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Cut off switch for VFD?

Lou Jacobs

Lou Jacobs

Joined
Jul 18, 2018
Messages
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Location
Baltimore, MD
Website
loujacobswoodturning.com
I recently acquired a new lathe with a VFD. The documentation suggests installing a cut off switch to isolate the lathe from the power grid, and prevent destroying the VFD in the event of a surge or lightning strike. I wonder if anyone has done this, and what kind of switch do you use, or do you simply unplug the machine when not in use? There is a power on/off switch on the main box on the VFD (separate from the spindle on/off on the front control unit), but my sense is that this is not a sufficient disconnect to actually isolate the machine. I’d love to hear your thoughts.
 
Both of my VFDs are plugged in to a switched outlet. Lots of surges and outages here. I leave them switched off until I use the lathes.
 
In the case of a nearby lightning strike, complete equipment isolation probably would be the best bet. That means no ground connection to the equipment as the ground circuit integrity can be damaged or destroyed and voltage imposed on it. A switched supply normally maintains a ground connection. On the other hand pulling the plug is not a clean electrical break as a switch would be, especially when twist loc connectors are used such as I have installed. So, a switch should be used to interrupt the circuit then unplug. Now what are the chances of a nearby lightning strike?
 
Joe Kaufman said:
In the case of a nearby lightning strike, complete equipment isolation probably would be the best bet. That means no ground connection to the equipment as the ground circuit integrity can be damaged or destroyed and voltage imposed on it. A switched supply normally maintains a ground connection. On the other hand pulling the plug is not a clean electrical break as a switch would be, especially when twist loc connectors are used such as I have installed. So, a switch should be used to interrupt the circuit then unplug. Now what are the chances of a nearby lightning strike?
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Joe I am curious . I unplug my lathe (twistloc) how is that not a clean break from the grid when there is no wire connected to lathe?

Also not mentioned but DO NOT use the breaker as a switch. Not designed as such and will weaken the spring in breaker.
 
Gerald,
Take my case where I have to wiggle the twist loc plug in the process to pull it from the receptacle. There is too much force required for me to simply pull it free. One or both of the hot legs can make and brake contact several times in the process even though it happens in a fraction of a second. Not good for the electronics. A similar situation can occur with a conventional cord cap, though less likely. The case I was making is a switch makes less electrical disturbance in operation than pulling the plug or inserting the plug for that matter. The switch alone does not isolate the equipment from the ground side of the electrical distribution. In the case of a close or direct lightning strike the ground circuit through the plug may incur a voltage potential and destroy electronics even though the power switch is off. Not unheard of in the rural mid-west. The best protection to the electronics is to shut off the power with a switch to minimize electrical disturbances in the process, then unplug to isolate the equipment from the ground side of the power distribution system.
 
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So, if my 220v plug is not twist lock, but simply three prongs (two hot prongs at right angles to each other) and the receptacle is conveniently located, is installing a switch just redundant if I’d be unplugging anyway when the lathe is not in use? The only reason I can see installing a cutoff switch is if it would save me from unplugging each time.
 
John King said:
Unplug all power tools (lathe, bandsaw, table saw, etc.) when not in use. Isolation of tools does not depend on any electrical device/switch. Failsafe. - John
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I totally agree....especially the lathe which has a VFD. Much more costly and difficult to replace.
 
Lou Jacobs said:
So, if my 220v plug is not twist lock, but simply three prongs (two hot prongs at right angles to each other) and the receptacle is conveniently located, is installing a switch just redundant if I’d be unplugging anyway when the lathe is not in use? The only reason I can see installing a cutoff switch is if it would save me from unplugging each time.
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Yes. I have a power on/off switch/button on my lathe....assume all VFD lathes do. Turn off - pull the plug.
 
Most Powermatic Lathes, except new 3520C, do not have a power disconnect switch for the inverter. AFAIK, Jet is similar. If the power cord it is plugged to a live circuit the inverter is powered. In those cases, the red stop button is a logic input function and does not disconnect input power.
 
Lou,
Looking at the 3520C manual on line, it appears (not indicated) that the power in should go to terminals 1,3 of the switch & ground goes to the ground terminal. Terminals 2&4 are the switched power to the inverter, the ground is not through the switch but continuous which it should be. The ground is the path to the inverter when lightning or whatever disrupts the integrity of the ground side of the system and voltage appears on the ground connection. Again, most common is a nearby direct lightning strike to the power distribution system. As I see it, your lathe is isolated from the power system if you first turn the power switch off (less induced disturbances than just pulling the plug) and unplug. If your lathe is hardwired, no cord and plug, maybe the cutoff switch Powermatic recommended is for safety, to enable lockout and tagout procedures generally used in an industrial application. I looked at the manual briefly and did not see a reference to a separate cutoff switch. Does the Powermatic switch on your lathe have provision for a padlock to lock the off position?
 
Joe,
Thanks for your research on this. No, there’s no capacity for a padlock, but that kind of security is not my primary concern. I have been switching the power switch off, and then unplugging. I’d been thinking of the external power cut-off switch to avoid the need to unplug, and risk of forgetting, but I’ve posted a large “unplug lathe” sign on my shop light switch, and so far it seems I’m getting into the habit, so I’m less concerned.
As an aside, a large ash tree was taken down in my neighborhood yesterday, and I made off with a couple of sections. I’ve just finished cutting a dozen bowl blanks ranging from 16-20”. I’m looking forward to getting them on the lathe to rough turn them, and possibly finish turn a couple and see where the drying/warping process takes them, a la Robo Hippy.
 
I wonder if anyone familiar with NEC or electrical codes will point out any potential code violations in using the 3 pole Leviton switch as a disconnect including the ground? If you mounted it on the lathe seems there would be less chance of code violations.
 
I have several machines in my shop powered by VFD's and they all are protected by a switch.

The picture is the switch for my lathe. I thinks its probably a AC disconnect. I found some 240V led's are ebay and wired one in so I wouldn't forget to turn the switch off at night.DSCF8939.JPG
 
Larry, I like your set-up. My shop is in a converted detached garage behind my house. I can’t see the door from the house, so I’ve also wired an LED into the window frame facing the house that shows me if I’ve left the door open, (or if the shop has been broken into - which never happened, but once someone tried to jimmy the door, which actually was the motivation for the lights). A second one is wired into the lighting circuit to show when I’ve left the lights on.
 
The main concern is opening all of the conductors going to the VFD controller, when you get a lightning strike that hits the power lines or your shop all of the conductors power, neutral and ground can be energized with a high voltage spike which eats VFD's and sensitive electronic equipment for lunch. Disconnecting all (3) wires for a 120V or 220V outlet is critical in preventing a high voltage spike from reaching your equipment. Some newer equipment can have (4)-wires on the plug which would require a 4-pole disconnect to isolate the equipment from the premise electrical service. A direct lightning strike can still cause problems with all electrical equipment that is bonded together as per required by the NEC electrical code, when you consider the voltages that are imparted onto a building from a lightning strike it is difficult to prevent damage from these types of events. A voltage spike coming in on a utility line is a lot easier to mitigate damage by using surge arrestors at the meter main, surge arrestors at the power distribution panel and power strip and outlet arrestors at the equipment.
Unplugging the equipment from the wall will guarantee no damage if you were to take a direct hit from lightning.
 
Mike, forgive me, I’m not an electrician, but I don’t believe there is any bonding here as the machine is only plugged in with a three prong 220V plug (3 conductors including ground). Are you saying this is in violation of code? Or that it would be if it were bonded? What’s the difference between installing a three pole cut-off switch and unplugging the machine when not in use?
 
Mike Johnson said:
equipment.
Unplugging the equipment from the wall will guarantee no damage if you were to take a direct hit from lightning.
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Mike has it right. Once again....hit the power off button on your lathe and pull the plug. Doesn't get any simpler! :rolleyes: If you have trouble with a twist-lock plug just swap it out with a matching 3-prong 220v outlet & plug.
 
Lou Jacobs said:
Mike, forgive me, I’m not an electrician, but I don’t believe there is any bonding here as the machine is only plugged in with a three prong 220V plug (3 conductors including ground). Are you saying this is in violation of code? Or that it would be if it were bonded? What’s the difference between installing a three pole cut-off switch and unplugging the machine when not in use?
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Lou,

All conduit (metal) and metal junction boxes, and receptacle and switch mounting straps are bonded to the electrical ground system to provide a path back to the electrical service. All of these components are part of the electrical system. When lightning strikes a house or work shop directly all of the metal components including the ground wire system become energized at a high potential for a second or two until the voltage can be dissipated to earth. If you have plastic switch and junction boxes you still have a ground wire system that runs from each box all the way back to the electrical service which then bonds to the service entrance and meter which then bonds back to the utility transformer neutral. The average lightning strike starts at about 10 million volts and can exceed 120 million volts on a large lightning strike. With that level of voltage potential the voltage can gap several meters jumping from one metal surface to another trying to find its way to earth. You will see scorch marks on screw connections, metal conduit connections, metal covers, switch plates, outlet plates, phone plates, coax plates, any and all electronic equipment plugged into an outlet will be toast on a direct hit to any building, unless it is protected by a well designed lightning rod system capable of taking the strike to earth.

Here is where the NEC code 250.106 calls out for lightning protection systems to be bonded onto the electrical ground system of a building structure. I have argued this point with a number of people that sit on the NEC code panel board. They want all system grounds bounded together electrical, phone, coax, plumbing systems, gas lines etc.so there is no possible way to energize a system and not have it go back to the same ground which should trip a breaker or blow a fuse if an electrical fault were to occur on any of the systems. If all system grounds are bonded to the same ground network there is no potential for a voltage differential between the systems. This keeps a person safe from an electrical shock potential but exposes the electrical systems to electrical damage from a direct lightning strike to a building. .
 
I get it Mike. Thank you for this very thorough explanation. I think we’ll simply unplug the machine when not in use.
About 20 years ago we chartered a sailboat on the gulf coast of Florida for a week. Our second night out we were at anchor and were struck by lightening. The boat, I believe had a bonding system connecting all the rigging from the top of the mast down to a plate on the bottom of the keel. While we were not hurt, all the electronics on the boat were fried, and the wind vane at the top of the mast was blown off and landed, scorched, on the deck. We took the boat back to port the next morning, and finished our vacation camping on land, as the charter company claimed our security deposit, apparently equal to the deductible on the boat’s insurance. They demanded another deposit for us to continue the charter. We decided against it.
 
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