IET
Decrease font size
Increase font size
Topic Title: Neutral bar
Topic Summary:
Created On: 03 January 2018 11:26 AM
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.
 03 January 2018 11:26 AM
User is offline View Users Profile Print this message



leepeth

Posts: 138
Joined: 30 March 2005

Good morning all,

It has been many a moon since I last posted on here, but I am looking for some advice from all you very knowledgeable fellas.

It concerns the neutral bar being a live conductor, and whether this needs to be isolated before terminating to it. My thoughts are that in a TN system it is securely connected to earth, and therefore does not need to be isolated, but certain other people in my company who pull more weight than I just read the EAWR as neutral is a live conductor and therefore it would be classed as working live if connecting onto it.

In addition, would any of you be aware of a distribution board that could be terminated into without having to isolate the entire board? With a suitable method statement of course....

Thanks in advance!
 03 January 2018 12:26 PM
User is offline View Users Profile Print this message



ArduinoXR

Posts: 92
Joined: 16 August 2017

If it's the RCD neutral bar just knock it off and it's isolated. If it's the main neutral bar I would think you need to open the switch fuse. if there is a break in the neutral further upstream the potential between it and ground could be up near line voltage, obviously this is extremely unlikely but better safe than sorry in my humble opinion. I would consider it needlessly risky.
 03 January 2018 12:37 PM
User is offline View Users Profile Print this message



alancapon

Posts: 7121
Joined: 27 December 2005

I was caught out by a neutral bar in a commercial property many years back. I had opened the double pole switch fuse supplying the board, and proved all connections dead. During the course of my work, a time clock operated to switch external lighting on. The timeclock tried to switched a contactor, which had its neutral connected into my (supposedly isolated) board, making everything including the neutral bar live. . . It was a bit of a surprise.

I guess what I am saying, is that if you do isolate the neutral from the incoming supply, how do you know it won't come live during the work?


Regards,

Alan.
 03 January 2018 12:51 PM
User is online View Users Profile Print this message



broadgage

Posts: 2683
Joined: 07 August 2007

My personal view is that the neutral bar should be isolated before performing any extensive work that could result in a dangerous shock.

However for simply terminating an extra conductor in a vacant terminal, I would probably not bother., presuming the use of an insulated screwdriver.
The chances of the neutral bar unexpectedly becoming live during the few seconds that it takes to make the connection is minute.
And even if it DOES become live, the chances of any serious consequences are even more minute.
 03 January 2018 04:46 PM
User is offline View Users Profile Print this message



mapj1

Posts: 10414
Joined: 22 July 2004

Neutral is not really as 'live' as the phases, despite international pressure to declare it so. In many ways it is 'nearly earthed'
Certainly in a TNC-s supplied system it ought to be at the same potential as the earth, and generally we don't disconnect that.
So personally I'd isolate if it's easy, like a DP or 4p switch, but not too worried if it stays attached, and not to bother if isolation would require a lot of unbolting to do, which is in itself introducing more work and new risks. I'd be a bit nervous about isolating neutral bar unless I was quite sure all of the phases were isolated first and remade after neutral was reconnected- if we were paranoid we could always short it to earth before starting work, but I don't know of anyone who recommends that sort of precaution

I suspect the official modern answer ought to be 'treat as live, and isolate' ; while the more pragmatic answer is just check its not live enough to register on the tester and then get on with it. Though it is more than a slack handful of volts off earth then stop and find out why.

-------------------------
regards Mike
 03 January 2018 06:01 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 16619
Joined: 13 August 2003

To my mind, operating an appropriate isolating device provides adequate isolation - and reg 537.1.2 makes it quite clear that for TN-S and TN-C-S supplies with a suitably low impedance to earth (e.g. complying with the ESQCR) that isolating N isn't required.
- Andy.
 04 January 2018 08:55 AM
User is offline View Users Profile Print this message



aburgar

Posts: 31
Joined: 11 March 2008

I hate it when I read these discussions and everyone adds their own two pennies worth. Anyway without wanting to teach anyone to suck eggs, as described above ideally the neutral does need to be isolated, however in practice it is normally considered as connected to earth, of course it can become live if not on on a low impedance connection and there is a fault or unbalanced three phase load eleswhere on the transformer load. There is always the consideration of local temporary earthing adjacent to the point of work, guidance on legal status of this are given in HSG85 paragraphs 57 and 58.

Of course you need to be really careful in the sequence of testing that these are removed prior to energisation otherwise the danger of arc flash is increased.

-------------------------
Andrew Burgar BSc CEng MIET
 04 January 2018 12:21 PM
User is offline View Users Profile Print this message



Parsley

Posts: 1363
Joined: 04 November 2004

I think what's is actually being asked is, can a new circuit be added to the DB without switching off the DB. Rather than the DB is switched off via the incoming 3P switch-disconnector and then removing the incoming neutral link.

If your writing your own RA you may be satisfied the risk from the neutral bar is acceptable and that the adjacent live conductors don't pose a risk and you carry on without isolating the DB. If your writing the RA for your team you may not be as comfortable. What do the manufacturer's instructions recommend?

BEAMA's forms of separation guide states if your going to be adding circuits when adjacent circuits are live then a form 4 assembly should be used.

Schneider powerpact panel board info below.

Degree of protection IP3X Form 3b type 2 As standard
Form 4 type 2 & 6 Can be achieved by use of individual disconnectable neutral links adjacent to breakers or by the use of 4 pole breakers . Outgoing terminals should be shrouded with long terminal shields . The main neutral bar either side of the incomer should be removed and discarded together with the connecting copper bar . The incoming breaker should be a 4 pole breaker.

Regards
 05 January 2018 12:47 PM
User is offline View Users Profile Print this message



ArduinoXR

Posts: 92
Joined: 16 August 2017

Long story cut short here is the safety officer won't sign off on any method statement which states somebody will be working on a live board.
 05 January 2018 09:49 PM
User is offline View Users Profile Print this message



mapj1

Posts: 10414
Joined: 22 July 2004

We do allow live working indoors for setting up or debugging faulty equipment where it is unavoidable , but no lone working, two persons present, and they have to agree (as a risk assessment ) that what is to be done is both sensible and necessary. Of course live working on something on the load side of 13A fuse or 16A ADS protection is likely to blow a rather smaller chunk out of the end of your screwdriver, compared to a box with busbars and a sub main, so some thought is needed about the degree of enclosure/ exposure that is acceptable depending on how much of what is exposed. An NE short is likely to be far less of a fireworks display than an LE or LN one.

-------------------------
regards Mike
Statistics

New here?


See Also:



FuseTalk Standard Edition v3.2 - © 1999-2018 FuseTalk Inc. All rights reserved.

 
..