SSR relay fails closed

[Davebones]

One of the technicians showed me a ( Continental SSR RVDA/6V40 ) that failed in the closed position . This relay feeds a hydrogen oven so we have a concern with it failing closed . I see some new SSR that have have integrated thermal protection in them . Not sure this is the answer to our concern of the SSR failing closed . Wonder if we should consider Electro-mechanical relay instead ...

 

[grich]

Some 30-years-ago, I got to install the first production model of a 10kW FM radio transmitter. The HV (about 7kV) was controlled by a combination of SSR's (one for each phase) and a contactor (used to bypass the step-start resistors). One of the SSR's failed closed (shorted) during a lightning event. The transmitter overloaded, which dropped the SSR's and step-start contactor. Current continued to flow through the shorted SSR and the step-start resistor, which was only supposed to have current flowing through it for maybe 300 milliseconds, turned white-hot and set fire to the main wiring harness before it burned open.

The first of several factory field-modifications made was to replace the three SSR's with a contactor. A heat shield around the step-start resistors was another.

 

[kwired]

One of the technicians showed me a ( Continental SSR RVDA/6V40 ) that failed in the closed position . This relay feeds a hydrogen oven so we have a concern with it failing closed . I see some new SSR that have have integrated thermal protection in them . Not sure this is the answer to our concern of the SSR failing closed . Wonder if we should consider Electro-mechanical relay instead ...

Failing closed would likely be that it is shorted for some reason, too much current, poor heat dissipation, a little of both...

Mechanical relays can weld themselves closed as well, if anything you need other safety monitoring and/or additional shut down methods in case normal control fails if the failure to shut down hazard is high risk. An additional safety or master relay, even shunt trip breaker on the supply.

 

[Jraef]

SSRs have only three failure modes; shorting, melting or exploding. If it's exploded or melted, you can usually tell pretty quick.

That was a joke really. Failing shorted is what they do, which is one of the drawbacks of using an SSR without some sort of isolation device.

 

[petersonra]

One of the technicians showed me a ( Continental SSR RVDA/6V40 ) that failed in the closed position . This relay feeds a hydrogen oven so we have a concern with it failing closed . I see some new SSR that have have integrated thermal protection in them . Not sure this is the answer to our concern of the SSR failing closed . Wonder if we should consider Electro-mechanical relay instead ...

The thermal protection only covers overloads and can't tell if the thing is shorted or not.

You might be able to put some kind of a circuit in to detect whether the solid state relay is on or off and compare that to the desired state and if it's not in the desired state trip the main for the equipment.

There are all kinds of ways to readily fix this problem but it really depends on the design of what you have controls wise.

 

[BillyMac59]

My experience has been that SSRs short when they fail. I've had luck with monitoring the current through the relay and compare with the relay's "on' signal. If signal to relay = 0v, disable current thru relay and send message to HMI, turn on a pilot light etc. As I've written this, i cannot recall seeing an SSR fail "open".