P.V survey

sambotc · Jan 9, 2012

Had to survey a job today for the first time, normally I install but times are hard so i'm on the ground!

Its a 4kw system, fitted onto a corrugated steel outbuilding roof approx 40 metres from the main house. SWA cable to be trenched in by customer, TNCS earthing from an over head supply.

My intention was to take a sub main off tails, into a 60A switched fuse, then 3 core 16mm SWA out to the outbuilding for minimal voltage drop and use 1 core to export TNCS, bonding the array and steel roof. Sheath to be earthed back at house only.

Is this acceptable given that the DNO ask for max 3 metre on tails?

Also, the pitch of the roof is very shallow approx 15 degrees, and I informed the customer that for optimal performance the array might have to be jacked up at the back to raise it up to more like 25. He didn't sound overly happy as it would cost more and was trying to tell me he is happy to put them on as is.

Where do I stand with the reg's on pitch, at the end of the day it's his investment at stake?

Being my first survey, i'm a little bit sceptical I have made the right decisions so suggestions welcome.

Cheers, Sam

JD6400 · Jan 9, 2012

You would run into further hasle from planning if you were to try and jack it up at the back as you would almost definitely be going out of the ( I think it is ) 200mm allowance .
15 degrees is not as bad as it sounds and some makes /models work better than others at it , although do be careful on the warranties as know some will void it under 10 degrees .
There is a bit of a argument that thin film is best placed for this type of application , but i prefer a good quality ploy personally .

yellowvanman · Jan 9, 2012

sambotc said:
My intention was to take a sub main off tails, into a 60A switched fuse, then 3 core 16mm SWA out to the outbuilding for minimal voltage drop and use 1 core to export TNCS, bonding the array and steel roof. Sheath to be earthed back at house only.

Is this acceptable given that the DNO ask for max 3 metre on tails?

As you've put the supply through a 60A switched fuse then no problem with the DNO as long as the switched fuse is within 3m of the head. Although you shouldn't need a 60A fuse more like 20A.

Why export TNCS earth, TT it at the outbuilding and bond your steel and array to your rod? You're no supposd to bond the array to TNCS anyway and only bond the steel roof if its extraneous. By TTing it you only need a 2 core SWA, should be a little cheaper!

sambotc · Jan 9, 2012

Hi yellowvanman,

The reason for a 60a was so that if the customer wanted to add any circuits to the new C/U then there was scope for future work. The circuit will be covered by a 16a MCB on the sub main board so protected adequately.
Also the customer is supplying the SWA and digging the trench so I thought it would probably be easier than trying to get a low resistance with rods! as the panels are directly in contact with the roof and the roof is supported by steel pillars, I thought the whole lot would be in the equipotential zone so would require bonding?

Haven't come across having to bond any type of array before so this is all a bit of a minefield in the reg books to me, could you explain why you shouldn't bond back to tncs in this scenario?

Cheers for the help

Sam

yellowvanman · Jan 9, 2012

There is nothing in the regs to say you can't export the TNCS, but I avoid it if I can most of the time.

There is certainly something in the DTI guide that says the array should not be bonded to a PME earth, in which case you've got to put a rod in anyway, so why not bond everything to the new rod. Putting in a rod is easy, unless you've got poor earthing through the soil etc., in which case you may have to put longer rods in or >1. Just by using the building steelwork maybe enough for your earth without having to put rods in!

I know your proposing to use 16mm SWA and of course one the cores will be big enough for extending the equipotential zone, but if you use 2 core that could save a bit of money as well.

sambotc · Jan 10, 2012

I thought you shouldn't take PME outside of the equipotential zone (hence not bonding array in DTI) but since the array is part of structure which falls inside the equipotential zone, then there is nothing you can do.

Otherwise you could argue that an incoming service pipe, water for example, shouldn't be bonded if PME where an outside tap is fitted due to same issue, or even if it leaves the property in copper as you can not guarantee that the pipe would not be accesible at some point elsewhere, or am I barking up the wrong tree?

I see where you are coming from with the TT it should make saving on materials, but i've never got involved in earth spiking so maybe i'm shying away from it for that reason. Also I've no idea what the ground is like or how long it may take to install a rod and get a good Ze and whether or not the labour cost would outweigh the saving on materials exporting.

Would welcome any comments

yellowvanman · Jan 10, 2012

There are occassions where the PME is exported and you've highlighted some examples. I just try and reduce the risks where possible. If the neutral is lost on a PME then it floats up (with the earth), inside an equipotential zone thats not a problem, but if your remote building hasn't got a concrete floor then whilst the neutral/earth rises in potential you touch the steelwork and your in contact with true earth then ouch!

As mentioned previously you could use the building steelwork itself. I was working on a job today and got a 7ohms earth loop impedance reading by just bonding the the frame of the building.

I know if you haven't done an earth rod before it seems like a bigger job than it is.

Generally poor earth conditions are usually assocaited with dry soil conditions, so sandy or chalky soil can cause a problem. Other problems are caused by underlying rock, whwre you can't drive a rod in anyway!

I'm not saying your ideas are wrong or even dangerous in the end we all have our own preferences.

sambotc · Jan 10, 2012

No, thanks for the input, certainly gives me more ideas to think about and may help with situations in the future. If the job goes ahead then I will consider my options when next on site.

Thanks

Grumpy · Jan 11, 2012

My somewhat sketchy understanding is that you should not export PME and in the circumstance you describe you should TT. If you've a week or so to spare do a search of "exporting PME" on the IET forum that'll give you something to read! I'd be interested in your decision. Banging in a rod is quite straightforward and you're unlucky if you don't get a reading under 100 ohms and as long as you're under 200 you'll be OK.

sambotc · Jan 11, 2012

There is another issue I have thought about. Where TT is concerned specifically with P.V and that is where do you stand with RCD protection on switches?

Also this specific install is on an outbuilding roof of a farm, would this fall into a special location therefore requiring RCD protection to switches etc? If so would it have to be 30ma, which may cause an issue with the P.V and trip times etc as talked about on here?

I've not got my big green book handy, so I cannot cross reference to the relevent regs at the mo and I won't try to pretend I know everything in it off the top of my head!

If I remember there was also an existing power circuit in the outbuilding but the SWA feeding it would be undersized for the solar, if I TT'd the new circuit, I could have 2 circuits in the same building with different potentials?