Electrical component order

[joshual002]

I am wiring up a new enclosure and was curious if I should change it around and make it go Breaker -> SSR -> Contactor. Or Breaker -> Contactor -> SSR. All the images I see online show the SSR in the middle. Any thoughts and the electrical reason behind it would be great.

I have attached an image of the way I am thinking of laying it out.

 

[doug293cz]

I always design with Breaker--->Contactor-->SSR. The contactor is the switch that actually de-energizes things, unlike an SSR which leaks current (so doesn't remove voltage) when off. I think it's best practice to have your disconnect remove power from as much of the circuitry as possible, which means putting it as close (circuit wise) to the incoming power as possible.

Brew on

 

[joshual002]

Does it matter if I am using a craftbeer pi hat on my RaspberryPi? I was not planning on having any switches on the front. Totally automated.

 

[doug293cz]

I'd still do it the same. The relay driven by your Pi that activates a contactor performs the same function as a front panel switch that activates a contactor.

Either order will work. Do you have a reason that you think the contactor should come after the SSR?

Brew on

 

[joshual002]

Mostly space reasons. I want it safe and not too cluttered with long wire runs. Most diagrams and enclosures I see out there are for two elements and two pumps. I am adding an extra 110 and 240 outlets. My enclosure is only 20x20x4. So it is a tight spot and the outlets jut in quite a bit.

 

[Bobby_M]

You should have at least an E-stop on the front panel that kills power to the contactor coils. Relying completely on software is almost like not have a contactor at all.

 

[joshual002]

I have an E-Stop installed.

That is what makes me wonder. Should I even use the contractors at all? I have one on the main. If I have no switches am I just wasting space and wire?

 

[d3track]

I, like your plan, don't have more than a main power switch, but I do have contactors for the elements. I wanted to be able break the circuit without depending on the SSR. I felt it was a smart safety/peace of mind aspect of the build. I have mine going contactor>SSR

 

[Bobby_M]

There are three scenarios where a contactor per element output makes sense.

1. Your supply cannot tolerate more than one element powered at a time. Essentially the contactors are used for the"element select" function. In the case of front panel selector switches having a center off position it would double as an "element enable" function as well.
2. You have enough supply to run all your elements at the same time. The contactors are purely for "element enable" so that you can disable them and prevent a dry fire.
3. You don't have a main power contactor. This is pretty common. If you have a panel with 2 element outputs, each on their own contactor for element enable or selection, the only thing an upstream main contactor does interrupt power to the pumps and PID/controller electronics. You can just as well leave those devices on full time or create a low current switch function that just interrupts line power to the PIDs. That doesn't have to happen through a high current contactor.

No, I don't think you need both a mains contactor and one per element output. Pick one or the other.

 

[doug293cz]

There are three scenarios where a contactor per element output makes sense.

1. Your supply cannot tolerate more than one element powered at a time. Essentially the contactors are used for the"element select" function. In the case of front panel selector switches having a center off position it would double as an "element enable" function as well.
2. You have enough supply to run all your elements at the same time. The contactors are purely for "element enable" so that you can disable them and prevent a dry fire.
3. You don't have a main power contactor. This is pretty common. If you have a panel with 2 element outputs, each on their own contactor for element enable or selection, the only thing an upstream main contactor does interrupt power to the pumps and PID/controller electronics. You can just as well leave those devices on full time or create a low current switch function that just interrupts line power to the PIDs. That doesn't have to happen through a high current contactor.

No, I don't think you need both a mains contactor and one per element output. Pick one or the other.

If you want to implement a "Safe Start" interlock, then you will need a main power contactor (and some extra contact pairs in the relays controlling elements and pumps.) A safe start interlock prevents you from turning on the main power if any elements or pumps are on. If you trust your programming skills, you can implement safe start logic in your control software, but software bugs can give you some unpleasant surprises.

Brew on

 

[itsnotrequired]

when an ssr fails, it invariably fails in the 'on' position, contactor provides that means of easy isolation. without it, you could fire up your panel and have an element with a failed ssr cause problems.

 

[Bobby_M]

If you want to implement a "Safe Start" interlock, then you will need a main power contactor (and some extra contact pairs in the relays controlling elements and pumps.) A safe start interlock prevents you from turning on the main power if any elements or pumps are on. If you trust your programming skills, you can implement safe start logic in your control software, but software bugs can give you some unpleasant surprises.

Brew on

If your element outputs are hard enabled/disabled via dedicated contactors and you don't have any main power switch, there really is no concept of safe start (and that's okay). If you don't want elements to fire, disable them. The only case where I think the extra complexity of a separate main power contactor is worth it is if your panel is in a place where untrusted people can put their curious fingers on the controls and you want to have a key switch. A panel mounted E-stop switch can easily be integrated into the element contactors just by running the coil circuits through.

I think my point was that you don't necessarily need "cascading" contactors, but I'd much rather one per element and no main than a main and no element contactors.