Battle with Student

[donf]

Another battle with a student.

If you look at the appended jpeg you will find two schematics for a motor control circuit. “A” is my drawing. I had been taught that you cannot use Neutral as a switched conductor. I believe it is buried somewhere in code, but darned if I can find it. It is my understanding that you bring your L1 through the Overload to the Switches. In the event of an Overload trip, Line 1 voltage is cut to the circuit.

Drawing “B”, brings L1 to the switches and uses Neutral to the Overload. In the event of an Overload, the contacts open and Neutral drops out, opening the circuit. But that is effectively using Neutral as a switched circuit, Is that allowed in the whacky world of motor starters?

In practice, both methods will work. However, I believe that “B” violates the NEC but I cannot prove that statement.

If I were using a transformer for 208 to 24V or even using a 208v coil from the L1 & L2 conductors I don’t think it would really matter since I am not connecting A2 to Neural.

By the way, Article 100’s definition of “Neutral” is silent regarding Neutral as a switched conductor.

 

[LarryFine]

We have discussed this before. They've been wired like B from the factory forever, which leaves us out of it. They were often wired line-to-line, which matters less.

 

[donf]

We have discussed this before. They've been wired like B from the factory forever, which leaves us out of it. They were often wired line-to-line, which matters less.

Is the code silent on using Neutral as a control switched circuit?

 

[LarryFine]

Is the code silent on using Neutral as a control switched circuit?

That was our conclusion, if I remember correctly.

 

[wwhitney]

Is the code silent on using Neutral as a control switched circuit?

430.38 says "Motor overload devices, other than fuses or thermal protectors, shall simultaneously open a sufficient number of ungrounded conductors to interrupt current flow to the motor."

My conclusion is that if you have a fuse or thermal protector as the motor overload, it may open only the grounded conductor (for a 2-wire circuit). All other motor overload devices may open the grounded conductor, but must open enough of the ungrounded conductors so that current to the motor would be interrupted even if the grounded conductor were maintained.

E.g. a non-fuse, non-thermal motor protector that opens only one wire may not be used in the grounded conductor of a 2-wire motor circuit. Likewise, a non-fuse, non-thermal motor protector that opens only two wires of a 3-wire motor circuit must be installed in both ungrounded conductors on a corner grounded delta.

Cheers, Wayne

 

[roger]

For controllers see 430.85

 

[Tulsa Electrician]

Is the code silent on using Neutral as a control switched circuit?

Grounded conductor. I know what you mean though.

Give 430.74 a read.

 

[tortuga]

I believe the diagram is of a 3 phase 3 wire motor circuit that is using L1 and a neutral as the control circuit (4 wires to motor controller instead of a control transformer).

If that is the case I believe 430.75 requires all control circuits to be disconnected when the disconnect is open so you'd have a 4-pole disconnect and it would not matter if you used A or B.

 

[kwired]

Grounded conductor. I know what you mean though.

Give 430.74 a read.

A problem I was asked to look into last summer involved center pivot irrigation machine, a particular manufacturer that commonly utilizes the grounded control conductor (120 volt control circuit) as a safety loop in line with the holding circuit would not shut down when customer had a system installed that can control the machine via cellular phone app. The installer of that was stumped and the owner called me out to look into it. Installer though there was some underground wiring fault. I eventually figured out everything was fine but there was too much capacitive coupling going on and even though they were opening the grounded conductor there was enough capacitive coupling to allow it to hold in the low VA coil that the holding circuit was carrying current for. Placing additional load parallel with the coil fixed the problem. I never liked how this particular manufacturer uses the grounded conductor as part of the holding circuit. A ground fault, which this coupling kind of was in a way, could cause unintended results, where if they used an ungrounded conductor for the holding circuit a ground fault would be blowing the control circuit fuse.

I don't know why NEMA motor starters have always placed the overload contact in series with the grounded conductor connection to the contactor. Though I kind of don't have a problem with it as long as it is the only contact in the grounded conductor and it is in very close proximity to the coil, as it usually is. You would need to have a ground fault on only a 6-12 inch piece of conductor depending on size of starter in order to have undesired operation if there were a ground fault in that section of the circuit.

 

[retirede]

A problem I was asked to look into last summer involved center pivot irrigation machine, a particular manufacturer that commonly utilizes the grounded control conductor (120 volt control circuit) as a safety loop in line with the holding circuit would not shut down when customer had a system installed that can control the machine via cellular phone app. The installer of that was stumped and the owner called me out to look into it. Installer though there was some underground wiring fault. I eventually figured out everything was fine but there was too much capacitive coupling going on and even though they were opening the grounded conductor there was enough capacitive coupling to allow it to hold in the low VA coil that the holding circuit was carrying current for. Placing additional load parallel with the coil fixed the problem. I never liked how this particular manufacturer uses the grounded conductor as part of the holding circuit. A ground fault, which this coupling kind of was in a way, could cause unintended results, where if they used an ungrounded conductor for the holding circuit a ground fault would be blowing the control circuit fuse.

I don't know why NEMA motor starters have always placed the overload contact in series with the grounded conductor connection to the contactor. Though I kind of don't have a problem with it as long as it is the only contact in the grounded conductor and it is in very close proximity to the coil, as it usually is. You would need to have a ground fault on only a 6-12 inch piece of conductor depending on size of starter in order to have undesired operation if there were a ground fault in that section of the circuit.

I think if you go back to ancient times, 3 phase NEMA motor starters rarely had a control power transformer and the coil was line voltage. In order to have the OL contact wired from the factory, it was on its own side of the coil, so to speak. As CPTs became more common, they just left the OL contact there for consistency.

 

[texie]

The way I have always understood this is that you can only use the grounded conductor to open the control if it does not leave the controller enclosure per 430.74. Of course that still begs the question of why do it even within the enclosure but we all know that NEMA starters been this way since dirt was invented.

 

[jim dungar]

The way I have always understood this is that you can only use the grounded conductor to open the control if it does not leave the controller enclosure per 430.74. Of course that still begs the question of why do it even within the enclosure but we all know that NEMA starters been this way since dirt was invented.

It is much easier to factory wire a reversing or multi-speed contactor if the overload contact is after the coils on a common wire.

In general the NEMA overload relay contact was not good at handling short circuits. Putting the contact after the coil meant there was always some impedance reducing the fault current, unless the fault occurred in the small factory wire 'internal' to the starter. This was considered to be a very low risk.

The NEC does not apply to UL Listed 'factory' wiring.

 

[donf]

I took the position that the O/L's purpose is to Open the contacts that allow current to flow to the motor.
I cannot find any mention in code where a Neutral can be used to control a circuit. Logically then, using the circuit grounded conductor to open the circuit of the starter is incorrect. Placing the N/C Overload contact in front of the Stop switch will cause the circuit to drop all power to the OL circuit, Using Neutral, however, does not block power for the rest of the holding circuit, it just takes Neutral away. Would not the power to the rest of the circuit still be applied?

 

[don_resqcapt19]

I took the position that the O/L's purpose is to Open the contacts that allow current to flow to the motor.
I cannot find any mention in code where a Neutral can be used to control a circuit. Logically then, using the circuit grounded conductor to open the circuit of the starter is incorrect. Placing the N/C Overload contact in front of the Stop switch will cause the circuit to drop all power to the OL circuit, Using Neutral, however, does not block power for the rest of the holding circuit, it just takes Neutral away. Would not the power to the rest of the circuit still be applied?

While there is still power on the circuit up to the point of the overload relay contact, the load drops out as there is no complete path for current flow when the overload relay drops out.
There is no issues with having the overload contact in the neutral conductor and that has been the standard design of NEMA starters for many decades.

 

[retirede]

I took the position that the O/L's purpose is to Open the contacts that allow current to flow to the motor.
I cannot find any mention in code where a Neutral can be used to control a circuit. Logically then, using the circuit grounded conductor to open the circuit of the starter is incorrect. Placing the N/C Overload contact in front of the Stop switch will cause the circuit to drop all power to the OL circuit, Using Neutral, however, does not block power for the rest of the holding circuit, it just takes Neutral away. Would not the power to the rest of the circuit still be applied?

Yes, the purpose of the OL is to shutoff the motor. That is accomplished by wiring the OL contacts in series with the contactor coil. It doesn’t matter which side of the coil it’s on in order to accomplish that.

The NEC tells you what you must do, and what you can’t do. It doesn’t tell you what you can do, unless as a modifier to one of those. In this context, since the NEC doesn’t say you must wire the OL on the “hot” side, and doesn’t say you cannot wire it on the neutral side, it’s permissible to wire it on the neutral side.

In a series circuit, there is always power up to the first open control element. Irrelevant. The circuit will function regardless of the sequence of the control elements.

 

[tortuga]

While there is still power on the circuit up to the point of the overload relay contact, the load drops out as there is no complete path for current flow when the overload relay drops out.
There is no issues with having the overload contact in the neutral conductor and that has been the standard design of NEMA starters for many decades.

If they do run a neutral from the branch circuit for the control circuit they need to show a 4 pole disconnect on the drawing.

 

[don_resqcapt19]

If they do run a neutral from the branch circuit for the control circuit they need to show a 4 pole disconnect on the drawing.

Most of the time, the starters I worked on were combination starters with control power transformers, but in one older part of the plant, they had used external control power. They did not use a 4 pole device, but added an auxiliary switch to the starter branch circuit and ground fault protective device. It was shown as a switched contact with dashed line to the breaker on the drawings.