I am currently redoing the controls on an extruder and trying to decide the best way to pull in the 208Vac heating elements. It is currently wired with one side always with 110V on it while the other side is pulled in through a SSR to make the full 208V to energize the elements. Is this common practice or should you in fact pull both sides in when you want the elements on.

I have found some dual output SSRs but I have only seen them in panel mount versions. I would much rather have din rail mountable SSRs. If anyone can shed some light on this for me I would greatly appreciate it.

Thank you

You can turn only one side off, but it's not recomended. I've had instances where the other side of a heating element shorted to ground and the heating element would heat continuously because it was not being controlled with both phases. Also, both sides need to be fused. Not just the side you are controlling. I would recommend getting a good 3-phase SSR and only using two of the phases. Continental makes a really good 3-phase solid state relay that I've used on several machines for heating both 3-phase and 2-phase heating plates.

http://www.eurothermonline.com/product/RVD3-6V75TH

Just use L1 and L3 for two phase heaters and you will be good. And they are dinrail mountable. I've had whole heating plates short to ground and not take out these SSR's, they hold up very well.

Even if you turn both side OFF, you need to turn all of them OFF "Upstream" (i dont know how to say it in english) with a magnetic contactor.

I don't think that's true. You can cut the power to the SSR with a contactor if you wanted to but as long as the SSR was fused or you put a circuit breaker (like an ABB or something) in line with it, it should be ok. I do put contactors inline just for a second line of defense if I had to cut the power in case of a runaway heating plate but I don't think you HAVE to do that unless there was a safety issue. If you are working on the machine then there needs to be a disconnect that would cut all power to the machine.

Thank you guys for the info. I thought that both sides should be "off" while the heater was not being used. I just wanted to make sure of it.

I recommend an upstream contactor, with the upstream contactor hardwired in with the over temperature protection device (required for FM approval). A shunt-trip breaker can be used instead, but that usually costs a lot more. SSRs nearly always fail in the ON state. That means runaway heating when one does fail. The upstream contactor is usually switched on/off when the SSR is off so that it is not switching load but it still needs to be rated to switch the full load current. With the upstream contactor you can switch just one leg to control the flow of heating current.

Yes, one of the reason is safety. One other is SSR failure in ON state as said by Alaric.
You dont need 1 contactor for each SSR. Only a "main" contactor, before the distribution and fuses to each zones.
I often saw machines that needed power for maintenance (moving press platen for mold change) and SSR are not a safe way to cut power to elements.