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I could use some advice please.

I am looking to use a set/trip relay as part of a safety critical circuit. Both set and trip supplies are interlocked via other contacts but I am in the process of doing a FMEA, and was looking at what effect it would have on the relay if both coils were energised at the same time under a fault condition.

The relay is an Arteche BF-4BB latching relay with coil over voltage protection.

I believe the relay would not chatter and the circuit with the most impedance will eventually win. The supplies to the coils are both driven by timer relay contacts and are asymmetrically off-set. (that is, the set coil is driven from a 2 second constant supply and the trip coil from a 4 second constant supply.)

enter image description here

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  • \$\begingroup\$ What does the relay data sheet tell you? \$\endgroup\$
    – Andy aka
    May 1 '20 at 9:29
  • \$\begingroup\$ Data sheet link please. || You cannot guarantee any result if you present conditions not defined by the data sheet. Your condition may or may not be covered. I would EXPECT that if both conditions were applied that the relay would "most likely" remain in its last state while both remained applied - BUT this is uncertain. | It is more certain but not certainly certain that if one remained applied for longer than the other that it would produce the final result BUT it is possible that the dual application MIGHT put the device into an undefined state that locked it. \$\endgroup\$
    – Russell McMahon
    May 1 '20 at 12:03
  • \$\begingroup\$ Safest approach is to build circuit in a fail safe manner such that dual applicatiion is not possible. This is harder than not doing it but hardly hard. \$\endgroup\$
    – Russell McMahon
    May 1 '20 at 12:04
  • \$\begingroup\$ @Andyaka Nothing :-). Page 12. \$\endgroup\$
    – Russell McMahon
    May 1 '20 at 12:25
  • \$\begingroup\$ That's what I expected but, ya never know. Good answer. \$\endgroup\$
    – Andy aka
    May 1 '20 at 12:27
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The manufacturer's page gives minimal technical information and given the evident significance of the application and the intended use of the relays then a query to the manufacturer looks to be by far the most reasonable solution. I may be wrong :-).

Data sheet of sorts here - page 12, second column. No information on dual activation.

You cannot guarantee any result if you present conditions not defined by the data sheet.

EITHER your answer is in the data sheet
OR we, like you, can only speculate.

I would EXPECT that if both conditions were applied then the relay would "most likely" remain in its last state while both remained applied - BUT this is uncertain.

It is more certain but not certainly certain that if one remained applied for longer than the other by more than the maximum time to operate/release that it would produce the final result
BUT it is possible that the dual application MIGHT put the device into an undefined state that locked it.

The safest approach is to build a circuit that operates in a fail safe manner such that dual application is not possible. This is harder than not doing it but hardly hard. You can then decide what action you desire in any combination of dual coil activation attempts.

Examples only:

  • If a secondary activation attempt occurred during an initial one the secondary activation should be ignored.
    or
  • If a Reset and Set occur simultaneously then the Reset should be accepted.

Note - you may need to add eg

  • If a Reset and Set occur simultaneously and the Set persists for longer than the Reset then the Set should be ignored until it is deasserted.

There are likely to be other "maybes" which you meed to examine the implications of.


The problem i have is that i am having to do the whole circuit via relay logic rather than plc, i have a number of interlock auxiliary contacts in the circuit and the circuit is controlled by sprung load rotary switch. But I was more curious than concerned about the circuit design as it will always fail safe.

Depending on how important it is to you, you could eg lockout the set coil when reset_in is asserted, using a relay to do it.

For "extra points you could have a relay which continues to lock out access to the set coil for as long as set_in remains asserted if set_in is ever asserted when reset_in is asserted. May take 2 or 3 extra relays - only worthwhile if the result is important to you. Without such, if set and reset are asserted together you'd expect the last man standing would win.

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  • \$\begingroup\$ Thank you for your reply, i have contatced the manufacturer and am awaiting a response, the problem i have is that i am having to do the whole circuit via relay logic rather than plc, i have a number of interlock auxliary contacts in the cirucit and the circuit is controlled by sprung load rotary switch. but i was more curious than concerned about the cirucit design as it will always fail safe.. \$\endgroup\$
    – davey345
    May 1 '20 at 15:19
  • \$\begingroup\$ @davey345 Depending on how important it is to you, you could eg lockout the set coil when reset_in is asserted, using a relay to do it. || For "extra points you could have a relay which continues to lock out access to the set coil for as long as set_in remains asserted if set_in is ever asserted when reset_in is asserted. May take 2 or 3 extra relays - only worthwhile if the result is important to you. Without such, if set and reset are asserted together you'd expect the last man standing would win - which may be OK. \$\endgroup\$
    – Russell McMahon
    May 2 '20 at 0:02
  • \$\begingroup\$ Anser back from manufacturer states they have no protection for dual coil operation, they are lookking to put an inhibitor in for later designs. so i have chnaged the dsign and used two seperate relays without the latching and put a current sensor linked to low ohm MCB, to disconmnect the circuit on a fault \$\endgroup\$
    – davey345
    May 5 '20 at 13:57

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