Earthing

Bruthal

i would of thought neutralizing was essential. If metal work in a house -eg kitchen sink comes in contact with a live cable then the earth rod alone would`t be a good enough connection to trip even a 10 amp mcb. And also the fault current path to earth only exists because the centre star point of 10kv transformer to 220/400v secondary side which is the neutral point is connected to earth with earth rods or earth grid. An individual earth rod at a house does little on its own - its connection to earth is quite poor on its own, but it is connected to all the other earth rods of other houses by means of the nuetralizing connection at each house, ie - all the earth rods are connected to each other in parallel by being connected to the neutral at each house and so they act as an earthing grid. And also the earth fault resistace is a lot lower by connecting the earth system in houses to neutral at the supply point.
It does add a bit of a problem when neutral breaks at a point outside the house though.

Bruthal

i was in house recently where there was 350 volts between phase and neutral. there`s a brain teaser

davelerave

robbie7730 wrote: »

i would of thought neutralizing was essential. If metal work in a house -eg kitchen sink comes in contact with a live cable then the earth rod alone would`t be a good enough connection to trip even a 10 amp mcb. And also the fault current path to earth only exists because the centre star point of 10kv transformer to 220/400v secondary side which is the neutral point is connected to earth with earth rods or earth grid. An individual earth rod at a house does little on its own - its connection to earth is quite poor on its own, but it is connected to all the other earth rods of other houses by means of the nuetralizing connection at each house, ie - all the earth rods are connected to each other in parallel by being connected to the neutral at each house and so they act as an earthing grid. And also the earth fault resistace is a lot lower by connecting the earth system in houses to neutral at the supply point.
It does add a bit of a problem when neutral breaks at a point outside the house though.

I think all the circuits are prot ected by an rcd in the uk system to detect fault current

fishdog

i would of thought neutralizing was essential

In some rural areas in Ireland the TT earthing system is used i.e. neutralising is not permitted.

If metal work in a house -eg kitchen sink comes in contact with a live cable then the earth rod alone would`t be a good enough connection to trip even a 10 amp mcb

This is not correct. If it was the ETCI would simply not allow it. What you have to look at here is the earth fault loop impedance. If this is low enough the protective device can operate within the specified time (0.4 seconds for sockets or 5 seconds permenently connected equipment)

By neutralising it can often be easier to get this impedance low enough. Sometimes multiple earth rods must be installed in a TT system to achieve the required earth fault current for the protective device being used, but it is possible to do it safley within the regulations. The ESB will determine if you neutralise or not.

An individual earth rod at a house does little on its own - its connection to earth is quite poor on its own,

This is why multiple earth rods are sometimes required in a TT system.

And also the earth fault resistace is a lot lower by connecting the earth system in houses to neutral at the supply point

This makes up part of the earth fault loop impedance and is the reason that most installations are neutralised.

It does add a bit of a problem when neutral breaks at a point outside the house though.

As you correctly point out neutralising has some serious disadvantages too. "Neutral inversion" can cause touch voltages in one installation to raise to dangerous levels if there is a neutral fault in an entirley seprate installation.

Bruthal

earthing systems have to be connected to the neutral at some point, if not then rcd`s would`t work properly, because the earth fault current only flows because it eventually ends up flowing back to the star point on secondary side of ESB`s 3 phase transformer or to the neutral of single phase ones, which become neutral by the act of earthing one connection on secondary side. When a phase contacts the ground or earthed object, current flows not because its the ground, but because the earth forms pert of the neutral circuit.

if a transformer was not earthed at secondary side it would be an isolated supply, like a shaver socket and there would be no earth fault current even if phase touched earth, might be tiny currents due to capacitance but thats more prevelant in medium to high voltage

in practice the neutral is earthed at multiple points including at house meter position.

Steve

Not true robbie - RCDs work on the difference between the phase current and neutral current. It doesn't matter where it goes, it'll still trip.

Edit: sorry - I read that wrong, if the traffo star wasn't neutralised yes, there'd be no path but the line voltage could float up to high KVs and it would arc everywhere.

Bruthal

well obviously its the difference between current in and back out that trips them. but to say it doest matter where it goes is a bit sort of vague,,, where do you think it goes



well it does matter where it goes. go and buy a shaver socket. the secondary side wire through an rcd. and get either pole of the outgoing side of rcd and touch to earth, no chance it will trip


and why is that?????

Bruthal

did`t notice you had seen i said about not earthing star point,

but not earthing it would`t affect line voltages, there are plenty of installations with 3 phase isolated traffo`s with the secondary side star not earthed. last week a fella called asking why the earth loop impedence tester was`t workin properly, and thats exactly what it was,

Steve

if you don't earth some point on the system then the line voltage will float wherever it likes though.
It's interesting because in theory you can touch a phase and nothing happens - unless someone else touches another one at the same time!

Bruthal

quote
This is not correct. If it was the ETCI would simply not allow it. What you have to look at here is the earth fault loop impedance. If this is low enough the protective device can operate within the specified time (0.4 seconds for sockets or 5 seconds permenently connected equipment

it is correct for sure. no single earth rod on its own provides much of a connection, its the collection at a group of houses. they form a collection by being connected to the neutral at each house.

hammer one on its own into the ground in summer and try it,
And the ETCI would`t allow it if was true???? well if the neutral breaks just to that house then the earth rod should keep the house working especially on light loads, but it wont, because one rod on its own does little

Bruthal

no the line voltages will remain constant whether star point is earthed or not, only the voltages across single phase loads will vary if they are not balanced loads and if no neutral wire is provided back to the star point from the loads, the neutral of all the single phase loads is a common point to that star point, if they are inbalanced and without a connection back they feed through each other and the voltages across each appliance or load will depend on their resistance. \if a shower was connected between L1 and neutral and a 100 watt light bulb between L2 and neutral, but there was no connection from that neutral to the supply star point then you`d have less than 10 volts across the shower and about 390 across the bulb, but the line voltages of supply would remain at 400 phase to phase

Steve

robbie, you can press the button under someone's post if you want to quote them.

Bruthal

the quote button, a sure its hardly a life changing event,

have you tried the theory of touching a phase,

Steve

sorry man, just trying to help.

Yeah but only on a small s/p traffo - I made damn sure it was isolated secondary first!!

Bruthal

a i was`t havin a go at ye,

if you touch a phase you will feel nothing in dry conditions with boots or runners on, in damp conditions will feel a bit of a tingle or a good one, but if up to ankles in water then its dangerous, thats why the 110v traffos are around, centre tap earth so 55 volts to earth,

the real dangers are phase and neutral hand to hand, or 2 phases,

DublinDilbert

robbie7730 wrote: »

no the line voltages will remain constant whether star point is earthed or not, only the voltages across single phase loads will vary if they are not balanced loads and if no neutral wire is provided back to the star point from the loads, the neutral of all the single phase loads is a common point to that star point, if they are inbalanced and without a connection back they feed through each other and the voltages across each appliance or load will depend on their resistance. \if a shower was connected between L1 and neutral and a 100 watt light bulb between L2 and neutral, but there was no connection from that neutral to the supply star point then you`d have less than 10 volts across the shower and about 390 across the bulb, but the line voltages of supply would remain at 400 phase to phase

There's another very subtle problem with not earthing the star point in the 3 phase transformer.... If you don't earth the start point, and say one of the 3 phases some how connects to earth, no current will flow, but the following will happen:-
1) all the neutrals in the system will rise up to 220v above ground
2) the other two phases will also rise up 415V above ground

The phase to neutral voltage will still be 220V so everything will still function.. but there would be serious safety issues...



[From memory with a TT earthing system, do you not need to have a 300mA RCD protecting the entire board? (i've never ever seen this... ) ]

Steve

If it's isolated though you could sit in a bath and touch a phase, nothing would happen.

Yeah the site traffos have saved many a life with the CTE.

Bruthal

yes thats more or less what i said, although you could say its actually the ground thats rising 220 v above the neutral, the ground is actually becoming that phase that touched it, albeit not a great connection to it

Steve

DublinDilbert wrote: »

[From memory with a TT earthing system, do you not need to have a 300mA RCD protecting the entire board? (i've never ever seen this... ) ]

Yes, that's true afaik.

Bruthal

yes its true, and i also never seen it, im not sure if you seen my earlier posts, i was saying what you said, neutralising is used for that exact reasons, if a line contacted the ground it would`t show until another phase contacted, or if neutral did, or someone got a bang from neutral,

im at a loss as to how the other 2 phases to earth would go up to 415 though.

in 10kv ESB system that is an isolated supply, if it was`t then every branch of a tree making contact would cause trip outs, but you would still be getting blasted bt touching a phase, theres one for ye

DublinDilbert

robbie7730 wrote: »

im at a loss as to how the other 2 phases to earth would go up to 415 though.

Well the star point on the trafo will go to 220V above ground. The other two phases will then also rise up to what ever the phase to phase voltage is above ground...

The other way of thinking about it is that a phase voltage is typically 220V greater than the star point. If the star point moves to 220V, the the phase voltage with respect to ground must also increase...

Bruthal

well if line voltage is 400 between phases then phase to neutral is 230 or 400 /root of 3

if one phase touches ground on non neutralised neutral system then the ground is now 230v away from the neutral, and still 400 from each other phase assuming a good contact with the ground. Even if was neutralised the phase would still just sizzle on the ground, it would`t make a good enough connection to trip anything just lying on the ground assuming its an overhead ESB line etc

the ground has no relevance and if a phase touched a person instead of ground then the person would be 230v above neutral and would be 400 volts difference still between the person and other phases.

Bruthal

actually i just realised you wrote the other 2 phases go 415 above ground, your right,,, your taking a line voltage of 415 to start with,,, i was for some reason thinking you meant they would increase from 400 - 415

my mis understanding, if you know what i mean

fishdog

Robbie:

no single earth rod on its own provides much of a connection, its the collection at a group of houses. they form a collection by being connected to the neutral at each house.

I never suggested that a single earth rod was enough for anything. You seemed to say that neutralising was essential, in fact it is not.

i would of thought neutralizing was essential. If metal work in a house -eg kitchen sink comes in contact with a live cable then the earth rod alone would`t be a good enough connection to trip even a 10 amp mcb

My point is:
1) Neutralsiing is not essential. The ESB decide if you neutralise or not.
2) Without neutralising a 10 amp MCB can operate as it should within the correct time.
3) Without neutralising it is still possible to comply with the ETCI regulations.
4) I never said that just one earth rod would be enough for a TT system, in fact I said

Sometimes multiple earth rods must be installed in a TT system

Robbie:

And the ETCI would`t allow it if was true???? well if the neutral breaks just to that house then the earth rod should keep the house working especially on light loads, but it wont, because one rod on its own does little

I dont understand what you are saying here. My point about the ETCI is that they would not allow any TT systems if protective devices would not operate properly. With a TT system if the neutral breaks everything using that neutral simply stops working, end of story. This is one of the advantages with a TT system.

If you have a TN system (neutralised) and neutral inversion happens, you can then have a dangerous situation.

Bruthal

on tt systems the entire installation is required to be protected by an rcd of 300ma differential.

now if you have a live and neutral into a house on its own and the earthing within the house is connected to nothing but the earth rod, and a phase happens to contact an earth within an appliance, what is the earth fault path?? its path is down the earth rod and through the ground
and back to the neutral via the neutral connection to earth at the ESB transformer. This fault path will not have a high current. The ten amp mcb was an example of the fact an earth rod forms a poor connection to the ground and a direct phase to earth fault will not draw a high current in a tt system. Thats why a tt system has RCD covering entire installation

Bruthal

if tt system is so good at tripping devices on earth fault why did those installations have 300ma RCD covering entire installation as a requirement.

and if any ESB transformer is installed the 3 phase star point is always earthed. On single phase ones the 2 ends of windings has one connected to earth, the one deemed to be the lagging phase according to the 10kv 2 phases feeding it.

Neutralising is simply connecting the neutral to earth. I have not seen any installation in my time being connected that doest have traffo neutral earthed.

TT system refers to the actual house, and so depended soley on earth connection to the actual ground for its earth fault path. Its not a good low resistance path, so tn system came in,

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. 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.

And the reason??? an external light had too big a bulb and melted the live into the earth. And the woman in the house was receiving small shocks from the sink. When i got there and connected neutralising cable between earth bar and ESB main fuse neutral the lighting circuit immediately tripped. And it took a while to then find the faulty light fitting.

Now why did`t the lighting mcb trip before the neutralising was done??

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 a direct earth fault did`t trip a thing, which is why TT systems had 300ma rcd.

All neutralising does is reduces earth fault path impedence.

Its true on tt that if neutral breaks the earthing in house will not be as likelely to go up to the broken neutral voltage. But if the more likely phase to earth occurs as in house above, then tt will not trip except if has 300ma RCD covering it. Why the ESB would do it on one house and not another is new to me. And why not go with tt everywhere so. and where am i,

and no neutralising is not essential for electrical systems, But it does help to show up faults. And as i said, try your earth loop impedece tester with no neutralising, it wont be up to ETCI standards for a new installation

fishdog

on tt systems the entire installation is required to be protected by an rcd of 300ma differential.

You may be correct here, to be honest I cant remember for sure.

if you have a live and neutral into a house on its own and the earthing within the house is connected to nothing but the earth rod

(known as aTT earthing system)

and a phase happens to contact an earth within an appliance, what is the earth fault path??

It is as you correctly said:

its path is down the earth rod and through the ground
and back to the neutral via the neutral connection to earth at the ESB transformer.

This fault path will not have a high current.

Why???
The fault current will depend on 2 things, the voltage with respect to earth (230 volts aprox) and the earth fault loop impedance.

The earth fault loop impedance will depend on:

1) The impedance of the phase conductor from the traffo to the fault (same as a TN system)

2) The impedance from the fault to the earth bar (same as a TN system)

3) The impedance of the earth cable to the earth rods (low because a 10 square is used)

4) The impedance of the earth rods to earth (low because earth rods are being added until a suitable value is reached)

5) The impedance from the earth rod through the ground back to the earth rods at the traffo. Again low because,

= (resistivity of earth)x(distance)/(cross sectional area of earth)

Because the CSA is so large this impedance value is very low.

5) Impedance from earth rods at traffo to the star point, also very low.

The ten amp mcb was an example of the fact an earth rod forms a poor connection to the ground and a direct phase to earth fault will not draw a high current in a tt system

If the 10A MCB will not operate under fault conditions, why install it??

You are correct in saying that a single earth rod can form a poor connection with the ground. I am saying that this is why with a TT system you often have to put down multiple earth rods to achieve a good connection with ground. This is why a TT systems are used in rural areas,you simply need space for lots of earth rods!

Bruthal

Well your statement says your memorising stuff rather than analysing. A 10 amp mcb will trip on any short circuit fault, thats why its there. The earth fault current will take the path to earth rod if not neutralised as you say. But you can have 240 square to the earth rod and it wont help an earth rod make good connection to earth. The 300ma RCD is there for that reason, earth fault path is not reliable with just earth rods. Ask yourself why the 300ma RCD is used,,,, because its known the earth fault current wont be good. And why do the ESB use neutralising at all, becuase neutralising gives very low earth fault impedence.

You can hammer down 50 earth rods and still wont be as good as a neutralising connection particularly if have weeks of dry weather.

fishdog

if tt system is so good at tripping devices on earth fault why did those installations have 300ma RCD covering entire installation as a requirement.

If this is a requirement (and I think it is) I would guess that it is that if the earth cable were to break the RCD would offer some protection as a back up.

if any ESB transformer is installed the 3 phase star point is always earthed

True. But that does not mean a TT system can not be used!

I have not seen any installation in my time being connected that doest have traffo neutral earthed.

Me neither! That has nothing to do with TN or TT earthing system though.

TT system refers to the actual house

Correct

Its not a good low resistance path, so tn system came in,

Incorrect. It can be a very low resistance path. With the TN system it is simply easier to get a low resistance path, but it suffers from the danger of neutral inversion that the TT system does not suffer from. Why do you think the TT system is sometimes used??

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.

With 1 earth rod only there is a high chance of this yes.

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.

Because as you said the earth fault loop impedance was too high. The ESB decided that in that installation they wanted the TN system to be used. Just as in other areas they will say they want the TT system to be used. It is not the norm, but it happens.

Now why did`t the lighting mcb trip before the neutralising was done??

Because the earth fault loop impedance was too high. If this was low enough the MCB would have operated regardless of the earthing system.

So this house was in effect in tt,

Yes, without a good enough connection to earth!

Why the ESB would do it on one house and not another is new to me.

This is decided on several factors, but as you know they want TN in most places because multiple earth rods are often not an option.

and no neutralising is not essential for electrical systems

This is the opposite to what you said earlier, whice is why I responded in the first place!

And as i said, try your earth loop impedece tester with no neutralising, it wont be up to ETCI standards for a new installation

Then why do they allow it at all?? There are pictures of TT installations in the ETCI regs!

fishdog

Well your statement says your memorising stuff rather than analysing

I am doing both! I am more than 10 years out of my time so I admit I can't recall every regulation! I did say that I thought you were correct about the RCD!

I am in my final year of an electrical engineering degree, so I have to analyse a bit!

I do know that the MCB will operate if the fault current is large enough and this depends on the voltage and the earth fault loop impedance. As you know, Ohm's law.

A 10 amp mcb will trip on any short circuit fault, thats why its there

No it wont. The fault current must be large enough. Not any current will do. In fact a B type MCB can take a current 1.5 times its rating and not trip for many hours. Look at the fusing characteristics and you will see.

But you can have 240 square to the earth rod and it wont help an earth rod make good connection to earth

You are 100% correct. This is why multiple earth rods often have to be used to achieve a good enough connection to earth in a TT system.

You can hammer down 50 earth rods and still wont be as good as a neutralising connection particularly if have weeks of dry weather.

In some situations it will not be as good as you say, but it will be good enough to comply with current regulations. TT also has another advantage as discussed earlier that we agree on.

Are you suggesting that all TT systems in Ireland do not comply with the regulations???

I agree that TN is the norm. But TT is used at times, is allowed, is shown in the ETCI Regulations and can give the required protection.

Bruthal

yes i said as you keep quoting,,, i would of thought it essential, then said its not essential for electrical system to work. thats not the point here.

you just said why are tt systems used,, have you seen any new instalations use it?

and why is tn used at all so

and earth is a low impedence because large csa,, but thats no good with very small earth rod area. You would need equally large surface area on earth rods,

and why is 300ma rcd used

and in that house i talked about fault in, it did`t trip on the light fitting earth fault because as you siad it did`t have a good low earth loop impedence. Is that not the point here. you wont get a reliable low impedence.

The large earth cross sectional area sounds exactly like the stuff i heard as apprentice. High area low impedence. You should actually get a few earth rods and try it yourself. Get an earth rod and grip ameter and connect a 10 amp protected feed to top of the rod. If ground nice and wet you may trip it, but nice summers day not a hope

Again why use RCD protecting the tt entire installation<<<

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.