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Topic Title: Why is it forbidden to earth the neutral in TN-S systems?
Topic Summary: Even though both N and PE are connected to the midpoint of system?
Created On: 15 January 2016 06:14 PM
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 15 January 2016 06:14 PM
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Nakhil

Posts: 46
Joined: 11 April 2014

What is the reason behind not being allowed to earth the neutral when PE conductor and neutral are separate? After all, the PE conductor is connected to the midpoint of the power system just like the neutral conductor is, and also the neutral and PE are already together as PEN in the TN-C part of TN-C-S system (and PEN is even recommended to be earthed multiple times), so why once they are separated it is not allowed to earth the neutral anywhere?
 15 January 2016 06:36 PM
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rocknroll

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Joined: 03 October 2005

Because in 1975ish .gov (DTI) legislated that "Consumers may not combine the functions of neutral and protective conductors in their installations, i.e. consumers must not operate TN-C systems or use CNE cables within their installations," it then got inserted into the ESQCR around Regulation 8 or 9ish.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
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"Oh! The drama of it all."
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"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
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 15 January 2016 07:30 PM
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sparkingchip

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Because the neutral is a live conductor and the earth isn't.

Andy
 15 January 2016 07:39 PM
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rocknroll

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Originally posted by: sparkingchip

Because the neutral is a live conductor and the earth isn't.

Andy


Partly right.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 15 January 2016 07:46 PM
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OMS

Posts: 22435
Joined: 23 March 2004

OK - the distributor must retain the neutral at or around earth potential.

Combining the neutral and earth functions in one conductor means that the cable costs are reduced and any fault between conductors will be line-to-line or line - neutral so that, due to the low(er) value of earth fault loop impedance, the protective device will operate quickly to remove the fault.

Compare this to a TN-S distribution arrangement where a neutral-to earth fault could exist for a long(er) period.

This is distributor stuff - so multiple earthing keeps the neutral at or around earth potential and combats against the broken neutral conductor that can raise the MET to dangerous levels (although bonding in accordance with BS 7671 will partly mitigate any risks)

In BS 7671 land, combining neutral and earth will allow both fault and load current to circulate and resultant voltages will be present on any Class 1 system. Equally, circulating currents in different size conductors can lead to overheating and fire.

Also it's probable that current division between neutral and earth may well render RCD's useless

Don't do it - ESQCR prohibits it for good reason

Regards

OMS

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 15 January 2016 07:47 PM
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sparkingchip

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Well, they would both be live conductors if interlinked within the installation.

Andy
 15 January 2016 07:50 PM
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OMS

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See Above Andy - the supplier must maintain that "near earth" potential to allow us to use 3 pole devices in 4 wire systems - ie other than at home, we don't need to switch the neutral

ESQCR prohibits TN-C in a consumers installation - it doesn't prohibit the TN-C distribution for the supplier - 'cos suppliers are capable of dealing with the risks

Regards

OMS

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Let the wind blow you, across a big floor.
 15 January 2016 08:44 PM
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sparkingchip

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It doesn't stop the Americans doing it though.


Andy
 15 January 2016 11:17 PM
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mapj1

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The Americans have a system that is on fault likely to give you about half of 110V to ground, assuming both lines have equal impedance which is less likely to kill you. They restore the risk balance back up to an acceptable danger level by having a different view of what we used to call EEBADS, and a less rigorous testing regime.
They have more fires.

The correct answer to why is embedded in OMS's answer, but it is possible to summarise that it is
1) because if you do this you cannot easily do ADS for low current earth faults at the origin, as you cannot tell small fault currents from legitimate load current
2) a high resistance in a combined neutral and earth path immediately makes all your 'earthed' metalworks semi live
3) having more than one N-E connection will mean you will divert some of your neutral current down your earth paths that appear in parallel with it - which may mask broken neutral, and may heat up earth wiring and things like gas pipes etc, never intended to carry a steady current.

Yes the DNOs can do it, but only under strict conditions, and they have to preserve the double fault to danger principle, so you will see redundant clamps, double grub screws, and various engineered solutions that make a loss of PEN quite rare.

-------------------------
regards Mike
 16 January 2016 04:41 PM
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sparkingchip

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Alsi you can't use RCD protection.

Andy

Edit- OMS has already mentioned that.
 16 January 2016 08:28 PM
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AJJewsbury

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Why is it forbidden to earth the neutral in TN-S systems?

Because it then ceases to be a TN-S system and becomes TN-C. TN-C as a number of disadvantages:
1. Neutral currents can then flow through protective conductors, exposed- and extraneous-conductive-parts and so potentially through N (or CNE) conductors of other circuits. You then can't assume that a fuse (or single pole MCB) in the line conductor will protect all N conductors from overcurrents (as a N may be carrying currents from other circuits in addition to its own). (DNOs typically refuse to give multiple tennants of a steel-framed building PME earthing facilities because of this problem.)
2. Voltage differences caused by voltage drop along the N (CNE) within the installation can then be imposed on different exposed- (or extraneous-) conductive parts - and so can be perceived by someone touching those different parts. Normally the voltage would be well below dangerous levels from a shock point of view (e.g. 50V), but can often still be felt, especially in damp conditions (bathrooms or kitchens) and can be disconcerting or possibly dangerous if the shock results in an involuntary movement (falls, slips and so on).
3. Should a high impedance or open circuit fault occur on the CNE, everything connected downstream (load-side) of the break/fault goes up to close to line voltage - including everything connected to the earthing system on that portion of the CNE. Typically that means a single-fault-to-danger - normally we expect at least two simultaneous faults before danger can arise.
4. As a concequence of 1. (as others have already mentioned) RCDs can't be used.

- Andy.
 22 February 2016 05:22 PM
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Nakhil

Posts: 46
Joined: 11 April 2014

Thank you everyone for your informative replies.

Not sure what to make of this then (from Legrand power guide):

In order to avoid differences in voltage between neutral and earth (in particular in widespread installations or in the event of lightning overvoltages), it is possible to use regular interconnections (removable for measurements) between the PE conductor and the N conductor: at the source (neutral point of the transformer), upstream of the main protection device (in the main LV distribution board), upstream of the protection devices on the load circuits (secondary distribution boards), at the point of use (power socket outlet).


Picture of page

PDF, p. 54
 22 February 2016 09:36 PM
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mapj1

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Well, this is more or less what happens in the American system, and is still to be found in some parts of Eastern Europe. However, it is not permitted in the UK regulations, and for the following reason - you are in effect confusing the roles of current carrying and protective conductors, and by design, allowing your circuit currents to be handled partly by the wiring, and partly by a random mixture of building steel and bonded services. It then becomes just as dangerous to disconnect an earth connection ,as it is to introduce an open circuit in the neutral on a system while energised. In effect all neutrals need to be incapable of interruption - in effect to be treated as joints in a PEN have to be treated with more respect - so for example double pole switching is not permitted, upstream of the last NE bond before the load.

And in the UK the DNOs are allowed to do this, and do regularly, but they make their CNE joints very carefully. (and even then there are a few hundred problems a year per DNO)

-------------------------
regards Mike
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