Earth Rod

Diggerdunne

I swapped out my fuse board the other day. All is fine but noticed that there is no earth rod.... I know the reason for an earth rod encase the supply nuetral breaks, but how safe are all the houses in the estate that are in the same position with old fuseboards and no earth rod????

2011

If I were you I would install an earth rod in your house first, then worry about the other houses in the estate. What you have is not a safe situation. Exposed conductive parts within your home could end up being at a potential above earth.

Diggerdunne

yeah i am in the process of installing an earth rod... Trying to figure out best place for it as i have a concrete jungle....
The question is then has the house been extremely unsafe since the day it was built back in the 70's

Bruthal

When a neutral failure happens, the earth rod at a house in an estate goes from being connected to all the other earth rods of houses fed from the same transformer, to being isolated on its own, and on its own, it will do little to keep the open neutral at neutral/ground potential, with anything but the very smallest of loads.

So the earth rod at a neutralised house in an estate, is actually part of a grid of earth rods with the other houses, mini pillar earth rods etc.

So it will not really have been majorly unsafe if it had a poor earth rod, or none.

Bruthal

2011 wrote: »

If I were you I would install an earth rod in your house first, then worry about the other houses in the estate. What you have is not a safe situation. Exposed conductive parts within your home could end up being at a potential above earth.

The reality is, items in the house will end up above earth potential anyway, with anything but a neon light probably.

Bonding is the backup against this broken neutral scenario, not the earth rod.

maxfresh

More than likely there will be no 10mm sq earth to bond the pipes in the hot press or gas pipe

Diggerdunne

I ran a 10sq earth to the hot press to bond the pipes just the earth rod needs to be sorted now...

Bruthal

Diggerdunne wrote: »

I ran a 10sq earth to the hot press to bond the pipes just the earth rod needs to be sorted now...

You can put it through concrete beside the house, just keep it at least a foot out from the wall to avoid the foundation. In similar situations I have chased out a block the shape of the green inspection box, then drill through the rest of the concrete depth and hammer in the earth rod. Fit inspection box, and a little chase over to the house for the earth wire, and in through the wall etc.

2011

robbie7730 wrote: »

The reality is, items in the house will end up above earth potential anyway, with anything but a neon light probably.

Which is OK to a point.

In a situation were the neutral is broken the earth rod will keep exposed metalwork at a potential that is closer to earth potential. This reduces the touch voltage.

I find it hard to think of a good reason not to install an earth rod.

Bruthal

2011 wrote: »

Which is OK to a point.

In a situation were the neutral is broken the earth rod will keep exposed metalwork at a potential that is closer to earth potential. This reduces the touch voltage.

I find it hard to think of a good reason not to install an earth rod.

I never recommended not installing one.

But the single earth rod will not keep the exposed bonded metal work down toward earth potential at all. Not a hope.

I have been to many houses with failed neutrals, and not once was there any sign the earth rod did anything to keep the open neutral at ground potential. If the earth rod did anything of the sort, then items in the house would work as normal.

The single earth rod will do almost nothing to keep the open circuit neutral below 230v above the 0v neutral, unless the load is tiny, and the earth rod is very good.

It is fundamentally flawed to believe or assume the earth rod will perform as such. But it is a common belief that it does. It is simply one of the earth rods to connect the neutral to the ground so that the neutral is at ground potential relative to the phases.

2011

robbie7730 wrote: »

I never recommended not installing one.

I know.

But the single earth rod will not keep the exposed bonded metal work down toward earth potential at all.

Why? I am not saying that it will keep exposed bonded metalwork at earth potential, but it will keep it closer to earth potential.

I have been to many houses with failed neutrals, and not once was there any sign the earth rod did anything to keep the open neutral at ground potential.

Did you measure the current flowing through the earth rod?
If there is a current flowing then there is a corresponding volt drop. This volt drop will cause the potential difference between the bonded points and earth to reduce.

Bruthal

2011 wrote: »

Why? I am not saying that it will keep exposed bonded metalwork at earth potential, but it will keep it closer to earth potential.

In my experience they do little to do this. I seen them where a 100 watt bulb barely lit. So most of the voltage was dropped on the earth rod path.

Did you measure the current flowing through the earth rod?
If there is a current flowing then there is a corresponding volt drop. This volt drop will cause the potential difference between the bonded points and earth to reduce.

Thats the very point. It is the voltage drop on the earth rod path that decides what the voltage is on the broken neutral/earth bar. Since the typical earth rod impedence is quite high, even a small load will see most of the 230v dropped on the earth rod path, and bigger loads increase it. The intact neutral would have very little volt drop, and so the neutral and earth bar will be at 0v or very close. So you might have it backwards there saying a voltage drop will reduce the pd between the bonded metal and earth/intact neutral.

This is why bonding is necessary in neutralised installations, because typically the earth rod will have too high an impedence to do much to keep the open end neutral voltage down. So bonding keeps all metal at equal potential at least. If the earth rod had perfect low impedence matching the intact neutral, then a broken or failed neutral would go un-noticed.

It is also why tt systems have a main RCD. The earth rod would likely not operate even a 10 amp MCB directly connected to it. And I have seen this tested out.

An earth rod is really going to form a potential "pool" around itself in this scenario, more so than keep the open neutral down much lower than 230v.

I suppose saying the earth rod has no effect would be wrong as well. But it is minimal (depending on the load on in the affected installation, ground moisture etc) as to have little safety benefit in neutral failures.

2011

robbie7730 wrote: »

In my experience they do little to do this.

Yes, but you agree it makes a difference.

I seen them where a 100 watt bulb barely lit. So most of the voltage was dropped on the earth rod path.

Well a 100W bulb has a resistance of about 500Ω.

I measured individual earth electrodes recently and they typically had a resistance to the general mass of earth of about 150Ω (using the Wenner method).

Therefore the current flowing would be about 230 / (500 + 150) = 0.35A

So the PD between the neutral (and therefore bonded metalwork) and earth would be 0.35 x 150 = 50V

If the current flowing was say 1 amp, then the PD between the broken neutral and earth would be 150VAC, so as you suggest as the current increases the touch voltage does too. But it is still reducing the touch voltage from 230 to 150VAC.

I accept that all of the numbers above are very approximate.

Bruthal

2011 wrote: »

Yes, but you agree it makes a difference.

In terms of electrical shock potential to people in a house with a broken neutral, often not. But I already said it can help with small loads. Its simplicity itself.

Well a 100W bulb has a resistance of about 500Ω.

I measured individual earth electrodes recently and they typically had a resistance to the general mass of earth of about 150Ω (using the Wenner method).

Therefore the current flowing would be about 230 / (500 + 150) = 0.35A

So the PD between the neutral (and therefore bonded metalwork) and earth would be 0.35 x 150 = 50V

If the current flowing was say 1 amp, then the PD between the broken neutral and earth would be 150VAC, so as you suggest as the current increases the touch voltage does too. But it is still reducing the touch voltage from 230 to 150VAC.

I accept that all of the numbers above are very approximate.

Yes, it is good in calculations, but in real life situations, it is often a bit different. Nice dry conditions for a few weeks, and that 150 ohms can fly up. This year earth rods may perform better than the usual average in terms of connection to earth.

Switch on the immersion with its 17 ohm impedence, its heading for 210v touch now with the 150 ohm earth rod. As i said, only with small loads will it show much sign of doing anything.

With current actually flowing, the earth rod connection to ground can possibly deteriorate, and starting at 150 ohms, id say its likely it may go up with sustained current flow on it. Electrolysis possibly occurs, likely increasing the impedence of its connection after a while.

I accept it can reduce the touch voltage in ideal conditions. But in real life conditions, it has been minimal in houses i have seen. It is the luck of the draw as to what load is on, when a good earth rod is in place.

So overall, my point would be that often, the earth rod does little to protect persons in broken neutral situations. Cross bonding is the main protection for that I believe.

2011

robbie7730 wrote: »

But I already said it can help with small loads. Its simplicity itself.

Sure, but even at 1A the touch voltage was reduced to 150V.
Far from perfect, but I would rather that than full 230VAC.

Yes, it is good in calculations, but in real life situations, it is often a bit different. Nice dry conditions for a few weeks, and that 150 ohms can fly up.

No doubt, swings and roundabouts. It could also be argued that on a wet day it may also drop to 50Ω. It will depend on luck of the draw, soil resistivity etc...

50Ω may sound unrealistic, but I have witnessed tests of not very large earth pits where values have been <10Ω.

This year earth rods may perform better than the usual average in terms of connection to earth.

I witnessed the earth rod tests referred to above this year

Switch on the immersion with its 17 ohm impedence, its heading for 210v touch now with the 150 ohm earth rod. As i said, only with small loads will it show much sign of doing anything.

Sure, but if you had a very wet day and your earth electrode was 50Ω to earth the touch voltage would be around 170VAC.

With current actually flowing, the earth rod connection to ground can possibly deteriorate, and starting at 150 ohms, id say its likely it may go up with sustained current flow on it. Electrolysis possibly occurs, likely increasing the impedence of its connection after a while.

Yes, but hopefully common sense would prevail and people would turn off everything electrical when they realise that something is very wrong before this occurs.

I accept it can reduce the touch voltage in ideal conditions.

Phew! :D

But in real life conditions, it has been minimal in houses i have seen.

It can be, but there is no real down side to installing an earth electrode.

Although it is no a "silver bullet" the OP would be best to install it.

M cebee

if everything had 30ma protection a theoretical max of 1666ohm
limits touch voltage to 50v
The new uk boards are dual rcd(for example) so it might come into play in that case

2011

M cebee wrote: »

The new uk boards are dual rcd(for example) so it might come into play in that case

Once the I Δ n < 30mA the RCD will not trip.
The bonded metalwork that Robbie and I were talking about is upstream of the RCD.
Therefore an RCD is not going to operate due to a broken neutral on the ESB side, so from that prespective no number of RCDs on the customer side will be of any benefit.

Bruthal

M cebee wrote: »

if everything had 30ma protection a theoretical max of 1666ohm
limits touch voltage to 50v
The new uk boards are dual rcd(for example) so it might come into play in that case

A 30ma RCD does not limit the shock current to 30ma though. All it does is makes sure the duration is very short, and will trip at the low level a person does conduct.

Bruthal

2011 wrote: »

Once the I Δ n < 30mA the RCD will not trip.
The bonded metalwork that Robbie and I were talking about is upstream of the RCD.
Therefore an RCD is not going to operate due to a broken neutral on the ESB side, so from that prespective no number of RCDs on the customer side will be of any benefit.

Yea they wont be tripping from contact with items becoming live due to a failed neutral

2011

The picture below provides a good overview of how a 3Ph RCD works:

2011

robbie7730 wrote: »

Yea they wont be tripping from contact with items becoming live due to a failed neutral

I always get worried when you agree with me this easily :P:D

Bruthal

2011 wrote: »

I always get worried when you agree with me this easily :P:D

Incorrect:D

M cebee

yes i wasnt thinking there-theres no imbalance in that scenario
Is it feasible to use the steel in a raft-ive read some
posts on this somewhere

M cebee

apparently the concrete gives good conductivity

2011

M cebee wrote: »

Is it feasible to use the steel in a raft

Do you mean the steel in reinforced concrete?

It would be normal to bond this to local earth bars in industrial installations, generally with a 70 or 95 sq. Cu/PVC cable.

I have not seen this on a domestic premesis.

M cebee

robbie7730 wrote: »

A 30ma RCD does not limit the shock current to 30ma though. All it does is makes sure the duration is very short, and will trip at the low level a person does conduct.

Those figures for tt are for disconnection of a fault causing touch voltage over 50volt to persist
Not direct contact

M cebee

think ive read about it on uk forums

Bruthal

M cebee wrote: »

think ive read about it on uk forums

For 50v touch voltage on metal in a tt setup, it would be a different fault than an open supply neutral.

With the 1666 ohms, it would require a fault from the 230v to the metal through a resistance of 6k ohms to leave 50v on the metal. That would be unusual.

What it is showing is, any sort of an earth rod, and a 30ma RCD will function properly.

With a tncs and failed neutral, the touch voltage will be depending on the value between the load resistance, and the earth rod to ground resistance. A high earth rod resistance will mean the touch voltage is higher for any given load resistance.

So the load will be between the live and the touch voltage metal.

M cebee

hmm!
1666 by 0.03=50v
Ensures a touch voltage exceeding 50v can't persist(rules speak) for 30ma protected TT does it not

M cebee

i know those calcs dont apply to open neutral tncs-i was mistaken there

2011

M cebee wrote: »

hmm!
1666 by 0.03=50v

Yes, you are correct. If 0.03A were to flow through a resistance of 1666Ω then there would be a potential difference of 50V across it.

Robbie's point if that for 0.03A to flow the total resistance would have to be 230/0.03 = 7666Ω of which your electrode connection would be 1666Ω leaving the rest at 6kΩ