Checking Proper Grounding for Small Shop

[EmptyHead]

My wife has just built a shop and the electrician came round today. I asked if he will put an earth rod in the ground and use an RCD mainswitch. He said he could do it, but had a funny expression on his face like he may not be sure. And you know how things are here. They say yes but mean no.

Then my GF says, "I think it doesn't matter..just a small shop, very safe". So it doesn't help that she thinks like this. (She seems to think farang are too cautious)

Basically, i want to know if the guy does the job right, especially with the earth rod.

So what should i be looking for to make sure its done right?

I don't mind if he doesn't use an RCD. I can put one in myself if he doesn't. But the grounding the most important thing.

He will be coming off the overhead line, by the way. The shop is too far away from the house to come off the house consumer unit.

Thanks for any advice!

Oh and one more thing. What size of cable should be coming in? and what size for earthing? It's a tiny shop (probably 2 lights and 2 sockets, lol)

Thanks again!

Edited November 26, 2014 by EmptyHead

[EmptyHead]

Anyway, today we saw that he bought a consumer unit with a built in GFCI and a 1 metre (ish) ground rod, with 2.5mm earth wire.

He says it is safe, but is it really? I mean, the shop is tiny. There are only 2 appliances (a fridge and a freezer), plus a couple of lights.

Any thoughts would be great. I just want it to be safe.

He plans to come off the overhead line with a 2.5mm twin-core (PVC sheathed) cable. This sound ok?

[ChrisB87]

I think CROSSY should be able to answer this for you. He's the local expert

[Crossy]

I wouldn't use smaller than 10mm as the incomer, purely on a mechanical strength basis (this is the smallest that MEA/PEA would use). If mechanical strength isn't an issue (no unsupported overheads) 2.5mm will be more than adequate as far as current rating is concerned.

With 2.5mm the main breaker on the CU should be 20-25A max.

With an RCD (GFCI) front end the 1m rod should be adequate if you're not on sand although PEA would want to see 2,5m.

[EmptyHead]

Thanks Crossy! Much appreciated.

I will look into the mechanical aspect of it. Although, there are poorly supported overheads. He also brought a load of plastic conduit so we wil see if he does anything smart with that when dropping the cable down into the shop.

I believe it may be a 32A mainswitch that cam with the unit. I will see about replacing it with a 20-25A.

Glad to hear the earthing and protection should be adequate. That was my main concern.

Thanks again.

[Crossy]

If you can't replace the breaker for a sensible cost use 6mm cable as the incomer

EDIT You may be able to swap the whole CU for one intended for a 5/15 meter

[EmptyHead]

I saw a 20A mainswitch in ThaiWatsadu for about 400 baht. I think i can probably go with that.

Thanks again

[Anthony5]

I think the size of the main breaker should be in relation to the total of the CB for the individual circuits, unless you don't mind the main breaker to trip frequently.

[ChrisB87]

I have to admit, i'm a little confused about the main switch thing.

I thought 2.5mm cable can handle 32A? In Scotland, usually socket circuits are wired in 2.5mm and protected with 32A circuit breakers. Or is this just ok becuase they are done in ring main, rather than radial? Or is it an ambient temperature thing? (Thailand is hotter, lol)

Sorry, for adding this to the mix, but better to be safe than sorry and downgrade the mainswitch or upgrade the incoming cable.

Edited November 28, 2014 by ChrisB87

[Anthony5]

I have to admit, i'm a little confused about the main switch thing.

I thought 2.5mm cable can handle 32A? In Scotland, usually socket circuits are wired in 2.5mm and protected with 32A circuit breakers. Or is this just ok becuase they are done in ring main, rather than radial? Or is it an ambient temperature thing? (Thailand is hotter, lol)

Sorry, for adding this to the mix, but better to be safe than sorry and downgrade the mainswitch or upgrade the incoming cable.

[ChrisB87]

Ok, I see. So, according to that table, roughly between 15-55A for 2.5mm cable, depending on whether it's chassis wiring or power transmission. Interesting data, I hadn't considered it would be that much.

[Crossy]

Ok, I see. So, according to that table, roughly between 15-55A for 2.5mm cable, depending on whether it's chassis wiring or power transmission. Interesting data, I hadn't considered it would be that much.

NO!

The 2.5mm cable is actually a cross-sectional area of 2.5mm² (about 13AWG) so 7-35A depending upon use, it's getting pretty warm at 35A mind.

Normal installation methods rate 2.5 at 20-25A, stick it on a 20A breaker and it will be fine.

Our UK ring mains (correctly called ring finals) can cause confusion, 32A breaker because each outlet has two feeds round the 2.5mm ring. Current from each outlet limited to 13A by the fuse in the plug.

[Crossy]

I think the size of the main breaker should be in relation to the total of the CB for the individual circuits, unless you don't mind the main breaker to trip frequently.

Nope.

The main breaker protects the supply. The individual breakers protect each circuit. I hate to think what our breakers add up to, but I know it is several times the 50A incomer.

The incomer has never opened.

You have to consider that you will rarely, if ever, have everything loaded to the max, this is known as diversity, there are formulae available to help you calculate a realistic maximum demand.

In our home, even with all the lights and aircon on (and one water heater) I haven't seen a supply current over about 45A.

[ChrisB87]

Cheers Crossy

[Crossy]

The main breaker protects the supply. The individual breakers protect each circuit. I hate to think what our breakers add up to, but I know it is several times the 50A incomer.

The incomer has never opened.

I just added up all our individual circuit breakers, grand total --- wait for it --- 418A, main incomer is 50A.

That said they are distributed over two sub-mains which are individually fused at 40A, so in reality at total of 80A is actually fed from the incomer.

[Anthony5]

The main breaker protects the supply. The individual breakers protect each circuit. I hate to think what our breakers add up to, but I know it is several times the 50A incomer.

The incomer has never opened.

I just added up all our individual circuit breakers, grand total --- wait for it --- 418A, main incomer is 50A.

That said they are distributed over two sub-mains which are individually fused at 40A, so in reality at total of 80A is actually fed from the incomer.

That is indeed quiet impressive, you must have a massive house.

I have a large 4 bed/5 bath house with pool, irrigation systems, 7 water heaters and more. Each room has a dedicated breaker for plugs, lights and aircons, waterheater,while some rooms have even 2 separated plug circuits and the total of my breakers adds up to 568A. My incomer is 100A and it split up in 3 circuits with 2 x 63 A and 1 x 40A RCBO. I had a electrical system inspection performed by a European qualified electrician, and he advised I should add another 63A RCBO.

[EmptyHead]

We got a 6ft (1.8m) copper ground rod, instead of the 1m steel one. To my surprise, it went in by hand very easily in under 2 mins (with some water to loosen the dry Isaan earth).

Also bought the 20A main switch. It hasn't been changed over yet though.

Thanks again for all the advice! This forum really provides invaluable information, which without may cause difficulty, even danger, in Thailand.

Starting to feel much better about my daughter rummaging around in the fridge at her mum's new shop! (of course, the fridge will be earthed through a 3-pin plug... once i buy the plug, lol)

Edited November 28, 2014 by EmptyHead

[Anthony5]

I have never seen a massive ground rod?

Even in my home country they are always steel, with a clad layer from copper. They look like copper, but have a steel core. It is IMO very difficult to hit a copper rod in soil. It will bend or even "crease" (not sure if this is the right word) a few cm under the surface.

But maybe you are right?

Arjen.

Solid copper rods are available at least in Thailand, but of course come at a price. There are methods to drill the rod into the soil without the risk of bending it, however it may take some longer time to reach that result.

[EmptyHead]

So, the installation is done. Everything is done as i mentioned above, except unfortunatly the electrician couldn't seem to remove the 32A main switch to replace it with the 20A one i bought. It seems to be stuck in there. I tried also with no luck. It seems odd that it wouldn't come out. Maybe i can get it changed in the future.

Still, got the RCD and the 1.8m ground rod. And earthwire at all socket outlets. They were laughing their heads off when they saw all the protective stuff I bought (which wasn't on their list). Bloody cowboys.

The incomer was ran individually with bamboo posts for support (about 4m high).

Pity about the main switch though. Still gonna work on that.

Cheers for eveyone's help on here!

Edited November 30, 2014 by EmptyHead

[Crossy]

Your 32A incoming breaker may actually be OK, a couple of questions:-

1. How long is the incoming 2.5mm cable?
2. What is the total of the beakers in the CU (except the incomer)?

[EmptyHead]

Your 32A incoming breaker may actually be OK, a couple of questions:-

1. How long is the incoming 2.5mm cable?
2. What is the total of the beakers in the CU (except the incomer)?

The run is somewhere between 20-30m. (I can "pace it out" tomorrow if you need a more specific figure.)

There are only 2 breakers being used, both are 10A. So, 20A in total being used.

Cheers.

[Crossy]

With 2 x 10A breakers there is no danger of overloading the 2.5mm incoming supply no matter what the rating of the main switch.

In the event of a short the current will be around 550A (on 30m of 2.5) which will open the 32A breaker 'instantaneously'.

You should really replace that 32A incomer just in case someone in future installs bigger breakers than the current 10A units, but there's absolutely zero danger with the system as-is.

[EmptyHead]

Thanks Crossy! Great help and peace of mind.

I'll work on changing that incomer eventually (either the mainswitch down to 20A, or the cable up to at least 6mm).