additional cu cable size

[Maff]

When installing a 'garage cu' adjacent to the main ccu to accomadate an RCD for connecting a TL inverter, what cable size would you use to link the cu's , less than 400mm run

 

[SolarCity]

It depends. Why not run it off a breaker inside the garage CU and it size it accordingly?

 

[Guest123]

When installing a 'garage cu' adjacent to the main ccu to accomadate an RCD for connecting a TL inverter, what cable size would you use to link the cu's , less than 400mm run

How & where you intend connecting to the existing install will govern what size cable you will need to use i.e what rating is the overcurrent protection at the point you are connecting to....is it the suppliers fuse or an MCB etc??j

 

[Maff]

I was thinking of replacing the Henley block for an isolator, and wondered if I could feed it in there and bypass the CCU, using the same size tails (25mm/100A incomer)

 

[malcolmsanford]

What size KWp is this system. As it is domestic I assume you are staying within the 4kw range of things then 16amp protection is all that is needed.

So is there a spare way on the existing CU, if there is use that as it will be cheaper for you client. Take a supply from the spare way to the new enclosure, protected by a 16amp MCB. Fit a Double Pole RCD, as you said on a previous post it is transformless type Invertor so you need a B type RCD.

If there is not a spare way, why do you want to remove the Henley Block ? Why do you want to feed the enclosure that is going to be maximum 16amp with 25mm tails? Are you planning feeding both the existing CU and the enclosure via this isolator you intend to fit?

 

[SolarCity]

Fit a Double Pole RCD, as you said on a previous post it is transformless type Invertor so you need a B type RCD.

Not necessarily. Most TL inverters that I have used don't require B type RCDs.

 

[malcolmsanford]

Then they must have another form of separation other than a transformer.

 

[Maff]

it a 2.6KWp system. We are doing a ccu change, so was going to put in an isolator switch, its an SMA inverter which according to its specs says a type A RCD, from reading the DTI guide I got the impression that a, you have to oversize the cabling, b) you must keep the ac supply seperate from any other circuits

 

[malcolmsanford]

it a 2.6KWp system. We are doing a ccu change, so was going to put in an isolator switch, its an SMA inverter which according to its specs says a type A RCD, from reading the DTI guide I got the impression that a, you have to oversize the cabling, b) you must keep the ac supply seperate from any other circuits

So you now going to change the CU completely, and still fit the garage type CU?. So again are you going to fit the one isolator for both CU and garage type CU ? or are you going to fit Isolator then a set of Henley's and feed the 2 points from there.

Again yes agree if you wished to fit 25mm tails to the main CU, but you may not need 25mm tails to the Garage CU have a look at Reg 434.2.1

point 1 then if the manufacturer instructions say A type is fine then good. Point 2 ...............hmmmmmm had this before.

 

[Maff]

the design is thus- From the meter 25mm tails to iso switch then 25mm to henly, 25mm from henly to CCU, and 25mm from henly to RCD in seperate enclosure+ lockable ac isolater etc,
the proposed 25mm cable in total will not be greater than 2 metres

 

[malcolmsanford]

That is a good design and how I would do it, though you will only really need 4-6mm cable for the RCD enclosure, makes life much easier than terminating 2 x 25mm plus your CPC into it.

Also you need overload protection remember, not just RCD protection.

 

[BruceB]

You can probably miss out the henley; just take both cables from the output of the isolator. The second cable only needs to be 4mm/6mm which is likely to fit in the terminals with a 25mm. It is protected from fault by the DNO fuse (unless the DNO has specified a minimum cross sect area).

The SMA installation instructions do NOT require a rcd. They say if an rcd is required then it must be a minimum of 100mA. And type a rcd is fine for SMA TLs.

Also, as there is only the one device on the circuit, the inverter, then there is no need to protect against overload from the mains side of it, only fault.

Regards
Bruce

 

[malcolmsanford]

You can probably miss out the henley; just take both cables from the output of the isolator. The second cable only needs to be 4mm/6mm which is likely to fit in the terminals with a 25mm. It is protected from fault by the DNO fuse (unless the DNO has specified a minimum cross sect area).

The SMA installation instructions do NOT require a rcd. They say if an rcd is required then it must be a minimum of 100mA. And type a rcd is fine for SMA TLs.

Also, as there is only the one device on the circuit, the inverter, then there is no need to protect against overload from the mains side of it, only fault.

Regards
Bruce[/QUOTE

 

[SolarCity]

Then they must have another form of separation other than a transformer.

I'm sorry, but I don't think that is correct. I'm ready to stand corrected however. Why is another form of seperation required?

 

[malcolmsanford]

If you don't fit an RCD B type as fault protection then you have to install an Inverter that will not allow DC fault currents back into the AC side.

So the use of simple separation by a transformer was the way round that.

If the SMA Inverters are transformerless, and do not need a Type B RCD protection, then they must have either another form of separation, or the capability of not to allowing those DC fault currents to pass through.

 

[BruceB]

If you don't fit an RCD B type as fault protection then you have to install an Inverter that will not allow DC fault currents back into the AC side.
If the SMA Inverters are transformerless, and do not need a Type B RCD protection, then they must have either another form of separation, or the capability of not to allowing those DC fault currents to pass through.

It's the latter with SMA inverters. The document the states it has been cited here a few times.
http://files.sma.de/dl/7418/RCD-UEN103120.pdf page 8

"All SMA inverters with transformer and all transformerless SMA inverters mentioned in the following are not
capable of feeding DC residual currents to the grid due to the circuit design. They fulfill this requirement in
accordance with DIN VDE 0100-712 (IEC60364-7-712:2002)."

Regards
Bruce

 

[malcolmsanford]

Well as said obviously the TL inverter must accomplish sepration by another means as I posted.

What worries me about this is that SMA are advising on German installation procedures in a UK environment. Though we have some harmonisation additional protection in the UK for instance cable that is buried and not to Reg 522.6.6 needs 30mA protection and if your installing as to reg 551.7.2 on the load side then again you need 30mA protection.

So though I won't go as far as to say that the SMA TL Inverter should still be protected by a B type RCD, I would rather be reading a manufacturers instruction that is based on UK installation methods and regulations.

Seems there are a lot of practices within the PV industry that is taken directly from Germany, another is the rod procedure for earthing an array frame if the earthing system is TNC-S. Here in the UK there is no problem with our stable PME system and I personally can not see an advantage of fitting a rod when you have a much more stable earthing system available to you.

 

[BruceB]

Well as I have stated in another thread I do not believe you should be connecting PV on the load side.

I am not sure you should blame the Germans - it was our dear DTI that published their guide. Personally I agree with you and think connecting TL framework to TNCS MET is arguably less risky than putting an additional rod in. But I would not do it that way whilst I have MCS assessors looking at my work.

Regards
Bruce

 

[malcolmsanford]

The DTI seems to have literally copied the German practices and fair do's if that is the way they want to go and you need to dance to that tune you have no option.

I'm not blaming the Germans they are doing the installs to their standards, I expect if the German Department of Trade and Industry were forcing UK standards on German electricians there would be a couple of Germans berating this as we are, an such a forum.

It still seems to me that it is the wrong thing to do, unless of course we intend to adopt more European practices, or because the industry is still in it's infancy, they may review installation practices at a later date, and for now adopted their methods.

What is worrying me is that it is putting this magical 100mA RCD in installers minds as an accepted norm, where here in the UK additional protection is by a 30mA RCD. I'm just surmising that perhaps not so savvy installers will not realise that they are installing systems that require additional protection by 30mA devices and fitting just 100mA as the SMA catalogue advises.

 

[SolarCity]

If the SMA Inverters are transformerless, and do not need a Type B RCD protection, then they must have either another form of separation, or the capability of not to allowing those DC fault currents to pass through.

I think that's correct. Seperation isn't needed by some kind of mechanism to prevent DC fault currents certainly is.