OT magnetic safety sensors e-stop

Hi there,


I have a machine with approx. 30 covers that can be removed without tools. So they have to be protected by sensors to stop the machine in e-stop.

My mechanical guy doesn't want mechanical door switches. He prefers contactless (magnetic) safety switches. So I found several, from Schmersal or Pilz etc.

The question is. If I have mechanical door switches I can put a multiple of switches in serie on one pilz safety relais. Can this also be done with the contactless (magnetic) safety switches? Is there a limit because they have a higher resistance? Or do I need a safety relais for every contactless (magnetic) safety sensor?

I'm not sure on the answer to your question.

What I would say though, if your wiring in series, you will want to consider as well how you would identify which switch is open. Could be annoying if a switch failed and you had to search each switch one at a time.

PeterW said:

I'm not sure on the answer to your question.

What I would say though, if your wiring in series, you will want to consider as well how you would identify which switch is open. Could be annoying if a switch failed and you had to search each switch one at a time.

Click to expand...

The switches have a N.O. and a N.C. contact. so one contact would be in the e-stop loop the other contact will be wired on the plc. (So for e-stop the contacts are in serie, for plc every switch is wired seperate on an input, for scade/hmi purposes)

Are the covers being locked in position whilst the (assumed) rotating parts come to rest or have you got a very very quick e-stop time ?

It is direct switch of, so no delay times. When people remove a cover the parts will be standing still well before they can have theire hands in the machine.

So I learned from Pilz that there is a difference between mechanical switches and magnetic switches. Main reason is there is electronics in the magnetic switches. That's why Pilz has a special relay where 4 magnetic switches can be connected seperately. If you want more in series you can couple the relays. So it is not allowed to put these swithces in series. Mechanical switches (without electronics) are allowed in series.

Werner said:

My mechanical guy doesn't want mechanical door switches. He prefers contactless (magnetic) safety switches.

Click to expand...

Sometimes you just need to know which song to sing...

http://www.lyricsdomain.com/18/rolling_stones/you_cant_always_get_what_you_want.html

If you want go cheap, look into security system magnets and reed switches. I'm shure that you have seen them on entry doors & door frames. they are NO with out the magnet. Wire them in series with your E-stop ckt.

If you want go cheap, look into security system magnets and reed switches

Click to expand...

NEVER EVER. If this is an e/stop circuit you have to use the correct equipment. I don't know about over there but here we can be held personally responsible (legally) for failing to do the correct thing.
You should do a risk analysis, etc, work out the required safety category and then act. You will probably find you need to have a dual interlock system (cat 3 or cat 4) so those spare NC interlocks wont be spare.
Sorry if I sound a bit over the top.

I have used the reed switches for doors here and they meet specs. You can get them out of McMAster Car catalog. You can get them with NO and NC contacts.


But if "the big red screaming I am a safety switch" is the way you want to go. STI has what you are looking for. I have put 8 in series with no issues.

Here is their web page.
http://www.stimachineservices.com/
If you go the STI route I would break it down into several relays so trouble shooting can be set to an area. Their relays have a seperate output for triggering an PLC input and the other is set for a MCR.

Cannot believe people are suggesting "Door alarm type" switches in a safety circuit, is is dammed right dangerous, all safety circuits must use components specificaly designed for the job, the first thing is to assess the catagory of the risk i.e. 1 to 4, selsct components & design that are designed for that catagory.

Most reed type safety switches are magnetic reed & are just like contacts so can be used in series, however I suggest anything above catagory 2 you use dual circuit with a safety relay that monitors any motor contactors that in the event of contact weld could cause injury, also a main supply contactor that is also monitored

in the event of a contactor failing short (welded contacts) at least the main one would break the circuit, or if main failed others would remain off, the "pilz" will only energise if the monitored contacts of the contactors (N/C) are closed (all de-energised in the event of e-stop)
Dependant on country but any modification to a safety circuit should meet current legislation i.e. if the machine was built 30 years ago, to make modifications it would probably need to be re-assessed to put it in the correct catagory, failure to do so may result in prosecution if someone was injured or killed.
Any modification to machine safety should be done by a competent person who understands machine safety circuits.