AC Isolators

[paulsamuel1984]

Hi all,

I have a quick query, looking for a bit a assistance.

I am looking assist with the install of a 50kW PV system.

On the AC side we have a 3ph fuseboard with an 80A switch disconnector. This feeds a 35mm cable to the switchroom, located approx 50m away.

The RCD protection and circuit protective device is located at the switchroom. At the switchroom a 80A lockable AC isolator is proposed.

My question is, as well as the AC isolator at the switchroom, do I also require an AC lockable isolator local to the fuseboard? I don't see the advantage in having one at both locations but this has been requested on site.

Any help would be appreciated..

Paul

 

[Worcester]

The DTi Guildelines are quite clear, you will need an isolator need

A manual a.c. switch-disconnector must be provided located in an accessible position within the Customer’s installation as in G83/1 (e.g. adjacent to the consumer unit or main distribution board), which is:
* in accordance with BS 60947-3
* must switch all live and neutral conductors.
* must be securable in the OFF position only. It must be simple to secure using a standard padlock - devices that require a separate removable adaptor or special key to enable them to be secured are not acceptable.
* must clearly show the ON and OFF positions and be labelled as ‘PV system – main a.c. isolator’.

You can interpret that for 3 phase simply enough.

Other isolators are only required

If the a.c. switch-disconnector and the inverter(s) are not in the same room a local isolator should be installed adjacent to the inverter(s). This is to facilitate maintenance of the a.c. cable run and inverter(s)

And they do not have to be lockable

 

[platty1005]

Just on the AC mains side Does the fuseboard feed to the switchroom or does the switchroom feed to the fuseboard?
Where is the inverter(s) located ?
An Isolator local to the Inverter is required to allow the mains cable to be isolated while working on the inverter
An Isolator local to the fuseboard at which the feed in cable from the Inverter is connected is required. To allow the Inverter supply to be isolated from the fuseboard if work on the fuseboard is required
If the inverter and fuseboard where the feed in cable is terminated are in the same room Then I think just the one isolator is required
Remember the AC cable can be made live from 2 sources The mains and the Inverter and hence both ends need isolation

 

[paulsamuel1984]

Thanks for the reply Worcester / Platty,

The inverters are located externally and have local AC isolation. These in turn feed into a new fuseboard in a control room. The fuseboard contains 3No supplies from the inverters, then an 80A switch disconnector is connected to a 35mm cable that runs approx 50m in a duct before entering to the switch room connecting into the existing electrical system via the existing supply isolator.

I have an AC isolator at the switchroom, just before where I connect into the existing electrical system.

Basically, I have an AC isolator local to the inverter, on at the point of connection that can isolate the complete PV system, but I have not got one located adjacent to the the new fuse board. The question is, do I also require an AC isolator at the new fuseboard.



Thanks,

Paul

 

[Worcester]

Presumably the 80A dcisonnector switches all live and neutral conductors?

is this your configuration =>

Inverter(s) => AC Isolator => Dedicated Distibution board with MCB's (Fuseboard) => 80A Disconnector => Dedicated 35mm2 Cable => RCD => Lockable AC Isolator Adjacent to => Main Distribution Board / Incoming DNO Supply

So long as the Disconnector is capable of switching off under load, and it switches all live and neutral conductors then it can be considered an isolator and doesn't NEED to be lockable. (Though it may be a good idea in case people by practice use that as the primary disconnector)

Which rules apply that dictate the requirement of an RCD? - is the 35mm2 cable SWA?

 

[paulsamuel1984]

Thanks Worcester,

The switch disconnector is within the fuseboard, i.e not a separate rotary isolator unit.

I have the rotary isolator (separate unit) adjacent to the main DB, not adjacent to the new main fuseboard (although as, mentioned the fuseboard has an 80A switch incorporated.

We are proposing SMA Sunny Tripowers, and they require a 100mA RCD per inverter (3No inverters), plus it is on a farm so 300mA RCD is required for other circuits.

Yes, the cable is SWA.

Thanks,

Paul

 

[BruceB]

A lot of people misread the installation manual for the transformerless SMA inverters. Sunny Tripowers do not require a rcd. What the manual means is that if you have an rcd then it must be at least 100mA. (Extract from the Tripower manual below)

"If an external RCD or residual current device is strictly required, you must use a switch that triggers at
a residual current of 100 mA or higher."

In your case it sounds like you need a 300mA rcd because it is agricultural.

Regards
Bruce

 

[paulsamuel1984]

Thanks Bruce,

Point gladly accepted.

I have attached a quick sketch to help explain my query, I am sure the answer is no, I do not need the additional isolator, but some clarification would be appreciated.

Thanks

PS

 

[BruceB]

You already have an isolator in your 'fuse board', so the question is do you need a separate lockable one external to the fuse board? I agree with Worcester's answer - there is no requirement provided you have a lockable one elsewhere that meets the DTI Guide requirement.
Regards
Bruce

 

[paulsamuel1984]

Thanks guys,

My confusion was coming from the G83 requirement that it has to be adjacent to the consumer unit etc. Whilst I have not got separate isolator at the point of connection, I did not have one located within the fuseboard.

Thanks for the information.

PS

 

[Worcester]

@Samuel,
I could have missed it, however I couldn't find any such requirement in the G83 document. There are a couple of EXAMPLE drawings in the DTi Guide, one for Single phase and one for three phase.

Or have I missed it and need to go back and read the G83 document?

 

[platty1005]

Looking at your sketch you have 3 isolators local to the inverters to isolate each inverter from mains feed.
You have an isolator in the switchroom to isolate the switchroom DB from the PV feed so I would think you don't need anything else.
You appear to have satisfied the need for isolation at both ends