Do 'safe' (fail open) SSR, optos etc exist (like Y class capacitors) - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: September 30, 2020, 01:08:14 pm
content: Just wondering what circuit elements other than fuses, breakers & Y class capacitors can be relied on. It would be nice if there existed opto isolators and/or SSR guaranteed to never fail shorted at the output.

author: TheMG
author_rank: Frequent Contributor
timestamp: September 30, 2020, 01:25:36 pm
content: Not that I know of. Semiconductors by their very nature are more likely to fail short than any other failure mode.If it's critical then you need to design your circuit for fault tolerance.If you need to guard against SSR failure in your application, put a contactor/relay upstream of the SSR, so if a fault is detected you de-energize the contactor and isolate the circuit that way. In normal operation the contactor remains energized and you use the SSR for normal switching.

author: 741
author_rank: Frequent Contributor
timestamp: September 30, 2020, 01:41:36 pm
content: OK, what I guessed really.How about relays/contactors though, you still have to test for welded contacts? Is it reliable to do that test based on a 2nd set of contacts?One issue is the power to pull in and keep in a contactor. I can't scale up signal drive power because that scaling could fail, keeping the contactor on.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 30, 2020, 01:56:55 pm
content: You could have a SCR crowbar across the output along with a circuit to activate it when the SSR has had enough time to switch off.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 30, 2020, 02:12:15 pm
content: If you can rely on the source to have enough juice you can use a crowbar circuit to blow the fuse.Otherwise you could look at safety or monitored relays. Those notice a fault and the system can then act on them.Either by removing some other contactor or engaging an trip coil in the circuit breaker.

author: 741
author_rank: Frequent Contributor
timestamp: September 30, 2020, 02:24:21 pm
content: In general, anything can become disconnected/open if faulty. So the SCR part might fail.

author: 741
author_rank: Frequent Contributor
timestamp: September 30, 2020, 02:32:17 pm
content: What happens if a DC coil relay is (due to a power fault) driven at 50Hz?If the AC is sufficiently high in frequency (1kHz?), I'd guess there would be no harmful chatter due to mechanical intertia. To some degreee also coil inductance limits current, but this is not always given.For AC relays, there are shaded pole types and split winding/internal diode types. I understand that both aim to create a phase delay so the magnetic field is always present while AC power is applied. I am unsure whether shaded pole would be safer because there are no diodes, perhaps so.

author: jbb
author_rank: Super Contributor
timestamp: September 30, 2020, 05:17:55 pm
content: You can get safety rated contractors with auxiliary contacts for monitoring. Apparently such devices can be found with special mechanical designs such that if the main contacts are welded shut the auxiliaries stay closed too.As I understand it, to reach the higher echelons of safety you also need to self-test so that single faults are detected before they can turn into multiple faults.What’s your end goal with this design?

author: 2N3055
author_rank: Super Contributor
timestamp: September 30, 2020, 06:59:30 pm
content: For safety critical circuits, you double break with two relays, and use monitoring...Mechanical relays can have contacts welded, so you switch one first and other few ms later. That way only one is switched powered and will weld so other one will stay good and break surely...Also combine mechanical (static) and SSR (switching) to always have different failure mode...That is simplified version.Proper way is described in respective standards for safety critical equipment (for instance building elevator control standards-...)I highly recommendnot to playwith safety critical stuffbefore you really know what are you doing.As I said it is necessary to find standard that applies to exact situation at hand and try follow it to the letter.

author: razvan784
author_rank: Regular Contributor
timestamp: October 02, 2020, 10:17:44 am
content: Quote from: 2N3055 on September 30, 2020, 06:59:30 pmI agree.If safety is an issue, I suggest looking up components designed with safety as a primary goal, such as safety relays and safety PLCs.Pilz for example manufactures these, as do all the major automation companies.