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Topic Title: 200 amp three phase tt supply
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Created On: 22 July 2012 12:06 PM
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 06 November 2012 12:34 PM
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leckie

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Originally posted by: 21stcentury

Hi all, just to recap the 3 pole automatic disconnection set up



200amp 3 phase tt supply incoming



earth electrode installed

(6.8ohms at earth bar)



bonding to structural steel, gas and water



95mm metering tails in PVC trunking or double insulated in steel trunking



insulated 4 pole 200amp isolator



3 pole 200amp mccb with adjustable earth leakage relay incomer (set at 500ma) in metal clad dist board



Single pole rcbo at 100ma for general circuits

Single pole rcbo for 30ma for socket outlets



good to go... without the need for 4 pole automatic disconnection or double pole due to low Zs.



Thanks nick


I thought that for TT you needed the following:-

Class 2 or equiv for RCD at the origin not in a metal enclosure

RCD device at origin to be isolating all lives, inc neutral, so 4-pole

RCD at origin to be S type if you have other RCD/RCBO's upstream

and all rcbo's to be DP for single phase circuits to give discrimination for a N-E fault.

Why does the low Zs reading make a difference? Does double insulated tails count as Class 2?
 06 November 2012 01:00 PM
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AJJewsbury

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I thought that for TT you needed the following:-

Class 2 or equiv for RCD at the origin not in a metal enclosure

Unless it's class 2 all the way to the next tier, in which case you can omit an single RCD at the origin (e.g. all insulated CU with RCBOs).

RCD device at origin to be isolating all lives, inc neutral, so 4-pole

Agreed.

RCD at origin to be S type if you have other RCD/RCBO's upstream

(downstream?) Agreed (and at least twice the rating of the largest downstream device).

and all rcbo's to be DP for single phase circuits to give discrimination for a N-E fault.

Agreed.

Does double insulated tails count as Class 2?

Yes - see 412.2.4.1.

- Andy.
 06 November 2012 01:45 PM
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Parsley

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Does double insulated tails count as Class 2?

I raised this with the NIC tech dept a few weeks ago and was shot down.

I was looking at a TT street lighting/cctv cabinet that had a Eaton Memshield 2 DB c/w 100ma RCD main switch with a length of metal trunking on the top, the double insulated tails from the meter enter the trunking then drop into the RCD main switch of the DB.

I asked if this was OK as the tails were double insulated, the NIC tech guy said no way, and was quite adamant that it must be an insulated enclosure, I asked how a fault would develop in the double insulated cable in the locked cabinet and did it comply with 412.2.4.1 but he didn't want to discuss it.

There's a note on the figure 5.1 in GN7 that states consumer unit with insulated enclosure or further mechanical protection to meter and isolator tails
I would have thought the later part of the statement meant double insulated tails.

Regards
 06 November 2012 02:38 PM
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Delbot321

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2 points

Firstly - With regard to insulated cables passing into the metal enclosure - the NICEIC man is giving conflicting advice to that which was contained in one of their previous connections magazines. In their article it was using a single phase metal cald unit with an RCD main switch. They said there was no problem if the hole had additional bushing in the form of a pvc bush. In your case you are using paxulin but its the same principle. I think the NICEIC tech man needs to read their own historical advice (it was 5 years ago but the regs haven't changed significantly) - I'd go with you GN7 guidance as that is published in black and white and can be used in court as a defence.

Secondly:
Reg 537.2.1.1 STATES Every circuit sltall be capable of being isolated from each of tlte live supply conductor!. In. TN-S or TN-C-S system, it is not necessary to isoltte or switch the neutral conductor where it is regardcd as being reliably connccted to Earth by a suitably low impedance.
Personally I would argue that by default they are implying that a TT supply is not connected to a suitably low impedance and therefore the neutral conductor needs to diconnect, UNLESS, you have 2 pole RCDs on each circuit or similar in which case this would cover that requirement. (Bearing in mind that most RCBOs don't switch the neutral - they just monitor the current in it).
 06 November 2012 02:56 PM
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AJJewsbury

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Personally I would argue that by default they are implying that a TT supply is not connected to a suitably low impedance and therefore the neutral conductor needs to diconnect

Agreed.

UNLESS, you have 2 pole RCDs on each circuit or similar in which case this would cover that requirement. (Bearing in mind that most RCBOs don't switch the neutral - they just monitor the current in it).

Bear in mind that ADS isn't the same as isolation (although the word disconnection is perhaps overused in this context in the regs)
- the next sentence goes on to say that several circuits may be isolated by common means if service conditions allow - i.e. from the point of view of this reg, you can use single pole switching RCBOs (or indeed MCBs on a common RCD) provided you can still isolate elsewhere - e.g. a DP (for 4P) main switch.

- Andy.
 06 November 2012 03:29 PM
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leckie

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Does 412.2.4.1 mean double insulated tails count as class 2?

Note 1 says "Cable product standards do not specify impulse withstand capability. However, it is considered that the insulation of the cabling system is at least the equivalent to the requirement in BS EN 61140 for reinforced insulation."

So is this saying that double insulated cable, e.g. tails or twin and earth are considered reinforced and also the cable becomes equivalent of Class 2?

I find it amazing that you can use tails in steel trunking or directly into a metal enclosure with the incoming RCD in it for a TT installation. There is always a lot of attention drawn to the need for a class 2 enclosure for the incoming RCD. 90% of the time installers would be using double insulated tails anyway so why would there be so much mention of a class 2 enclosure if tails were ok? It would be easier to say "a class 2 enclosure or double insulated cables"

I also thought that using an insulated grommet or gland to seperate the tails from a metal encosure was not considered compliant,presumably in case the cable shorted to the case of a metal enclosure.
 06 November 2012 03:46 PM
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Parsley

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Originally posted by: leckie

Does 412.2.4.1 mean double insulated tails count as class 2?



Note 1 says "Cable product standards do not specify impulse withstand capability. However, it is considered that the insulation of the cabling system is at least the equivalent to the requirement in BS EN 61140 for reinforced insulation."



So is this saying that double insulated cable, e.g. tails or twin and earth are considered reinforced and also the cable becomes equivalent of Class 2?



I find it amazing that you can use tails in steel trunking or directly into a metal enclosure with the incoming RCD in it for a TT installation. There is always a lot of attention drawn to the need for a class 2 enclosure for the incoming RCD. 90% of the time installers would be using double insulated tails anyway so why would there be so much mention of a class 2 enclosure if tails were ok? It would be easier to say "a class 2 enclosure or double insulated cables"



I also thought that using an insulated grommet or gland to seperate the tails from a metal encosure was not considered compliant,presumably in case the cable shorted to the case of a metal enclosure.


The meter tails are double insulated tails, if installed correctly why would they short out, in the case I mentioned it was an exisitng installation in a locked cabinet? I was asked to try to resolve the issue of the 100ma main RCD switch nuisance tripping.

Regards
 06 November 2012 03:51 PM
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AJJewsbury

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A while ago a lot of steel CU manufacturers started producing "TT kits" which were basically bits of plastic to separate the incoming tails from the CU case.

I guess it's the old problem of double insulation (like any other measure) only being effective if it's suitable for the conditions. T&E is generally classed as double insulated - after all no other method of protection against shock would work. (There may be a c.p.c. present, but as it's not positioned to be between live conductors any any victim, it doesn't of itself provide shock protection). So the question becomes - is a PVC oversheath adequate protection inside a steel enclosure, or more to the point, going through the potentially sharp edges of a KO hole? If the edges of the hole are sharp, is that a failure of 'good workmanship' or an inappropriate choice of protective measure?

We don't doubt for a moment the sheath of meter tails when they're loose or clipped to a wall between meter and CU - so does the presence of a metal enclosure enhance or degrade that protection?

- Andy.
 06 November 2012 03:56 PM
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AJJewsbury

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plus of course, what happens where the oversheath is removed (for termination) - extreme case, if the cable termination came loose from the terminal, could the exposed end come into contact with the case?

- Andy.
 06 November 2012 04:01 PM
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Parsley

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I personally don't see a problem with it if the workmanships good.

I guess someone can turn up in the future and start adding to the install and make a pigs ear of it, which might compromise the original design!

Regards
 06 November 2012 04:03 PM
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rocknroll

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T&E is generally classed as double insulated


I think you will find that T&E is classed as 'insulated and sheathed' slightly different meaning where the insulation is to a particular standard and the sheath of a material suitable to protect the insulation against damage.

T&E does not qualify for classification as double or reinforced insulation, its class is 'basic insulation and earth'.

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!"
-------------------------
 06 November 2012 04:09 PM
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OMS

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They can do that with any protective measure though Parsley.

Personally, I wouldn't have any problem with insulated and sheathed tails being enclosed in metal trunking provided they were installed with good workmanship (bushes, grommets etc).

In fact, enclosing the tails in metal trunking may well be a requirement of BS 7671 and other standards in specific circumstances. 434.2.1 for example, 434.3 is another. Section 527 is another area of consideration.

regards

OMS

-------------------------
Failure is always an option
 06 November 2012 04:15 PM
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leckie

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Well for what its worth, I phoned NIC tech line about a year ago regarding an install in an industrial unit with a TT system. The double insulated meter tails came straight into a TPN dist boards with a 4-pole RCD incomer via steel trunking and a brass bush in the bottom of the board. It was an almost new install but I was adding a circuit and I queried if the arrangement was OK. They said no, the RCD had to come out and be installed in a class 2 enclosure. I got the contractor back who did the main install and they did as the NIC men said.

And so they should because the NIC know everything
 06 November 2012 04:16 PM
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AJJewsbury

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I think you will find that T&E is classed as 'insulated and sheathed' slightly different meaning where the insulation is to a particular standard and the sheath of a material suitable to protect the insulation against damage.

T&E does not qualify for classification as double or reinforced insulation


OK, my wording was slightly loose - may I re-phrase it as: "T&E (indeed most insulated & sheathed wiring systems rated 300/500V or above) would generally be accepted as satisfying the requirements of 412.2 - i.e. the requirement for both basic and fault protection for the protective measure double or reinforced insulation. (reg 412.2.4.1)."?

I agree that we shouldn't identify it with a double-square double-insulation symbol though.

- Andy.
 06 November 2012 04:17 PM
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Parsley

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Sound very similar to what they told me a few weeks ago.

Regards
 06 November 2012 04:26 PM
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rocknroll

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I will expand on that, UK domestic installations by law are required be '230/400v basic insulation and earth' and this is based on the theory of double improbability, which works because hardly anybody dies, double or reinforced insulation installations which generally does not require an earth is reserved for .gov or by special arrangement, BS7671 covers you from the cutout to the outlet which is the wire sticking out of the wall, ceiling or wherever and the earth must continue to that point, what is connected to the outlet, socket, switch, ceiling rose, transformer etc, etc is covered by a different standard the only stipulation being they must be suitable and safe to be connected to a '230/400v basic insulation and earth installation'.

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!"
-------------------------
 06 November 2012 04:37 PM
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AJJewsbury

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I will expand on that, UK domestic installations by law are required be '230/400v basic insulation and earth'

Is my doorbell illegal then?
- Andy.
 06 November 2012 05:06 PM
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OMS

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And so they should because the NIC know everything


LoL - you reckon ?

In my experience they have some guys who know lots and some guys who know little - what they are good at is coming up with a party line on a particular issue and sticking to it regardless of BS 7671

Is it a bad thing - maybe not, but it's a pain in the ***** if you happen to be the designer or installer of say the system you've just described.

regards

OMS

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Failure is always an option
 06 November 2012 05:34 PM
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Parsley

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Originally posted by: OMS

And so they should because the NIC know everything




LoL - you reckon ?



In my experience they have some guys who know lots and some guys who know little - what they are good at is coming up with a party line on a particular issue and sticking to it regardless of BS 7671



Is it a bad thing - maybe not, but it's a pain in the ***** if you happen to be the designer or installer of say the system you've just described.



regards



OMS


OMS so who's right? I don't see using double insulated as being an issue if it's installed correctly.

Regards
 06 November 2012 05:45 PM
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OMS

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Nor me personally - but if you have to comply with NIC edicts as part of being in thier club then that's one issue that often conflicts with design that is actually compliant with BS 7671.

I'm not in the NICEIC so it's easy for me to tell them to get stuffed - but it does waste my time resolving these issues when clients and contractors are questioning the design and looking for extras. Still, I've no trouble with having a good argument with them if needed.

I guess you decide based on where you sit in the contractual arrangements for any given job

Regards

OMS

-------------------------
Failure is always an option
IET » Wiring and the regulations » 200 amp three phase tt supply

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