Centroid Community CNC Support Forum

tblough wrote:We'll have to disagree on what's safer. NEC specifically states neutral conductors must not be switched and only gives very specific exceptions.

Can you give a reference for this?

Separate from that, can you describe any real-world scenario where disconnecting both hot and neutral with a 2-pole (or larger) switch can lead to a safety, reliability, or functionality hazard?

"Where a switch is placed in a neutral conductor, disconnection and reconnection shall be such that the neutral
conductor shall not be disconnected before the line conductors and shall be reconnected at the same time as or before
the line conductors."

Most 4-wire disconnects will enforce the neutral is disconnected last and reconnected first. Normal 3-phase disconnects do not do this when wired to disconnect single phase. This ensures that the hot wire is never energized without a return path other than the person touching the device.

On a machine with possible multiple sources, such as a light or a Digital readout plugged into a wall, it is possible (while not probable) that something could short in that device and search through the machine for a suitable return path.

Like most things in the NEC, this is to prevent a 1 in a million cascade of events.

Section 404.2(A) covers switch connections for three- and four-way switches. It specifically states that these combinations shall be wired so the switching is accomplished only in the ungrounded circuit conductors. Grounded conductors are specifically covered in 404.2(B), which states switches or circuit breakers shall not disconnect the grounded conductor of a circuit. In this case, there is an exception in 404.2(B) that states: A switch or circuit breaker shall be permitted to disconnect a grounded circuit conductor where all circuit conductors are disconnected simultaneously or where the device is arranged so that the grounded conductor cannot be disconnected until all the ungrounded conductors of the circuit have been disconnected.

There are two other locations in the NEC with similar text to the exception noted above. Section 430.85 states, “one pole of the controller shall be permitted to be placed in a permanently grounded conductor, provided the controller is designed so that the pole in the grounded conductor cannot be opened without simultaneously opening all conductors of the circuit.”

Similar wording is used in 430.105: “One pole of the disconnecting means shall be permitted to disconnect a permanently grounded conductor, provided the disconnecting means is designed so that the pole in the grounded conductor cannot be opened without simultaneously disconnecting all conductors of the circuit.” Grounded and neutral conductors generally must not be switched; doing so can be dangerous.

Thanks Tom,

As you stated, neither the Neutral or Ground (Earth) can be switched unless they are switched at the same time (and by the same mechanical operator) as Line(s).

In the electronics for my Taig, I disconnect L1, Neutral AND Earth with the disconnect switch. That switch does the same thing as pulling the plug from the wall. I don't want any possibility of having anything backfed through the Neutral/Ground line while I'm inside that box. If lightning were to hit the house, there would be no direct path to anything inside the box with the disconnect switch open. (Yes, I know that a direct lightening strike could easily span the air space inside a disconnect switch, but I'm using a little poetic license, after all, I live in a desert. What do I know about lightening?) I wire the Neutral and Ground through the disconnect switch on separate poles because Neutral and Ground are bonded in the circuit breaker panel. It wouldn't do any good to wire Neutral through the disconnect switch without also wiring Ground through the disconnect switch. You'll note that what I'm writing now disagrees with my post about grounding lugs. That's not a mistake or a typo. I'm aware of the electrical codes and I recommend that everyone follows the codes; but, in my own home, inside my own enclosure, I wire things so that I am safe even if code says it can be done a different way. I don't do things "my way" when I install process computers for customers. I require that the customer have his licensed electrician supply power to the box. I provide both DIN rail terminal blocks as well as a disconnect so that the electrician can connect the three wires in a 120V circuit or the four wires in a 1-phase 240V circuit. So far, no electrician has ever used the DIN rail terminal blocks. They've all used the poles on the disconnect switch to completely disconnect all power from the box just as if a wall plug had been pulled.

Wow, the topic of grounds can always start a healthy conversation, and all for the good. That was a good discussion, glad the tread got hacked.

I'm retired now but early in my career I was a Field Engineer for a large CNC manufacture (who ultimately gave in to a Japanese CNC company ) I started up many large machines with CNC's back in those days and one issue we ran into a lot was intermittent trips due to electrical noise. When wiring encoders, etc installers would often connect both ends of shielded cables to ground. This could cause circulating ground currents in the cables which could contribute to electrical noise. I solved many intermittent problems simply by snipping the ground connection on the far end of the shielded cable, leaving only the connection on the control end.

Granted most hobby machines are small and may not see the problem, but still a good practice.

I completely agree that disconnecting a neutral conductor while leaving a hot (ungrounded) conductor connected is potentially dangerous.

The exception you point out in NEC 404.2(B) supports my belief that there is no significant hazard to disconnecting the neutral, as long as the switch ensures that the hot wire(s) are disconnected at the same time.

NEC 404.2(A) applies to light switches ("three-way" and "four-way" not to be confused with 3-pole or 4-pole).

Every 120V single-phase machine I have worked with has been plug connected. If I just want to turn it off, a single-pole switch in the hot is sufficient, but there is no harm in using a 2-pole or 3-pole switch, and switching the neutral as well. If I truly want to disconnect it (e.g. in Mike's scenarios) I will unplug it.

Richards wrote: ↑Wed Oct 20, 2021 3:01 pm In the electronics for my Taig, I disconnect L1, Neutral AND Earth with the disconnect switch. That switch does the same thing as pulling the plug from the wall. I don't want any possibility of having anything backfed through the Neutral/Ground line while I'm inside that box. If lightning were to hit the house, there would be no direct path to anything inside the box with the disconnect switch open.

There may be some value in disconnecting neutral. It is not uncommon to have hot and neutral swapped on a 110V outlet. This leaves some uncertainty about whether you really switched hot with only 1 pole. However, I can't think of a situation where I would want to switch ground. Assuming the hypothetical lightning came in through the wiring and jumped the disconnect, I would hope it arced to ground. A disconnected ground would make the voltage more likely to to try to find a path through a body. On a more practical level, if you are working with ESD sensitive equipment, it is best to ground yourself. Regularly touching a ground in the cabinet is a good way to discharge yourself if not using a wrist strap.

I agree with cncsnw. Unplug the cabinet when you don't want a ground. This has the added benefit of a large air gap.

This is what I do as well. My machines have an 18" pigtail with a twistlock plug. Disconnect completely when not using and easy for lockout-tagout.

However, when I have power running into my cabinet, and I'm referring to power inside the cabinet regardless of the disconnect position, I want the redundancy of having both neutral and ground connected in case the incoming hot chafes or somehow shorts to the metal cabinet.

eng199 wrote: ↑Thu Oct 21, 2021 10:50 am However, I can't think of a situation where I would want to switch ground.

Ground and Neutral are bonded (connected together) in the breaker box in your home/shop. Opening Neutral at the disconnect switch still leaves neutral and ground connected at the breaker box. Ground is a safety circuit. It is not meant to carry current unless all else fails. I disconnect all wires with the disconnect switch. Unplugging the box is fail safe. Doing that 100 times means that the plug/socket is loose. Switching a disconnect switch 100 times does not loosen the contacts in the switch. I'm nitpicking, but, I'm also somewhat a pessimist. I don't use circuit breakers to switch power on an off except in emergencies. I don't unplug a cabinet to kill the power except in emergencies. I use switches to switch power. That's what they were designed to do.

I should specify one other thing. When I turn on the Disconnect Switch, only the 24VDC power supply is turned on. An indicator lamp shows that 24VDC is on. The Acorn controls a separate 5VDC power supply and the 75VDC servo power supply through a contactor. The contactor only controls L1 to those power supplies. The Disconnect Switch is the only connection where I completely disconnect all power from the enclosure.