Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in

Switching neutral in 120v control

Status
Not open for further replies.
electromike

electromike

Electrical
May 22, 2003
28
0
0
US
Does anyone have experience with switching the neutral instead of the "Hot" conductor in control circuits except in motor starter buckets. I have encountered this practice on a DCS system in a chemical manufacturing facility.I am told this is a safer practice in that all conductors in the field are already a grounded conductor and any shorts to ground will only switch "on" the respective circuit. I would like some input on this concept if anyone knows some specifics. Thanks.
 
Replies continue below

Recommended for you

Sort by date Sort by votes
I have seen this practice occasionally in 24V DC control circuits (common positive rail with switched negative) but never on AC. Switching the neutral without switching the associated line using a multipole switching device would be a code violation in most locations.

The idea that switching on a circuit due to an earth fault is somehow safe is intriguing although in a system designed to fail 'safe' it is likely to be flawed.

When the switch is open there will be line voltage across the contacts, so it is foolish to regard it as a grounded conductor when in this state. It is very much live, and although the available current may be limited by the impedance of the contactor coil there will likely be enough current available to cause a fire or injury.

Where have you seen this scheme?


----------------------------------
image.php

If we learn from our mistakes I'm getting a great education!
 
Upvote 0 Downvote
In the United States. The two facilites are Japanese owned and we have had some isues with differing approaches as far a technical issues go(Code be damned). We do use some positive-common DC and I think this is where some inexperienced person who didnt understand the difference with AC and DC introduced the switching the neutral in AC control idea to the inhouse technicians thinking it the same as the positive-common DC.
 
Upvote 0 Downvote
Once upon a time, there were cars with positive ground and switches on both sides of devices. The thinking back in those days was mostly "make/break the current" and no one seemed to bother about common plus or minus. In effect - I don't think there was a 'common' at all during electricity's first decenniums.

The only application in relatively new AC control circuits where I see connection to neutral opened is where thermal protection sometimes is connected between 'lower coil' and neutral. But don't think that is done any more.

Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
Upvote 0 Downvote
Motor control circuits are a special case. When the neutral is switched by an overload relay, the neutral circuit is contained in the starter enclosure and the starter is pre-wired and as such is subject to different codes.
Switching a neutral is generally forbidden by the NEC unless the hot conductor is opened simultaneously.
I am sure that this practice will make trouble shooting more challenging.
Note that the intended operation of our grounding systems is to trip the supply breaker in the event of a ground fault, not unexpectedly start machinery.
You probably have serious safety violations with this circuit as well as code violations.
If management were aware of the possible legal and liability issues, they may over-ride the "Away from home" experts.
Despite that, be very careful approaching management. Such an action may be a career ending event in some workplace cultures.
s

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
Upvote 0 Downvote
GE: "...neutral opened is where thermal protection sometimes is connected between 'lower coil' and neutral. ..."

The over-temp 'fuse' that was added (several years ago, part of a safety update) to my home's ventilation system was installed on the Neutral side of the motor. I thought it was a strange choice. For every rationale justifying that choice, it seems that there's probably two rationales for the Hot side.

 
Upvote 0 Downvote
waross, you are right about the troubleshooting being challenging. It was a troubleshooting situation that an electrical contractor was working on, with dificulty, that brought the fact that there are 120ac control ciricuits that are switched on the neutral to my attention. To my supprise our inhouse electricians(Texas Journymen Licence holders)pointe out that switching the neutral was "better" and that was how they do it now. The practice will no longer occur in these facilites.
 
Upvote 0 Downvote
I am a Structural - so my two cents is worth less than 2 cents....

But I have done a ton of house wiring - usually without turning off the power... too big a hassle to find the right breaker, etc, etc

Anyway - imagine replacing a fan or light. Turn off at the swithc and you should be good to go. When somebody (usually my Dad) switched the neutral side I would grab the "black" wire thinking it was "dead". Well - I got knocked on my butt more than once!!
 
Upvote 0 Downvote
Mike,

Our house's previous owner was a fan of switching the neutral line... I thought I was safe killing the breaker (always do that anyways). Then I found some of his work where two circuits (i.e., two separate breakers) connected together through some odd patchwork of wiring in the attic. Now I have to trace everything back to the panel to make sure that breaker kills it completely.

Dan - Owner
Footwell%20Animation%20Tiny.gif
 
Upvote 0 Downvote
Yeah - fianlly went out and bought one of those sensor/tester things - about the size of a large fat pencil. Sells for about $10 at Radio Shack or Lowes. All you have to do is touch it near a suspected live wire. IF it beeps - its alive.

Unfortunately - when it doesn't beep - that does not guarantee its a dead wire. Learned that the hard way!!
 
Upvote 0 Downvote
While I don't do house wiring, I believe switching the neutral was the standard in the USA. The breaker is on the hot side, and the switch is on the neutral side (that's why you always shut off the breaker before starting service). This is supposed to keep it safer for the user; if the switch fails they are touching the neutral side of the load instead of the hot side.

I wire my industrial enclosures the same way and I've never seen the standards contradict this... I'll review the standards that I have tomorrow and see if I can post a definition.

Z
 
Upvote 0 Downvote
Zapped,

Please tell me how to identify your enclosures. No offence but I never want one of them on our site - non-standard wiring practices which don't meet our national regulations just aren't worth the trouble, however good they otherwise are. Sorry. [sadeyes]




----------------------------------
image.php

If we learn from our mistakes I'm getting a great education!
 
Upvote 0 Downvote
Which third world country are you in zapped? In Canada and I believe the US, switching neutrals has been illegal for over 50 years. It is only allowed if and when a multi pole switch opens the hot conductors simultaneously or ahead of opening the neutral.
While I don't do house wiring,
Click to expand...
You got that part right.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
Upvote 0 Downvote
zapped,

Opening the neutral at the switch means the socket is still live... that's always a bad thing. If I see a light go off when I flip a switch, common sense says that socket is dead, but that wasn't the case for my house (at least the stuff the idiot beforehand self-installed).

Dan - Owner
Footwell%20Animation%20Tiny.gif
 
Upvote 0 Downvote
Well how about that... I opened a wall switch in my house and the switch does interrupt the hot wire!

So you guys are convincing me I'm wired backwards, but where is this documented? The only thing that I can find in IEC 601010 and NFPA 79 state that "all current carrying conductors" need to be disconnected. That makes sense for the power inlet, but not the switch. Maybe I'll learn a few more things by reading the right standard!


Mho, the engineer formally known as ZappedAgain
 
Upvote 0 Downvote
I found it; 2008 NEC 70 Sections 380-2, and 230-42, and 100-A tie it all together:

"Switches or circuit breakers shall not disconnect the grounded conductor of a circuit"

"...the grounded (neutral) conductor."

The world will be a safer place now.

Mho
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

FrankieAugust
  • Locked
  • Question
Current Leakage at 95% SCR Phase Control in Vacuum Sintering Press Process
Replies
1
Views
6
LionelHutz
LionelHutz
AAEDSN
  • Locked
  • Question
Short circuit current for control panels 2
Replies
10
Views
15
jraef
jraef
kjoiner
  • Locked
  • Question
Switching mains power for Shcuko type plugs with no polarity in a control panel
Replies
4
Views
5
kjoiner
kjoiner
Tamer Mansour
  • Locked
  • Question
Control Relay overheat
Replies
9
Views
10
RRaghunath
RRaghunath
Phlapjak
  • Locked
  • Question
Wire Size for MV Switchgear Control Circuits
Replies
5
Views
11
wcaseyharman
wcaseyharman
Back
Top