Earthing Arrangements ?

[nightsurfer200]

I'm wiring up a wooden cabin and a double decker bus (been donated) for a local school.

I've ran 2 x 10mm swa cables from the local 3 phase consumer unit to the underground trench to the bus and cabin.

The question is would you make the Bus supply TT and the cabin (all wooden) and extension of the PME supply.

The bus is parked on a concrete base and is to used as an classroom extension, so socket and lights with a electrical heater.
The wooden cabin is going to be used as a library, so just a light,heater and a couple of sockets.

They are both about 20mtrs from the school building and 50mtrs (overall) from the consumer unit.

Any help or advice would be appreciated

 

[telectrix]

what is the current earthing arrangement in the school?

 

[nightsurfer200]

TNC-S (3 Phase Supply)

 

[telectrix]

TT the bus, the wood cabin can be on TNC-S or put it on the TT if that's easier,

 

[westward10]

Can't see 10mm complying with vd requirements unless the loads aren't much above 20A.

 

[nightsurfer200]

'vd requirements' - whats vd (Volt drop?)

 

[westward10]

Yes volt drop.

 

[nightsurfer200]

how do you get to 20amp max load ?

 

[westward10]

2% vd for the submain, 50m and a buried triple pole supply.

 

[nightsurfer200]

all that will on the circuits would 1 lighting circuit feeding a led 5 ft fitting, a couple of double sockets feeding a computer,printer and a 3rd circuit feeding a 2kw heater
The bus - a lighting circuit (LED fittings) , a heater and a couple of sockets to feed small science equipment

 

[davesparks]

all that will on the circuits would 1 lighting circuit feeding a led 5 ft fitting, a couple of double sockets feeding a computer,printer and a 3rd circuit feeding a 2kw heater
The bus - a lighting circuit (LED fittings) , a heater and a couple of sockets to feed small science equipment

What size ocpd are you using for the submains? What level of selectivity are you looking to achieve here?

Why do you think one should be on a TT and not the other?
What do you understand as being the reason for creating a seperating earthing system for an outbuilding?

 

[Lucien Nunes]

Before we can agree on the correct earthing configuration, we will have to agree on whether a stationary bus is a building or a vehicle...

 

[nightsurfer200]

I was original thinking that 32amp on the sub main would have been fine for the volt drop calc.

 

[westward10]

Volt drop is calculated from the source to the extremety of any circuit. The submain needs to take into consideration the drop after its point of connection hence 2% is often used as a guide allowing a further 1% for lighting and 3% for power.

 

[davesparks]

I was original thinking that 32amp on the sub main would have been fine for the volt drop calc.

OK, can you elaborate on how you calculated this?
Did you take in to account the existing VD to the board you are using to supply this and the VD of the final circuits connected to it?

 

[PEG]

Before we can agree on the correct earthing configuration, we will have to agree on whether a stationary bus is a building or a vehicle...

I would always consider the earthing of any structure,sat on rubber,as worth considering.
I do know of a pal,who now considers this,every time his feet straddle the gap between his metal step,and his previously unsuitably earthed butty van ?

 

[R-fur]

Electrically the bus is the same as a caravan/motorhome. I would supply a socket pillar adjacent to the bus and supply the bus via a plug and flexible lead.
Apply the regs as per the special locations.