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]

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.