Sub Main MCB supply cable

[Pmarsh]

Hi all,

I have a sub main installation 30 metres from source on a supply that is generated from solar with a TT earthing arrangement.

My question is would it be sufficient to protect the cable to the sub-main with a 40a MCB from the non RCD side of the plastic ( customer doesnt want an upgraded board fitting ) original board at the house. or is it absolutely necessary to have an RCD behind it also.
I've read about 100ma S type time delayed RCDs but would it be sufficient with just an MCB

The CU at the sub main end will be metal, the cable run SWA 10mm underground, an earth rod will also be used for earthing at the sub main end, I was going to earth the armour at the sub main end as it will be insulated at the plastic board at the house. the circuits will be 40A/30maRCD with two finals lighting 6A and radial 16A

Assessor out to see work to sign up, is overload protection on the supply cable acceptable?

thanks in advance

 

[Pmarsh]

I will bond the armour at both ends in both boards

 

[HandySparks]

Is the solar PV the only source of supply or is there a grid supply as well? (I can't be sure from your description.)

If two sources, is the grid supply also TT? If so, what is the Ze measurement?

 

[Pmarsh]

Its just PV and win generation to a bank of batteries completely off the grid so Ze will be low, spoke to someone today on it and they seem to think what i'm proposing is fine, 40A MCB in db, 10mm SWA underground at decent depth to sub main CU earth the armour both ends with 6mm and final ccts will consist of 40A-30ma-RCD 6A-MCB-lighting 16A-MCB-Radial.

Not going to bother with an earth rod at the Sub-main end incase it introduces a potential difference and negatively effects the RCD

10mm earth and armour earthing should be sufficient for the intended run.
What are your thoughts interested to know, 6 years of my career has been in substations therefore the 17th regulations are loosely followed within the network due to the age of the installations.

 

[Simonslimline]

Its just PV and win generation to a bank of batteries completely off the grid so Ze will be low, spoke to someone today on it and they seem to think what i'm proposing is fine, 40A MCB in db, 10mm SWA underground at decent depth to sub main CU earth the armour both ends with 6mm and final ccts will consist of 40A-30ma-RCD 6A-MCB-lighting 16A-MCB-Radial.

Not going to bother with an earth rod at the Sub-main end incase it introduces a potential difference and negatively effects the RCD

10mm earth and armour earthing should be sufficient for the intended run.
What are your thoughts interested to know, 6 years of my career has been in substations therefore the 17th regulations are loosely followed within the network due to the age of the installations.

The submain would be better off being protected by an HRC fuse for discrimination.

 

[Octopus]

Not going to bother with an earth rod at the Sub-main end in case it introduces a potential difference and negatively effects the RCD

You've lost me....

 

[HandySparks]

Its just PV and win generation to a bank of batteries completely off the grid so Ze will be low...

So is this submain supplied at 230V 50Hz via an inverter from the batteries?

Why is this installation TT (instead of TN-S for example)?

 

[davesparks]

Does this installation even have a Ze? With no external supply how can there be an external loop impedance?

The big question in my mind is what fault current is available? It's almost certain that the inverter will not supply enough fault current to operate an ocpd and so other means of earth fault protection will be needed

 

[HandySparks]

The big question in my mind is what fault current is available? It's almost certain that the inverter will not supply enough fault current to operate an ocpd and so other means of earth fault protection will be needed

Indeed. That's exactly where I was intending to lead the OP.

 

[Pmarsh]

http://www.solarcharge.com.au/docs/SB1200SB1700SB2500SB3000.pdf
this is the inverter, which wattage it is I'm unsure as it isn't written on it anywhere.
Theres also a bank of about 10 large batteries of which I'm not sure of the amp/hr without a re-visit which is looking likely.

Murdoch, nuisance tripping of the rcd in sub main board I was referring to with the Earth rod and possible introduction of a potential difference

might it be an idea to add the 40a to the rcd side of the house board then? Or seperate rcbo.

 

[Bobster]

It maybe an idea to hire someone who knows what they are doing to help you with this.

 

[Octopus]

http://www.solarcharge.com.au/docs/SB1200SB1700SB2500SB3000.pdf
this is the inverter, which wattage it is I'm unsure as it isn't written on it anywhere.
Theres also a bank of about 10 large batteries of which I'm not sure of the amp/hr without a re-visit which is looking likely.

Murdoch, nuisance tripping of the rcd in sub main board I was referring to with the Earth rod and possible introduction of a potential difference

might it be an idea to add the 40a to the rcd side of the house board then? Or seperate rcbo.

Hum... best you read up on how RCD's work then before making any changes....

 

[HandySparks]

...so Ze will be low...

Without an actual earth fault loop impedance measurement, I don't think that anyone can say whether the submain requires an RCD for earth fault protection.

 

[Pmarsh]

Without an actual earth fault loop impedance measurement, I don't think that anyone can say whether the submain requires an RCD for earth fault protection.

Going to get a ze reading and take a further look into the supply spec Monday.

 

[Octopus]

Going to get a ze reading and take a further look into the supply spec Monday.

And fingers crossed read up on rcd's.

 

[Pmarsh]

And fingers crossed read up on rcd's.

Yes re-visit my notes like I say 6 years in substations, rcds aren't something I come across often

 

[davesparks]

Going to get a ze reading and take a further look into the supply spec Monday.

How can it have a Ze of it is being fed via an on-site inverter?

 

[Pmarsh]

How can it have a Ze of it is being fed via an on-site inverter?

Surely still an external impedance measured at the board to take into account with what is before it in the installation

 

[davesparks]

Surely still an external impedance measured at the board to take into account with what is before it in the installation

External impedance is external to the installation, the inverter is part of the installation.
And even despite that what loop are you measuring? Is one pole of the inverter referenced to earth?

 

[Pmarsh]

I have found that the inverter is capable of tripping a maximum of 16A B type MCB under overload conditions, anything of a higher rating requires an rcd for fault protection.


worth reading up on the inverter installed if anyone else comes across an installation that is independently off the grid like this thanks to those who gave useful answers.

 

[davesparks]

I have found that the inverter is capable of tripping a maximum of 16A B type MCB under overload conditions, anything of a higher rating requires an rcd for fault protection.

How quickly does it trip it and at what current? Just saying it can trip an mcb under overload could be very different to fault conditions.

An RCD will not work without one pile of the supply being referenced to earth, otherwise you have two floating poles with no earth reference and nothing for the RCD to work from

 

[Pmarsh]

How quickly does it trip it and at what current? Just saying it can trip an mcb under overload could be very different to fault conditions.

An RCD will not work without one pile of the supply being referenced to earth, otherwise you have two floating poles with no earth reference and nothing for the RCD to work from

It trips in the desired times now that it is connected to the rcd side of the installation and the installation is referenced to earth through a rod (TT) the manual for the solar installation requires that it be installed with a TT earthing arrangement.

The equipment is supplied by SMA if you'd like to take look in to it.