Regarding the issue of the handover and interfaces between automatic control signals and electrical signals in the SIS system, how should relays be selected?
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As titled: 1. Automatic start signal: The automation team provides the normally open contact – that is, the contact which closes when the conditions are met – to the electrical team; it is a passive dry contact with a closing time of 2 seconds. Interlock shutdown signal: The automation team provides the normally open contacts, which close when the conditions are met, to the electrical team; these are passive dry contacts with a closing time of 5 seconds. The above is accomplished through an intermediate relay. Why is the pulse signal 2 seconds for startup and 5 seconds for shutdown? 2. If it is the handover method of 1: then after the interlock stops the pump, and 5 seconds have passed with the pulse signal no longer present, can the on-site control panel still start the pump? Can it be reset without using the SIS at this time? 3. I remember that in the past, if a pump was stopped due to interlock mechanisms, it was necessary to reset the SIS system; otherwise, the pump could not be started on-site. In such cases, the relay should remain continuously closed and powered, so what damage could this cause to the relay? That is, why is the intermediate relay a pulse signal rather than a continuous power signal? Manager Discussion Group
Handover of control signals and electrical signals
It seems that the way in which this automatic control signal is connected to the electrical system needs to be discussed together, or conditions need to be set, and then the people in charge of automatic control will handle it
That above is the result of their discussions. I want to know why they did this; there must be a reason and a need for making such changes, right?
1. For SIS, it follows the fail-safe principle: regardless of whether interlock is activated or deactivated, a 0 is outputted; therefore, normally open contacts must be connected； I have worked on projects before; for the start signal of power supply, the DCS sends pulses, and to stop it, the DCS sends a hold signal ； As for whether to keep the pulse or not, this is entirely determined based on the discussions between the two teams; it simply comes down to whether it will be implemented in MCC or in DCS. 2. If the pulse signal disappears after the interlock stops the pump (indicating that the cause of the interlock has returned to normal), it is possible to first reset it on the SIS before starting the pump manually on site. 3. If it is a pulse signal, the relay certainly will not remain closed all the time ； But in some cases, continuous closing of the relay is inevitable; as for the potential damage to the relay, it would be necessary to consult the specifications provided by the relay manufacturer. I believe that with today’s design of relays, this aspect has already been taken into account.
1. Automatic start signal: The automation team provides the normally open contact, which closes when the conditions are met, to the electrical team; it is a passive dry contact with a closing time of 2 seconds. Interlock shutdown signal: The automation team provides the normally open contacts, which close when the conditions are met, to the electrical team; these are passive dry contacts with a closing time of 5 seconds. The above is accomplished through an intermediate relay. Why is the pulse signal 2 seconds for startup and 5 seconds for shutdown? ----It seems there is no fixed rule for this. In one of our projects, it was specified that the start and stop signals sent to the electrical system should be 5-second pulses, with the electrical system handling the self-maintenance function. 2. If a different switching method is used, then after the interlock stops the pump, and after 5 seconds when the pulse signal disappears, can the on-site control panel still be used to start the pump? Can it be reset without using the SIS at this time? ---If there is a reset in the logic, it must be reset; otherwise the start signal is useless, and the device will not start even if the operation button is pressed. Moreover, the control lever that allows the pump to be started directly on-site is likely an electrical control lever. When setting up circuits in electrical engineering, it should be possible to choose whether to enable or disable one or two circuits; those with interlocks should have only one circuit enabled. If the instrument is not reset, the output from the flip-flop will remain at the shutdown signal, resulting in continuous pulse-based shutdown; in this case, it will be impossible to start the device. Therefore, it is necessary to reset on the SIS; that is the correct logical approach. 3. I remember that in the past, if a pump was stopped due to interlock mechanisms, it was necessary to reset the SIS system; otherwise, the pump could not be started on-site. In such cases, the relay should remain continuously closed and powered, so what damage could this cause to the relay? That is, why is the intermediate relay a pulse signal rather than a continuous power signal? ---Your result is the same as my answer 2, and it’s also correct. It’s not a problem if the relay remains powered on all the time; the main concern is that continuous power supply will accelerate the aging of the relay. As for why pulse signals are used, I don’t think there’s any specific requirement for this – sometimes, in discussions with electrical engineers, step signals are what they need. I think it’s just to enable self-sustaining operation for the electrical system, that is, to have the electrical department take overall control of the electrical signals
Regarding question 3: From a fail-safe perspective, the pump-shutting down interlock signal from the SIS is the normally open contact of a relay; the best approach is to connect it in series with the power supply line of the motor control circuit. The working principle is as follows – when the SIS does not issue an interlock command, the output is 1, the relay becomes energized, the normally open contact closes, and the pump can operate； When SIS interlocks, the output is 0, the relay loses power, the normally open contact opens, and the pump stops operating. Therefore, when the pump is operating normally, this relay remains powered.