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Topic Title: Disconnecting both poles in a TT installation.
Topic Summary: A old discussion rehashed.
Created On: 04 February 2013 09:01 AM
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 05 February 2013 05:17 PM
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weirdbeard

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Originally posted by: OMS

less than £100 in parts



You could do it for half that


How much is half of less than £100
 05 February 2013 05:26 PM
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AJJewsbury

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Hmmm.... does ESQCR apply within a BS7671 installation, and an electrode would be only necessary in a TN system if there were no bonded extraneous conductive parts to provide an earth reference?

I think the idea is that the local electrode provides a true earth reference - if there's a significant voltage difference between the supply CNE and true earth (as indicated by a certain current flowing through the resistor) then the RCD trips and disconnects both L and CNE (or L, N and supply earthing conductor if downstream of the N-PE link - requiring a 3-pole device of course).

Having switching contacts in the earthing conductor is a bit frowned upon of course.

- Andy.
 05 February 2013 05:27 PM
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OMS

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In a TN system the neutral can be considered to be reliably connected to earth so you don't need to switch it

In a TT system, it cant be so assumed and thus you need to switch it

regardless of earthing type, a domestic or similar installation supplied single phase requires all pole switching of the supply at the intake position

All RCD's are required to be capable of switching all line conductors

For discrimination, a downstream RCBO switching in single pole, may operate as a result of, but not isolate, say a N-E fault - that N-E fault will trip the upstream RCD. And alan has alrady given you the scenario where that neutral may we well above earth potential

So, there are a number of different conditions to meet - but none of them require double pole protective devices in a TT system unless that device is acting as the linked circuit breaker capable of switching, onload, both line and neutral conductors at the origin of a household or similar installation.

regards

OMS

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Failure is always an option
 05 February 2013 05:34 PM
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weirdbeard

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Originally posted by: AJJewsbury
]

I think the idea is that the local electrode provides a true earth reference



But if there are bonded extraneous conductive parts wouldn't they be performing the same function, ie in a TN system you would only need a separate electrode to give an earth reference if there were no bonded extraneous conductive parts?
 05 February 2013 05:34 PM
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Parsley

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It's all in table 53.4.

Regards
 05 February 2013 05:45 PM
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AJJewsbury

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But if there are bonded extraneous conductive parts wouldn't they be performing the same function, ie in a TN system you would only need a separate electrode to give an earth reference if there were no bonded extraneous conductive parts?

I don't have the circuit diagram entirely in my head, but I think the idea is that only the connection to the electrode, via the resistor, goes through the RCD sense coil - if you took the connection to extraneous-conductive-parts through it, it would be false-tripped by "normal" diverted N currents. The resistor value is chosen so that when say 50V exists between CNE and true earth, the rated residual current of the RCD flows - effectively turning an RCD into a voltage-operated ELCB.

- Andy.
 05 February 2013 05:48 PM
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weirdbeard

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Originally posted by: AJJewsbury


I don't have the circuit diagram entirely in my head, but I think the idea is that only the connection to the electrode, via the resistor, goes through the RCD sense coil - if you took the connection to extraneous-conductive-parts through it, it would be false-tripped by "normal" diverted N currents. The resistor value is chosen so that when say 50V exists between CNE and true earth, the rated residual current of the RCD flows - effectively turning an RCD into a voltage-operated ELCB.



- Andy.


A contactor costing a bit more than less than half of £100 seems more straightforward!
 05 February 2013 05:52 PM
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daveparry1

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effectively turning an RCD into a voltage-operated ELCB

I'm glad you put that bit in Andy, I was just thinking to myself this is sounding a bit like the VOELCB arrangement!

Dave.
 05 February 2013 05:55 PM
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OMS

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Originally posted by: weirdbeard

Originally posted by: OMS

less than £100 in parts

You could do it for half that


How much is half of less than £100


less than £50 I suppose.

regards

OMS

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Failure is always an option
 05 February 2013 06:01 PM
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weirdbeard

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Originally posted by: daveparry1
was just thinking to myself this is sounding a bit like the VOELCB arrangement!



Does it? the loss of neutral would cause the contactor to disengage the supply due to lack of voltage whereas a voelcb relys on a voltage appearing to earth.
 05 February 2013 06:01 PM
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sparkingchip

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"effectively turning an RCD into a voltage-operated ELCB "

a correctly installed voltage operated ELCB tripped at 30mA

Andy
 05 February 2013 06:05 PM
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alancapon

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Ultimately, I suppose it depends on whether you are happy ignoring what is a credible risk to life and property in a TT installation.

Regards,

Alan.
 05 February 2013 06:08 PM
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OMS

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Originally posted by: weirdbeard

Originally posted by: OMS

for example in a typical house you could probably protect against the dreaded loss of neutral for less than £100 in parts by fitting a bi-polar contactor in the tails, but no-one does this do they?


You could do it for half that with an RCD, electrode and a resistor - it would be illegal under ESQCR in the UK though [IMG][/IMG]



Hmmm.... does ESQCR apply within a BS7671 installation, and an electrode would be only necessary in a TN system if there were no bonded extraneous conductive parts to provide an earth reference?

Where does the resistor go?


If you are using the RCD as a loss of neutral detector it would go into the suppliers cable prior to the N_E bond in the cut out.

There's a sketch of it here which mapj1 knocked together when we were discussing it previously (it was deployed in South Africa back when the ANC though copper theft was a legitimate political protest

Lost Neutral detector

Regards

OMS

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Failure is always an option
 05 February 2013 06:10 PM
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weirdbeard

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Originally posted by: OMS

Originally posted by: weirdbeard



Originally posted by: OMS



less than £100 in parts



You could do it for half that




How much is half of less than £100




less than £50 I suppose.




That seems to add up to me, so, if it's such a cheap fix why is there such a percieved risk generally from the loss of a suppliers neutral?
 05 February 2013 06:14 PM
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OMS

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Beacuse generally you'd have to fit it in the suppliers cable - and as they are bound by statute to keep that PEN intact, then there is no risk and therefore no point is there

take a look at the link I posted, all will be clear

Regards

OMS

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Failure is always an option
 05 February 2013 06:19 PM
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weirdbeard

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Originally posted by: sparkingchip

"effectively turning an RCD into a voltage-operated ELCB "



a correctly installed voltage operated ELCB tripped at 30mA



Andy, if you tested a voelcb using a standard rcd test instrument I would suspect the test itself simply caused the voltage to earth to rise above the threshold for the device causing it to trip?
 05 February 2013 06:24 PM
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rocknroll

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That seems to add up to me, so, if it's such a cheap fix why is there such a percieved risk generally from the loss of a suppliers neutral?


Because the DTI said many years ago that because the risk of a lost neutral is so small there is no requirement to design for it.

Probably best left in Alice in IET forum land.

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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 05 February 2013 06:24 PM
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weirdbeard

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Originally posted by: OMS
, then there is no risk and therefore no point is there [



Agreed, which brings me back to my original point, within a BS7671 installation there is no requirement to consider faults that are external to the installation, unless it is an additional stipulation by the designer.
 05 February 2013 06:28 PM
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sparkingchip

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An example of a requirement to use double pole over-current devices is found in 721.43.1(caravans and motor homes) BS7671 page 254:

"Each final circuit shall be protected by an overcurrent device that disconnects all live conductors of that circuit"

Also from the same section 721.411.1 requires a RCD to disconnect all live conductors.

So that rules out single pole MCBs and RCBOs in caravans and motor homes, this doesn't appear to be a requirement in a house or similar installation.

Andy
 05 February 2013 06:36 PM
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OMS

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Originally posted by: weirdbeard

Originally posted by: OMS

, then there is no risk and therefore no point is there [


Agreed, which brings me back to my original point, within a BS7671 installation there is no requirement to consider faults that are external to the installation, unless it is an additional stipulation by the designer.


Well BS 7671 does require you to consider bonding, which clearly protects against faults external to an installation, and the size of the bonding provides protection from other faults

It bans PME supplies in a variety of applications, which again protects against external faults

It provides additional requirements where PME is used in certain applications to do more bonding to protect against external faults.

So, there are a few cases, but generally, no - BS7671 doesn't require it - although compliance with BS 7671 is a minimum level of standard, good engineering judgement and practice would take you above what BS 7671 requires.

regards

OMS

-------------------------
Failure is always an option
IET » Wiring and the regulations » Disconnecting both poles in a TT installation.

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