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Stopped on E-Stops
GERRIE ROCKWELL AUTOMATION GROUP - Industrial Components - Dec 2020
By Joe Daley
When configuring an E-Stop Button for a safety-systems there are a number of items to consider. The Environment, the type of E-Stop, the physical size of the button itself. There are guidelines in the E-Stop Standards to help a machine designer determine the best E-Stop for the application. Christian Bidner goes over these standards in his blog.
To learn more about these standards, read Christian Bidner's blog on Why you should consider Illuminated E-Stop Buttons for your next project BOM...
What about the contact blocks? Typically, most E-Stops applications require a minimum of 2 Normally Closed (N.C.) Contact Blocks. Rockwell Automation has 3 contact block offerings suitable for E-Stops. Standard N.C. Contact Blocks, N.C. Late Break Contact Blocks and Self-Monitoring Contact Blocks.
Which one do I use and why?
Let’s first look at each contact Block.
Standard N.C. Contact Blocks
Most Standard N.C. Contact blocks offered by Rockwell are all “Direct Drive” which means in the event of a contact weld there is enough mechanical force within the contact block to break the weld and open the contact. Direct Drive contact blocks will have this symbol on it. So long as the standard contact is a Direct Drive contact it can be used for an E-Stop Circuit
N.C. Late Break Contact Blocks (N.C.L.B)
N.C.L.B. Contact blocks unlike a Standard N.C. Contact Block this block will only open if the E-Stop is fully depressed. These blocks can help with inadvertent activation of the E-Stop. Even brushing up against and E-Stop it may exert enough force to open a Standard N.C. Contact Block. Thus, creating an E-Stop condition you didn’t want. An N.C.L.B Block will prevent this as it is a “Late Break” The Button must be fully depressed in order to open the contact.
Self-Monitoring Contact Blocks
Self-Monitoring Contact Blocks are unique. They have a Normally Open (N.O.) Contact internally wired in series with a N.C.L.B contact. Once installed and mounted to the E-Stop button. Enough fore is applied to the Contact Block to close the N.O. Contact but not enough to open the N.C.L.B.
How is it Self-Monitoring? Well, essentially it is a fail-safe device. In the event of a button failure such as physical damage to the E-Stop Button or should the contact block become separated from the button the N.O. contact will open thus creating an E-Stop situation. This prevents a machine from operating with a Damaged E-Stop Button.
Self-Monitoring Contact Blocks do have some limitations and should be considered when designing an E-Stop. In an 800T 30mm NEMA Buttons only two Self-Monitoring contact blacks can be used per button and must be on the first level and cannot be stacked together. In an Illuminated E-Stop you are limited to using only one Self-Monitoring Contact Block. You can stack other Standard Contact blocks in the second level. In an 800F 22mm IEC Button the Self-Monitoring Contact Block is not Stackable, you can mount up to three Self-Monitoring Contact blocks to an 800F E-Stop. Also, the physical size of the 800F Self-Monitoring Block is the same as two standard blocks stacked together you must ensure the enclosure you select is deep enough to accommodate the block.
So which Block do I use?
Most applications you can use the Standard N.C. Contact Blocks they are very flexible and can be used in all button designs same with the N.C.L.B. Contact Blocks They are also very flexible and can be used in all button designs. Things to consider, is the E-Stop in a high Traffic Area or in a location where the probability of an accidental operation is quite high. Using a N.C.L.B may be a good choice. Special considerations must be made when using Self-Monitoring Contact Blocks. They provide the safest E-Stops, but they are not as flexible as Standard Blocks you must consider the number of contacts required for the E-Stop and if it’s illuminated.
If you have any questions on Rockwell’s Contact Block offering, please feel free to contact you local Gerrie Branch We’d be happy to help.
And remember “Every Process Starts with a Push Button”.
Let's chat again soon.
Joe
About The Author
Joe Daley is our Industrial Control Product Manager and with over 20 years in the industry he is our encyclopedia of knowledge with Industrial Component applications.
Contact Joe Daley