Hi All,Thanks for that!!! Yes, agreed, RCD's are not the best!!! I looked at someones installation recently, [A three phase TT'ed supply] in a factory, and sure enough, an RCD at the origin, and sure enough, it did not work!!! Probably never even had its button pressed, never mind tested properly.
There may be a reason other than that of the RCD as to why it wont work, especially in a factory.
Why TT?? ; Because they were at one time, all separate units in the one tin shed with lots of supplies.
What sort of tin sheds are we talking about?
Where you have a "multiple occupancy" tin shed, the DNO will only PME one of them, if any....
Are these tin shed occupancy's?
Anyway, Zs at the DB's of the TT supply was very low though, as a result of being bonded to the same tin shed as the "other" 200A PME supply!!!! So all OCPD's would operate ok, just not cut off the neutral..... Still, i suppose the TT supply, is now a "TNCS" by default, just using the other supplies neutral/earth!!!
How and why are these tin shed bonded? It will always be a TT.
Anyway, the isolator for this TT supply, has the name of another, different firm on it, so you ignore that one, switch off the "right" isolator, [with the correct firms name on it], open a DB, stick your hand in and get killed!!!! Very recommended.!!!! [moral; check dead properly..]
I have told them that this is dangerous, [the whole system is dangerous!!!] and, as all the TT system operates is one welder, one small saw and a light bulb, i am going to disconnect the TT system altogether for them, and put it all on the other 200A supply. Save them paying two standing charges too!!
Anyway, back to the RCD's
As Zeespark says, i know there are various "views" as to whether on a TT system, a "protective device" has to disconnect all live conductors in the event of a fault [in the way that an isolator on such a system must] but what if you have the DP disconnection "facility" for want of a better term, provided by the RCD used as an incomer in the CU [lets hope it works!!] but just a single pole RCBO in series with it as "backup" ??? Bit of a "half measure" perhaps??
ADS only requires disconnection of the line, it depend on the designer as to its other purposes if any.
Agreed, without question that a DP one [RCBO in series with the RCD] would be a better job...
Again it depend upon what you need to achieve, level of discrimination, protection against fire
I know that a lot of people get all worried about TNCS supplies, and issues connected with them [we all know what they are] and would rather TT things, but it scares me a bit to be honest..... At least with TNCS an earth fault will disconnect everything very quickly, but with TT, if the RCD does not operate, a fault to earth will "liven up" every exposed conductive part in the entire installation. What is the most likely?? A DNO neutral fault, or an RCD fault??? I know which one i would bet on....
Well many RCD faults on commercial are to do with the incorrect RCD for that application.
Something i am working on at the moment, would, in my opinion, be MUCH better PME'd, but i am going to TT it all [lots of outbuildings in a field] purely as it is a more "conventional" way of doing things, hence the idea of RCD's in series..
Well your DB circuit ADS can be met by a TD RCD, so why not have discrimination too, have the TD as your back up
When it all goes wrong, much better to be "conventional" than "controversial" M'lud !!!!
Since you are all wondering, i will describe the layout, and you can all comment on what you think..
Right, Incoming supply is a three phase PME one. It, and the meter, are housed in a remote brick "Meter house" I am not going to use the DNO earth terminal.
And what are your reasons as to why your not using the PME?
Leading from this, is a 70 Metre submain that feeds two separate DB's in two, separate, externally mounted, metal, IP rated enclosures.
Back at the origin, immediately after the meter, will be a four pole RCD in a plastic enclosure. This will double as a four pole isolator too.
What Type are you suggesting?
Next to this, will be a three pole MCCB to protect the submain. I know it is a requirement to be able to disconnect the neutral, and in the past i have used a "neutral link" to achieve this, mounted next to the MCCB, but, in this case, as the RCD/isolator disconnects the neutral 6 inches away anyway, i do not think this would be neccessary.
The submain, [and in turn the metal enclosures and the other SWA cables leading from them] will all be TT'ed back at the origin. [Where the RCD is]
I have a great unease about this, because, in the event of an earth fault in the submain and a fault in the RCD, the enclosures will be live......
I would have preferred to TT the submain, BUT INSULATE the incoming SWA from the enclosures, and then fit rods adjacent to them, and fit the DB's with RCD incomers. Idea being, that even if the submain were damaged, and the RCD at the origin DID fail to operate, the enclosures would remain "dead"
Did you not say the submain was TT? Why not leave it PME?
BUT, what put me off this idea, is that sometime, some "know it all" is going to come along and say;
"Ah, the submain is protected from the origin, the metal enclosures have their own RCD and rod, BUT, if there was an earth fault in the few inches "jump" from the end of the submain to the incomer, the enclosure would be live, as the RCD cannot see faults before it"
Whats wrong with utilizing section 412?
Quite correct, but less chance of a fault in 6 inches of cable going through an insulating gland and then surrounded in a bit of plastic trunking entering into the enclosure than a fault in 70 Meters of submain. But there we are..
So, forgetting that idea, the submain will feed "ordinary" four pole incomers in the two DB's
The DB's will then feed [by means of more SWA] 5 little out buildings. What i am going to do, is to feed these with the SWA, but into PLASTIC consumer units, each with their own rod and RCD. [and maybe RCBO too]
Idea here is, to prevent an earth fault in one outbuilding [coupled with an RCD failure at the origin] transmitting itself to the other outbuildings, in the way that it would if they were all SWA'ed together, with just the one RCD at the origin.
In other words, a fault in one building, will not transmit itself to other parts of the installation, but will remain "in" that building, and not shock people that might be touching exposed parts in any of the other buildings....
Comments please!!!!!
john..
