How to determine whether the design of buttons, lights, and relays in a SIS configuration is reasonable?
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This post was last edited by qabcd on 2020-5-9 at 10:07. Basic information: Chemical industry; a contract has been signed for the SIS system. Several SIL loops are rated at level 2. The SIS supplier has provided the drawings, and the system CPU, AI/DI/DO cards, as well as the system software were all thoroughly verified before ordering (redundancy mode/certification certificates). Question for assistance: When placing the order, I didn’t pay close attention to whether the buttons on the auxiliary control panel (emergency stop and reset), the alarm lights, and the relays associated with the DO outputs were designed properly; I’m worried that there might be design flaws. 1. Buttons (emergency stop and reset), alarm lights, etc., are included in the auxiliary control panel set. The main details of the buttons as shown in the drawings are as follows: No specific model numbers are indicated in either the drawings or the configuration list. The drawings describe the emergency stop button as having a red push-button head with two positions (and a protective cover) ； Reset: Green momentary button head (with protective cover) that turns on and off ； Buzzer: Flashing buzzer ； Alarm indication: Indicator light (red, AC/DC 24V). I’ve heard of something called a “safety switch”; I’m not sure if it really exists. As an important DI input signal for the SIS system, it is used in important logical judgments. Is such a design reasonable? Or there might be a better solution. 2. In the configuration scheme, the relay is described as an ordinary relay, and it is clear that it is not a safety relay. As an important component involved in the DO logic of the SIS system, is such a design reasonable? Note: The standards that are commonly considered include IEC 61511, IEC 61508, and GB/T 50770. The other measuring instruments and final units of the system meet the requirements, so no further discussion is needed.
It is recommended to use an emergency stop button with three pairs of contacts and a three-out-of-two interlock mechanism, in order to improve reliability and prevent accidental shutdowns due to wiring faults. The reset button does not require a protective cover. Switches, relays, etc. with SIL certification – in my opinion, this is something designed to deceive Chinese people; they are just simple electrical components, and there’s no need for such things.
The relays we use here are indeed not what are known as safety relays; they are relatively inexpensive anyway. If you think they aren’t good enough, it’s sufficient to replace them every 2 years or during each maintenance session； For the buttons, we have chosen those with double contacts; both contacts are connected to the wiring, with the contacts being connected in parallel. To connect them to the SIS system, a single set of wires is used. The buttons feature normally open contacts that close when pressed.
Recent adjustments: 1. As shown in the figure below, the button has been changed from having one set of contacts to three sets of contacts (normally open + normally closed). Following a three-to-two logic, additional card components are required to meet the input requirements. 2. I took a close look at the \"Reset Button (with cover)\\" – the illustration shows it without a cover, while the textual description says it has a cover; this is probably due to the designers’* tendency to be lazy. 3. Other components such as the alarm light, buzzer, and relays associated with the DO output have not been adjusted.