "The switch disconnector MUST have a clear label affixed next to it indicating how array isolation is achieved, the array isolation MUST be labelled 'Do not disconnect under load' (or equivalent)"

So, a label or an isolator. Considering the potential fire risk of a d.c. isolator, would it not be considered best practice to avoid a d.c. isolator wherever possible?


What is the difference between an isolator and a switch-disconnector anyway? No difference according to BS7671 Part 2?? If there is no difference, why use two different terms...?

Edited: 07 December 2017 at 09:57 PM by stuff
 07 December 2017 11:31 PM
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alancapon

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The system including the inverter, the installation and labelling must also comply with G.83, which is the DNO's requirements for parallel operation with the public electricity network.

Regards,

Alan.
 08 December 2017 11:03 AM
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AJJewsbury

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What is the difference between an isolator and a switch-disconnector anyway? No difference according to BS7671 Part 2?? If there is no difference, why use two different terms...?

Er, that's can't be right. An isolator isn't necessarily capable of making/breaking load (and sometimes overload/fault) currents - that's a job of a switch. Isolator = disconnector (no switching function, just isolation); Switch-disconnector = isolating switch (combined switching and isolation).
- Andy.
 08 December 2017 04:12 PM
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stuff

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I'm used to installing appropriately rated d.c. isolators, that operate as both switch and disconnector, but I guess, as you say some isolators may not meet the definition of a switch. It was the switch being electronic, not mechanical, that was throwing me. But this is now clear. Once again thanks for all the helpful replies.
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