Just went back and looked at this again.

Sounds like what OP wants! GA.

I agree that this is probably what the OP is looking for so I gave you a GA, too.

The curmudgeon in me is bothered by the question itself. I believe any electrician capable of safely wiring an industrial exhaust fan will be able to design and assemble a simple relay toggle circuit using the secondary contacts of the contactors controlling the fans. Getting a reliable four hour timer circuit to trigger the toggle circuit is all that is required next. So I wonder what might be missed to make this a safe system.

Something like this and a couple of relays/contactors would work.

Why not use two timers powered from the same power source?

You would need a timer and an alternating relay. Depending on the fan motor size, you may need contactors for the motors. If you Google 'alternating relay', you'll find sources and wiring diagrams. A more elegant solution would be to use one of the programmable relays from Omron, Eaton, etc., but you'd need to do a little programming.

I may have mis-understood the problem. If you want one of the fans running all the time, usb's solution is good.

Why not use a flipflop timer connected to a couple of contacts! K.I.S.S!

I wonder about the overall strategy of alternation. Sooner or later, one fan will wear out. While it is then taken out for repair, the other fan has about the same number of hours on it, and thus is about ready to crap out, too.

Well put SE, with an economy of words. I like that.

An overlap should be incorporated to allow one fan to run up to speed before the second stops. A simple change over timer won't suffice.

Maybe a run up overlap will be needed to maintain a near constant flow. Let's remember that we have no idea at all what or why these mystery industrial blowers should operate in alternating four hour cycles. They could be ventilation fans for a large building, mine shaft, tunnel ventilation, green house ventilation of a marijuana crop, negative pressure bio-hazard containment control, or just a method to continuously dry figs in a production facility. Without knowing why these fans must switch out after four hours or even if this is a 24 hour seven days a week operating condition, all we can do is try to answer the question.

Once again at CR4, the only thing we know is that the OP has asked us to explain how to do a relatively simple task. We do not know why the task is needed, nor do we understand why the requested task seems baffling. I suspect that the unstated small details are actually why the task is baffling but I may just be overly generous.

Sounds like standard enviro greenie logic to me. If each fan only runs half the time then they are only using half the power!

I makes perfect sense if you don't think too hard about it.

Hi, use the timer to control the latch relay, which will alternate the running between the two fans. Hope it will help you.

This will work....

Solar Eagle is correct. What you are looking for is a; SPDT relay. Single pole double throw. Dont forget to install Disconnect switches at each fan. If ever you service them, power must be shut off at the equipment to safely work on them.

If you can understand the circuit, it is used for single domestic exhaust fan but you can connect 2 fans provided low power(with NC contact).

VR1 and C3 determines timing circuit to switch off. so the timing for 4 hours is not sure. The Reed switch(and magnet) in circuit is for European closet

Industrial to me sounds as if the fans are three phase, where all the answers so far (all be it good ones) deal with single phase fans. Can you clear this up for us, along with the technical detail of the fans to be controlled, before we can give you the correct answer.

The easiest way would be to use two swimming pool timer clocks. Just like hooking up a swimming pool filter motor and pool sweep motor to run at different times.

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