IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Disconnecting both poles in a TT installation.
Topic Summary: A old discussion rehashed.
Created On: 04 February 2013 09:01 AM
Status: Post and Reply
Linear : Threading : Single : Branch
<< 1 2 3 4 Previous Last unread
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 05 February 2013 08:32 PM
User is online View Users Profile Print this message



daveparry1

Posts: 6162
Joined: 04 July 2007

That's true!
 05 February 2013 10:32 PM
User is online View Users Profile Print this message



AJJewsbury

Posts: 11372
Joined: 13 August 2003

Originally posted by: alancapon

So, after changing the subject completely to PME, the original question was "do we need to disconnect both poles of a TT installation automatically". Discuss.



Regards,



Alan.

I suspect the answers are:
Is it a good idea? - yes.

Does BS 7671 require it? - no.

Discuss? OK - how about this for starters...

I suspect it boils down to: how dangerous is it to have N at a significantly higher voltage than E for a period of time?

I guess these days - with appliances designed for EU-wide markets and so safe for use with unpolarised plugs, unearthed N etc, a truely "live" N shouldn't be an issue.

BC lampholders aren't polarised anyway and modern ES lampholders have a recessed ring contact, so can safely be connected either way around too.

The other side of the effect - having L at say only 115V above true earth will right royally mess up ADS fault currents (e.g. half them), but as in practice sensitive RCDs are used and rod will be way below the 7666 Ohms operational limit anyway, the actual danger of ADS failing to operate doesn't seem significant. After disconnection the N would still be at an elevated voltage, which (depending on the nature of the fault) could be transmitted onto exposed-conductive-parts, but provided bonding is in place, the shock hazard inside buildings should be minimal (if it's a true TT system with the MET having a significant resistance to true earth then there bonding should be able to maintain a reasonable approximation to an equipotential zone). Outside might be different of course (but perhaps similar to exported PME earth used for garden appliances and outdoor lights?).

Having DP RCCB rather than SP RCBOs doesn't of itself guard against having a faulty RCD, so the risk of originating an undisconnected L-PE fault remains.

Personally I do like the idea of a 100mA S type backing up DP 30mA RCBOs - on the basis of being a good neighbour as well as providing a backup for (inherently?) unreliable single RCDs - but obviously not the cheapest solution.

- Andy.
 05 February 2013 11:32 PM
User is offline View Users Profile Print this message


Avatar for ebee.
ebee

Posts: 5673
Joined: 02 December 2004

"Personally I do like the idea of a 100mA S type backing up DP 30mA RCBOs - on the basis of being a good neighbour as well as providing a backup for (inherently?) unreliable single RCDs - but obviously not the cheapest solution. "

Well said that man!

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
IET » Wiring and the regulations » Disconnecting both poles in a TT installation.

<< 1 2 3 4 Previous Last unread
Topic Tools Topic Tools
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.