Safety circuits safe or not??

[Paul Osman]

Hi, Recently whilst investigating a fault on some equipment at work I noticed something I was unsure of about two different safety circuits.

Machine 1. (Elevator control panel)
All the safety elements (2x Door Switches, 1 Estop button and 3x Pull cords) are connected in series to the coil of a general purpose relay (Omron MY4). Then 2x NO contacts of the MY4 are then monitored by the safety relay. Surely a fault with the MY4 would then render the safety elements useless?

Machine 2. (Elevator control panel)
All the safety elements (4x Door Switches, 1 Estop button and 3x Pull cords) are connected in series to the A1 (24vdc) power input of a pilz safety relay.

My understanding and examples of safety circuits I have looked at have the safety elements monitored. I was just wondering if anyone had seen these types of configuration and why they are done like this.

 

[Darkwood]

Depends, in machine 1 is the supply to A1 of the safety relay (which as you suggests runs through 2no N/O contacts on the MY4) broken by the action of the aux relays alone or will operating any e-stop functions cut its power. What i mean is if the contactor jams in does operating an e-stop break the power supply going through the aux' contacts or have they got a maintained independant supply.
Also would need to know the safety relay type as they operate in different ways most have maintained power with monitor channels although i suspect not the case here as it drops out on power loss, more info please... plus do you know what safety cat' the machine is?

 

[Knobhead]

As above, more infornation needed.