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 (dc) 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 electrical 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
You don't need a 12-contact relay. Just power a 24V transformer from the juice feeding the dust collector motor, then run the 24V to contactors at each station.
To go the other way is a little trickier. Simplest would be to install a current relay on each station, then wire them in parallel to energise a contactor on the duct collector.
Thanks for the reply. Planer has 3HP 240V motor, everything else 2HP or less 240V motors. AS best I can tell all the motor switches are simple spring-loaded mechanical switches.Putting a relay on each machine, as I understand, would require dustproof box between switch and motor, dustproof fittings, and another pair of wires run to some central point.As it's an artisan operation in small building, have to be able to easily move equipment if making large item, so the relay wiring to the central point would have to be plug-connected (the equipment is all plug connected).
"The local electrical permit and inspection bureau "
Under what code is this required? It is certainly not NEC.
NICELY has for what code and section that requires this. Then read that section. Often there are alternates and exceptions.
"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 (dc) being on."
"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."
"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"
But that sequence of operation is EXACTLY what you said in the opening.
If you go this way you need to use 700-PK for 3 hp equipment and 700-PL if you have any 5 HP. The latter only has 6 contacts.
But also not that this is LATCHING relay. You need a start and stop button and have it also control DC. Not just have it on when the DC is on.
http://www.ab.com/en/epub/catalogs/12768/229240/229266/229661/3087128/229735/tab3.html
http://www.ab.com/en/epub/catalogs/12768/229240/229266/229661/3087128/229735/tab5.html
As far as mounting it you have the right concept. Metal utility box at a comvenate location and run the wiring through it and mount the relays.
Also you only need to break one leg of the 240. This is for control. Not disconnect.
If you go this way you might want to look at an alternate wiring system,
Get a high power contactor. Maybe 100 amp.
Then run a feed from the panel through the contactor to a sub-panel. Then run all of the equipment off the sub-panel
While, as Dan suggested you can use some 24 volt controls that brings up a couple of issues. First you don't know if all of the equipmet has 24 volt controls. Lots of them are LV starters or monentary switches with a hold in coil.
While this does meet the interlock as YOU describe it, I think that it would be much handyer to have the DC automatically operated by the equipment.
You can get current operated relay. don't know if any have contacts to operated the DC so you might need an auxillary contactor.
Run one hot lead for each piece of equipment through the current relay. Need to verify the size of the opening.
Or do the sub-panel thing. Run one hot of the sub-panel feed through the current relay.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Sorry for my mis-statement. The inspection bureau said turning on the table saw, ... would have to turn on the dust collector.The inspection bureau has never put in writing that the interlock would eliminate my building from classification as a Class III location. Verbvally, they said I had to have interlock. On the required elec drawings they wrote that the shop much comply with Article 503 (Class III). I'm going to try to clarify whether they are giving me an either/or ...Your comments are very helpful, although borderline beyond my comprehension. I plan to print all the disussion this evening and give to my certified electrician.Thanks.
The current relays that are readily available will switch 24V AC or 120V AC. If you go with 24V then you just need 24V transformer, one 24V contactor (for the dust collector), plus AT MOST one current relay for each device. It is tricky figuring out where to put the current relay, since the one I have an example of is a plastic box about 1x2x3" and needs to be enclosed in an box where its yoke on one end can wrap around one of the power wires.But there's no reason you can't use one current relay for several wires (I'd guess you can get 4-5 in the yoke), so long as you use the same leg of 240V from all circuits. So any common box with sufficient space will do.You can also use the 120V current relays and contactor and avoid the 24V transformer, but then you need to route a couple of 120V-rated wires to each relay position. This may be easier or harder than doing the 24V setup, depending on where you place the relays and how much conduit/raceway space you have.
If your view never changes you're following the wrong leader
All of the power orgininates at one location. Put the current sensing relay(s) near that point.No need to string any more wiring around the room..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Dan and Bill:This would be a very elegant solution, especially if inspector allows the current sensing relay in sub-panel feeding just the table saw, band saw, jointer, planer and edge sander. Would only have to install the current sensor to the supply wires from the main panel to the subpanel alongside.Any suggestions which manufacturer(s) might make current sensors, and cost?This idea I definitely will share with the electrician I hired.Thanks.
The cheapest current relays are the ones made by Research Products Corp, the folks who bring you Aprilaire humidifiers. They sell both a 24V model (model 50) and a 120V model (model 51). About $25 andd $35 respectively. Only question is whether these units are considered suitable for "industrial" use.AFAIK, to get a more "industrial" model you have to get up in the $100-200 range, more complicated, and about 5 times as large. And you still need to provide your own enclosure.
If your view never changes you're following the wrong leader
Something that occurred to me, though, is that I've seen dust collection systems advertised with the automatic switching stuff. I would think that whoever made your DC might have the optional gear to do this job, more neatly than something we could cobble up.In fact, this setup will power up the vac AND open a blast gate:http://ecogate.com/industrial/one.htm
If your view never changes you're following the wrong leader
I found two.http://www.crmagnetics.com/newprod/ProductView.asp?ProdName=CR4395McMasterCarr has them. About $71.http://www.mcmaster.com/Type current relay in the Find window.Grangerhttp://www.grainger.com/Grainger/items/6C057Did not see what the sensing current range is.http://web1.automationdirect.com/adc/Overview/Catalog/Sensors_-z-_Encoders/Current_Sensing_Switcheshttp://www.flex-core.com/relays-switches.htm
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.