Two hand start example

Hi all, I am looking to program my first two hand start sequence. I will be using the N/C contact on the safety relay for the PLC input to identify that the two hand start is operated. Has anyone got any ladder examples which uses this operations for a sequence please?

Here is a link I had stashed away.

http://forums.mrplc.com/index.php?showtopic=5427&hl=two+hand

You can also find alot of information on this forum on the subject by searching for 2 hand anti tie down or ATD.

BCS

You are not allowed to start with the safety switch, you must use two separate NO contacts.

shane_bac,

Since I don't know your application, I will state that in your area, the EU has certain rules and device requirements.
Please look them up and see what if anything has to change.
I used to work for a UK company but everything was made for the states.

regards,
james

James gives good advice.

If you're programming a two-handed start, I'd suggest that it's probably because a hazard exists, which you are trying to manage/mitigate by means of a two hand start, correct?

If so, this is a safety function. As a general rule (with very, very few exceptions), you CAN NOT have a PLC involved in safety functions. You need dedicated safety controllers, whether simple two-hand relays or a full-blown safety PLC. And it needs to be hardwired.

If it were my a$$ on the line, I'd be backing right up and starting again from the risk assessment.

My 2c.

Hi Typically I would use the N/C of the certified 2 hand relay as suggested to notify the PLC that the 2 buttons are pressed. Then use a N/O contact of the safety relay between the plc and the device.

That way the PLC is doing the control but the certified relay is taking care of the safety aspect

Edit... You need to check what you do satisfies your risk assesment

Try searching for the term "Poka-Yoke" control. Maybe you can find a few examples on the net?

I programmed an anti-tiedown 20 years ago. I was unaware that there even was a special unit for this at that time.
It had 2 buttons and a footswitch. The operator has to press the footswitch and then the buttons within 1 second with 2/10ths of a second between both buttons being pressed.
All was interlocked and timed for the exact sequence.
It might not be to the strict safety standards of today but is still working perfectly.
I wracked my brains for a long time and could not think of any possible way to defeat it.

A quick update......i have selected the correct safety relays with the help of Phoenix contact. One for the emergency stop with the two hand start safety relay hard wired to this. The two hand start will isolate all PLC outputs, excluding LED's on the panel, via duel reduntancy contactors using the 2 N/O contacts. The N/C contact will be used for the PLC status. This will have automatic reset where as the emergency stop relay will require a manual reset via a push button on the panel. Any critism or advice would be appriciated please.