What is it called when you have a primary circuit running a set voltage and when voltage goes over the maximum it is drained into a secondary circuit?

So like I have a theoretical circuit I want to maintain at 13.5 V any time a transient spike other over voltage condition occurs that exceeds the 13.5V it passes through some 'component' into a different circuit. But the primary maintains its regular voltage. What is that 'component' called?

It's hard to research a thing when you don't know what it's called, LOL

 Thanks

Sounds sort of like what a ground-shunt regulator does, except that rather than shunting the excess voltage to ground, you put it to use first.

If that's the case, then perhaps the easiest way of achieving that is to use a ground-shunt regulator off a small scooter, and route the line that usually goes to ground to your other circuit. Since the ground-shunt regulator senses what "ground" is off the ground line, routing the ground line to your other circuit will cause a floating voltage (it'll regulate the voltage on your primary circuit to 13.5 volts above the voltage of your other circuit).

If you want a steady 13.5 volts on your primary circuit regardless of the voltage on your other circuit, you'd have to separate the sensing line from the ground-shunt line, and ground the sensing line. On my scooter, that's impossible to do, since it's all potted. So you might have to build your own ground-shunt regulator... the circuit's pretty simple, though.

Well, zener diodes have a set voltage which when exceeded starts conducting the "over voltage current" to, say a tank circuit (a capacitor basically).
Then there's varistors which does the same job, only a varistor draws a little "stand by" current, something which you perhaps wouldn't want if you're looking into "harvesting" excess energy.
Other than that you could build a voltage comparator which when it's pre set voltage is reached a transistor opens which does the job of draining the excess voltage to whatever circuit you're dumping it onto.

There, a few ideas right off the bat.

So what I am seeing is that most likely I want to use something like a 13v 50w zener on the gate of a power mosfet which dumps the voltage and then just hope large spikes don't fry the zener eh? Maybe run a couple in parallel with some resistors???

Well if you want to go the "transistor-dumping" route, a voltage comparator would be the preferred way, atleast that's how I would build it.
You could use a simple voltage divider (I.E 2 equally sized resistors in series) and then connect the "divided" voltage to the non-inverting input of an OP amp and to the inverting input you connect a voltage reference consisting of 1/2 the desired voltage to be maintained, which in this case would be something like 6,9V then and let the OP output power the gate of the mosfet, which then would drain the "excess" voltage to the "dump" circuit the way you described it.

That way the mosfet would be what would take all the heat with regards to voltage spikes, etc.

If I find the time I could draw you a simple schematic of the circuit I'm thinking of, just to show you what I mean.

 Lynx , I would be interested to see the schematic. However, I have the words "Large voltage drop across the voltage divider" In my stack of unsorted information. My brain is as messy as my work area but I keep little snippets of information like that unsorted in my memory.
 I may have misinterpreted it, but is the voltage divider method particularly lossy? I mean seriously I don't know? I do realize the zeners are going to be losing energy as heat if I go that route but as always I am interested conserving as much of the energy as possible and minimizing losses. I was able to charge a battery with the system I am using in it's crude form until a large pulse destroyed my voltage rectifier. My test on the early prototype showed large gains on the meters but under actual load it got dragged down to minimal if any gain.
 So once again I am open to all options and I need to learn as many methods as possible to add to my knowledge base. Mostly i am just voicing my questions and concerns. I'm always looking to add other peoples understanding to my available resources. 

The voltage divider is not supposed to be lossy at all, in fact 2, say 1 meg or better yet 10 megaohm, resistors in series won't make for much losses at all, their sole purpose is to provide a voltage reference for a comparator, that's all. Same for the fixed voltage reference, a likewise resistor in series with a 6.8V zener diode reverse biased, where the cathode serves as a voltage reference, would do just fine, again it's not lossy and it's not supposed to be either as it's only for the sake of acting as a voltage reference to the comparator in question. Also, if you're looking into powering this, the "excess voltage draining" circuit, using the same source which you're looking into actually draining, in that case a simple voltage doubler could be used  to supply power to the actual draining circuit. I'll see if I can find and fire up Express schematics and tinker together some diagram, it's always easier to discuss details when looking at the schematics at hand.

In the good old days it was called a threshold detector or coherer and it's action was not unlike a clipper circuit.
http://www.electronics-tutorials.ws/diode/diode-clipping-circuits.html

As well we can have the perspective that electrical forces are some lifeless pseudo mechanical automation taught in textbooks or we can take it to the next level and see it as a sort of universal music as many great minds have in the past. As such most of the circuits and explanations you are looking for can be found at music electrical websites such as this ... http://www.generalguitargadgets.com/how-to-build-it/technical-help/articles/design-distortion/. The clipping section is near the bottom. Another very interesting concept is to attach preamp probes to the circuit or an external tickler coil and head phones then simply "listen" to your circuit as many have in the past... what will you hear?, how many thought of listening to their circuit?. Better instruments yield better results.

I may be an Engineer however I prefer to think like an artist at the bench because we cannot solve our problems using the same kind of thinking which created them and a little creativity is required. Think of nature as your canvas, components your brush and fields and flow your paint then create your own masterpiece unlike any other. Perspective is everything.

Alright, a quick and dirty schematic.
As for a voltage doubler circuit I asked my friend Google and got a few simple schematics, they should work (haven't tested them myself).
Hope you get the general idea Ragnor.