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??????

 

[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?

 

[domestic spark]

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?

very kind of you to ask! top man! and yes, ive been looking into exporting, earthing and bonding alot, and sat night looking on the web, just confused me!! its full or untrue rumours! engineer54 once again came to the rescue with his post and cleared it up and set me back on track, so thankyou!

 

[mart1711]

Im having a lazy sunday and have ended up reading countless threads on exporting PME!!! Never knew it was such a hot topic!!! To add my two pennies worth i would always run a PME supply to an outbuilding if possible. If in doubt use your books to confirm the cables/volt drop will be ok etc but invariably they check out fine.. I am always suprised at the relative csa of swa armour, always always more than you would think!!