Solid State Relays vs Electromechanical Relays 5

[whistlemeup]

I am not a controls engineer, but have been around electronics for several decades. The cost of Solid State Relays is three times that of Electromechanical Relays which have been around for ever. For this application, both must be Class I Div. II capable. My question is, why should I select solid state over electromechanical?

 

[jraef]

They never wear out, they switch much faster, they are generally smaller in small sizes (need heat sinks as size goes up) and they take less actuating power. There are significant down sides as well so there is no universally simple answer, all issues must be considered.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[benta]

The main downside to SS relays is that they don't tolerate abuse (overload) as well as the old-fashioned ones.

Benta.

 

[Skogsgurra]

If you are going to switch very often - like a few times every second - then solid state relays usually have an advantage.
If you are switching at a lower pace like once or twice an hour or so, and perhaps quite low loads (a few hundred milliamps) then electromechanical relays usually have a life that exceeds life of the application (thinking ten - twenty years or so).

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

Mech relays tend to have lives in two groups. 100k operations for larger/cheaper versions and 1million cycles for smaller/pricier versions.

Always do the math for your application. That's how many times a unit-of-time you expect your relay to cycle and how many unit-of-times you want the relay to last.

I see this cluelessly ignored frequently. You can rack up a million cycles with once a minute cycles in less than two years. Cycle the relay faster than that and ...

On the other hand SSRs can last even shorter times if they don't get their considerable thermal issues handled well.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DRWeig]

All great comments above. Since you mentioned Class I Div 2, the solid state relay can generally be employed in that area without special protection. The condition for doing this is that its surface temperature is well-managed by appropriate heat sinking to be certainly and substantially below the autoignition temperature of the gas in question.

A mechanical relay would have to be in an enclosure ($$) rated for Class I Div 1. It arcs and sparks in normal operation.

Of course, the best thing to do is mount the relay away from the hazardous area. Can you not stick it in the safe area and run conductors through a conduit seal into the hazardous area?



Best to you,

Goober Dave

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[Compositepro]

An SSR would also have to be in an enclosure in a hazardous environment as it is not rated for that use.

 

[DRWeig]

Composite,

That is true in a Division 1 hazardous location (explosionproof enclosure required), but the OP wrote Div 2. Items that do not present an ignition source in normal operation may be installed as if in ordinary locations in Div 2. A mechanical relay would need a Div 1 enclosure even in Div 2, but the SSR should need no more than a NEMA 1 enclosure.

Best to you,

Goober Dave

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