I need to source a SSR for a class 1 div 2 installation. The SSR will be installed inside a plastic enclosure. Fun part: I need to switch 30 amps /480 VAC
I wouldn't be that pessimistic. In this voltage and power range, the SSR is mostly two thyristors/phase and the forward drop is seldom more than 1.2 - 1.5 V peak. And considerably less during the rest of the cycle.
I would say 100 - 120 W total losses at full load. OK, that's also a lot of heat. But it should be possible to get it out of a plastic enclosure. It may have to be a little larger than you want it to be. But it should be possible.
I should have added that the low voltage control to the solid state relay has to go through an IS barrier if that control signal goes to that environment.
The first question is why does the SSR need to be in the hazardous area? The first principle of hazardous area design is to remove equipment from the area if at all possible, then provide suitable protection for those things which must remain. Why can you not provide switching at the origin of the circuit? Bring control circuitry out of the zone if necessary.
Why do you need an SSR? It is not uncommon to use an electromechanical contactor housed in an Ex'D' flameproof enclosure (explosion proof to the US). These are available more readily from commercial sources.
You will probably (i.e. almost definitely) find that the cost of a standard contactor in an Ex'D' enclosure is substantially cheaper than trying to gain one-off hazardous area approval for something you build yourself. In the UK this is time--consuming and expensive. I doubt it is any different elsewhere.
ScottyUK
The customer has indicated that an Explosion Proof enclosure is unacceptable. One train of thought was to employ a hermetically sealed switch/contactor inside the plastic/terminal enclosure. Locating a cost effective 22 amp hermetically sealed switch has proven to be difficult at best.
An SSR is going to dissipate approx. 1-1/2W/Amp going through it, so if you have a 1 pole relay with 22A going through, it will reject around 33W into the box. For a steel box, each sq.in. of surface area can dissipate .054W for every 10deg. C in temp. rise. So for instance, if your ambient is 30C and the relay can take 40C, you can only rise 10C. So for 33W / .054W/sq.in., 611 sq.in. of surface area are necessary to be available for free dissipation, i.e. not up against a wall or another box. If mounted on a wall then, you have 2xHxD + 2xxWD + HxW. So to start with, use 16H x 16W x 6D, so
2HD = 2x96 = 192
2WD = 2x96 = 192 HW = 256
= 640sq.in., roughly what you need in steel.
Plastic (assuming PVC) has roughly 60% of the heat transfer coefficient of steel, so the box exposed surface area needs to be 60% larger in that material:
640/.6=1066 sq.in. and if you go backwards through the steps from there you will end up with a plastic box that is 24x20x6 for that little 30A SSR.
After all that, you still need to find an SSR that has a Cl.1 Div.2 rating. I don't know of any. Notice BTW if your ambient is 40C and the relay is rated 40C, no box size will suffice! You will then also need to find an SSR rated for 50C enclosed, another difficulty.
Mercury displacement relays are available in Cl.1 Div.2 in that size range easily. They have virtually no heat dissipation and if already listed as Cl.1 Div.2 they can be put in any environmentally suitable box. RoHS may preclude you from using them now however, you would need to check into that.
You would still have to make power connections to the relay. They consider those connections have a potential to fail and cause an ignition source. That puts you back in an explosion proof box.
OperaHouse,
I think you might be thinking of Cl.1 Div.1 applications where the explosive vapors are present all of the time. Cl.1 Div.2 is only in unusual circumstances and hermetically sealed relays are widely acceptable there.
Have you asked the customer what they propose as an alternative and asked for suggested suppliers? It is all too easy for a customer to state what they don't want, and leave you with a spec which is either impossible or impractical to meet. I think you're in the latter position - technically possible, but impractical to implement.
Thank you jraef, scottyUK.
The customer is infatuated with the plastic enclosure. It is sort of their "brand". It is used in their hazardous and nonhazardous installations. In the past, for their 11.0 Amp application they have used a mechanically actuated hermetic microswtich. Now they are asking for the 30A model, no luck locating a commercially available/cost effective switch.
scottyUK your last statement should be my signature at work!
If you scroll down this web page, you'll see that the 35-60A relays can be ordered as Class 1 Div. 2 by adding an "X" to the part number. Just double check your group numbers too.
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