Exported earth

[Nicky Tesla]

oh!! that cable sheath.........you mean the sheath on a tn-s supply, sorry K

 

[welshwizard]

oh!! that cable sheath.........you mean the sheath on a tn-s supply, sorry K

That's better, I can take my ear defenders off now, you have stopped shouting, Nicky.

 

[Bclearnersparks]

On my 17th edition course earlier this week the lecturer asked how many of us made garage installations TT when house was TNCS. There were 25 electricians in the room, and a few of them with their own established companies who had quite a lot of sparks working for them. None of them had ever TT 'd a garage and they thought it was a waste of time!

 

[extension15]

As long as they keep exporting wine (here in the UK), so the earth can follow..



But seriously, makes me smile reading about this topic, brings so much interest and conflict!!

B)

 

[Zeespark]

Lots of info but you buggers have given me a headache now:_|

 

[Nicky Tesla]

On my 17th edition course earlier this week the lecturer asked how many of us made garage installations TT when house was TNCS. There were 25 electricians in the room, and a few of them with their own established companies who had quite a lot of sparks working for them. None of them had ever TT 'd a garage and they thought it was a waste of time!

As i mentioned before, it is the fact that it comes from a tnc-s system that is the issue, it also mentions this in that connections mag report mentioned earlier, but this was brushed aside.

The problem comes from a possible broken neutral and the problems that creates, nothing to do with all this rubbish above..

I just wish in these reports they would actually explain fully why they show to do things are to be done a certain way and not just because they said so, then it would not create all this confusion.

 

[Nicky Tesla]

Check out the thread by trecool, tt systems far sheds, on the IET forum

I rest my case.

 

[steptoe]

As i mentioned before, it is the fact that it comes from a tnc-s system that is the issue, it also mentions this in that connections mag report mentioned earlier, but this was brushed aside.The problem comes from a possible broken neutral and the problems that creates, nothing to do with all this rubbish above..

I just wish in these reports they would actually explain fully why they show to do things are to be done a certain way and not just because they said so, then it would not create all this confusion.

the fact it is a PME/TNCS system has absolutely nothing to do with it.

if it was a TT or IT system it would be perfectly acceptable, along with a multitude of other earthing system available,

it is the fact it is a TN system that makes it unacceptable to export the earth to a potentially different earth potential(following?)

that is the problem.

how can you be certain that the earth a possible 30/40/50/200 (?) metres away from the incoming cable is at the same earth potential.? (on a TNC)

or a possible 1000metre or more for a TNCS.?

the earth point of any system is always the star point.

or didnt you learn the basics of electricity on your 5 week course.?

 

[kme]

CORRECT COS A HIGH CURRENT FLOWING THROUGH A HIGH RESISTANCE WILL CREATE A HIGH VOLTAGE..SIMPLE ELEMENTARY STUFF

BUT WAIT A MINUTE YOU SAY IT HAS TO BE 230V MAKE YOUR CONFUSED MIND UP

If it`s simple, elementary stuff, why`d you get it wrong my friend. If your comment above were true, the National Grid wouldn`t run at 300KV+.B)

If (basic ohms law) V=IR (and I+V/R and R=V/I)

then for a constant voltage, there is an inverse relation between current and resistance. NOT a direct relation.

for example:

lets assume, for ease of calculation again, that we have a fixed voltage source of 100V

Next, we`ll calculate 2 current values; 10A and 100A

10A : R=V/I =100/10 = 10 ohms.

100A : R=V/I =100/100 = 1 ohm.

Special and I are trying, so please don`t shout in your replies.

I`m only taking one small topic per post; as you seem to lose the plot (no offence intended) with the longer posts.

And, oh yes. My calcs weren`t incomplete.

KME

 

[davetheglitz]

At 88 posts I'm not sure my re-input is going to help here - but my interpretation is that this is a matter concerning bonding. If there is bonding on the remote installation - then there is a chance of exteraneous voltage on the bonded metalwork meaning a lot of current needs to be taken to the MET. If it is a TNCS system the Ze will be very low - so the current may be in the kA range - so by the adiabatic equation the conductor size should be very large. As there is no fusing involved in this path - it is even questionable whether 5s is enough time for this current to be taken to maintain the 'Faraday cage' effect. So the solution is to TT the remote installation - makes things more managable.

Now there is a different debate with boding. It has been said that 40% of houses have no bonding whatsoever. It is assumed that if an exteraneous fault is introduced to an installation the fault will exist for under 5s - otherwise the Adiabatic calculation doesn't make sense. Now considering the number of seconds in a year - the chances of being in contact at the time - the chances of the fault in the first place - the chances of a bonding accident are less that being struck by lightening. No excuse for doing it wrong - but how do you explain the risks to the layman when there is no evidence of the hazards as an accident statistic.

Light blue touch paper - can we get to 100 posts!!!!

Dave

 

[kme]

At 88 posts I'm not sure my re-input is going to help here - but my interpretation is that this is a matter concerning bonding. If there is bonding on the remote installation - then there is a chance of exteraneous voltage on the bonded metalwork meaning a lot of current needs to be taken to the MET. If it is a TNCS system the Ze will be very low - so the current may be in the kA range - so by the adiabatic equation the conductor size should be very large. As there is no fusing involved in this path - it is even questionable whether 5s is enough time for this current to be taken to maintain the 'Faraday cage' effect. So the solution is to TT the remote installation - makes things more managable. Now there is a different debate with boding. It has been said that 40% of houses have no bonding whatsoever. It is assumed that if an exteraneous fault is introduced to an installation the fault will exist for under 5s - otherwise the Adiabatic calculation doesn't make sense. Now considering the number of seconds in a year - the chances of being in contact at the time - the chances of the fault in the first place - the chances of a bonding accident are less that being struck by lightening. No excuse for doing it wrong - but how do you explain the risks to the layman when there is no evidence of the hazards as an accident statistic.

Light blue touch paper - can we get to 100 posts!!!!

Dave

I like it Dave. Very, very good. I agree (obviously) with TT on the remote install; for reasons I`ve already covered. Statistics? We aren`t concerned with statistical probability generally, as sparx. That statistical analysis doesn`t quite hold up, for the following reason:

The extraneous fault is likely to be intruduced to the circuit by some action on the part of the user (plugging in, switching on, or hammering a nail). If you calculated the probability that they may be in contact with an item of exposed / extraneous metalwork; you`d be looking at a different scenario.

The fault voltage, at the time of maximum current flow, between the ends of the earth rod, could be expected to be >200v. It also doesn`t have to be pme; tns will provide a similar scenario.

The ONLY safe way to do this is for the exporting cable to be earthed AT ONE END ONLY; and the remote installation on a physically & electrically seperate earth.

n.b. the phrase "remote installation"? What is the definition of an "installation" in 7671?????????

KMEB)

 

[welshwizard]

Good Morning. Just got in & even though there is a high wind out, it is very humid.

 

[Nicky Tesla]

If it`s simple, elementary stuff, why`d you get it wrong my friend. If your comment above were true, the National Grid wouldn`t run at 300KV+.B)

If (basic ohms law) V=IR (and I+V/R and R=V/I)

then for a constant voltage, there is an inverse relation between current and resistance. NOT a direct relation.

for example:

lets assume, for ease of calculation again, that we have a fixed voltage source of 100V

Next, we`ll calculate 2 current values; 10A and 100A

10A : R=V/I =100/10 = 10 ohms.

100A : R=V/I =100/100 = 1 ohm.

Special and I are trying, so please don`t shout in your replies.

I`m only taking one small topic per post; as you seem to lose the plot (no offence intended) with the longer posts.

And, oh yes. My calcs weren`t incomplete.

KME

I will start by apologizing for the capital letters, i was not shouting, its just that using capitals makes it easier to see my response when mixed with the quote.

Sorry but ive not got a clue wot yor on about wiv the power station bit.

K the above calc is correct but the resistance is fixed ie the r1+r2 (which will increase when it heats up) and the Ze and the earth rod.

you showed above that v=ixr so why dont you do the calcs for different current values, the same as ive already said, you would do when calculating volt drop, im sure youve done it before, as you have to calculate the volt drop on the phase conductors and the neutral conductor to get the end voltage at the appliance for its given power rating ie its CURRENT draw.

10A x Ze of 0.1 = 1v.......10A x Rod res of 100 = 1000v

of course if the two were joined at the met then they would be in parallel and an equivalent resistance would be shown. The voltage at the met would then be found by multiplying this resistance by the 10A

1/r = 1/r1 + 1/r2......1/r = 1/0.1 + 1/100......1/r = 10 +0.01....1/r=10.01...r=0.0999

V = I x r......V = 10A x 0.999......V = 9.99V at the met at a fixed amperage of 10A

The SWA AND earth need to be disconnected when supplying the garage in a PME system to prevent normal neutral currents being drawn to the garage and going down the garage rod putting a voltage on the garage met and then onto any of the class 1 fittings.

 

[davetheglitz]

Hi Kme

I appreciate the fault you have described below - and agree:-

The extraneous fault is likely to be intruduced to the circuit by some action on the part of the user (plugging in, switching on, or hammering a nail). If you calculated the probability that they may be in contact with an item of exposed / extraneous metalwork; you`d be looking at a different scenario.

However - the fault described there is a user induced fault - and if there is any form of supplementary bonding in the system then this will not be an issue as the currents will always be (comparatively) small due to the fusing. In my experience supplementary bonding is often present in some of the most unlikely properties either by design or accident with boiler cross bonding, under sink connections, shower installations/water boilers etc - and this will catch the faults described - as indeed would an RCD.

With main bonding the customer can be faced with a major bill - gas and water don't necessarily come into the property in the same place, and to get a wire to them can involve a lot of work. As a customer my questions would be:-

Why do I need it?

What could happen if I don't have it?

What are the chances of the fault happening anyway?

As a customer I would probably reason that the chances of an accident are very small - more chance of being struck by lightning - and rather than spend a couple of hundred quid on main bonding - I'd just as soon put it towards my holiday!

If anyone has got some statistical information and pictures of what happens if main bonding isn't in place it would make my selling job somewhat easier!

Thanks

Dave

 

[SPECIAL LOCATION]

Lots of info but you buggers have given me a headache now:_|

Really sorry bout the headache M8..

I heard rumours Weshie has some medication hidden in that family circle tin in his workshop??

Been trying to keep thing easy for most people to follow

 

[SPECIAL LOCATION]

extract from post #86 Nicky Tesla

 

[SPECIAL LOCATION]

KME post #89I`m only taking one small topic per post; as you seem to lose the plot (no offence intended) with the longer posts.

That indeed is a very good idea sir

 

[SPECIAL LOCATION]

Nicky Post#77option 2) two conductors

 

[SPECIAL LOCATION]

Quote from nicky post #79ALSO 230V CAN ONLY BE PRESENT AT EACH CCTS CPC WHEN A FAULT OCCURS IF THE MET IS DISCONNECTED FROM EARTH AND NO EARTH ROD OR EQUI BOND EXISTS

so it is only possibly where the earth path is left open circuit?

Lets consider an illustration..

just for fun make it a bit like a question in a electrical class..

{ again like KME this is done in stages so everyone.. no matter what qualifications..

should be able to follow and/or correct if required? }

A domestic semi detached property with internal garage which backs onto the kitchen.

The consumer unit, supply head & meter are all mounted on the garage wall.

The central heating boiler is also mounted on the garage wall.

The supply is TN-S with a Ze of 0.72ohms.

The nominal 230v supply has an actually reading value of 249v

Part of the installation includes four radial circuits.

1] Garage light, 6a circuit breaker, 1.0mm T&E, 3m from CU to light fitting.

2] Boiler supply. 10a circuit breaker, 1.5mm T&E, 3m from CU to 3amp FCU boiler connection.

3] A radial supply or freezer in kitchen, 15a circuit breaker, 2.5mm T&E, 5m from CU to double socket outlet on kitchen wall for fridge/freezer.

4] Oven supply, 32a circuit breaker, 6.0mm T&E, 8m from CU 45amp double pole oven isolator switch (no integrated socket outlet).

Question?

During a direct phase-earth short circuit at each of the respective accessories, please answer the following

 

[davetheglitz]

Have we tipped 100 yet Dave??

?We have now!