TT system

[sparks1234]

I am quite interested to know how people would go about this installation.

Install a 3-phase supply from a house to the middle of a field and install a distribution board with main switch. approx 150 mtrs
Then from one phase run away to various outlets for caravans the furthest being 300 mtrs away. this cable would be downrated to a lowly 40 amps but would be quite sufficient for the voltage drop, probably a 35mm SWA.
Each of the caravan outlets has its own 30ma RCD and earth stake so will work on its own TT system.
The origin would have a time delay RCD so as not to trip out before any of the outlet RCD's.

My question is this.
What earth connections would you make at the 3-phase board, would you divorce the earth?
Would you divorce the single phase 40 amp outgoing way from the 3-phase but connect it with the 3-phase SWA.
The time delay is to give the SWA some protection.

 

[Deleted member 9648]

I would use the house earth to protect the 3phase SWA and DB and break the earth there to the outgoing circuits...this would mean the time delay RCD would be in the field 3phase DB as a main switch,with a rod at that position serving the outgoing circuits. I would not necessarily provide a rod for each outlet.I would not provide RCD protection at the origin,as if it tripped access for resetting may not be possible if the house was not occupied. As you rightly state each outlet must have it's own 30ma RCD AND overcurrent protection.

EDIT.....are you looking at 708.553.1.14 as the reason to have each outlet on it's own rod?..as I dont interpret it as individual rods but just a rod and TT.

 

[piccollo]

sparks, your main problem [ & as you've already mentioned is volt drop]. If we say a 63amp 3-phase & neutral supply then with a 4core 35mm2 xlpe supply cable your vd will be 10.87 volts at the db. Max allowed 20volts, leaving 9 volts for single phase supplies. You say 300mtrs to the last point & current of say 40amp, to keep within the 9 volts you will need a 70mm2 cable [ volt drop of 8.04 volts]. A suggestion install cable to 1st 2? points & second to last 2? points, say one cable 150mtrs & second 300mtrs. You could run a 16mm2 for 150mtrs [say 20amp v.d. 8.7volts ] & 35mm2 for 300mtrs [ say 20amp vd 8.1 volts]. Regarding earth I would treat DB as TT with 100mA RCD, caravans already have 30mA rcd. Main supply would have 100mA time delay RCD in insulated enclosure feeding 63amp 3 phase & neutral fused switch. Main supply swa cable to distribution board would be eathed at fused switch only, [ insulated box below distribution board to save exporting earth. Distribution board with own earth stake & distribution swa's earthed at the distribution board only. Caravan points have own earth rods so swa cables bonded at insulated boxes at the caravan outlets. Hope this is understandable & of help, regards peter

 

[Deleted member 9648]

Picollo you need to look at section 708 as there are very specific requirements for RCD protection of caravan outlets which your scheme will not comply with.

 

[piccollo]

wirepuller I agree Time delay RCD at origin not neccesary, but has sparks mentioned it, didn't know what supply arrangement was.[ was trying to digest all at the same time] regards piccoll

 

[Deleted member 9648]

It wasnt that I was referring to but that each 16a outlet intended to supply a caravan must have it's own 30ma rcd and overcurrent protection....708.553.1.12 and 13.

 

[piccollo]

wirepuller, trying to think back, BRB in van,slightly different on residential caravans that have a meter, rcd & own earth rod. regards piccollo

 

[sparks1234]

The volt drop is not an issue here, its the earthing arrangement.
I have had different opinions from very good sources, the time delay RCD at the source was from the NIC, access to the house and that sort of issue does not come into the equation, its just the best way of earthing

 

[Deleted member 9648]

Dont see a problem with RCD at origin...as long as access is not an issue.
I would still use the house earthing system up to the 3 phase DB as long as it is a TN and separate beyond that.

 

[sparks1234]

So wirepuller, If you have the RCD at the origin, what would you do with the armour of the outgoing circuit, connect it to the dis board or divorce it and connect it to the incoming which would protect it by the time delay

 

[Deleted member 9648]

Outgoing SWA's would be insulated from the DB earth and connected to a rod at the same location...the origin RCD would provide fault protection....(would still prefer it at the DB end though)...The rod would also serve the caravan outlets,each of which would be provided with overcurrent and 30ma RCD protection...not an RCBO though as the RCD must disconnect both poles.

 

[sparks1234]

Good discussion, thanks chaps

 

[yellowvanman]

You can get 2P RCBOS (Hager) if you want to consider those, makes the DB easier to wire, but a little more expensive to buy the parts.

 

[Engineer54]

Dont see a problem with RCD at origin...as long as access is not an issue.
I would still use the house earthing system up to the 3 phase DB as long as it is a TN and separate beyond that.

No-ones said anything about the supply for the DB being TN, so perhaps the OP can clear that one up?? I would also try to include a CPC core in the SWA, as the armouring in certain conditions will cease to be capable of being a CPC in longer runs of cable... ie, 5 core SWA cable.

As for the individual caravan pitches, a rod needs to be driven at each one. Nothing to stop you connecting all the pitch earth rods in a string, this would in all likelihood also bring the overall Ra down to a respectable level too, where you would not be totally relying on the RCD devices!!

I would also go for DP RCBOs at the DB, for each caravan 16A supply point, depending on how the pitch supplies are being provided for. Some connection units will cover say 2 caravan pitches, located centrally.

 

[Deleted member 9648]

No-ones said anything about the supply for the DB being TN, so perhaps the OP can clear that one up?? I would also try to include a CPC core in the SWA, as the armouring in certain conditions will cease to be capable of being a CPC in longer runs of cable... ie, 5 core SWA cable.

As for the individual caravan pitches, a rod needs to be driven at each one. Nothing to stop you connecting all the pitch earth rods in a string, this would in all likelihood also bring the overall Ra down to a respectable level too, where you would not be totally relying on the RCD devices!!

I would also go for DP RCBOs at the DB, for each caravan 16A supply point, depending on how the pitch supplies are being provided for. Some connection units will cover say 2 caravan pitches, located centrally.

This is not correct....reg 708.553.1.14 only states that the CPC of each socket outlet shall be connected to an earth electrode complying with the requirements for a TT...not that each socket shall be connected to a separate earth electrode....this technically only applies where the supply is TNCS as well,a TNS supply can be used for the entire install without the need to separate and TT anything,although IMO it would be preferable to TT beyond the DB regardless of the source earthing system.

The usual provision is power boxes incorporating the fault protection devices at the caravan pitches, each serving no more than 4 sockets/pitches. 4 separate rods at one location would not be practical or necessary....there would of course be advantages to providing one rod at each power box location rather than relying solely on a single rod at the DB end....and yes DP RCBO's would be fine but not single pole.