Sharing power between two systems

One of my customers made a request I've never heard before and I'm not sure if I should agree to do it. They say that at their facility, there is not enough power (electrical utilities) for their existing machinery and the new system that I'm working on. They want to give my PLC an input that means "it's safe to run at full load amps". (Meaning, their other machinery is not running). When my PLC does not see this input, my system can not run all its equipment. This way they can "share" power between two systems (their existing system and my new system.)

I've never heard of anything like this before. I guess my concern is, what happens if there is something wrong with their signal/my PLC input, or the way I have it programmed, and my system runs when it's not "supposed" to, and the customer's transformer gets overloaded? It seems risky to depend on just this one PLC input to determine whether my system should run or not.

Any advice is appreciated! Thanks in advance!

I have interlocks like this between 2 air compressors where both of their fused disconnects
are wired to the same buss bar fused disconnect. my switches are on the individual disconnects for each compressor so only one can be energized. Worst case scenario is the fuses in Buss Disconnect would blow if both compressors were running together.

Seems odd but I don't see anything too dangerous about it. No different than any other permissive IMO. If the transformer is fused correctly, there is no hazard present if they both come on, just an inconvenience.

No issues there. Swap it around though so that your machinery has preference not the older plant

It’s done all the time, often called “load shedding”. The trickier part of this is not the PLC permissive interlock, it’s the sensing of when to make that decision. You/they will want a Load Monitor Relay looking at kW doing that function, not just a simple current monitor, because current alone can be deceptive when it comes to actual load. kW always represents the actual load.

The main "gotcha" I see is that your system (you) is the only one accountable in this scenario. The other system could be started while yours is running at full load and it will be your fault.

Thanks guys!

This is extremely common where you generate your own power (i.e. ships or offshore drilling platforms). In my experience you would get the available power as a value (like 2000kW) and use that to determine what device gets how much power based on priorities. Obviously, it helps when the VFD has a power limit capability (basically tell it not to exceed X) and that's how it is usually done.

As Shawn_75 referred, power management is a system in its own right, so I would want to know more about how said signal will be determined and what happens when your system is running.