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Posted By Topic: Electrical Distribution board

superman22
Jul 25 2016 09:05

Why Distribution board supply from MEN main switchboard has no link between neutral and earth bar
   

Sarmajor
Jul 25 2016 10:53

Because of this clause of AS/NZS3000:2007

Section 8.3.8.1(a)
   

mf51to1
Jul 25 2016 18:33

Why did they scrap the linked busbar switchboards?

Many moons ago when I studied as an apprentice, we learnt that removing a link in the linked busbar switchboard could cause dangers - typically potential rise between N and any bonded metal, protection might not operate due to a poor EFLI....how is it different from a DB? My old boss used to rant on about how DB\'s should in theory have a link.

Is it simply to prevent current flowing in parallel Neutral and, more importantly, Earth conductors?
   

ShaneR
Jul 25 2016 19:10

I always thought it was because of the introduction of RCD\'s?
   

AlecK
Jul 26 2016 09:41

Correct an existing link in a DB - such as the old \"linked busbar switchboard\" from \'97 Regs, or any DB from earlier when all swbds ad links - should not be carelessly or unthinkingly removed.
For a variety of reasons including possible poor EFLI (though difference in potential between N and earthed metal is not one of them).
And especially where the incoming PEC is connected to the N (as in old caravans and some other DBs) where removing link results in complete loss of earthing functions.


The primary reason for not putting links in new DBs is to maintain the integrity of the earthing system.

Simplified:
Ns are generally sized equal to As (can be reduced in some cases where N current can be shown to be significantly lower).
Es are generally sized at half the A size.
That\'s permitted because when an E carries a fault current, its temp rise starts from ambient (compared with a N, which starts from anything up to and including fully loaded and therefore could be running at max temp already).

There\'s also testing, where having multiple links makes it difficult (bordering impossible) to test the insulation resistance of the live wiring(including Ns).

ONE point of connection does all that\'s required, and allows for testing, and ensures normal load current is not flowing on the PECs.

And when you\'re installing a new DB, you will know that the EFLI is good (else your installation work is non-compliant)


This has NOTHING to do with RCDs.
   

EricB
Aug 02 2016 20:14

I know from working in TV many years ago that we would never had any Television service in NZ, if the full use of the Regs before 1960 were applied. All NZBC TV Stations (today TVNZ) were given exception for the required link between the N&E.
If the link connection was enforced, then we would have seen all sorts of lines over the pitcures. Sometimes our rules and regulations mean we are left behind the rest of the developed world.
   

pluto
Aug 02 2016 20:51

EricB Aug 02 2016 20:14
Your comments
I know from working in TV many years ago that we would never had any Television service in NZ, if the full use of the Regs before 1960 were applied. All NZBC TV Stations (today TVNZ) were given exception for the required link between the N&E.
If the link connection was enforced, then we would have seen all sorts of lines over the pictures. Sometimes our rules and regulations mean we are left behind the rest of the developed world.

My comments
This is one of the many reasons that the linked switchboard was removed from the regulation book it also cause havoc in some computer suites for the same reasons the TVNZ could not use linked switchboards Telecom New Zealand was another organization which had problems with linked switchboards.

   

BrianW
Aug 03 2016 21:01

Wow EricB, you must be really old. I can remember watching Dr Findlays Casebook around 1963 on a brand spanking new PYE Black and White TV that took 3 days to warm up and had that many lines, and rolling pictures, that a few more wouldnt have been noticed.
   

dbuckley
Aug 04 2016 11:40

EricB notes:

> I know from working in TV many years ago ... All NZBC TV Stations (today TVNZ)
> were given exception for the required link between the N&E. If the link connection was
> enforced, then we would have seen all sorts of lines over the pitcures.

That\'s interesting, as the problems of ground loop currents flowing in the screens of unbalanced video cables was a well known issue, and still is.

In some countries the TV frame refresh was synchronised to the mains, so as to prevent rolling bars on the picture from video cable ground currents, as well as from lousily built TV receiver power supplies; was that not the case here?