Earthing for bathroom water heater, how to ?

[Sean87s]

The electrician gave the on demand water heater in our bathrooms its own earthing rod instead of hooking it up to the main earth in the CU box. Is this done correctly?

Also , i never saw how long the main earthing rod actually was before it went underground but the cable itself is 4mm. Is this sufficent ? If not i still have time to change it before the floor tiles goes on. Been reading on this boeard that it should be atleast 10mm.

[lostinisaan]

I made the mistake and let an electrician connect the 2 mm wires to a rod outside the house that's only about 20 cm in the earth.

Please Google it there's a lot of good stuff to watch on youtube.

P.S. If there's a proper ground connection in the CU box, why not? But i really doubt that.

[Sean87s]

Im digging it out first thing tomorrow and replacing the wire and the rod. Its placed directly under where the CU box is underneath the floor so ill have to cut a piece of the wall (chap) and hammer up the concrete.

[Crossy]

Individual rod for the water heater, if it's a retrofit then OK, not ideal but OK. Cable to that rod, same size as the supply cable but no smaller than 4mm² purely to limit potential damage to it. Rod at least 1.5m long.

Main earth, >2m rod, 10mm² or bigger to the rod from the earth terminal.

Do you have an RCD / RCBO (Safe-T-Cut)?

Unless the cable is thermally welded to the rod you should leave the connection accessible for inspection (use one of the PVC/brass pipe access thingies).

[Sean87s]

Alright thanks. Gonna get a new rod and pull a new 10mm cable to outside the home before they put plaster on the front wall.

Ill make a small service hole with a 5" pvc pipe where ill shove the rod down and propably put a stone tile on it as lid.

And yes ive got a RCBO installed that trips at 30ma leakage. I asked the builder and he said the rod he used was barely 1m...

[Sean87s]

Can i use 10mm aluminium cable as the grounding cable ? Couldnt find any 10mm copper wire at the local store.

[Crossy]

No. You won't be able to get a reliable connection to the copper rod. Al cable can be a nightmare due to it oxidising readily and having a tendency to work harden.

10mm copper should be readily available almost anywhere.

[Sean87s]

Drove 60km fort and back to get the 10mm copper wire. Also got a 2m copper rod with 2 clamps. Im gonna tell him to rewire the ground cable for the water heaters to the main ground rod as well.

Here is how it turned out so far.

[Crossy]

If you are on a construction supply (8 Baht a unit) you will need a PEA inspection to get a normal meter.

You won't pass as you are not wired MEN. Get out the instructions for the consumer unit and you'll see what I mean. The inspector will expect the incoming neutral to go via the ground bar (yes it sounds counter-intuitive).

They will not accept TT installations (which you have now)

Have a look at PDF page 10 (pamphlet 12 and 13) of this PEA document http://crossy.co.uk/Handy%20Files/groundwire.pdf

Also, Thai standard wire colours are the same as the US NEC, Black-live, White/Grey-neutral, Green-earth. Since you are just starting I would stick to those colours.

[Sean87s]

It does have a MEN connection ? The green thick wire inside goes from the Neutral bar ( to the left of the RCBO) to the ground bar. Does it make any difference where the N cable goes where first ?

[Crossy]

It does have a MEN connection ? The green thick wire inside goes from the Neutral bar ( to the left of the RCBO) to the ground bar. Does it make any difference where the N cable goes where first ?

Yes, it makes a massive difference.

Your RCBO won't stay engaged like that, worse, if it stays on it will trip randomly dependant upon load.

MEN needs a N-E link on the SUPPLY side of the RCBO. Nothing to stop you moving that green link from the neutral bar to the supply side of the RCBO, electrically the same. BUT your PEA inspector may not see it that way.

What do the instructions that came with the CU say??

[Sean87s]

Instructions say to put the N cable into the top ground bar and then pull another wire to the N socket on the RCBO.

If this is correct, ill tell him to change it tomorrow.

[Crossy]

Yes, that's the Thai way to do MEN and will keep the PEA chap happy.

[Sean87s]

Is there other ways to do MEN ? Trying to check your info site but it wont load.

Also what size should the MEN cable be ? Same as the 10mm copper wire or the 25 mm aluminium wire ?

[Sean87s]

Looking at your diagram, you have also put the N feed directly into the N socket on the RCBO and then another eire into the same socket that goes to the Ground bar.

So the only thing i should move is that green 10mm cable from the N bar to the N socket on the RCBO.

it still feel like its the same setup as i have now just different sockets. *confused*

[Crossy]

That diagram is well out of date, I should fix it I suppose. That's how it's done in Aus, in the UK (where it's called PME) the link is in the service-head where the user can't access it and there's no local rod needed. Like this the N-E link should be at least 10mm², preferably 16mm² (but you'll probably struggle to get the two in the breaker terminals). The drawings in Forkinhades' pinned thread are much better http://www.thaivisa.com/forum/topic/693630-how-to-make-your-thai-electrical-system-safe/

As I noted above, moving the green to the incoming neutral is electrically identical, but your PEA inspector may not agree.

Are you feeding the Al directly into the breaker? Are the breaker terminals certified for Al cable?

Personally I wouldn't have Al cable inside the house, pigtailing Cu to the Al before the wall penetration is the way to go.

Aluminium cable has a nasty habit of work-hardening and cold-flowing leading to loose terminations which can fizz and make nasty smells as well as potentially starting fires.

If you are confused just follow the manufacturer's instructions and you'll be just fine. Draw what you have, then what is in the diagrams and you should immediately see the difference.

[gandalf12]

They should perform an earth resistance test. THat will tell you how deep it needs to go. In general I say at least 1 metre but this can vary depending on the resistivity of the soil

[Crossy]

In an ideal world I agree totally with Gandalf. But even in the UK if a rod is required on a domestic (TT supply) installation, 99.9% would bash in a 2m rod and measure it, if it's not low enough impedance bash in another until it is. Of course I'm not talking about industrial installations with large and properly sized earth mats.

Meanwhile, Thai regulations are prescriptive in the same was as the US NEC regulations, i.e. "do it like this and it's 'to code'" rather than the UK regs which are proscriptive i.e. "this is what you must achieve, within reason how you do it is up to you". that's why there's so much lively discussion on the IET "Wiring and the Regulations" forum.

Thai requirement is a 2.4m rod, no mention of resistances or the like. Have a look at this PEA document http://crossy.co.uk/Handy%20Files/groundwire.pdf (20MB download) it gives a good idea what the Thai regs require for a domestic installation to pass a PEA inspection. It's in Thai of course, but lots of illustrations.

[GaryB7471]

What would be a satifactory ground resistance measurement?? A measurement that would satisfy the master witemen that frequent this forum???

[Crossy]

What would be a satifactory ground resistance measurement?? A measurement that would satisfy the master witemen that frequent this forum???

The lower the better (of course).

If you are TT and have an RCD / RCBO getting the rod below 100 Ohms would be fine, anything higher is unlikely to be stable. IIRC our 2.5m rod is around 20 Ohms but the ground is always damp (we have a river outside the fence).

If you're TNC-S with MEN (or PME) as required for new installations then your rod is in parallel with everyone else's as well as the multiple neutral grounding rods provided by MEA / PEA so, as long as it meets the requirement of being 2.4m you're good to go.

It's also worth checking the ground resistance of your building steel (Google Ufer Ground), ours is rather better than the rod, I've linked the roof steel (convenient) to the MET in our CU.

Do you have an earth resistance tester or earth-loop impedance tester? You won't be able to measure the ground resistance with a regular multimeter.