Safety relay - diffrence between power on and emergency stop

James is correct, warning signs & risk assessment are essential,
Here in the UK the supply of machinery (Safety) Regulations state:

It must be possible to start machinery only by voluntary actuation of a control device provided for the purpose.

The same requirement applies:

when restarting the machinery after a stoppage, whatever the cause,
when effecting a significant change in the operating conditions.
However, the restarting of the machinery or a change in operating conditions may be effected by voluntary actuation of a device other than the control device provided for the purpose, on condition that this does not lead to a hazardous situation.

For machinery functioning in automatic mode, the starting of the machinery, restarting after a stoppage, or a change in operating conditions may be possible without intervention, provided this does not lead to a hazardous situation.

Where machinery has several starting control devices and the operators can therefore put each other in danger, additional devices must be fitted to rule out such risks. If safety requires that starting and/or stopping must be performed in a specific sequence, there must be devices which ensure that these operations are performed in the correct order.

Thanks for all the input!

It is a unmanned water treatment machine and it takes a long time for technicians to reach the site, meaning loss of water at the output side of the plant after power failure.

It's my customer who wan't to know if it is at all possible. If doable they will be doing riskassesment and so on.

I like the idea about a pulse relay at start but I'm not comfortable with having a normal time realy involved in the safety circuit. The PLC idea is something more to work on, but I have never used a safety PLC and I think that this would be a requirement.

We have also thought of using a UPS for the safety circuit, this could be enough

/Tim

rQx said:

The PLC idea is something more to work on, but I have never used a safety PLC and I think that this would be a requirement.

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Compared to a lot of PLC skills it's not too difficult. I'm an amateur compared to most of the people here and I got one up to spec after watching a few Youtube videos.

Puddle said:

Compared to a lot of PLC skills it's not too difficult. I'm an amateur compared to most of the people here and I got one up to spec after watching a few Youtube videos.

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I'm sure I can handle it but to do it "correct" and how I can use it in a safety manner and do this thing, I'm not sure it's so straight forward. But maybe it is

This can be easily achieved with a timed relay contact on power up in parallel with the pushbutton reset, however, depending on the application this could be dangerous and might land yourself in some trouble if things go wrong.

It's industry standard for motors 0.37kw and greater not to automically restart after power failure (given some exceptions), would motors now be able to start if the safety relay automatically reset?

Why do you need to do this, what's the harm in having someone just pressing reset button?

After just reading what your application is I'd use a UPS on the safety relay supply or remote reset providing there's no risk of starting up unmanned, if there is a risk of starting up unmanned then there is no solution to this except for controlling them risks also as part of the project

mass89 said:

After just reading what your application is I'd use a UPS on the safety relay supply or remote reset providing there's no risk of starting up unmanned, if there is a risk of starting up unmanned then there is no solution to this except for controlling them risks also as part of the project

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We have also the UPS solution in mind and it lean towards that it seems , since there is no easy way of doing this "correct" if there even is one