The local electrical permit and inspection bureau is requiring that my artisan woodworking business have an electric interlock, so that I cannot operate any equipment (table saw, planer, et cetera) without the dust collector being on. Several years ago there was an article in FW describing of a home-made circuit board providing some sort of electric interlock. Such a solution would never pass what is sure to be a rigorous application of the National Electric Code (NEC) during final inspection for my electric permit.
An electric engineer told me of a 12 contact relay (Allen Bradley 700-P series) with a single coil. If the dust collector circuit was used to energize the coil, the 6 pair of contacts would then be closed. One of these coils could therefore “interlock” the dc with 3 pieces of equipment (if they are each on separate 240V circuits) by having each of the two hot wires of a 240 circuit pass through one pair of these contacts. In my shop, there would have to be two of these relays so that I could interlock both hot legs of five pieces of equipment.
There are two issues with this otherwise elegant solution. First, the “interlock” would be opposite from what the local inspection bureau described – instead of having the table saw circuit turn on the dust collector, the solution requires that the dc be in operation in order for the table saw to be turned on. Hopefully they would not object to this reverse implementation.
The larger issue is how to install such a relay in the building wiring so that the installation will meet a strict application of all the various rules and requirements of the NEC. My strong preference would be to mount the pair of (parallel) relays in a separate utility box alongside the circuit breaker panel for the building. This way, short wires could come off the breakers, pass through a connecting conduit and connect to the relays. Matching short wires could come off the relays, pass back through the conduit, and be connected to the wiring out to the various pieces of equipment. The relays would be alongside the circuit panel, there would be an absolute minimum additional wiring required, and wiring runs would be kept as short as possible. One electrician I explored this concept with however thought the installation would involve “raceway” conflicts in the NEC.
Are there any people with much more detailed understanding of the relevant sections of the NEC than my limited knowledge that can address the compliance of this approach with the NEC?
The second question is whether there is an equally elegant and simple interlock method that would in fact energize the circuit for the dust collection when another piece of equipment was turned on? For example, installation of a simple, traditional 50 amp subpanel alongside the main panel, with all the equipment to be interlocked with the main panel coming off breakers in the subpanel. Is there is a simple, off-the-shelf UL listed device that could sense any level of current flow in the wiring from the main panel to the subpanel, that could in turn through a relay of something energize the the circuit for the dust collector?
I was quoted a price of about $290 for an Allen Bradley 12 contact 700-P relay, so a pair, utility box 8″ deep, and some wiring might involve a total materials outlay of about $750.
Thanks. Don Stephan
Replies
If you haven't already, take a look at Ecogate.
-Steve
I'd ask over on Breaktime. There are more electricians over there.
I second Steve's suggestion of an EcoGate "Green Box".
I've had the setup for 5 years and wouldn't be without it. A small coil is attached to the power feed for each tool; when the tool is turned on, the appropriate blast gate opens and the DC starts. The DC runs for about 30 seconds after the tool is turned off. The green box handles (i think) 8 machines, 7 automatic and one manual.
Frosty
"I sometimes think we consider the good fortune of the early bird and overlook the bad fortune of the early worm." FDR - 1922
"The local electrical permit and inspection bureau is requiring that my artisan woodworking business have an electric interlock, so that I cannot operate any equipment (table saw, planer, et cetera) without the dust collector being on."
Since when do they have the authority to do this?
They haven't confirmed, but apparently they are saying I will be a Class III hazardous location because I will produce "fibers and flyings" (I kid you not), subject to protected wiring and dustproof box requirements, among others, if I do not have an interlock. I.e. they would not be "requiring" the interlock, just suggesting I couldn't afford the alternative.
I visited the Ecogate web site. It's great to have an off-the-shelf option. However, the warranty is voided if the basic 8 station box ($269) is used for commercial locations, and the next alternative is their GreenBox Pro ($2980). A total cost before S&H of $4855 is quite high for a one-person artisan business with 5 pieces of equipment and artisan levels of income.
Uh, so who needs a $4,500 warranty? You could buy 18 of the basic, non-warranted units for the price of the pro unit. If your anything like me, all you need to do is to buy two of something, so you have a spare, and the first one lasts for freekin' EVER, warranty or no.
;-)
Mike HennessyPittsburgh, PA
I don't know why you need the warranty. I haven't had any problem with mine - and - they can't tell from California if you are commercial or not!At first I thought the vibration sensor would be the way to go - it would be easier to hook up. But I found that if I dropped a board on the saw table, the DC started. That got old in a hurry. Then I bought the coils and mounted a small electrical box on each machine. I ran the power feed through the box, splitting off one of the wires to run through the coil. You can also put the coil in the wall box that contains the duplex power plug.Frosty"I sometimes think we consider the good fortune of the early bird and overlook the bad fortune of the early worm." FDR - 1922
Don, I am not an electrician, but I have a pretty good understanding for a few things, and thought I might suggest another approach. It seems that maybe there could be some sort of coil which mounted in your breaker box, which would be somewhat like a amp-meter which clamps around a wire that reads the amps, only here, all of these wire circuits to those tools, would pass through the coil, and when any of these tools were turned on, this would energize a solenoid that turned on the DC.
But then again, most electricians are pretty diligent about making everything very tidy in the breaker box, so there may not be room to separate them into one location which would allow the extra wire length to get them through the coil, but I think it should be worth a look.
"It seems that maybe there could be some sort of coil which mounted in your breaker box, which would be somewhat like a amp-meter which clamps around a wire that reads the amps, only here, all of these wire circuits to those tools, would pass through the coil, and when any of these tools were turned on, this would energize a solenoid that turned on the DC."
That's how the aforementioned Ecogate works. They actually have two types of sensors: one detects current flow, the other detects vibration. The vibration sensor is easier to wire, since you don't have to go inside the breaker box, but won't work with tools that don't vibrate very much.
-Steve
Yea, Steve, I didn't go to your link until after I posted mine. <one detects current flow, the other detects vibration. The vibration sensor is easier to wire, since you don't have to go inside the breaker box, but won't work with tools that don't vibrate very much.> Maybe I don't understand the logic that would make monitoring the vibration easier. It seems like there would have to be something on each tool if you go that way. On the other-hand, since everything comes out of the breaker, including the DC, it seems like this would be the easiest place to monitor when any of these tools was in use. All of their leads are right in there together. But maybe that is just my dumb logic.When I wired my shop quite a while ago, I wired start / stop switches all around the shop by most of the tools , except the table-saws. There, I dropped a pigtail out of the starter boxes with a receptacle to plug a shop vac into, which lives under the outfeed tables behind each saw. I picked up the hot lead behind the contacts, before the heater protection for the motor. My wiring to each saw is greater than required for the saw, and vac. I did mine this way, because the TS is turned on and off a lot more than the rest of the tools, and my DC has a 7.5 hp motor, so I think I am saving energy. But then, I just had to laugh at myself, and checked on the shop vacs. They are both Sears, 16 gal. The older one says 5 hp, and the newer one says 6.5 hp. Ha, I wish I knew how they could manage to get that kind of power through a 14 ~ 16 ga wire. I don't know what the real hp is on those vacuums, and don't really care, except that they do move a lot of air for what they cost, and that is OK for me.
"Maybe I don't understand the logic that would make monitoring the vibration easier. It seems like there would have to be something on each tool if you go that way."
Right. The point is that that's a low-voltage signaling circuit, and is therefore "easy" in the sense that you don't have to worry about all of the various safety precautions and regulations as you would working with line-voltage electrical equipment. You basically can't hurt yourself, start a fire, etc.
-Steve
Check with your dust collector's tech support people for the effect of frequent start-ups. Some cyclones may not start the third or fourth time you turn on your TS in a short time period; they take a while to get up to speed and like to run constantly, but start-up puts excess load on the motor.
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