IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Position of devices for overload protection
Topic Summary: Position of switches
Created On: 11 August 2015 05:38 PM
Status: Read Only
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 11 August 2015 05:38 PM
User is offline View Users Profile Print this message



20did99

Posts: 12
Joined: 24 June 2012

Hi all,

I am looking for some advice with the above regulation (433.2).

I am sure we have all heard of the 3m rule when it comes to overload protection. But I have some issues, let me give you an example.

Busbar system being supplied with 95mm 200a supply. From the BB there is DB being supplied with 25mm 6491X cable and there is no overload protection. Regulation 433.2.2 states that overload protection can be installed upstream depending if it is within 3m of where the reduction in current capacity of the cable has occurred.

So to rectify this I would change the main switch of the DB from a BS 60947-3 to a BS 60898. Problem solved.

But here comes my issue - (and the position of the protective device has neither branch circuits nor outlets for current using equipment). Surely a DB has branch circuits, or do they mean something else?
 11 August 2015 05:47 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 15838
Joined: 13 August 2003

But here comes my issue - (and the position of the protective device has neither branch circuits nor outlets for current using equipment). Surely a DB has branch circuits, or do they mean something else?

They mean no branches before the overload device - i.e. so the device 'sees' all the current that flows through the conductor it's protecting.

There is a school of thought that says if the sum total of the ratings of the outgoing devices in a DB is less than the cable's rating then the supply cable is still protected from overload - even though that's not spelled out in the words of the regs.

BTW - the 3m rule isn't really to do with overload - 4332.2 (ii) is just re-iterating the fault protection requirements. If the upstream devices provide adequate fault protection then the overload can be any distance downstream from the reduction is c.s.a. etc.

- Andy.
 11 August 2015 05:50 PM
User is offline View Users Profile Print this message



20did99

Posts: 12
Joined: 24 June 2012

Thanks for clearing that up, that makes perfect sense now.
 11 August 2015 07:12 PM
User is offline View Users Profile Print this message



johnboy6083

Posts: 54
Joined: 25 July 2008

If the primary supply to the busbar chamber is a 200a MCCB, has it been turned down? If not, then I would. Usually it's an Io or Ir setting. Check the MCCB instructions if you're unsure.

If you want to fully utilise the 200a supply, maybe by taking another feed from the chamber, then I would install an MCCB to the incoming of the DB, or if the DB is rated at 200A +, upgrade the tails size.
Statistics

New here?


See Also:



FuseTalk Standard Edition v3.2 - © 1999-2017 FuseTalk Inc. All rights reserved.

..