Second earth rod for detached garage?

landcrzr

Hi all,
Firstly, I will get an electrician to do this work but I have a question.
I am building a detached garage. There is a 6 sq SWA already running from house cu to the garage, it has an earth core. I intend putting an ip65 cu in the garage for sockets, light, roller door etc
My question is if I should have an earth rod for the garage cu, connected to the earth bar only of course?
I will move the house boiler to the garage later and this will have bare copper pipe that needs bonding.

Bass Reeves

The Electrician will know best but I think you will need a seperate earth bar for garage and both the earth from the house over the SWA and the earth from the earth bar are both bonded in your electrical supply box.

landcrzr

Bass Reeves wrote: »

The Electrician will know best but I think you will need a seperate earth bar for garage and both the earth from the house over the SWA and the earth from the earth bar are both bonded in your electrical supply box.

I suppose my concern is that the earth core in the SWA is 6sq or less and I thought you needed a 10sq earth due to exposed metal pipework. Also the SWA from cu to cu is probably pushing 20m long. Pulling a new earth core between CU's isn't really an option either.

Bass Reeves

landcrzr wrote: »

I suppose my concern is that the earth core in the SWA is 6sq or less and I thought you needed a 10sq earth due to exposed metal pipework. Also the SWA from cu to cu is probably pushing 20m long. Pulling a new earth core between CU's isn't really an option either.

No an earth bar is the answer. AFAIK you use the earth in the swa to bond both the house and the garage. You use an earth bar rather than a 10sq eth as if the 10sq ever got cut your garage would be unbonded.

landcrzr

Bass Reeves wrote: »

No an earth bar is the answer. AFAIK you use the earth in the swa to bond both the house and the garage. You use an earth bar rather than a 10sq eth as if the 10sq ever got cut your garage would be unbonded.

I think the spark is linking the earth bars between the CU's with the SWA earth core anyway. I probably should have said this in the first instance.
My question is if it would be good practice to have a second earth rod linked to the garage cu earth bar with a 10 sq cable. The same garage earth bar being linked to the house cu earth bar also.

Bass Reeves

landcrzr wrote: »

I think the spark is linking the earth bars between the CU's with the SWA earth core anyway. I probably should have said this in the first instance.
My question is if it would be good practice to have a second earth rod linked to the garage cu earth bar with a 10 sq cable. The same garage earth bar being linked to the house cu earth bar also.

IMO and i am not an electrician no . The advantage of using the earth in the SWA is that if it is guarded by the metal shield on the SWA and if ever cut you will lose everything. A separate earth could be cut and you would never detect it normally. The purpose of bonding them together is to prevent the live voltage floating. He will be bonding them at the common earth point in both power supply boxes not at the earth bars.

2011

Bass Reeves wrote: »

The advantage of using the earth in the SWA is that if it is guarded by the metal shield on the SWA and if ever cut you will lose everything.

True, it is a better job when possible.

The purpose of bonding them together is to prevent the live voltage floating.

There are a number of reasons for earthing one of the main reasons is to provide a low resistance return path for fault currents.

He will be bonding them at the common earth point in both power supply boxes not at the earth bars.

Not really sure what you mean by this, but it would be best to connect the earth bar in the shed to the earth bar in the distribution board that the shed is fed from.

Tuco88

I won't personally place a second earth rod unless it's a generator. You are creating a potential difference between the two, depending on soil resistance imo.

If you are earthing pipe work in the shed it must be 10mm^2. So you will have to run a separate conductor. Maybe it's possible to bond the pipes at the house side?

The steel armor of a SWA cable must be earthed at a point with a cable gland and banjo. It's not used as a cpc anymore.

Risteard81

Tuco88 wrote: »

The steel armor of a SWA cable must be earthed at a point with a cable gland and banjo. It's not used as a cpc anymore.

You are correct that the armouring of an SWA is an exposed conductive part and therefore MUST be earthed. It is incorrect to suggest that it cannot be used as the cpc though, as this is expressly permitted by ET101.

2011

Risteard81 wrote: »

You are correct that the armouring of an SWA is an exposed conductive part and therefore MUST be earthed. It is incorrect to suggest that it cannot be used as the cpc though, as this is expressly permitted by ET101.

Agreed, the armour can be used as a CPC if the overall earth fault loop impedance is low enough. However relying on this alone as the CPC is considered as bad practice by many as the resistance can increase dramatically due to corrosion or glands loosening. In many installations the designer / engineer would not sign off on it. It does not comply with ET105 either.

Tuco88

2011 wrote: »

Agreed, the armour can be used as a CPC if the overall earth fault loop impedance is low enough. However relying on this alone as the CPC is considered as bad practice by many as the resistance can increase dramatically due to corrosion or glands loosening. In many installations the designer / engineer would not sign off on it. It does not comply with ET105 either.

Have not seen armour or conduit used as cpc since stuff from the 80s.

Tuco88

I'd like to hear views on the second earth rod...

Bruthal

There is not a lot of point in a second remote earth rod for the purpose of earthing the earth bar of a sub board in a neutralised installation. It will be far less effective than a cpc out to the second board. Earth rods are not really a fault path reliably capable of operating mcbs. And is not really their intention.

And if such an earth rod is installed, and there is neutral failure outside the house, you now have 2 points distant from each other within the house boundary with possible potential pools.

Imo there should be just 1 earth rod at a metering point in neutralised installations. Adding more is pointless in general. Just my own opinion though.

2011

I agree with Bruthal's post above with respect to the ability of an earth rod to providing a fault path with resistance low enough to provide an earth fault path that would trip a protective device.

However earth rods provide a local reference point. What I mean by this is that in a more extreme situation where the shed has only one earth rod at the main distribution board that is several hundred meters away from the shed there may be a potential difference between the earth bar in the shed and the ground below it. This would not be desirable, so an additional earth rod (or pit) would be best. That is unlikely to be an issue in this example.

2011

Rules and regulations are important and can not simply be ignored. Having said that I think many blindly follow them without understanding their intent or the science behind them. ET101 is not supposed to provide all of the electrical design solutions, rather it is there to provide minimum standards.

Bass Reeves

Bruthal wrote: »

There is not a lot of point in a second remote earth rod for the purpose of earthing the earth bar of a sub board in a neutralised installation. It will be far less effective than a cpc out to the second board. Earth rods are not really a fault path reliably capable of operating mcbs. And is not really their intention.

And if such an earth rod is installed, and there is neutral failure outside the house, you now have 2 points distant from each other within the house boundary with possible potential pools.

Imo there should be just 1 earth rod at a metering point in neutralised installations. Adding more is pointless in general. Just my own opinion though.

2011 wrote: »

I agree with Bruthal's post above with respect to the ability of an earth rod to providing a fault path with resistance low enough to provide an earth fault path that would trip a protective device.

However earth rods provide a local reference point. What I mean by this is that in a more extreme situation where the shed has only one earth rod at the main distribution board that is several hundred meters away from the shed there may be a potential difference between the earth bar in the shed and the ground below it. This would not be desirable, so an additional earth rod (or pit) would be best. That is unlikely to be an issue in this example.

There is another reason for a local earth as I explained earlier. There is always a risk that a cable carrying an earth from the house could be cut by accident. Therefore if you were bonding anything in the garage if this happened you would have bonding failure. A local earth bonding point reduces thsi risk.

2011

Bass Reeves wrote: »

There is another reason for a local earth as I explained earlier. There is always a risk that a cable carrying an earth from the house could be cut by accident. Therefore if you were bonding anything in the garage if this happened you would have bonding failure. A local earth bonding point reduces thsi risk.

Bonding is about connecting extraneous conductive parts together so that they are all at the same potential, hence the term "equipotential bonding". So it is possible to bond equipment together without having any reference to earth (or earth rod).

Tuco88

What's a good test value for a earth rod 100 ohms?

What's the shed less than 50m from the house? Imo tn-c-s should have just the one earth reference point more so in a built up area.

Personally as an example of this. I'd run a 10mm^2 SWA and future proof the shed and place it on a rcbo. No I never had tripping issues, I have a small inverter welder/bench grinder/air comp and bench pillar drill and other gear. The only tragic thing is my welding efforts...

Is this a regulation now? The second earth rod for a shed?

All in all, there is nothing wrong with second rod I'm just not for it in this situation.

2011

Yeah 100 ohms is a good value for an earth rod in isolation in dryish ground. If the soil is wet it will improve.

Multiple earth rods is quite normal for many TNCS installations.

Tuco88

2011 wrote: »

Yeah 100 ohms is a good value for an earth rod in isolation in dryish ground. If the soil is wet it will improve.

Multiple earth rods is quite normal for many TNCS installations.

Medium to large scale yes. Even apartment blocks will just have one sole earth pit, well from what I have worked on.

In a domestic set up tho? Each to there own I guess.

2011

Tuco88 wrote: »

Medium to large scale yes. Even apartment blocks will just have one sole earth pit, well from what I have worked on.

Yes, but even a large apartment block has a small footprint.

In a domestic set up tho?

As I said above in this example no, I feel one is enough.

Each to there own I guess.

Small installations can have multiple earth rods too. For example farm yards. It is many moons since I wired a milking parlours but at the time there was an ETCI or RECI document floating around that recommended a minimum of two earth rods just for the grid in the floor. I don't know if that document is still around. Earthing has to be taken very seriously in farm yards.

Risteard81

2011 wrote: »

However earth rods provide a local reference point. What I mean by this is that in a more extreme situation where the shed has only one earth rod at the main distribution board that is several hundred meters away from the shed there may be a potential difference between the earth bar in the shed and the ground below it. This would not be desirable, so an additional earth rod (or pit) would be best.

That is my view on it also - more earth electrodes are not undesirable for a local earth reference. Relying on one earth electrode with a high impedance is not necessarily going to be very effective at tying the PEN conductor to Earth potential.

Bruthal

2011 wrote: »

However earth rods provide a local reference point. What I mean by this is that in a more extreme situation where the shed has only one earth rod at the main distribution board that is several hundred meters away from the shed there may be a potential difference between the earth bar in the shed and the ground below it. This would not be desirable, so an additional earth rod (or pit) would be best. That is unlikely to be an issue in this example.

Earth rods provide a reference for the 3 phases to earth by bonding the neutral to the general mass.

Take a fly lead from the main earth bar to 10 meters out of the premises(simulate earth bar of shed), voltage to the ground will be zero. take it 50 meters, it will be the same, because the general mass of earth will be very stable due to its size. It only varies close to earth rods, going from zero, to the earth rod potential.

If the voltage on the fly lead to ground was for example 10v, this means the the main neutralizing point/earth bar is 10v above general mass ground potential.

Now place an earth rod at the end of that fly lead to bond 10v difference, what happens? You get the earth rod and the ground in contact with it at the same potential as the end of the fly lead/earth bar of shed, but the earth rod will not be pulling the earth bar potential to ground level, the earth rod will rise to the earth bar potential, creating a circular potential pool, dissipating to general mass potential within a few inches to feet, depending on earth rod resistance to the ground. So the 10v difference would still exist, unless a grid is put down. Pointless at a house.

Therefore the best place for any additional earth rods would be at the neutralizing point. There is no advantage to putting them at the end of a cpc ran from a house to a shed.

Risteard81 wrote: »

That is my view on it also - more earth electrodes are not undesirable for a local earth reference. Relying on one earth electrode with a high impedance is not necessarily going to be very effective at tying the PEN conductor to Earth potential.

Connect the extra rod to the pen then, not at the end of a cpc to a shed sub board. Connecting it to the CPC is in effect connecting it to the pen, but with a long lead.

You are not relying on one earth rod. In a housing estate, there are many, all along the neutral system, at minipillars and transformer etc. Neutral failure at a house and suddenly you have 1 earth rod alright, add another at the shed at the end of the garden, and a nice set of potential pools are there at 2 positions now, not all that desirable when the overall benefit will be little or none. A few cattle on the farm and earth rods at a few sheds, imo not great. If a few are required, keep them at the neutralizing point area.

A pole transformer will have them at the pole, and the house.

Bruthal

Tuco88 wrote: »

Have not seen armour or conduit used as cpc since stuff from the 80s.

No, and just because a rule says its permitted, does not mean its a good method.

Some will rigidly follow what may be the whim of some fella in an office though.