Metal clad distribution boards and TT

[gavindrummond]

Ok thanks sidewiser

 

[professional]

Right ok so can you correct me please If I'm wrong. Deep breath here goes lol double insulation is applied to appliances where cpc is not required? Insulation around a conductor is basic protection and the sheathing is purely for mechanical protection?

No, the sheathing MAY be purely for mechanical protection - it depends on the manufacturers spec.

Oh, and did I miss a good "debate"? ut the kettle on

 

[Manator]

gavin,That is the gist of it.

Please also see the IET CoP for In Service Inspection & Testing of Electrical Equipment.

There are several product standards that you will probably have less chance of access to, and this is covered in BS7671, part 412 to some extent.

If you wish to get more detailed info, perhaps again it would be good to start another thread, then others can add info to explain the details of Double, or Reinforced insulation of equipment?

Again, Mods?

If you wish to start a new thread on these topics then please do, if posted in the students area perhaps more use could be made of the debates and instruction.

 

[steptoe]

Oh, and did I miss a good "debate"? ut the kettle on

NO.

Ive only just got here.

 

[kme]

Oh dear.

I missed the debate

Life as we know it has altered irrevocably!!!

And THAT has to be "word of the day" - we haven`t had one of those in a while.

OK - serious head please mr. Gummidge.

We did touch on this in a recent solar thread, where someone thought that stripping the brown & blue cores from a piece of T+E, and covering them with coloured sleeving, made them "double insulated". In fact, to my mind, they are LESS well insulated than they were!

There seem to be two separate points here, if I`m reading this right:

1) There would appear to be a suggestion that the difference between "double insulated" cable; and "insulated and sheathed" cable, is purely an exercise in semantics. If so - I believe they are completely different things. The fact that someone discussing it doesn`t have access to a potentially relevant standard (6004) does not make that standard irrelevant, IMO.

2) I assume there is a disagreement as to the suitability of M/C enclosures within, or more specifically, at the origin of a TT installation.

What I would say is this: For a typical domestic TT installation, it is going to be difficult to comply with 7671 and "best practise", by installing metal-clad switchgear at the origin. I do not preclude its use under specific situations and circumstances, but it generally won`t allow compliance.

My twopence worth

p.s. Play nicely - we`ve already got 3 mods and an admin in here. If the other one arrives; I`ll deny all knowledge of you all!

 

[apprentice87]

Hi Unphased,

I am glad you are not banned, and everything is smoothed over!!!

You see, Sidewinder is HUGELY knowledgable, HUGELY experienced, and a HUGELY respected member of this forum.

In addition, Sidewinder has SERIOUS qualifications in this sort of thing, not to mention other areas of engineering too.

He does not go around "blowing his own trumpet" as it were, cos believe me, he has no need to....

Mr Sidewinder is one clever bloke, take it from me......You will soon come to realise this yourself...

john...

 

[unphased]

Ok. Good morning to all. If I may (gulp) attempt to explain my thought processes. I find the regulations cumbersome and awkward to read. I admit that. I do not enjoy reading them. That said, I try. I also try to step outside the regs first and apply thought. In this case I discovered the insulated and sheathed terminology. thank you for that. My mind said PVC on PVC, double insulated. I also pointed out in an earlier post, probably before the switch to the new thread, that I understood the protective nature of the sheath for the primary insulation on the conductor. It is accepted, I think, that, for all intents and purposes, there two layers of insulating material, (PVC/PVC) but for various reasons the outer layer cannot be relied upon for insulation, only mechanical protection. 100% understood.

Thank you.

---------- Post Auto-Merged at 08:27 ---------- Previous post was made at 08:18 ----------

Now for the other topic. The metal clad fuseboard on a TT system. Sidewinder pointed me at the reg 531.4..... So I read it. What I read told me this (it how my mind works I'm afraid). :_|

IF an installation.... is protected by A SINGLE RCD then it shall be placed at the origin. That is what I read. So my mind said, well, I have a 30mA RCD protection on all circuits by a series of RCBOs or maybe TWO Rcds on a dual RCD set up and even possible a third RCD (100mA or 100mA type S) on the main switch. Therefore I have more than a SINGLE RCD so it is okay. That is what I deducted. So please enlighten me. I am humbled.

 

[Doc Hudson]

Manotor surely you can see how his response has wound me up. I did not deserve this. Please don't ban me I have only just started to use the forum.

Thanks for the link Admin 3. It is clear to me now that the terminology of double insulated cannot be applied to 6181Y and that it is insulated and sheathed. . I stand corrected. But it is all a matter of terminology. Know ing that has not made any particular difference to how I install the stuff.

I will make two observations, Firstly, I have to disagree with your comment "I have only just started to use the forum" as you actually joined in Feb 2008. Almost at the start of the forum and you have posted in September 2008, see posts 20 & 21 here. http://www.talk.electricianforum.co.uk/electrical-inspection-testing-forum/1292-earth-loop-inpeadance.html So you have had a reasonable time to read the posts, even if not contributing much, to get the feel of how the forum operates. However, welcome back to the world of posting, the forum encourages healthy debate, especially as this industry is full of grey areas requiring interpretation. But is will not tolerate name calling, bad language and other such disrespectful posts.

Secondly, with regard to meter tails, the term "double Insulated" is widely used within the industry when describing 6181Y, as such it is not surprising that electricians call meter tails double insulated. A quick google will bring up links such as;

6181Y Double Insulated Meter Tails

6181Y Double Insulated Cable | Building Wire

Prysmian Meter Tails 1-Core Grey 25mm

 

[kme]

Good morning.

So, envisage a (for simplicity) generic domestic setup. You have a "17th edition" metalclad DB (or even a hi-integrity, full of RCBos). At some point your supply cables (typically tails) will enter that enclosure, to connect to the main switch. All of the distribution wiring before the RCDs (or RBCos), is not provided with fault protection.

The only way to satisfy the regs is to have an RCD upstream of the enclosure; but you`ve then got the problem of their being ONE residual current device covering the whole install - which we`re trying to get away from (314.1(i)).

Does that help?

edit: Doc. I`m pretty sure the symbol is only the appliance.

I have some "double insulated" cable; which is primarily used for 100V line P.A. systems Anyone familiar with it would immediately recognise the differences in cable contruction.

That cable does NOT have the square within a square symbol...........

 

[professional]

Interesting reading :

IET Forums - Site applied insulation

412.2.1.3 Electrical equipment having uninsulated live parts shall have reinforced insulation applied in the process of erecting the electrical installation, providing a degree of safety equivalent to electrical equipment according to Regulation 412.2.1.1 and complying with Regulations 412.2.2.2 and 412.2.2.3, such insulation being recognized only where constructional features prevent the application of double insulation.

 

[apprentice87]

Hi unphased,

There is NOTHING that says you cannot have a metalclad CU, DB or any other accessory on a TT system.

What the regs ACTUALLY say, is;

"If an installation which is part of a TT system is protected by a single RCD, this shall be placed at the origin of the installation unless the part of the installation between the origin and the device complies with the requirements for protection by the use of class 2 equipment OR equivalent insulation"

Class 2 equipment is defined as;

"equipment in which protection against electric shock does not rely on basic insulation only, but in which additional safety precautions such as supplementary insulation are provided, there being no provision for the connection of exposed metalwork of the equipment to a protective conductor, and no reliance upon precautions to be taken in the fixed wiring of the installation"

So, at first sight it appears that you CANNOT use metal enclosures as the regs do say "and no reliance upon precautions to be taken in the fixed wiring of the installation"

BUT, then again, they have, in the first paragraph already stated "or equivalent insulation" by which i take it they mean, in their own words "additional safety precautions such as supplementary insulation"

What form this supplementary insulation is to take is not specified. I know that at least one maker produced kits for where the cable entered the enclosure. I also know that all equipment is supposed to conform to british standards or equivalent, so what standard were these kits produced to?? Is there a british standard for insulating gland kits or bushes?? i do not know....

If we go back to the regs, they state "If an installation which is part of a TT system is protected by a single RCD, this shall be placed at the origin of the installation unless the part of the installation between the origin and the device complies with the requirements for protection by the use of class 2 equipment or equivalent insulation"

"Origin" is defined as "The position at which electrical energy is delivered to an electrical installation"

So, if the RCD is right next to the cutout, perhaps it could be in any enclosure you like...

So, where does all this leave us?? [Apart from knowing that they cannot write the regs so that they make sense]

Well, who better to tell us than the same people that wrote them!!!!!!! So let us look in the OSG a "condensed" version of the rules PUBLISHED BY THE SAME PEOPLE so they cannot really argue.... Does it not say on the front cover "The IEE On-site guide BS 7671:2008 IEE Wiring regulations 17th edition, and at the bottom; BS7671:2008 Requirments for electrical installations.

Anyway, back to the OSG..What do they say in there?? Well, the only WRITTEN info i can find states;

"The enclosures of RCD's or consumer units incorporating RCD's in TT installations should have an all insulated or class 2 construction, or additional precautions recommended by the manufacturer need to be taken to prevent faults to earth on the supply side of the 100mA RCD" [presumably the reference to 100mA refers to "front end" RCD's installed at the origin]

On several illustrations in the book though, [showing pictures of a domestic CU] there is a label stating "For TT installations insulated enclosure or further mechanical protection to meter tails"

Now, people rather more competent than I, have already pointed out that the sheath of meter tails is merely to be regarded as "mechanical protection" and not insulation, so presumably, all we have to do to comply PERFECTLY with the regs, is to provide something additional that gives the same degree of "mechanical protection" to the sheathed tails as does the actual sheath itself.

I know they do make insulated glands, so perhaps one of those would be suitable if using SWA, if not, and just "ordinary" tails, then just something that gives additional mechanical protection, piece of trunking perhaps, or maybe just a piece of plastic screwed to the enclosure for the tails to pass through?? You could always stick each tail through a nylon compression gland, provided of course you made sure that you complied with the bit about conductors being surrounded collectively, [and NOT individually] by ferromagnetic material.

Use your "engineering judgement" ....

john...

 

[jameseye12]

He does not go around "blowing his own trumpet" as it were, cos believe me, he has no need to....

He could of at least told us he had a trumpet..... :C

 

[unphased]

Hi Doc Hudson. Indeed I did enrol on the forum back in 2008? but I have not contributed to the forum since a few early posts. It was welshwizard I believe who set the whole forum up and, as I can see, it has grown in to quite an established community. Unfortunately I have spent far too much time on the srewfix forum and brought my bad habits straight on to here. Sorry Doc. I understand how it must have looked ...but lets not go back there eh I have learned an important lesson. I am enjoying reading the replies and hopefully sidewinder will elaborate on latest posts.

Ta B-)

---------- Post Auto-Merged at 18:46 ---------- Previous post was made at 18:08 ----------

Apprentice 87

Thank you for your explanation. I often find myself questioning regs and the meaning behind them Why do you think it is so important, for example, to treat TT systems so onerously? The path to earth is still there via the main earth conductor and the electrode (rod ususally). I look at ohms law and understand the high resistance and low current. So an earth rod at 10ohms impedes fault current to 23A at 230V. Obviously higher resistance is even worse. This affects the tripping times of short circuits to earth. But, why is the reg there? If I am reading between the lines correctly it is guarding against a loose live wire shorting to the cabinet casing if metal? But the reg appears also to apply irrespective of the casing material so it still stands for insualted boards. So what is the reason for the reg? Tails feeding a metal fuseboard could short on to the casing? If this is the only reason then a 300mA RCD could be used at the origin to protect the tails from earth faults and would be ample descrimination in the event of the 30mA RCDs on the circuits tripping. This would comply with the reg. But why the reg for insulated fuseboards? Whilst on the subject I question why a 100mA, traditionallyrequired on the incomer under 16th edition, is wholly necessary now that 30mA can be used on dual RCD or RCBO loaded boards. Anyway, lets see what ensues. Sorry this is introducing further questions. Lets stick to the metal enclosure stuff.

 

[steptoe]

unphased, you have got it more or less, the simpliest and usually most cost effective method of protecting a metal clad board in a normal domestic installation is to fit a 100mA RCD in an insulated enclosure before the MC CU.

this is not the ONLY method, but as I said, usually the easiest and most cost efficient,

why 100mA?

protection of property,

special conditions normally apply to >100mA protection which would not normally apply in a normal domestic property,

industrial, and especially agricultural installations make much greater use of 300 & 500 RCD protection, although this is for different purposes than the 100mA protection that is regarded as the minimum required for a standard domestic installation.

 

[apprentice87]

Hi unphased,

To answer this, you must first look at why RCD's are required on TT systems. Simple answer is, to achieve required disconnection times. All the rest about "protection from fire" etc, flows from this..

The methods of protecting people from being electrocuted in the first place, is obviously [normally] insulation. But you have to provide against failures, so, this is where EEBADS comes in [Earthing, Equipotential bonding, and, Automatic Disconnection of Supply.

You see, as on a TT system earth loop impedance is so high, you cannot rely on an over current protection device [OCPD] to disconnect at all, [and it is the OCPD that you would usually hope would carry out the ADS function for you] never mind in the required time.

Say you had a B32 MCB, to achieve disconnection in the required time would take 160 A....

Using ohms law, you can see that this is simply not feasible, you would never get Ra low enough.

Even a B6 MCB would require 30 A.... They would not disconnect at all, as you can see.

So, we get around this by fitting an RCD, this provides us with an alternative means of providing ADS and also achieves our disconnection times for us.

So to answer your question; If you put the RCD in a metal enclosure and the incoming cable was to touch the metalwork, the RCD would not work. An RCD cannot "See" faults that occur before it, so it would not disconnect, you would not have ADS.

It gets worse than this...... You would presumably have "earthed" your metal enclosure....

Remembering that ALL cpc's are connected to the MET, and that your protective bonding conductors are also connected to the MET, in the case of the incoming cable touching the enclosure, ALL EXPOSED CONDUCTIVE PARTS AND ALL EXTRANEOUS METALWORK THOUGHOUT THE ENTIRE INSTALLATION WOULD ALSO BE LIVE.

Every class one appliance would be live too.. If it is bonded, or earthed, it would be live....and, as i have shown, it would be EXTREMELY unlikely that the OCPD would operate, EVERYTHING would be live, and would remain so, until such times as the building burnt down, or your electricity ran out!!!!

This is why you have to be so careful about these things....

Now, in fairness, i have to point out that i am no expert at this sort of thing, so perhaps one of the more experienced forum members could read this post and maybe add to it...

john..

 

[Sidewinder]

unphased,

Sorry I have not been able to enter into the debate tonight, I got in from work at about 18:00, then immediately had to rebuild the wife's car!

Fit new coolant radiator, thing is it has to have a load of the front end taking off to get the old one out, so it all had to be put back together.

Just, & I MEAN just finished now, so we'll have to take a rain check, but it seems the other guys have been posting well.

 

[unphased]

Look forward to it, sidewinder, sorry to hear of your plight.

I had a phoncall today from one of my customers whom I have yet to do the job. He told me his wife has died today!!!!! omg. Awfull news. Oh well.

 

[Sidewinder]

American!

 

[steptoe]

just when you thought you werent missing good old BL.! :slap

 

[Sidewinder]

^Did my time there mate!