Choose TT or TN-C-S earthing?

[markocosic]

Should I use TT or TN-C-S earthing?

 

 

Rural site with "7 kW" 3-phase supply. 4-core supply (3 phases and neutral) with the neutral notionally at earth potential.

 

Yes that's 7 kW split over three phases and not a typo.

 

 

There's a borehole for water. It is probably the most locally earthed thing on the build.

 

There are 50+ groundscrews into damp sand/clay as the foundations. These are also very well earthed.

 

Consumer unit is all RCBO. (Hager 30 mA Type A)

 

 

The transformer is small and/or the incoming cable is a long piece of wet string.

 

Flip a kettle on and you'll drop from 233 VAC  to 229 VAC from phase>neutral. You notice the lights dim slightly.

 

If the kettle is on a different phase you'll notice the lights brighten up slightly.

 

The neutral is moving.

 

 

Should I use the imported TN-C-S earth or TT this?

 

 

If TN-C-S

 

Borehole pump should be earthed. For safety (although the supply pipe is plastic), for EMC (inverter driven), and for lightning (try divert nasties to earth before they hit the motor windings)

 

If I connect the incoming neutral to the earth used by the borehole pump this is going to result in currents flowing to earth via the pump as the neutral moves. Motor probably won't like that.

 

If I were also to tie it to the groundscrews to reduce currents in the borehole pump/provide some redundancy against cable damage it will result in currents flowing to earth via the screws. The galv probably won't like that either.

 

 

Am I being daft in thinking that a TT earthing arrangement might be more appropriate here?

 

 

Use the borehole pump AND the groundscrews as the earth reference for the house. Allow the incoming neutral to float about as it pleases whilst supplying the loads.

 

Pop some SPDs between incoming phases and neutral. This will be downstream of the main incoming MCB (they fit a 13A Type C after the meter, for power limiting purposes, that's not ours to touch) and hopefully sends anything nasty on the incoming line back to the transformer whilst also dropping the connection to the house; which shouldn't be an attractive earth because that neutral line isn't connected to the borehole pump casing etc as it would be if the installation were made TN-C-S.

[ProDave]

I wouldn't like to give an accurate answer as I don't know your local regs, but if the supplier provides a TNC-S earth I would use that and perhaps add your own earth rod as well, what we know here and PME Protective Multiple Earthing.

 

And we complain at low power supplies, you have available just 10A per phase.  Is that really regulated by 10A fuses?  What a lousy offering (sorry)

[markocosic]

It's currently setup TN-C-S with bonding to the borehole pump and one of the legs of the house. I'm wondering if neutral currents flowing through the borehole pump and the founds are a good idea though.

 

 

In answer to capacity yes.

 

 

I asked for:

 

22 kW (32A) 3-phase to the boundary, 32A 3-phase cable laid across the plot to where the access road / parking spot for an EV would be; then 32A 3-phase cable laid to the house location from there; but only a single phase 32A 7kW supply to be made for now because you pay for connected capacity and as yet we no EV and I haven't won the argument with SWMBO about where to put 22 kW of PV...

 

 

Instead I got:

 

"7 kW" 3-phase cable laid only to the house using a long piece of aluminium string that's not far off the price of twin and earth in copper.

 

https://online.depo-diy.lt/product/34510

 

It's actually 10 kW given that it's a C13 breaker that trips at 1.13x13A protecting it but that's only if you can load the phases equally.

 

 

Can't complain too much. What the wife lost in translation between myself and the contractor working for the electricity board she won by allowing the contractor to be sexist pigs expecting any old excuse for being late to stick. 50-50 split between electricity board and ourselves for connection cost and €X per day late she notes. Duly sets diary date for the point late fees would exceed contract value. Makes an offer to agree that the connection works were complete on the date that late fees would mean the contract was still worth 100% of the original value conditional on a goodwill gesture of their eating our 50%.

 

 

Could fix it with a supply upgrade and a cable upgrade. (shouldn't melt at 32A / 22 kW but I think voltage drop is a touch much over the length involved) Too mean to pay the €3 per kW per month capacity fee for 22 kW.

 

 

Heat pump. Induction hob. Oven. Borehole pump. Quooker. White goods. It'll be a test of circuit assignment and diversity in practice!

[ProDave]

Yup I will stop complaining.

 

I was offered a 12KVA single phase supply, expecting a 40A fuse, but I got the same cable size as any other supply and a 100A fuse, so the 12KVA means nothing.

 

If you are worried about earth currents, put a clamp meter on the earth to see if anything is actually flowing that way.

 

At least you have got a circuit breaker, I hear stories from places like Spain where it is a small fuse and you get your diversity wrong and you are off until the supplier comes and replaces it.

[markocosic]

Ouch.

 

"3 kW" with a C16 breaker in most apartments. Kettle and oven ok. Kettle and washing machine or dishwasher ok. You get used to using one white good at a time though; and "auto resumes after power failure" is a feature that the shops highlight on white goods!

 

7 kW to allow induction hobs is spendy to retrofit unless the entire staircase does it at the same time.