3 phase supply to classroom, unsure to TT?

[mpoy222]

Hi I'm after some advice, ive been asked to connect up a new outbulding at a school, there was a building there before and the 4 core 16mm SWA was already in place. It will be a kind of extra classroom/club room and will have several sockets, lights and a small water heater in the toilet. Heating for the rest of the building isnt decided yet. I plan on fitting a 4 way 3 phase board and running everthing on seperate RCBOs. Probably 2 for lights, 2 for sockets, 1 for water heater and 1 for a fire alarm system. All circuits will obviously be 230v
My question is about the earthing. I thought about exporting the earth (main building TNCS) but im not sure the armour of the cable will be sufficiant. The main cable has also been joined including the armour. There are some main utilitys running under the building so if i TT it i know im going to drive the earth rod in to something! It also means putting a main RCD switch in and this will also mean ill have to try and find a double insulated gland to run the SWA into the board.
Any advice on this would be greatly appreciated.

 

[westward10]

Why do you think the armour is insufficient. Is it required to support main protective bonding.

 

[cliffed]

If it’s for Main Earth & Main Earth bonding I doubt if swa complys

 

[westward10]

It will not be a main earth it will be a cpc.

 

[Wilko]

Hi - with 16mm 4 core swa it's likely the armour will be sufficient for the CPC of the distribution circuit. But if there are any extraneous conductive parts in the 2nd building then it won't be big enough for protective bonding back to the installation MET in TNCS.

 

[Spoon]

Maybe a silly question but why 3 phase?
Would single phase not do for what you are intending to install?

 

[Strima]

Maybe a silly question but why 3 phase?
Would single phase not do for what you are intending to install?

This is a good option depending on the phase balancing of the installation. Then if required you could use the spare conductors for bonding etc.

 

[TJ Anderson]

This is a good option depending on the phase balancing of the installation. Then if required you could use the spare conductors for bonding etc.

I like that. As he only has single phase loads, this could be a great solution if bonding required.

 

[Yozza19]

Tt is gland swa onto paxolin plate then bolt to base of dB and run earth straight from dB usaly 1 rod will get you around 30-40 ohms

 

[buzzlightyear]

the only option for the op is run a 5 core 16mm SWA cable out to the outer building .

 

[telectrix]

as previous. wtf do you want 3 phase over there ?

 

[westward10]

Tt is gland swa onto paxolin plate then bolt to base of dB and run earth straight from dB usaly 1 rod will get you around 30-40 ohms

You then have two different earthing systems in close proximity. The gland or end of The cable should be placed within an all insulated enclosure.

 

[westward10]

This is a good option depending on the phase balancing of the installation. Then if required you could use the spare conductors for bonding etc.

Assuming the spare conductors are the correct csa.

 

[davesparks]

the only option for the op is run a 5 core 16mm SWA cable out to the outer building .

How will that help? The chances of 16mm being big enough for a main bond on a large installation are pretty slim.

 

[davesparks]

Tt is gland swa onto paxolin plate then bolt to base of dB and run earth straight from dB usaly 1 rod will get you around 30-40 ohms

That creates a dangerous situation because you have metal connected to two different earthing systems exposed inside the DB.

1 rod? How long, what material, what type of soil, whereabouts in the country, will that be ok for the available fault current?

 

[westward10]

We do lots of school work and I have yet to see a separate building with a single phase supply.

 

[LankyWill]

Do the adebiatic equation to see if the armour is sufficient for cpc, i would also consider using single phase only so you have the spare conductors to export the main earth at the main school.

 

[westward10]

You are not exporting a main earth, it is a cpc.

 

[Spoon]

It will be a kind of extra classroom/club room and will have several sockets, lights and a small water heater in the toilet. Heating for the rest of the building isnt decided yet.

We do lots of school work and I have yet to see a separate building with a single phase supply.

But 3 phase for the above seems OTT. What would you do? Put 1 phase for the sockets, 1 phase for the lights and 1 phase for the small water heater?
Or is the OP leaving spare phases for the 'heating, which isn't decided yet'?
I would be interested to know.

 

[westward10]

I tested a small what they call a mobile class week before last, consisted of two class areas, WCs and entrance lobby. Twenty two circuits.