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Topic Title: TT System Consumer Unit
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Created On: 03 August 2008 02:47 PM
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 03 August 2008 02:47 PM
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Inrush

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Im just after a bit of advice regarding the switching of the neutral in a TT system.

I dont have my 17th to hand so the reg numbers below are taken from the 16th edition.

As the supply neutral on a TT (and TN-C-S /TN-S) system has to have provision for isolation /switching: 460-01-02.

However my question is is it OK use single pole RCBOs on a TT system as the individual circuits can still be isolated by the common (main) switching device; however this will mean that only the phase conductor will be disconnected in a fault and not the neutral which could have a potential to earth?

Edited: 03 August 2008 at 03:18 PM by Inrush
 03 August 2008 03:07 PM
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ChrisCrane

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As far as I am aware there is no difference in the 17th Edition as against the 16th on this subject.

In practical terms, now that all circuits require RCD protection (assuming no-one is going to be pedantic), there is no difference in the configuration of a Consumer Unit no matter what the incomming supply arrangement. So the quick answer to your question is yes it is standard practice to use single pole mcb's and RCBO's on a single phase circuit.
 03 August 2008 03:38 PM
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Testit

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I think the means of doule pole isolation refers to the main incoming switch of the unit.. thereafter single pole devices can be utilised for fault and overcurrent protection. Domestic..

537.1.4
A main linked switch or linked circuit breaker shall be provided as near as practicable to the origin of every installtion as a means of switching th supply on load and as a means of isolation...

A main switch intended for operation by ordinary persons, eg household or similar, shall interrupt both live conductors of a single phase supply...

537.2.1 - tns or tncs i snot necessary to isolate the neutral conductor...

Provision may be made for isolation of a group of circuits by a common means.... I tink this covers TT..?

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 03 August 2008 04:06 PM
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GeoffBlackwell

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There are a couple of issues here.
First isolation - this is switching to allow work on conductors that would normally be live. The general requirement [537.2.2.1] for anything you call an isolator is that all live conductors should be switched. However, for TN provided the neutral is reliably earthed we need not switch it.

This would allow the main switch of an installation to be single pole but 537.1.4 rules this out for 'ordinary persons'. So in a domestic installation your main switch has to switch all live conductors - including the neutral.

Now your main switch must provide the isolation function but this is for the whole installation. If you install any other isolators in the TT installation they must also switch all live conductors. Whether you do or not would depend on its layout. BS 7671 says very little about where isolation should be provided. It is not uncommon to find that the main switch of an installation is the only 'proper' means of isolation.

Next the issue of RCDs
In the past we always fitted a 'front end' RCD on TT installations. This would be time delayed if the installation had downstream RCDs. So typically we would get the 100 mA time delayed front end device on a split board with a 30 mA device for the socket outlets etc.

I have recently seen advice from a respected organisation that suggests that we may now have a plain main switch followed by, say, two RCDs, each controlling a number of mcbs. Now on the face of it this appears to be OK, but IMO there is a problem with shock protection. I have taken this up with the organisation concerned and they are going to discuss the issue in September. I don't want to say anymore on that at the moment because I want to give them time to review their advice. I would advise anyone considering not fitting a 'front end' time delayed RCD on a TT system to confirm with the manufacturer in writing that all shock protection measures are met, particularly between the outgoing terminals of the main switch and the input terminals of each RCD.

Finally the use of single pole RCBOs (double pole types are possible - you can fit an RCD module to a MEM (single slot) double pole mcb but its expensive.

A problem will arise if you have RCDs in series (such as using a 'front end' device and downstream device). A single pole RCBO will detect neutral to earth faults but - even through it will operate - it will not remove the fault because it does not open the neutral. This fault will then be detected by any upstream RCD and that will cause it to operate - often isolating the whole installation.

Regards

Geoff Blackwell
 03 August 2008 04:25 PM
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Testit

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Well I wouldnt usually put a main switch in a TT CU.... you could argue I suppose that within the CU prior to main switch then nothing is protected against fault/shock other than L/N short circuit through the main fuse..

A single CU with dual RCDs having a main switch is arguable.. I suppose if you used double insulation tails to go from the switch to the RCD incomers within a TT CU then there would be no difference in the fault/shock protection afforded by any of the earthing arrangements.

..just some passing thoughts..

Edit: actually that double insulation bit doesnt sound right to me.. have to think on it a little more..

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Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.

Edited: 03 August 2008 at 04:27 PM by Testit
 03 August 2008 05:45 PM
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davezawadi

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RCBOs are certainly unsatisfactory on circuits which may develop a neutral - earth fault in a TT installation, because they do not isolate the fault from upstream RCDs. However they do provide additional shock protection, and this is a desirable feature.
So on a balance of nuisance I would use them if I didn't have a convenient circuit available with a 30mA RCD for sockets. As the neutral - true earth potential in a TT installation is not well controlled I am more wary of using an RCBO in an area of enhanced risk because a second fault could raise the neutral potential to dangerous levels without tripping an overall RCD. It is much better that a fault be fully isolated by a double pole RCD.

David

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David
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myurl
 03 August 2008 06:40 PM
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osgood9

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Geoff,

Just to clarify what I think you seem to be advocating;

In TT systems, it is prudent to use double pole isolators and upstream Rcds?

Thanks.
 03 August 2008 07:00 PM
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GeoffBlackwell

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Any switch you call an isolator must open all live conductors in a TT system. That means double pole for a single phase system. However, this only applies to switches we are calling an isolator - that is switches installed to allow work to be carried out on conductors that would otherwise be live.

In a simple domestic installation the main switch is often the only 'proper' isolator.

On the question of RCDs - I recommend that a TT system has a 'front end' device (usually time delayed). This is for the reasons specified above. There is an additional reason if the TT system is actually derived from a TN-C-S system - examples included an out building supply or it could be a petrol filling station.

The reason is that if a neutral earth fault occurs and is not disconnected by a single pole RCD we have effectively left the installation connected to a TN-C-S source.

Regards

Geoff Blackwell
 03 August 2008 09:28 PM
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Testit

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The reason is that if a neutral earth fault occurs and is not disconnected by a single pole RCD we have effectively left the installation connected to a TN-C-S source.


Ok follow that.. ... a need to keep an S type rcd for discrimination then, though some may argue the danger involved is minimal..

trying to envisage the circuit in my head.. think I'm ok in saying that individual circuit discrimination in such a setup where E/N faults occur is through the window, as it will shut off the entire install, ok for line faults but neutral faults = shutdown....

Wonder how the safety issues of each balances, most likely the focus of discussion on the issue..

edit: probably confusing myself... just back from watching space chimps and love guru...

-------------------------
Online Services - http://propertydevelopment.org.uk

Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.

Edited: 03 August 2008 at 09:32 PM by Testit
 03 August 2008 10:16 PM
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John Peckham

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Geoff

Are you saying there is an issue with a high integrity board with a BSEN 60947-3 main switch and 2 double pole BSEN61008 RCDs on a TT system?

I see the issue with single pole RCBOs.

John Peckham

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John Peckham

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 03 August 2008 10:20 PM
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sparkingchip

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I thought (16th I will have to read this up in the 17th)it was only caravans where we had to use two pole mcb's or rcbo's to completly isolate individual circuits, caravans of course being banned from PME so installed TT.

Andy Betteridge
 03 August 2008 10:23 PM
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sparkingchip

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Am I right in thinking the Irish use single Pole Main switches so the Neutral always stays connected allowing faults to run out of the system as with the earth?

Lets face it with PME the neutral and earth are at the same potential so why the need to isolate one of them?

Andy Betteridge.
 04 August 2008 12:17 AM
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daveparry1

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>Are you saying there is an issue with a high integrity board with a BSEN 60947-3 main switch and 2 double pole BSEN61008 RCDs on a TT system?<

John, this does seem to be what Geoff is saying but I must say I don't really understand why?
Geoff, could you confirm this and if we're correct maybe explain a little more,
regards,
Dave.
 04 August 2008 05:25 AM
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GeoffBlackwell

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Well first let's look at a similar example.

If we have a TT system under the 16th Edition we had to comply with 531-04-01. The reason for this is so as to protect against the, admittedly rare event, that a fault between a phase conductor (remember we are in 16th edition speak ) and a cpc might occur - behind the RCD. That is between, say, the incoming tails and an earth bar.

This regulation requires that the RCD be at the origin and that any part of the installation that is behind it must meet the requirements for class II so as to provide indirect contact protection.

This led to the famous 'insulating kits' for metal clad CCUs.

The 17th Edition does not change this - we now have 531.4.1. However, the regulation refers to a single RCD so some may question how, or if, it should be applied to a CCU with multiple RCDs.

The fact remains however that such a fault is possible, and that it is potentially very dangerous, and that it would not be detected. It is possible that such a fault would raise the potential of all exposed-conductive-parts and extraneous-conductive-parts to 230 volts with respect to 'true' earth.

Now consider a shiny new plastic CCU with a plain main switch and, say, two split bars each protected by 30 mA RCDs - so no 'front end' device. If we use this on a TT system - suppose we manage to allow a cpc (which only has thin sleeving on it) to make contact with a line conductor (17th Edition speak ) installed between the outgoing terminals of the main switch and the input terminals of our RCD. How will this be detected and how will we be protected .

Now all would be well if ALL manufacturers could confirm that the area within the CCU where this might happen is either constructed to meet class II or is what they call a fault free zone. If not IMO we have a problem.

Note that the RCBO issue mentioned above would still require an all pole 'front end' RCD to eliminate it.

Regards

Geoff Blackwell
 04 August 2008 07:03 AM
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normcall

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Fitting RCD main switches solves all the problems and provides all the protection required by all editions.

Worked OK for years, so what has changed?

It's my age ya no!

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Norman
 04 August 2008 07:07 AM
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GeoffBlackwell

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I agree with you Norman and I think the French do too - that's a bit worrying init .

Regards

Geoff Blackwell
 04 August 2008 02:09 PM
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AJJewsbury

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suppose we manage to allow a cpc (which only has thin sleeving on it) to make contact with a line conductor (17th Edition speak ) installed between the outgoing terminals of the main switch and the input terminals of our RCD. How will this be detected and how will we be protected .

Geoff, even if we have a single RCD incommer, is there not a similar possibility of a CPC coming into contact with incomming meter tail or RCD supply terminals? (For some reason most CUs seem to have both the earth bar and the incomming terminals towards the top of the unit...)

- Andy.
 04 August 2008 02:31 PM
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GeoffBlackwell

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Yes but provided the tails remain 'double insulated' the risk is much lower. So much so that the manufacturers were able to make insulating kits for metal CCUs. These often consisted of little more than a robust cable bush for the entry hole.

Many of the so called 17th Edition CCUs are just adapted 16th Edition models. All it would take is for the connecting tail to the RCD incoming terminals to have a bit of copper showing and for a cpc to foul it.

Regards

Geoff Blackwell
 04 August 2008 05:43 PM
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AJJewsbury

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but provided the tails remain 'double insulated' the risk is much lower

Indeed. But then a pity the outer sheath often gets removed for a couple of inches before the tails meet the incommer - makes the tails easier to bend ... and I suppose allows the core colours to be seen (and hence the correct connections identified) - maybe that's why the DNOs tend to use cable makers marked L1/N etc.

- Andy.
 04 August 2008 08:21 PM
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dmulholland

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Geoff

are you saying that we need an rcd just in case we dont terminate our cables correctly in the consumer unit ?
IET » Wiring and the regulations » TT System Consumer Unit

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