designing a circuit to energize a 5hp motor when current is sensed to include a time delay (delay on release)
You're on the right track -- time to DIY an industrial control panel

The requisite parts for this aren't too terribly hard to find if you know where to look.  You'll need a:


suitable delay-OFF relay (the Macromatic TE-8816U is relatively cheap, UL listed, and rather versatile)
motor contactor rated for 5HP @ 240VAC 1ph with a 24VAC coil (the Square-D 8910DPA22V14 will do the trick, although you may be able to find a used-but-serviceable one cheaper)
current sensing switch (such as a Functional Devices RIBXKTA)
standard 240VAC to 24VAC HVAC control transformer (you can get this anywhere HVAC supplies can be found)
suitable wire for inside the control panel (I'd just use 14AWG THHN for the control circuits within the panel with the wire to the current sensor being 14/2 NM, while the existing wire for the dust collector circuit can be reused)
NEMA enclosure and length of standard DIN rail to hold it together (you can get these at any electrical supply house)


Electrically speaking, what you're dealing with looks like this:







simulate this circuit &ndash; Schematic created using CircuitLab

In it, XFMR1 is the control transformer with its primary connected to the L1 and L2 for the dust collector circuit, and SW1/R1 represent the current sensing switch as there's no current controlled switch part in CircuitLab -- one end of it goes to the same end of the 24VAC transformer as A1.  RLY1 is the time delay relay with A1 connected to one end of the 24VAC secondary, S connected to one end of SW1, A2 to the other end of the 24VAC secondary, 15 to the same end of the 24VAC supply as A1, and 18 connected to the coil of RLY2.  Set RLY1 to delay-off (function code D on the TE-8816U) and for the time delay of your choice.

RLY2, in turn, is the contactor -- the other coil end on RLY2 (silver terminals btw) goes to the same end of the transformer as the A2 terminal on the time delay relay, while the contact terminals (brass terminals) connect in line with both legs of the 240V circuit to the dust collector.  SW2 and SW3 represent the existing breakers, with SW2 being the breaker for the dust collector and SW3 being the breaker for the feeder to the rest-of-the-shop subpanel.

Mechanically speaking, the current sensor can live clamped around one leg of the feeder in the subpanel for the shop tools, while the transformer and the contactor can be mounted to the back of the NEMA enclosure with screws.  The time delay relay chosen goes on a DIN rail, which also screws to the back of the NEMA enclosure.  The existing cables for the dust collector can simply run into the NEMA box, while the cable connecting the current switch to the rest of the stuff can be 14-2 NM (it's a bit overkill, but hey! we don't really care if this is a Class 1 or a class 2 control circuit).

As to explaining this to the electrical inspector, it's a simple example of an industrial control panel under NEC Article 409 -- you have a power circuit (the dust collector), a control circuit (the loop running out to the current-switch), and control devices (contactor, time-delay relay, and transformer), all wrapped up in a neat box.  Article 409 of the NEC expressly allows industrial control panels to rely on the upstream branch circuit protection as ours does here and also provides for the time-honored tradition of field-fabricated control panels.

Of course, if you feel one bit uncomfortable putting this together yourself, this is something that shouldn't pose a problem to put together for any electrician with industrial controls experience.