Fused neutral, industrial control panel

[ControlThis]

Looking for insight on the neutral being fused inside a control panel that runs a couple servos, plc, and some other control stuff. It's acctually protected with a thermal magnetic breaker. The panel is 208vac, 3ø, 20A supply with neutral conductor also being supplied. We have quite a few of these systems. They were deisgned prior to my involvement but I maintain them. A plc, dc power supply, power on lamp, and cooling fan are the loads that this neutral supplies. My concern is over safety. I would never design this into a system. Why do you think someone did this? Is it safe? Should I remove these breakers from these systems?
Any insight greatly appreciated!
Kevin

 

[davidbeach]

In the neutral is on a single pole breaker you have a significant safety hazard. If the neutral is one pole of a multi-pole breaker with common trip so that any time the neutral is open you also have all of the phase conductors open there is no problem.

 

[ControlThis]

Thanks for the response! It is on a single pole breaker. Is there a situation where this would be considered appropriate? I'm trying to figure out why this was done and repeated on 30 systems.
What would be the best way to describe why this is a significant hazard?

 

[dpc]

I'd suggest talking with the people who built the panel to get their explanation. What you are describing sounds very odd.

Where were the panels built?

 

[ScottyUK]

If this supply is via a control transformer it is normal practice to earth one end of the secondary winding and nominate this as 'neutral' within the panel. A breaker in the neutral of a control supply seems strange, and a breaker in the neutral of any supply when that breaker is not mechanically ganged with the breaker(s) for the phase conductor(s) is both dangerous and very likely a code violation.


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If we learn from our mistakes I'm getting a great education!

 

[jistre]

Heck, I only took Volts for Dolts in college, and I know enough to say "YIKES!" on this one. The first maintenance guy who assumes that the equipment is de-energized because the breaker's open is going to be very shocked to find out he's wrong.

 

[bklauba]

. . . . which is why putting a NC Overload contact 'tween a contactor coil and neutral sure looks like tortured logic. (Sorry if you guys think this is a subject change, but, not really.) Putting it "in front" of the coil, so the coil gets NO JUICE when the NC Overload contact is open seems just so right!

BK

 

[itsmoked]

Are there also breakers, (other breakers), for the line conductors? If not, you WILL need them before you remove the neutral breakers. As they are still providing some fire protection - if not electrocution prevention.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ControlThis]

There is a 3 pole breaker for the 3 phases before the control panel at the bus that supplies this panel, but not on the panel itself. The neutral breaker is on the panel. The 3 pole breaker at the bus supplies power to only this panel. That should be suffiecient, right? It is sized to handle the drop cord size and panel conductors.

 

[dpc]

bklauba,

You're exactly right - we do think it is a subject change, so please start a new thread.

 

[jistre]

I just had another thought after reading the post about the 3 pole breaker at the bus. I can't say whether or not the arrangement is technically acceptable; that's for you EEs to discuss.

However, from dealing with maintenance and plant workers in general, I think you have to assume that the majority of them don't know the intricacies of breaker positioning and will always assume that a breaker at the panel is a way to de-energize the panel. If they have to walk across the room to pull the bus breaker or they can just flip the panel breaker, there are some workers who will always choose to flip the panel breaker no matter how much you've trained them that it doesn't safe the equipment. Just from a human factors viewpoint, I'd say that a single pole neutral breaker living by itself on a piece of equipment is always dangerous. To those who don't know better, there is the impression that it safes the equipment.

Recipe for disaster.

I stumbled on a real interesting discussion you sparkies are having here. I'm learning all kinds of stuff!

 

[dpc]

Part of being "qualified" is knowing how to de-energize equipment and place it in an "electrically-safe" condition.

Part of the process is a required voltage test.

It is never a good idea to rely on the position of the breaker handle to determine if a circuit is energized or not.

The breaker on the neutral is weird,a safety concern, and most likely a code violation, if in the US. But since there are 30 panels built like this, it seems that someone thought they had a good reason for putting it there. I'm not 100% convinced he is looking at a neutral, at this point.

 

[LionelHutz]

To me this sounds like 3-phases and neutral come in to the panel and then a breaker is connected to one of the phases to create a phase-neutral control power circuit (likely 120V).

Having 3-phases and a neutral come to a panel and then taking control power (likely 120V) from one phase to neutral with a neutral breaker for protection is wrong.

If I have interpreted the description of what you have wrong then please correct me.

 

[jraef]

It is wrong wrong wrong. Fusing (or any SCPD) on a neutral point in a 4 wire system will NOT protect the control circuit from a grounded fault on the line side!

If I had to guess (and my insatiable curiosity makes me do so), I'd say they did it out of a warped sense of thinking they were protecting the control circuit. I say this because I had a junior engineer once propose this to me out of genuine concern, yet flawed logic. The circumstances will help to explain.

We had a 50A 3 pole breaker (and a neutral landing lug) feeding 208Y120V power to a control panel. He was told to use one phase and neutral from the source for the 120V control power. His logic was, the 50A breaker was way too big to protect the control circuit, but he was always told not to have 2 SCPDs on the same circuit (bear with me for a minute here). So he came up with his idea that if he put a 10A fuse on the neutral side of the control circuit, it is protected and he didn't violate his perception of the "no dual SCPD" rule.

I had to question his "no dual SCPD" rule and found that he had once used two identically rated breakers on a circuit; one in a panelboard outside of the cabinet and another inside the cabinet. He was told (by someone else) it is not a good idea to have two SCPDs in series, because you will never know which one had tripped. He took that to mean NEVER having two protective devices in series, even if one was a branch! So the neutral fuse was his way of "solving" the perceived problem.

 

[automatic2]

Is this an ungrounded circuit?

 

[davidbeach]

If the circuit is ungrounded there is not a neutral.

 

[automatic2]

not true. Neutrals are however in most cases (but not all)pulled to ground. Visit a drilling rig and look for grounded neutrals

 

[waross]

I have seen several floating (ungrounded) 120/208 volt wye systems on ships. The lighting circuits are basically identical to shore systems EXCEPT the neutral is not connected to ground.
Any ship that may ever be supplied with shore power must be ungrounded when on shore power to avoid possible serious hull corrosion issues. Some ships ground the system through a knife switch that is supposed to be open when on shore power.Most run ungrounded rather than take a chance that the switch will be left closed on shore.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Per the NEC; if the "neutral" isn't solidly grounded it can't be used. One end of a single winding transformer may be grounded, but that isn't a neutral either (now that the 2008 NEC actually defines a neutral. If the conductor connected to the wye-point of an ungrounded transformer is used in applications where the NEC does not apply, it needs to be switched with the phase conductors; not independently as in the OP, but in a four pole breaker.

 

[waross]

I agree with you David. The shipboard systems I mentioned are not subject to the NEC. As you said, the neutral should be switched, and often it is, however i have seen cases where it is not switched.

Bill
--------------------
"Why not the best?"
Jimmy Carter