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Topic Title: Combining a tt and tns or tncs to provide an equal resistence?
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Created On: 29 June 2012 08:09 PM
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 29 June 2012 08:09 PM
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MarkScotia

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As an example, supplying a garage with a extra.. Conductive part incoming and providing a tt system to prevent a rise in voltage etc if lost neutral with the implications with regards to a tncs system but why seperate both and not combine?

I'm probably not explaining correctly but why not have both, we link all extraneous parts/services to create an equal potential within a building so why can't we export the safety zone to external buildings or lamp posts etc??
 29 June 2012 08:46 PM
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spinlondon

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I think you have misunderstood why outbuildings are often TT'd.
It's primarily due to cost.
Where PME conditions apply, bonding conductors are required to have a minimum CSA of 10mm², or to have the equivilent conductnce if not copper.
As such, you have to weigh up whether it would be cheaper to run a 10mm² conductor to the outbuilding, or install a rod.
 29 June 2012 09:59 PM
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MarkScotia

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If only it were that simples!!

Copper is quite pricey these days though, i believe theres a bit more to it

What about outside the equipotential zone with regards to street furniture as an example, why does guidance indicate a tt system installed?
 29 June 2012 10:14 PM
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stateit

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The outbuilding in your average urban semi garden may have a similar PD w.r.t. earth as the house, but an outbuilding out in the sticks may be 50m or 100m away from the PME earthed pole, and have quite a different reference to earth.

That's my take on it anyway.

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S George
http://www.sg-electrical.com
 29 June 2012 11:09 PM
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spinlondon

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With street furniture, different DNOs have different requirements.
The old LEB required an electrode with a maximum resistance of 20Ω, before connection to their PME was allowed.
Whereas the DNO next door, Seeboard, only required that an electrode be installed, there was no maximum resistance specified.
 30 June 2012 12:45 AM
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alancapon

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You also need to remember that with TNC-S, the conductor everyone calls earth, is not really an earth. With TNC-S, what you are presented with is a connection to the System Neutral made at the cutout, that the DNO has allowed you to use instead of a real earth. The System Neutral is a current carrying conductor which is connected to earth within the DNO's network; as such, it is subject to a voltage difference to "true earth" under both normal load and fault conditions.

Regards,

Alan.
 30 June 2012 06:34 AM
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ebee

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What is"True Earth" ?

The "True Earth" out by me might well be at a different voltage to the "True Earth" out by you.

Wherever you go on this planet of our, between any two points there is always a difference in potential.

just in the same way that wherever you go you always meet someone from Milton Keynes

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 01 July 2012 10:47 AM
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sparkingchip

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I ordered some of the new Guidance Notes (GN) to the Wiring regulations from the IET, these have been arriving in dribs and drabs, hot of the press as they have become available over the last few months.

The original post asks about " providing a tt system to prevent a rise in voltage etc if lost neutral with the implications with regards to a tncs system but why seperate both and not combine? "

This is dealt with in GN5: protection against electric shock 14.5 page 100- Additional earth electrode for PME supplies.

The gist of it is if the PME PEN conductor is open circuit on the distributors side of the neutral where it forms the earth connection for the consumers installation then the touch voltage on earthed equipment and extraneous parts of the installation will be raised, so anyone touching these parts may experience a touch current passing through their body to earth and touch voltage.

To quote the GN "The value of the resistance-to-earth necessary to limit the touch voltages to a given value depends on the load and the network parameters" as the current passes through the load in circuit onto the neutral from where it is diverted into the consumers installation earthing and earthed exposed metalwork.

There is a explanation of the formula then a table 14.1 giving examples of the additional electrode maximum resistance to earth, RA, necessary to reduce touch voltages to 50V and 100V

To reduce the touch voltage to 50V for a given load the electrode resistances are:

7kW = 2.1 ohms
3 = 4.9
2 = 7.3
1 = 14.6

Now read this in conjunction with Legh's post from a bit back http://www.theiet.org/forums/f...&threadid=47051


Now bear in mind this discussion is based on a reprint from the same GN 5 and the same desire to prevent death from electrocution by limiting voltage, current and time when exposed to fault conditions, also bear in mind one of the contributors to Legh's post also contributed to the GN, so I'm playing with the big boys here

So if a 9.8kW shower was running when everything went pear shaped and the PEN conductor was lost in the distributors installation you would need a very good earth electrode to save you without a effective RCD to limit time and the voltage (up to the point the RCD tripped) as well if you went to use the outside tap to water the garden whilst wearing your flip flops and the outside tap doesn't have a section of plastic plumbing to reduce the risk.

Andy

Edited: 01 July 2012 at 10:56 AM by sparkingchip
 01 July 2012 10:54 AM
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sparkingchip

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Given what is detailed in my prior post then the post by Spinlondon seems reasonable that a street lamp with a low current lamp requires a additional electrode of no more than 20 ohms to limit the touch voltage under fault conditions.

Andy
 01 July 2012 02:04 PM
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sparkingchip

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If I just got the maths correct if a lighting column has a 70 watt lamp then it has a load resistance of 755.71 ohms, so requires an earth electrode of no more than 209.9 ohms would keep the touch voltage of the column within a range of up to 50 volts, does that sound right?

Andy
 01 July 2012 03:14 PM
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ebee

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Yep.

I must admit I was not sure what was being asked at first (not having the guidance note) but then realised you meant say a purely resistive load connected to phase and to earth electrode of x ohms gives a touch voltage of 50V at the N terminal which is floating from system N.

.


using 240v instead of 230v (Because it is more likely to be the real voltage) and ignoring any Ph conductor impedance then I tried the same loads with rods of 200/150/100 & 50 ohms to give

7KW = 230/228/222/206
3KW = 219/213/202/173
2KW = 210/201/186/152
1KW = 186/173/152/112

Frightening isn`t it ?

A lost PEN is a potentially very nasty condition

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 01 July 2012 04:41 PM
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alancapon

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Originally posted by: sparkingchip
If I just got the maths correct if a lighting column has a 70 watt lamp then it has a load resistance of 755.71 ohms . . .

You would really need to measure it. Certainly with an incandescent lamp, its "cold" resistance is significantly lower than its "hot" resistance. This is why they usually fail on switch-on, because that is when the largest current flows. I have never measured the "cold" resistance of an HID lamp plus control gear, so I couldn't tell you what it was - apart from going outside and dismantling a street light to measure it!

Regards,

Alan.
 01 July 2012 10:13 PM
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sparkingchip

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True, however the figures are more about illustrating a point that if you have a very small load the risk of a broken PEN conductor where the neutral currents are diverted into the installation earthing system extraneous and exposed earthed (to the installation earth) are no where near as serious as when there is a large load in circuit giving high touch voltages and high touch currents, which without a RCD to limit time and touch voltage until the RCD trips could be fatal to contact.

Mark asked why we don't combine the two systems TNCS and TT, well if you did you would need RCD protection to the whole installation and/ or a amazing good consumer earth electrode, BS7671 does not rely on the use of a RCD in every circumstance and earth rods are not always amazing performance wise.

Having said that some countries such as I believe Australia do have a consumers earth electrode connected through the consumer unit with an unbroken main earth conductor to the suppliers earth terminal. I don't know what the requirement RA the electrode earth resistance, but I assume given desert conditions in some parts the Australian requirement cannot be extremely low.

Andy
 01 July 2012 10:34 PM
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alancapon

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Originally posted by: sparkingchip
True, however the figures are more about illustrating a point that if you have a very small load the risk of a broken PEN conductor where the neutral currents are diverted into the installation earthing system extraneous and exposed earthed (to the installation earth) are no where near as serious as when there is a large load in circuit giving high touch voltages and high touch currents, which without a RCD to limit time and touch voltage until the RCD trips could be fatal to contact. . .

I agree. However, there is a further problem that the RCD will probably not operate for a broken neutral (unless it drops out on no voltage) as the "shock path" follows the normal current carrying path through the rcd, through the load, and back through the rcd to the neutral terminal on the cutout.

Regards,

Alan.
 01 July 2012 10:48 PM
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MarkScotia

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Andy,

Thanks for all your posts, strange that you mention australia as i worked there for a few and found there methods quite reasonable and certainly ahead of the curve where additional protection is concerned.

My initial query was based on my confusion as to why we separated the cpc to an outbuilding supply, i believe you answered it though.

Cheers
 01 July 2012 10:50 PM
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sparkingchip

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Again true, in fact very true.

The shock path after it has passed through the neutral terminals of the RCD could include pipe work such as the incoming water main.

Things appear to be getting potentially dangerous and we are still under first fault conditions aren't we?

Andy
 01 July 2012 11:04 PM
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alancapon

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Yes. That is why the main bonding is so important in helping ensure that everything metallic will rise to the potential of the MET, minimising the risk of high touch voltages. The earth rod idea is to try and hold the MET voltage down, and whilst that is a good idea, it is also important that everything else is at the same voltage (whatever it is). Another argument for a plastic insert in the pipe to the outside tap!

Regards,

Alan.
 01 July 2012 11:52 PM
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Legh

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Originally posted by: alancapon

Originally posted by: sparkingchip

True, however the figures are more about illustrating a point that if you have a very small load the risk of a broken PEN conductor where the neutral currents are diverted into the installation earthing system extraneous and exposed earthed (to the installation earth) are no where near as serious as when there is a large load in circuit giving high touch voltages and high touch currents, which without a RCD to limit time and touch voltage until the RCD trips could be fatal to contact. . .


I agree. However, there is a further problem that the RCD will probably not operate for a broken neutral (unless it drops out on no voltage) as the "shock path" follows the normal current carrying path through the rcd, through the load, and back through the rcd to the neutral terminal on the cutout.

Regards,

Alan.


I can't see that myself since the RCD is not voltage dependent so will detect the current flowing through the phase conductor and back via the MET to the earth electrode.

Legh

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 01 July 2012 11:59 PM
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alancapon

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I am afraid not. With PME, the "shock path" in this instance is:

Cutout fuse --> meter --> rcd --> load --> rcd --> cutout neutral --> cutout "earth" --> MET --> "unlucky" person --> earth.

As the "shock" current is flowing through in through the line and back out of the neutral connection of the rcd, it will not record an imbalance.


Regards,

Alan.
 02 July 2012 12:17 AM
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sparkingchip

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You can purchase voltage dependant RCD sockets, but I doubt these would drop out with a broken PEN conductor on the distributors network as the consumers installation does not see the fault.

Andy
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