PLC for Anti-Tiedown Control

Hello Thomas,

The engineer that you speak of could find him/herself in some trouble. I have to be honest though, I've used the PLC for anti-tie down once in my early days of PLC (when the price of safety relays meant no profit!). But let's keep that between you and I.

It's not safe though...at all. The main reason is that you are probably going from a transistor or relay output/input on the PLC to another mechanical relay. The PLC will not be able to detect a welded contact. When done through the PLC it's usually based on timing. I would say to test your system you could use it, but to install into a plant, NO. Look at this PDF for Safety Controls.

Here are some of the types of safety regulations from the ISA:

Is standard A, B, or C valid?

•Type A is basic safety. EN292-1, Safety of machines, is the current standard.

•Type B is covered by group safety standards, both general safety
(B1)—EN954-1, Safety relevant parts of controls, and EN60204-1, Electrical systems on machines—and special safety
(B2)—EN418, Emergency stop fixtures and two-hand control.

•Type C refers to the technical standards and encompasses special safety considerations by machine or industry, including these:

•EN81-1, Elevators
•EN201, Injection molding machinery
•EN691, Woodworking machinery
•EN692, Mechanical press machinery
•EN1010, Printing and publishing machinery
•EN1921, Automated assembly machinery

Here is a site where the safety controls is done through mechanical relays. I'm not sure if it fits the definition of a safety control system, but it seems logical however I would have to really study all the consequences for each case of a welded contact.

Better safe than sorry, so quote the cost of a Safety Relay into the project.

Safety Relays using mechanical relays.

Greenja.