There are accepted standards for basic design irrespective of the safety level required, a risk assessment will also include the consequences of component failure ie 'is it fail safe' and what measure have been undertaken or what extra components have been put into the design to give the required level of fail safety, you also mention using the Start button to do 2 functions, first as just a start then also as a reset if an E-stop is operated, this would not be a good design, you should incorporate a seperate device be it a key, button or similar to reset the system usually by a deliberate action, your reset should not energise the circuit at the same time, this must be by the use of another device again be it a P/B or K/S etc, even colour coding of indicator lamps, buttons etc have guidelines, if the red stop is on the same station as an E-stop then a red button would not be permitted, some of these points are not statutory but a written agreement and risk assessment with the customer must be drawn up say if the customer wanted a non standard button colour or indicator, this may be because of existing systems say on workshop machinery follow an older code, it reduces the chance of confusion if everything is the same.
Is the fact that the stops being on a N/C circuit not fail safe enough, if anything else fails then the system won't be able to be re energised as they are all N/O?
Why shouldn't the reset energise the circuit, surely a spring return key switch with reset push button, is a deliberate action as you have to keep the key turned while pushing the reset?
There will be no emergancy stop on the same station as the stop.
Given the limited info I would as a basic have all the E-stop buttons as key release only, this way the students cannot mess about with the E-stops then quickly twist and release and then deny all knowledge, another way around this is to have an indicator system to identify the operated E-Stop...all this would be discussed on-site, I have seen so many poor designs when I worked through the schools and comments about this issues they have with them such as the one I just hi-lighted should be avoided in the design stage.
I agree, if a light was installed abouve each emergacy stop this would prevent the need to walk round 8 stops to find out which one, which regularly happens at college at the moment, however we don't really get students messing about with them.
I also ask the question why you cannot change the voltage, you are allowed to run different voltages in the same containment as long as all cables are sufficiently insulated to that of the circuit and or controls of the highest voltage.
You are right, it could be done using a 2 core insulated but it would need to be small so that each could be looped in and out the 20mm conduit, might causes more issues if a light was incorporated? Is the fact that they are at 230v an issue even if they are enclosure in metal conduit and the emergancy stops are of disaster construction?
Yes I know it is not a machine but you have to consider what may be plugged in will be energised when power is restored so there is no real relaxing away from standards that cover machinery control, it is still a safety system and by the fact it is controlling power to gas valves, possible extraction fans etc it would fall within the realms of the standards required for machinery thus the system should be design to meet it as such, even if you don't actually fall under the BS60204, you will fall under other standards and given they are all derived from the same blueprint it would most likely follow the same basic design requirements, if you have any knowledge of the BS60204-1 you will see many standards are actually repeated in the BS7671 especially that of O/L protection of motors etc... you will find this overlapping of regulations very common across the many different areas of British Standards.
The gas valves will shut off a oxy acetaline line to the booths, the extraction system should not be on the same D/B as the emergancy stops, I except that consideration should be taken to what is plugged in, hopefully this has been thought of by the design consultants!
