Someone clear this up asap!

[domestic spark]

"Generally speaking the exportation of a TN-C-S supply is the decision of the local DNO and only done with their permission. In the absence of such permission it would be recommended to use a TT system and RCD protection. However most DNO's will allow such exportation to an outbuilding provided it is 2 meters or less in distance from the main
building.

I advise that you check with your local DNO as to their requirements."







2 meters or less in distance from the main
building.

OPEN TO CORRECTIONS BUT, this is stating that, if you want wiring done in garage, conversion etc further than 2 meters from your house, it has to be a TT system??????

 

[Deleted member 9648]

Quite right Sintra....I guess there is a case for having an upfront 100ma T/D RCD on TT's to allow for this scenario.
Generally though a TT is used where the DNO dont provide a means of earthing.There's not really much that can be done in that case to provide a low resistance earth fault path.

 

[Sintra]

It has all come down to sticking RCD's onto circuits which is all fine and dandy when they work, but they are never tested regularly once fitted. Gone are the days of bringing the Ra down by adding additional electrodes, so long as the 200 ohm figure is met.

Again the omittance of supplementary bonding in bathrooms just because there is an RCD or RCD's on the circuits of the location but who knows when they will fail. The only way that these devices could be relied upon is if they were manufactured to be FAILSAFE.

 

[somersetsparks]

Our lot (DNO) would insist the size of the earth is the same as that of the supply earth if pme is exported - i.e. even if you only took a 4mm 32a supply out you would have to have a 16mm2 earth, obviously not to much of a problem for the domestic lot, but when you have a 70mm2 main earth can become impractical!

Forgot to say as far as im aware (in our area) 2m is a myth.

 

[Guest123]

tested secondry board where i ran i to and got a reading of 36ohms on ZE. ???????????????????????????????????

thats not good enough? now what?

max is 0.35 right!

An installation has ONLY ONE Ze, which is obtained at THE ORIGIN.

What you measured there was the Zs of the distribution circuit or Zdb as it is also known.

The 0.35 max is for the Ze measurement at the origin and has no bearing on the result you got.

 

[Engineer54]

Still confused here, what is the current Bonding cable size for a PME domestic installation, 16mm or 10mm??

 

[Sintra]

Table 54.8

 

[Engineer54]

Afraid your numbers are no good to me here, still waiting for my copy of the 17th to arrive, .....and that seems to be already out of date before i get it !! haha!!!

Been working for the past 4.5 years with NEC and DIN standards, coupled with the 16th edition...

 

[tony mc]

Neutral conductor up to 35mm bonding 10mm

 

[Engineer54]

Thanks Tony..... i've just been searching myself on the web, and come up with the same result...

 

[Elidor]

Throwing a spanner in the works here but an RCD is additional protection only right? Primary protection must come from the main circuit protection device which if exported to a TT scenario must now have a disconnection time of 0.2 seconds?

 

[telectrix]

depends on what you are using for the " main circuit protection device".

 

[malcolmsanford]

Look at table 41.1 note 2 of the Regs. Then go to appendix 3 and look there. That is why a 30mA RCD complies with the 0.2 second rule on a TT circuit that is less than 32 amps.

It's all down the IdN x2 times.

 

[telectrix]

but what elidor seems to be saying is that you should achieve 0.2 disconnection without reliance on RCD. with TT systems this is impossible

 

[malcolmsanford]

To be honest reread it twice now and it's got me a little fuddled, I thought he was aking how a RCD conforms to the 0.2 disconnection time as IdN x1 is 0.3 seconds. But at IdN x2 it is 0.15 seconds which brings it into compliance ...............

 

[Elidor]

Im very new to this trade so i wouldnt take anything i say as reliable information hehe, I was following the thread with interest and thought it was implying that RCD protection alone was a suitable circuit protection device. I was under the impresion that any electrical circuit (TN or TT) must provide fault protection by automatic disconnection of the supply within the 0.4s for TN and 0.2s for TT for all circuits up to and inclusing 32A. As the RCD offers no protection for overcurrent or short circuit protection between live conductors so it would still be necessary to provide conventional circuit protection with disconnection times to table 41.1 (P46). If the disconnection times cant be met then I was told the circuit must not be installed. Is this correct?

I'm new to the trade so please go easy on me hehe, im another product of the intensive training programs designed specificaly to confuse the hell out of everyone who signs up lol.

 

[Sintra]

If You have a look at the graphs on pages 244 - 251 it shows the characteristics for the different forms of overcurrent protective devices. These have to be installed in all system types for short circuit and overcurrent protection.

Eg a BS 60898 32A type B MCB will trip with an overcurrent of 160A in 0.1 seconds and will therfore comply to both a TN 0.4 & a TT 0.2.

 

[Elidor]

The original post was regarding an outbuilding where it was suggested that it may be necessary to provide its own earth electrode as it may not be permitted to share the PME from the original supply. In either case if the disconnection times cant be met then is it forbiden to install the circuit? or are there any other ways of installing the circuit and still be complying with the regulations?

 

[malcolmsanford]

The disconnection time of a protection device is determined by the Zs of that circuit. Not sure how far you have gone in your training with this but the Zs is the maximum value of resistance a circuit can have to allow that protection device to function. So what it is saying, say we take a BS 60898 type B 6amp MCB, for that device to trip in 0.1-5 seconds it will take 30amps to trip it. that is fig 3.4 in the regs page 249. To allow that amperage to be achieved in a circuit you go to table 41.3 on page 49 and it tells you that the maximum ohmeric value the Zs is 7.67 ohm. again I don't want to bog you down with rule of thumbs and such so let's say your circuit when you test it must be below 7.67 ohms for that 30amps to realised on that circuit to trip that device.

Now if as your saying you can't obtain that 7.67 ohm for whatever reason there are several ways that can be over come.

1. increase the size of your cable or run a seperate CPC that will help you achieve that Zs
2. Another is supplementry bonding as you do in bathrooms this will help as well
3 RCD protection.

There should never be a situation where you are forbidden to install a circuit becasue of a high Zs value, there are always options to achieve your disconnection times

 

[Deleted member 26818]

The original post was regarding an outbuilding where it was suggested that it may be necessary to provide its own earth electrode as it may not be permitted to share the PME from the original supply. In either case if the disconnection times cant be met then is it forbiden to install the circuit? or are there any other ways of installing the circuit and still be complying with the regulations?

It would be very rare to find a case where disconnection times cannot be met, especially at the design stage.
More likely where the earthing sytem on an existing installation has been changed, or it is found after installation that Ze is higher than recommended.
In such instances, supplementary bonding would be required.

 

[Elidor]

Thanks for the information, appologies if i have steared the thread off track a little. Does the thread starter have the informaiton he originaly requested?