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I've got a Crydom CKRD2420 SSR wired in series with a ABB S201-C6 circuit breaker, supplying power to a 1kW 240VAC heater. This is one of several identical lines, which are all supplied via a ABB S201-C16 circuit breaker.

The system worked great for a couple of months, but recently there was a short, and after hours of inactivity, as 0.1% of power was starting to flow, both the C6 and the C16 circuit breakers cut the power with a flash within a few seconds. This happened 2 or 3 times as I was trying to debug the system, and then the short became an open circuit (melted the heater).

The solid state relay works fine now that the load is fixed, but how can I be sure that it hasn't been weakened? I have read that the I²t of the relay must be higher than that of the breaker, but ABB's curves don't go any lower than 4000A unless I can't read the charts properly.

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The short answer is that you cannot be sure that the thyristors have not been damaged to some degree. The consequence of complete failure is probably the SSR sticking 'on', but it could also be 'off' or intermittent.

If it appears to be operating properly and the consequences of failure are not an issue, then you can proceed with caution to use the current unit.

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  • \$\begingroup\$ Thanks for the advice. Assuming the datasheets of the manufacturers are right, is it possible to tell if the relay is supposed to have survived? \$\endgroup\$
    – user42875
    Jan 12, 2016 at 18:04
  • \$\begingroup\$ Usually the I^2t of any circuit breaker won't meet the requirement, so it depends on other factors such as the actual fault current. Other than recreating the fault I don't see how you'd determine that. \$\endgroup\$ Jan 12, 2016 at 18:27
  • \$\begingroup\$ I didn't know it wasn't a reliable figure. Thanks. \$\endgroup\$
    – user42875
    Jan 13, 2016 at 11:00
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Protecting Thrysisters against fault currents is no mean feat because prospective fault currents can be thousands of amps .We were concerned about this on an industrial light dimmer project 22 years ago .Oversizing the thrysisters and adding some inductive reactance will sort this out .If you dont add the reactance you may find the SSR that can do the peak fault unobtainium like we did then .SCR type technology has not got much better over recent decades.Your inducter will be large and it must not saturate under faults .Anything with a closed field is a nono.I prototyped an air cored coil wound on a transformer bobbin .When you now look at total cost and size its now time to consider an IGBT based system . If you turn off the IGBT in less than generally 10 microseconds it wont die .

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