100ma front end rcd and metal clad ccu

[soulman]

o.k spent a lot of time reading and the conclusions i have come to are the following it will fine to leave the intergral 100ma rcd in place as long as reg 411.5.3 are met. and also the tails although not perfect as they are neither glanded or protected up front by an rcd device they are double insulated. so a risk assesment would indicate to me that no danger exists. I presume this is correct, as said in a previous post i understand that on farms etc that rcd's are used for fire protection, i just misunderstood after having been taught at college that 30ma rcd's were essential on tt systems in domestic properties. After leaving college and undertaking small jobs working in the real world, you need a little guidance from forums like this one, sometimes fresh out of college sparks will ask stupid questions, like asking for the wrong thing at the suppliers can be embarrassing, but i can take as i want to learn. So if my conclusions are wrong please point me in the right dirrection as you have on many occassion, unfortunately when working to complete my nvq 2356 the sparks i worked with never explained nothing and done a lot of things that i know now were wrong. For example neither of the sparks i worked with would touch a storage heater, so when someone asked me to have a look at a storage heater i was little out of my depth, however prodaves explanations on how to repair storage heaters were spot on taking me back to basics from what i learned at college. i have since fixed 6 storage heaters, from what i have learned on here.

Cheers

 

[Doc Hudson]

I have a TT system on my own place, outbuildings etc so am understandably interested in all things TT. However, whenever it came up pretty much every lecturer would say something on the lines of "Oh, that's mainly seen in rural properties!", virtually skip it and it was hardly touched upon. Any time TT came up I would ask the most questions and it became almost a joke when I did. TN-S and TN-C-S got much, much more time devoted to them.

That is a sign of how poor the quality of teaching is for "electricians". Are these courses advertised as inner city small domestic no TT partial training courses. If what you describe is common with training across the country, it is no wonder that candidates come out thinking they are qualified yet soon struggle because some of the essential basics have been omitted from the course. TT earth arrangements are not purely a rural thing. In fact is does appear that earthing in general and the electrical principals and science relating to it are not covered much at all. But it is only one of the most important safety measures we apply to our electrical installation in this country.

Doc H.

---------- Post Auto-Merged at 10:14 ---------- Previous post was made at 10:01 ----------

, i just misunderstood after having been taught at college that 30ma rcd's were essential on tt systems in domestic properties. Cheers

I would guess that part of the "training" now assumes you are only working on new domestic installs where 30ma protection is required for most buried domestic cables. Which is why a blanket statement of 30ma RCD for everything comes along. But installations installed to earlier versions of BS7671 did not have this general coverall 30ma RCD concept. Older domestic TT installations typically had 30ma for sockets and external work, but a 100ma time delayed front end protecting other circuits. Also there were longer disconnection times (5s) permitted on lighting circuits which had implications on permitted methods of protection. Don't forget that a 100ma & 30ma RCD in series would not offer any discrimination unless the 100ma is time delayed. Unfortunately the training providers are just wanting to fill you with enough information to pass a few regulations exams, which doesn't help people wanting to get a broad understanding of real world electrical installations.

Doc H.

 

[Ash]

Why is 30mA used in domestic?

because 30mA+ is what will kill a human

Why would you need "human killing" current protection for fire prevention of a structure?

 

[apprentice87]

Hi there!!

Firstly, the regs are NOT retrospective, so there is nothing I am aware of, that says you have to have an RCD at all, no matter what the system.

Secondly, everyone says [including the regs] that "an RCD is used [or to be used] on TT systems for fire protection" and yessssss, it is, as, in the event of a phase/neutral fault, normally enough current would flow to operate the relevant OCPD, but, owing to high earth loop impedance on a TT system, in the event of an earth fault, usually, not enough current will flow to operate any such device, so in the event of an earth fault, yes, there very well could be a fire.

HOWEVER, I would say that, that is NOT its primary use.....

Looking at things from a more technical viewpoint, I would say that on a TT system, the PRIMARY use of an RCD is to achieve required disconnection times..... [which will of course protect against fire too!!]

There is of course the requirement that sockets etc etc etc are protected with a 30mA device, so that in addition to achieving the required disconnection times for the circuits concerned, the severity of any shock that may be received, is limited in terms of current, to a much lower level than it would be with say, a 100mA device.

Finally, as others have said, there is NOT NOT NOT anything that says you cannot have a metalclad CU, DB or any other accessory on a TT system.

What the regs do say, [and this is not word for word, as i cannot be bothered to go and look it up!!!!] is that, "where an installation [or part of presumably] is protected by a single RCD, that RCD is to be located at the origin of the installation" [and if not, or in any case] "that all parts of the installation UP TO THE POINT WHERE THE RCD IS INSTALLED shall be of class 2 construction or the equivalent"

Soooo, yes you CAN have a metal enclosure, but the cables entering it need to pass through an insulated gland, or whatever other means that you can devise, to protect against the possibility of a fault to earth occuring. [The conductors touching the metal enclosure!!] thereby making the enclosure [and possibly the entire installation] live......

john..

 

[NozSpark]

and also the tails although not perfect as they are neither glanded or protected up front by an rcd device they are double insulated. so a risk assesment would indicate to me that no danger exists.

I would say that this would be a C2 if you were doing a PIR and therefore should be glanded in properly. Saying that I would hope that there is at least some grommet strip around where the cables enter, that still wouldn't be right though

....and BTW, tails are not normally double insulated,,, they are insulated and sheathed (big difference)

 

[ADS]

Hi Soulman,

Apprentice87 has given you the most useful info so far - not all correct, but nearly there.

I think your confusing RCD requirements for 'additional protection' (i.e. 30mA) with RCD requirements on systems with high EFLI (e.g. TT) for earth fault protection.

RCDs used for 'earth fault protection' would be selected dependant on the EFLI of the circuit - if the circuit also required 'additional protection', then you would opt for a 30mA anyway.

In the case of an up-front RCD protecting the whole of a TT installation, you would select the rating of the RCD based on your Ra (or Ze) - ensuring that you comply with 411.5.3 (i) & (ii) and the disconnection times given in table 41.1.

If the installation contained 30mA RCDs for additional protection, then the up front RCD would need to be time-delayed.

If the CU was metal, then the up-front RCD would have to come within the tails feeding the CU - or the alternative steps (described earlier) be taken.

I hope that's a help - I think it's accurate..............but don't worry, they're quick to jump down my throat if I get something wrong, so you'll soon know.

 

[steptoe]

now now son,

stop teasing.

 

[ADS]

Sorry, Soulman, I might be wrong with one of those statements.

If my thinking is correct, if the up-front RCD was to offer 'earth fault protection' to the whole installation - i.e. the final circuits - then it's selection would have to be based on the worse-case Zs and compliance with table 41.1 would apply.

If it was only offering fault protection to the tails, then a 1 second disconnection time would be allowed and could be selected based on Ze.

I think that's right.....we'll soon find out.

If your wondering why I haven't responded to canoeboy, he's still on my ignore list.

 

[steptoe]

^^