Earthing

Bruthal

does a 50 year old house with old rubber wiring and 2.5 square feed comply with regulations

Bruthal

so your telling me multiple earth rods are better than a solid copper path??

how can that be incorrect

im amazed you say thats incorrect,

so when i say a neutralizing connection is a lower impedence path than multiple earth rods you reckon thats incorrect, well im amazed at that,

and did i ever mention regulations about tt system, its just in my 21 years at it i never seen one installed, and some of that time was in the esb. Thats not to say it has`t been done, i just have not seen it, mostly industrial stuff i do, but lots of jobs on remote farms etc, ye learn something new every day

fishdog

does a 50 year old house with old rubber wiring and 2.5 square feed comply with regulations

I think we both know the answer to this.

so your telling me multiple earth rods are better than a solid copper path??

I am telling you:
1) The ESB decide on the earthing system.
2) The lower resistance path depends on several factors mainly length of run, resistivity and cross sectional area.

To answer your question, sometimes multiple earth rods are better than a solid copper path. But generally a TN system will have a lower earth fault loop impedance.

did i ever mention regulations about tt system

Yes you said:

If you do a new installation and there is no connection between the house earth bar and the neutral at metering point then the earth loop impedence test will fail.

If it fails it does not comply with the regs.

so when i say a neutralizing connection is a lower impedence path than multiple earth rods you reckon thats incorrect

Look at what I said. I am saying that it can be a lower impedance path! Often it is not I am sure we both agree.

The important thing is that I am saying that it can be low enough for the regulations!

its just in my 21 years at it i never seen one installed

I have not either and I have been involved a similar length of time! But that is beside the point!

Thats not to say it has`t been done

Good to see we agree on this!

Bruthal

See what i mean, you quote me on saying any short will trip a 10 amp mcb, and you say no it wont it has to be a high enough current,

now i think most would assume a short circuit would draw a fault current high enough to trip its breaker. And this 1.5 times the mcb`s rating, sure that may be true, but whats that got to do with anything.

The cold hard fact is i said neutralising was used because its seen as the best method for achieving high current earth faults, thats all, its not rocket science.

If tt earthing system was as good at having very low impedence then the 300 ma rcd would`t be needed. That system with RCD is a good one, but do you know why a tn is used, and why earth rods are used at the house, does anyone, does it matter

Bruthal

yes but have you seen it done, thats the thing, ive been at plenty of remote installations and yet to see one,

but i have got earth rods and done the experiments with them,

and yes earth rods can be made to have a low impedence in groups, but will in be the same in the summer, or after a month of sunshine and they are sitting in dry baked earth, I`ve seen earth grids go in outside switch rooms and they sometimes are not great, but they are installed for other reasons also,

Bruthal

have you done earth loop impedence on a non neutralised installation??

i cant see how earth rods can ever have a lower impedence than a solid piece of copper, are you going by testing or reading,

if you stand in your kitchen and touch end of a live wire will you feel it

Bruthal

whats your definition of neutral inversion, im just curious now, not testing or anything

durin this year a neighbour called and said stuff tripping in house. neutral was showing up as live, and was 370v out of phase with the live, neutrals from 2 houses had come away from the neutral bar in a mini pillar. But if the earth rods were a solid low impedence connection to earth then the neutral bar voltage would of stayed down close to zero, by being connected to the neutral in each house and the houses would of continued on working away, but they did`t

fishdog

you quote me on saying any short will trip a 10 amp mcb, and you say no it wont it has to be a high enough current

It is simply down to Ohm's law, not you or me!

Can we agree that the fault current will depend on the voltage and the impedance??

If so, can we agree that a high enough impedance will ensure that the fault current will be too low??

If the current is high enough it will trip, if not it will not.

If the a short circuit fault current is 1.5 times the rating of the MCB due to a very high earth fault loop impedance such as the house you described earler the MCB will not trip!

now i think most would assume a short circuit would draw a fault current high enough to trip its breaker

I think most would agree that Ohm's law applies. For a fixed voltage only if the earth fault loop impedance is low enough will the fault current be high enough.

The cold hard fact is i said neutralising was used because its seen as the best method for achieving high current earth faults

In most situations, yes it is that is why it is used most. I have always agreed on this.

My point simply is that sometimes (not often) TT is used and it can be safe and it can comply with regs. Therfore nuetralising is not essential. I only responded because you said the opposite, but now we agree on this.

If tt earthing system was as good at having very low impedence then the 300 ma rcd would`t be needed

As I said before, what if the earth cable breaks???? I would guess that is why.
This is beside the point.

Steve

robbie7730 wrote: »

I was actually at a house earlier this year where the neutralising point was left out, the ESB where there, and in their presense i had to neutralise the earth bar on the mcb panel.

I'm curious about this - if the ESB were on site, why didn't they neutralise at the meter - I thought that's the only place the link is allowed??

So this house was in effect in tt, but only because someone changed the MCB board and the neutralising wire did`t reach the earth bar in new board and they did`t even bother extending it.

And this is exactly why the ESB are the only ones that decide which system is to be used in an installation. Whoever changed the board should have spotted the neutralising link on the old one and queried it with the ESB - shoddy work to say the least.

Bruthal

no the 300ma rcd is used because an earth rod wont give a low impedence connection, you may believe it will, but it wont, go and hammer one into your back garden and measure resistance with a multi meter, try it, doing actual experiments is far far better than quoting

Bruthal

yes they do neutralise at the meter into the neutral of the main ESB fuse, but where do you think the other end of the neutralising earth wire goes,

and when that mcb board was installed and fella left out neutralising connection, the result spoke for itself, direct earth fault. No trip. ESB fella standing there, he said he was reading voltage on earth rod, i turned and said right, when i neutralise this something may trip as sounds like an earth fault. And thats what happened. Do you think i could find the light, the fella livin there was away, only installed the light a few weeks earlier and 150 w bulb in it

Steve

robbie7730 wrote: »

no the 300ma rcd is used because an earth rod wont give a low impedence connection, you may believe it will, but it wont, go and hammer one into your back garden and measure resistance with a multi meter, try it, doing actual experiments is far far better than quoting

As you said earlier - the impedance of TT is affected by weather and soil moisture content so 300mA main RCD there to provide backup to this.
If a TT installation fails an impedance test then you need to keep adding earth rods till it passes. I don't think I've seen any TT system that relied on a single rod.

Bruthal

yes but why hammer in 5 earth rods when can neutralise. and if one earth rod is in baked dry soil, will 10 make a big difference, ive seen many instalations with neutral fail at mini pillar, like that one this year, but luckily its usually only that house neutral

but i have yet to see one with an earth rod good enough to maintain the neutral through earth, if the earth rod was indeed a low impedence connection to earth then the broken neutral curent would flow down the earth rod and back to the mini pillar neutral via its earthing rods, but that never happens, which shows the earth rods are not a good connection, but i dont think you get what im saying

fishdog

yes but have you seen it done, thats the thing, ive been at plenty of remote installations and yet to see one,

No, as I admitted earlier I have not. But I did discuss it with a RECI electrical inspector that had on a course he was running years ago.

I wired a milking parlour once as an apprentice and I had to put down many earth rods as part of it. I ended up geting a very good connection, I cant recall how low.

and yes earth rods can be made to have a low impedence in groups

Glad we agree!

but will in be the same in the summer, or after a month of sunshine and they are sitting in dry baked earth,

That is a good point. In theory, the quality of this connection should be tested fom time to time.

i cant see how earth rods can ever have a lower impedence than a solid piece of copper, are you going by testing or reading,

Ok lets assume the copper has a small cross sectional area and it is very, very long. Lets further assume that I have installed an enormous earth pit with 500 earth rods. Can you see where this is going??

I have always agreed that in general the TN system provides an easy way to get a low earth fault impedance that will generally be lower than a TT system.

whats your definition of neutral inversion, im just curious now, not testing or anything

Ok. Suppose the neutral conductor to the house next door breaks between the ESB minipillar and their distribution board. The return curent from that house will then flow from their board to their neutralising point. It will then flow down their earth rod and into the ground. Because electricity like to take the path of least resistance this may well be up your earth rod, to your neutralising point and back on your neutral. There will now be a potential difference between your neutral point and your earth rod.

This voltage = (Current flowing from your earth rod to neutral point) x (Impedance of your earth rod to neutral point)

With your TN system your sink is connected to this neutral point, so it is now at this potential above earth.

Suppose Intel was the house next door! This current would be enormous, therefore your touch voltage at the sink would be enough to kill you!

fishdog

yes but why hammer in 5 earth rods when can neutralise

Because:
1) you must do what the ESB tell you
2) No danger of neutral inversion

and if one earth rod is in baked dry soil, will 10 make a big difference

Yes, think of resistors in parallel.

ive seen many instalations with neutral fail at mini pillar, like that one this year, but luckily its usually only that house neutral

There you go, a serious danger of neutral inversion there! The best example yet of why to use a TT system rawn from your own experience!

if the earth rod was indeed a low impedence connection to earth then the broken neutral curent would flow down the earth rod and back to the mini pillar neutral via its earthing rods, but that never happens, which shows the earth rods are not a good connection, but i dont think you get what im saying

Well it can happen and it has happened. This is exactly what can cause neutral inversion. If it rained heavily you would find that the earth rods connection would improve alot!

fishdog

SteveC:

I'm curious about this - if the ESB were on site, why didn't they neutralise at the meter - I thought that's the only place the link is allowed??

Things have changed, now the electrical contractor neutralises. The contractor connects up to his side of the ESB meter now and in doing so neutralises.

SteveC:

And this is exactly why the ESB are the only ones that decide which system is to be used in an installation.

Exactly

Bruthal

fishdog wrote: »

No, as I admitted earlier I have not. But I did discuss it with a RECI electrical inspector that had on a course he was running years ago.

I wired a milking parlour once as an apprentice and I had to put down many earth rods as part of it. I ended up geting a very good connection, I cant recall how low.


Glad we agree!


That is a good point. In theory, the quality of this connection should be tested fom time to time.


Ok lets assume the copper has a small cross sectional area and it is very, very long. Lets further assume that I have installed an enormous earth pit with 500 earth rods. Can you see where this is going??

well think now, the neutralising copper is only from fuseboard to meter, the rest of the path is down the same copper neutral as your assumption, and current is also free to travel down the same earth rod you depend on, as well as the short neutralising wire, think carefully about it,

I have always agreed that in general the TN system provides an easy way to get a low earth fault impedance that will generally be lower than a TT system.


Ok. Suppose the neutral conductor to the house next door breaks between the ESB minipillar and their distribution board. The return curent from that house will then flow from their board to their neutralising point. It will then flow down their earth rod and into the ground. Because electricity like to take the path of least resistance this may well be up your earth rod, to your neutralising point and back on your neutral. There will now be a potential difference between your neutral point and your earth rod.

This voltage = (Current flowing from your earth rod to neutral point) x (Impedance of your earth rod to neutral point)

With your TN system your sink is connected to this neutral point, so it is now at this potential above earth.

Suppose Intel was the house next door! This current would be enormous, therefore your touch voltage at the sink would be enough to kill you!

and intel wont be connected in this way, it has its own 110kv supply, and even moderate commercial premmises have 10kv supply into their own traffo

and current flow to a sink makes absolutely no difference to the touch voltage, as i said, that house with neutralizing connection left out had full 220 at sink,

if you have a 1000 amp cable from a panel, you sill still only get the same shock from it by touch as from your house lighting circuit, the real danger from high current conductors is if you drop a spanner in

Bruthal

hey whats your definition of neutral inversion and what causes it

Steve

fishdog wrote: »

SteveC:

Things have changed, now the electrical contractor neutralises. The contractor connects up to his side of the ESB meter now and in doing so neutralises.

Thanks, didn't know that - I'm into control systems nowadays and not current on installation regs...:)

fishdog

and intel wont be connected in this way, it has its own 110kv supply, and even moderate commercial premmises have 10kv supply into their own traffo

Yes it would not be connected that way. I was just trying to illustrate a point that if the installation next door had a large load the problem would be more serious.

I connected some large traffos abroad, so I should know!

But even Intel has transformers that have a star connected secondary side whose star point is connected to earth. If just 0.5% of their neutral current came up your earth rod, you are in big trouble!

current flow to a sink makes absolutely no difference to the touch voltage

I think you are geting confused here.

If the floor you are standing on is at 0 volts and the resistance from the earth rod to the earth bar has a resistance of 20 ohms and it has 10 amps flowing through it the earth bar is then at a potential of 10 x 20 = 200 volts

Because your sink is connected to this earth bar it will now be at a potential close to 200 volts above earth. This is the touch voltage.

fishdog

hey whats your definition of neutral inversion and what causes it

The example I gave with Intel. Sorry I forgot to say that when I described that situation.

It can be caused by using a TN earthing system and a broken neutral.

Bruthal

no think very very carefully, its actually you getting confused, its not the resistance to the earth bar that decides what voltage is on it, its a voltage appearing on the earth bar from a fault situation and the impedence from that earth bar back to the transformer that decides the fault current

and as for you saying.05 or .5 or whatever it was percent of neutral current from intel going up my earth rod, well think very carefully again, if you got a phase from ESB transformer and connected it directly to an earth rod in the ground, and that phase had unlimited current, what current would flow???

well if earth rod was 10 ohms then 22 amps, and thats direct from mains feed, do how would a high neutral current flow from another installation up earth rod,

Bruthal

yes and inversed neutral can happen in a 3 phase system irrespective of neutralising or not,
but in tn the earthing system comes up the the broken neutral voltage.

no one is arguing that before you quote again,

my arguement is earth rods are not good as a fault path,simple,
you said yourself they would be bad in dry conditions,

as i said in house earlier this year i had 370 volts phase to neutral, nothing to do with neutralising,

but you are arguing stuff im not talking about,

Bruthal

contractor always connected the neutralising earth wire at the MCB board end, its the meter end contractors now connect the mains and neutralising wire

Bruthal

its one thing to quote and memorise regs and ohms law, another to understand it and work on your own understanding,

Bruthal

there`s nothing to agree or disagree about really,

think of something else to test theory on now

have a look at my youtube videos, maybe you will know me ha ha

http://www.youtube.com/user/robbie77300

fishdog

its not the resistance to the earth bar that decides what voltage is on it

In the neutral inversion example given the voltage at the earth bar =
currnet flowing through it x the impedance between it and earth
Im sorry that is Ohm's law not mine

and as for you saying.05 or .5 or whatever it was percent of neutral current

I did not say that, I said

If just 0.5% of their neutral current came up your earth rod, you are in big trouble!

To illustrate that if this amount of current were to flow it would be a very large current. Intel is simply an example to illustrate a point as is 0.5% only an example of how much current may flow.

if you got a phase from ESB transformer and connected it directly to an earth rod in the ground, and that phase had unlimited current, what current would flow???

I fail to see your point here.
If the ESB could supply unlimited current, again Ohm's law would apply.

Current = Voltage/Impedance

well if earth rod was 10 ohms then 22 amps, and thats direct from mains feed, do how would a high neutral current flow from another installation up earth rod,

Sorry I dont follow, where are you getting 22 amps from?

If there was say 100 volts at the bottom of your earth rod and the impedance from there to the neutral point back at the traffo was 20 ohms the current that would flow would be 100/20=5 amps. The volt drop across this would be 5 x 20=100 volts. Most of the volt drop would occur from the earth rod to the earth bar so lets say the earth (and therefore neutral bar) bar has a potential relitive to earth of 90 volts and the remainder of the volt drop occurs between there and the star point. Your sink is connected to this earth bar so much of this voltage would appear between the sink and earth.

Note: I have only used figures I picked to illustrate a point.

How would you describe neutral inversion??

fishdog

its one thing to quote and memorise regs and ohms law, another to understand it and work on your own understanding

Sure, but Ohm's law becomes pretty important when you look at earthing.

I am under no illusions, I still have pleny of learning to do! In fact I am trying to get my head around power distribution problem right now!

Bruthal

what is the problem

fishdog

PM me an email address and I will email it to you! Any assistance would be great!!!